Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Measurement of the Changes in Flow Regime at
Poole Harbour Entrance Following Channel
Deepening
For
Poole Harbour Commissioners
Report No. 06/09/10047/1
October 2006
Job No 131084
Compass Hydrographic Services Ltd
91 Olive Avenue
Leigh on Sea
Essex
SS9 3PX
United Kingdom
Tel: (0870) 0132390
Fax: (0870) 0132391
Report No. 06/09/10047/1/JAT
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
EXECUTIVE SUMMARY
This report describes the analysis of the flows prevailing within the entrance to Poole Harbour prior to
and following on from a major capital dredging program to deepen the approach channel to the port of
Poole.
Surveys were undertaken on both Spring and Neap Tides pre and post construction using a vessel
mounted Acoustic Doppler Current Profiler with bottom tracking correction determined using RTK
GPS positioning data.
Both Spring and Neap tide surveys were undertaken on tides of similar range however in all cases
there were however significant differences in the shapes of the tidal curves for each tide. These
distortions or ‘stands’ in the tidal curve are typical of the area and are the result of the interaction of
shallow water constituents (1st and 2nd harmonics of the M2 semidiurnal tide) in an area where the M2
tide is weak.
These stands of water level are prevalent on Neaps but are also present on Spring tides and make it
difficult to undertake a direct comparison of the data since slight differences in the shapes of the tidal
curves can result in very different tidal streams being measured since flow is largely influenced by the
local sea level gradient which is complex in this area due to the small amplitude of the M2
constituent.
In order to overcome these limitations a comparison has been undertaken of range corrected transect
points as well as the flows across the measured transect line itself.
A transect point may be thought of as a point in space which is visited as repeatedly over the course of
the survey such that a time-series of the prevailing currents can be built by averaging the data from
each pass through the point into a single value of speed and direction, each pass through the point
being referenced to a fixed point in the tidal curve, in this case local low water.
An attempt to compensate for variations in the shape of the tidal curves on the survey days chosen has
been made by interpolating the time-series from each transect point before re-sampling it onto a 10
minute time-base. The resultant time series has then been scaled in terms of the intra-tidal range
variation through comparison with a theoretical mean Spring or Neap tide using the range factors
detailed in the Admiralty Tide Tables.
Comparing the two Neap tide data sets for the transect line 1, located outside the harbour, clearly
shows the dominance of the ebb flows from the harbour with the currents on the ebb attaining around
1.5 m/s compared with flood tide peak velocities of order 1.0 m/s whilst both Spring tide data sets
show flows of order 2.0 m/s to be attained on both flood and ebb.
The Spring tide time-series suggest that water flows into the harbour between LW-4.5 and LW-2.5
with the main flood occurring between LW+0.5 and LW+5.5 with the flood tide velocity reaching it’s
maximum around 2 hours after LW. Two phases of currents leaving the harbour are also observed the
first between LW-2.5 and LW when currents are at their maximum with a second phase occurring
between LW +5.5 and LW-4.5.
Comparing these data pre and post dredge suggest that velocities during the main period of ebb flows
are slightly increased with respect to their 2004 values, order <0.05 m/s (3%) but are reduced during
the second period of weaker flows (order 0.30 m/s -15%). On the flood tide following dredging the
strength of the peak flood currents also appear reduced (order (0.2m/s - 26%) compared with the pre
dredge conditions although the earlier period of flooding currents is slightly elevated order (0.2 m/s 12%).
As at the transect point 1 Neap tide data from the second transect located just inside the entrance the
show widely different behaviours with ebb tide conditions the prevailing between LW-3 and LW
during the pre dredge surveys when flows reached their maximum of 1.2 m/s with but no obvious
flood tide maxima. The 2006 data set on the other hand shows peaks at LW+2 and LW-1
corresponding to the flood and ebb tide maxima respectively.
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
The pattern observed on Spring tide shows flows to exceed 2 m/s on both the flood and ebb currents
with the ebb currents being the stronger. Inspection of the time-series shows that this occurs as 2
phases with the flood tide peak velocity occurring 2 hours after LW with the Ebb maxima occurring
between LW-1 and LW-2.
Comparison of the data for the Spring tide show both ebb peaks and the main flood peak to have
reduced following channel deepening with only the first phase of flooding at around LW-3.5 to be
elevated with respect to it’s 2004 value.
The magnitudes of the velocity differences observed between the pre and post dredge surveys is
0.25m/s in the case of the peak ebb currents and <0.05m/s in the case of the flood corresponding to a
reduction of 14% and 3% of their 2004 values respectively.
Data for the transect line 3 located inside the main harbour between Brownsea Island and the
Sandbanks Spit to the north of the North Haven Beacon shows flows to be generally slower than the
flows within the main harbour entrance and influenced by a large eddy feature which forms in the lee
of the Sandbanks Spit. In this region relatively small changes of velocity <0.05 m/s were predicted in
response to channel deepening.
At this site the 2006 Neap field data set shows peak values of flood currents to have a maxima of
0.75m/s whilst the main ebb current on Neaps attain 0.90 m/s. On the Neap tide surveyed in 2004
however only a single maximum with a flood orientation is visible with a peak reaching 1.40 m/s.
The corresponding Spring data set again 4 exhibits peaks (2 flood 2 ebb) with the peak flood
occurring approximately 1 hour after LW with the main peak occurring around LW-2. On the 2004
Spring survey the flood and ebb peaks are similar in magnitude reaching 1.40 m/s but on the 2006
Spring show the more typical Ebb/Flood Asymmetry with the flood currents reaching 1.15 m/s and
ebb currents the 1.60m/s.
Even after scaling the data to an equivalent mean neap tide it is apparent that the two Neap tides
surveyed remain too different in character to be able to draw any meaningful conclusions. The Spring
tide data sets are however more comparable and since they provide the strongest current velocities are
the most interesting and important in terms their effects and the ability of the hydrodynamic models
used to predict the changes.
Comparison of the Spring tide data pre and post deepening of the basis of tidal diamonds computed
from range corrected time-series for a transect point on each transect line suggests that overall the
change in velocities as a result of the channel deepening has been negligible with average values of
Spring tide speed differences being <0.1 m/s (0.2 of a knot).
Subdividing this further into nominal ebb and flood values based on the time relative to LW suggests
that at all three sites the strength of flows on the rising tide may have slightly increased with respect
to their 2004 values whilst the strength of the flows on the falling tide has reduced.
On the rising tide the change is greatest outside the harbour whilst on the falling tide it is within the
harbour that the greatest change was observed.
As both the rising and falling tides include phases of flow both into and out of the harbour as a result
of stands in the tidal curves data has also been subdivided based on the actual direction of flow
(positive or negative with respect to the across transect flow component) These differences thus refer
to all phases of the flood and ebb tides not just the times of peak currents.
Presenting the data in this manner shows that following channel deepening the biggest change
observed was in the Spring flood tidal streams were at most 0.4m/s lower on the flooding tide
following dredging with the largest differences occurring within the Swash Channel.
On the ebbing tide the largest difference observed was an increase of 0.3m/s this occurring inside the
harbour on Transect Line 3.
On average however the differences between the 2006 and 2004 data sets are less than 0.5 knots with
the flood flow now being slightly lower at two of the sites (B & C) than was the case prior to dredging
whilst the ebb currents are now lower outside the harbour but inside may have been slightly increased.
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Since it is can be seen from the time-series that these differences largely represent variations of flow
behaviour during the stands of sea level the implication is that overall the differences in the magnitude
of the currents following the capital dredging program are a) small in comparison to strong currents
which prevail at the times of peak flood and ebb and b) are broadly comparable in both magnitude and
sign with the predictions made as part of the Channel Deepening Environmental Impact Assessment.
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
CONTENTS
1.0
INTRODUCTION & OBJECTIVES
2.0
METHODOLOGY
3.0
RESULTS
4.0
DISCUSSION OF RESULTS
5.0
CONCLUSIONS
6.0
REFERENCES
7.0
AUDIT INFORMATION
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
LIST OF TABLES
1.1
Tidal Levels at Poole Harbour Entrance
2.1
Details of the Survey Data Collected on each Survey
2.2
Details of the Tides Measured by Each Survey
2.3
Start and End Co-ordinates of the Transects Extracted from the ADCP Database for Each of the Four
Surveys.
3.1
Data Inventory Neap Survey 2004 Transect Line 1
3.2
Data Inventory Neap Survey 2004 Transect Line 2
3.3
Data Inventory Neap Survey 2004 Transect Line 3
3.4
Data Inventory Neap Survey 2006 Transect Line 1
3.5
Data Inventory Neap Survey 2006 Transect Line 2
3.6
Data Inventory Neap Survey 2006 Transect Line 3
3.7
Data Inventory Spring Survey 2004 Transect Line 1
3.8
Data Inventory Spring Survey 2004 Transect Line 2
3.9
Data Inventory Spring Survey 2004 Transect Line 3
3.10
Data Inventory Spring Survey 2006 Transect Line 1
3.11
Data Inventory Spring Survey 2006 Transect Line 2
3.12
Data Inventory Spring Survey 2006 Transect Line 3
3.13
Timing with Respect to HW of Peak Flood Currents on Each Transect Line
3.14
Timing with Respect to LW of Peak Flood Currents on Each Transect Line
3.15
Timing with Respect to HW of Peak Ebb Currents on Each Transect Line
3.16
Timing with Respect to LW of Peak Ebb Currents on Each Transect Line
3.17
Peak Ebb Currents Values on Each Transect Line
3.18
Peak Flood Current Values on Each Transect Line
3.19
Transect Point A Spring Tides Depth Averaged Range Corrected Velocity
3.20
Transect Point B Spring Tides Depth Averaged Range Corrected Velocity
3.21
Transect Point C Spring Tides Depth Averaged Range Corrected Velocity
3.22
Transect Point A Neap Tides Depth Averaged Range Corrected Velocity
3.23
Transect Point B Neap Tides Depth Averaged Range Corrected Velocity
3.24
Transect Point C Neap Tides Depth Averaged Range Corrected Velocity
3.25
Transect Point A – All Tides
3.26
Transect Point B – All Tides
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
LIST OF TABLES (Continued)
3.27
Transect Point C – All Tides
4.1
Speed Difference Pre-Post Channel Deepening - Spring Tides
4.2
Speed Difference Pre-Post Channel Deepening - Neap Tides
4.3
Speed Difference Pre-Post Channel Deepening Spring Tides Subdivided Based on the Direction of
Flow Across the Transect
4.4
Speed Difference Pre-Post Channel Deepening Neap Tides Subdivided Based on the Direction of Flow
Across the Transect
LIST OF FIGURES
1.1
Location Map of Poole Harbour with Study Area Shown in Red
1.2
Predicted Tides for Poole Harbour for a Neap – Spring Cycle illustrating the Highly Variable Nature of
the Tidal Elevation Curves. Times of HW are denoted by the Red Dots and LW by the Green
1.3
Predicted Changes in the Spring peak Flood and Ebb currents following channel deepening a) ebb tide
b) flood tide.
1.4
Predicted 3D Currents through the Harbour entrance, before and after channel deepening
1.5
Near Bed Residual Currents in the Vicinity of the Harbour Entrance before and After the Removal of
the Chapman’s Peak Bathymetric Feature.
2.1
Detail of Survey Area Showing the Positions of the Transects Analysed within the present study
2.2
Predicted Tidal Curves for Poole Harbour Entrance for the Duration of each Survey Period for the 4
Tides Surveyed
2.3
Transect Point A Depth Averaged Time Series Spring Tide 2006 Observed and Fitted Time Series.
2.4
Transect Point A Depth Averaged Time Series Spring Tide 2006 Showing Range Correction Applied
4.1
Neap Tide 2004 Transect Line 1: a) Peak Flood Currents Pass 13 13/04/2004 12:34 L+1.82 b) Peak
Ebb Currents Pass 6 13/04/2004 08:55 LW -1.83
4.2
Neap Tide 2006 Transect Line 1: a) Peak Flood Currents Pass 18 21/07/2006 14:26 LW+1.52 b) Peak
Ebb Currents Pass 12 21/07/2006 11:09 LW -1.76
4.3
Spring Tide 2004 Transect Line 1: a) Peak Flood Currents Pass 3 08/04/2004 07:26 LW+1.52 b) Peak
Ebb Currents Pass 22 08/04/2004 16:22 LW-1.88
4.4
Spring Tide 2006 Transect Line 1: a) Peak Flood Currents Pass 5 12/09/2006 08:28 LW+1.56 b) Peak
Ebb Currents Pass 24 12/09/2006 17:43LW -1.52
4.5
Neap Tide 2004 Transect Line 2: a) Peak Flood Currents Pass 12 13/04/2004 12:08 LW+1.39 b) Peak
Ebb Currents Pass 6 13/04/2004 09:05 LW-1.67
4.6
Neap Tide 2006 Transect Line 2: a) Peak Flood Currents Pass 19 21/07/2006 14:33 LW +1.64 b) Peak
Ebb Currents Pass 13 21/07/2006 11:18 LW-1.61
4.7
Spring Tide 2004 Transect Line 2: a) Peak Flood Currents Pass 3 08/04/2004 07:31 LW+1.61 b) Peak
Ebb Currents Pass 22 08/04/2004 16:35
LW -1.67
4.8
Spring Tide 2006 Transect Line 2: a) Peak Flood Currents Pass 5 12/09/2006 08:16 LW+1.36 b) Peak
Ebb Currents Pass 23 12/09/2006 17:24 LW-1.84
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
LIST OF FIGURES (Continued)
4.9
Neap Tide 2004 Transect Line 3: a) Peak Flood Currents Pass 12 13/04/2004 12:17 LW+1.53 b) Peak
Ebb Currents Pass 5 13/04/2004 08:43 LW -2.03
4.10
Neap Tide 2006 Transect Line 3: a) Peak Flood Currents Pass 20 21/07/2006 14:42 LW+1.79 b) Peak
Ebb Currents Pass 14 21/07/2006 12:04 LW-0.84
4.11
Spring Tide 2004 Transect Line 3: a) Peak Flood Currents Pass 3 08/04/2004 07:38 LW+1.73 b) Peak
Ebb Currents Pass 22 08/04/2004 16:45
LW-1.49
4.12
Spring Tide 2006 Transect Line 3: a) Peak Flood Currents Pass 8 12/09/2006 08:40 LW+1.76 b) Peak
Ebb Currents Pass 34 12/09/2006 18:33 LW-0.70
4.13
Observed Tidal Curves for the Spring and Neap Tide Survey Undertaken Pre & Post Dredging
Showing the Relative Timings of Passes Made on Transect Line 1
4.14
Range Corrected Depth Averaged Current Speed and Direction Time Series – Transect Point A; a)
Neap Tides b) Spring Tides
4.15
Range Corrected Depth Averaged Current Speed and Direction Time Series – Transect Point B; a)
Neap Tides b) Spring Tides
4.16
Range Corrected Depth Averaged Current Speed and Direction Time Series – Transect Point C; a)
Neap Tides b) Spring Tides
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
1.0
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
INTRODUCTION AND OBJECTIVES
This report details the results of a study commissioned by Poole Harbour Commisioners (PHC) and
undertaken by Compass Hydrographic Services Ltd (CHS) to compare the current velocities
prevailing and their variation in time and space at the entrance to Poole Harbour before and after the
recent channel deepening program.
Two periods of data collection were undertaken to enable the comparison covering both Spring and
Neap tides the first being undertaken as part of consent application procedure made prior to the
dredging works in April 2004 with a second post dredge set of observations collected in July and
September 2006.
Data from the first campaign has been previously used to calibrate and validate the numerical model
developed by HR Wallingford and used in the Environmental Assessment for the channel deepening
work, HR Wallingford (2004).
In both cases the flows were measured using a Vessel Mounted 600 kHz Acoustic Doppler Current
Profiler system mounted on the PHC workboat ‘Rough Rider’ with a sequence of survey lines
spanning the entrance run over a period of 13 hours on the Spring and Neap tide.
In it’s normal mode of operation the Vessel Mounted utilises a second bottom tracking pulse to
determine the translation of the survey vessel over the ground. However due to the high velocities
prevailing in the entrance channel it was anticipated that moving bed conditions would occur on
Spring tides at the times of maximum flood and ebb current.
Moving bed conditions arise from the mobilisation of bed sediment by the flow and are known to
cause the ADCP’s bottom tracking to be mislead resulting to an incorrect estimation of the survey
vessel’s course and speed over the ground and a resultant under-estimation of the flow speed.
In order to overcome this limitation a high accuracy Real Time Kinematic GPS/Glonas base station
was deployed at a known benchmark located on the roof of the Haven Hotel at Sandbanks to provide
high accuracy DGPS corrections to the receiver installed onboard the survey vessel thereby allowing
high accuracy GPS (<0.05m) positioning to be used in place of the conventional bottom tracking as
the method used to determine the corrections for vessel motion to be applied to each ADCP profile
recorded.
Comparison of data from the two sets of surveys in this area of highly complex and variable tidal
dynamics has proved a difficult task since the flows are subject to both tidal and non-tidal forcing.
Much of complexity in the tidal component of the flow arises as a direct result of the proximity of the
study area to a M2 degenerate amphidrome, located in the vicinity Corfe Castle, and which results in
tides at this site being heavily influenced by ‘shallow water constituents’ - harmonics of the M2 tide
which modify tidal curves to pronounced double high waters (HW) and a series of stands of sea level
over the course of tidal cycle.
These perturbations of the normal sinusoidal tidal curve heavily influence the patterns of flow into
and out of the harbour. The comparison must thus attempt to isolate variations in the magnitude of
currents as a function of the unusual tidal dynamics of the area and variations in residual flow from
the variations induced by the creation of a larger channel cross section through the dredging within
the approach channel as well as within the harbour.
Although the two surveys were undertaken on tides of similar range there are significant differences
in the shapes of the tidal curves which make it difficult to undertake a direct comparison of the data.
For this reason use has been made of pseudo time-series built from the passes made through defined
point in space and repeatedly transected over the course of each survey. The time-series extracted at
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
each point being scaled in terms of the intra-tidal range through comparison with a theoretical mean
spring or neap tide constructed from the range factors detailed in the Admiralty Tidal Atlas (NP201006 Volume 1). Full details of the methodology used is described in section 2.2 of this report
1.1
Study Area
Located in the north western corner of Poole Bay on the South Coast of the UK Poole Harbour is one
of the world's largest natural harbours, see figure 1.1.
Figure 1.1 Location map of Poole Harbour with study area shown in red
Within the harbour itself the extensive area of sheltered waters provides an important leisure amenity
for recreational sailing and water sports whilst the mudflats and salt marshes surrounding the harbour
are of great ecological value for feeding and roosting birds.
The harbour also acts as an important thoroughfare for commercial traffic with Cross Channel ferries
sharing the navigation channels with small craft and other cargo vessels proceeding to and from the
busy commercial port located in town of Poole itself.
The harbour occupies an area of some 3,600ha and comprises a main basin containing several small
islands and two subsidiary basins, Holes Bay and Lytchett Bay. It is dominated by intertidal
sedimentary flats and shallow subtidal shoals. Muddy shores are most prevalent within the relatively
wave-sheltered south-western quadrant of the main basin, and the recesses of Holes Bay and Lytchett
Bay, where the upper levels of muddy shores are typically colonised by saltmarsh.
The entrance to the harbour is through a narrow channel of 150m width flanked by relatively stable
sand spits, and wherein lies the deepest part of the harbour 18m below Chart Datum. This entrance is
naturally scoured by the strong currents which prevail in the entrance.
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
The main approach to the harbour entrance is through the Swash channel, which prior to the capital
dredging was maintained to a published depth of 6m below CD. This channel is flanked on it’s eastern
margin by a large sand bar (the Hook Sand), which is separated from the coastline by the East Looe
Channel.
The harbour is essentially a drowned river valley which was formed, along with Poole Bay itself,
when rising sea levels broke through the chalk ridge which had connected Ballards Down on the Isle
of Purbeck with the Needles on the Isle of Wight. The Harbour was formed at the end of the last Ice
Age, approximately 7,000 years ago. The mudflats and saltmarshes that characterise the present state
of the Harbour have developed since this time
Four rivers, the Frome, the Piddle (also known as the Trent), the Corfe and the Sherford, drain into
the Harbour from the west. The Frome and the Piddle Rivers flow into the Wareham Channel, the
Sherford River flows into Lytchett Bay and the Corfe River flows into Wych Lake.The total discharge
into the harbour is estimated to be order 10m3/s
The waters of the main harbour are reported to have a similar salinity to coastal waters of Poole Bay,
but further inland brackish areas are present. The salinity regime is generally less variable over the
tidal cycle than is typical the case for estuaries, Dyrynda (2003).
The tides prevailing in the Harbour have a range of approximately 1.6m on mean Spring tides and
0.5m on mean Neap tides. The highest astronomical tide is 2.6m above CD and the lowest
astronomical tide is at the level of CD. Chart datum at Poole Harbour is defined as 1.4m below
Ordnance Datum (Newlyn).
Table 1.1 taken from the Admiralty Tide Tables summarises the tidal levels at Poole Harbour entrance
Table 1.1 Tidal Levels at Poole Harbour Entrance
DATUM
CD
ODN
HAT
2.6
1.2
MHWS
2.2
0.8
MHWN
1.7
0.3
MWL
1.6
0.2
MLWN
1.2
-0.2
MLWS
0.6
-0.8
LAT
0
-1.4
As has previously been stated the tides at Poole Harbour are highly variable in shape due to the
proximity of a local amphidromic point (a nodal point where the constituent in question has zero
amplitude) of the M2 semi-diurnal tidal constituent. This semi-diurnal constituent generally is the
dominant constituent of the tide in the seas around the British Isles. The net effect of this is that as the
Amplitude of M2 is small diurnal constituents and the smaller, higher order (quarter-diurnal, sixthdiurnal) harmonic tidal constituents have a more significant effect combining with the semi-diurnal
species to produce features such as a double high water (i.e. two maxima in tidal height) during spring
tides and a very variable tidal shape on tides with smaller range.
Figure 1.2 shows an example of the predicted tide curve at Poole Harbour Entrance for part of a
Spring-Neap tidal cycle. The general shape of the Spring tides can be seen with the double high water,
although the relative height of the two high waters is shown to vary considerably. During the Neap
tide period the shape of the curve can be seen to be extremely variable as due influence of these
higher order tidal constituents.
As the tides are built of some 120 tidal constituents some with periodicities of as much as 18.6 years
it is very difficult in practice to find identical tides from month to month or even year to year thus
tides with ranges similar to those of a mean Spring and Neap tide were selected for the survey
operations.
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
1.50
1.00
Elevation (mAODN)
0.50
0.00
-0.50
-1.00
-1.50
01/10/2006
00:00
02/10/2006
00:00
03/10/2006
00:00
04/10/2006
00:00
05/10/2006
00:00
06/10/2006
00:00
07/10/2006
00:00
08/10/2006
00:00
09/10/2006
00:00
10/10/2006
00:00
Date/Time (GMT)
Figure 1.2 Predicted tides for Poole Harbour for a Neap – Spring cycle illustrating the highly variable
nature of the tidal elevation curves. Times of HW are denoted by the red dots and LW by the green
1.3
Channel Deepening and It’s Anticipated Effects
The approach channel deepening undertaken for Poole Harbour comprised the capital dredging of the
Swash Channel lying outside the main harbour with additional dredging in the Middle Ship Channel,
within a portion of Little Channel and within the Turning Basin located adjacent to the berths.
The total area dredged was approximately 227ha of which 120ha was in the Swash Channel with
some 107ha within the Harbour itself. Prior to the channel deepening program the declared depth of
the approach channel and turning Basin was 6.0m below CD, with the capital dredging designed to
achieve a sustainable declared depth of 7.5m below CD by dredging deeper. The overdredge being
0.5m below the proposed published depth inside the harbour and 1m below the proposed published
depth in the Swash Channel to allow for accretion (i.e. to control the need for maintenance dredging).
In order to maintain the channel width at the proposed depth of 7.5m below CD, and to ensure
continued safety of navigation, localised widening of the Middle Ship Channel was also carried out,
with a proposed increase in width of 20m, from its existing 80m to 100m.
Prior to the capital dredging the Swash Channel was 150m wide; with the width maintained at the
increased depth by adjusting the geometry of the side slopes of the channel without making the total
width of the channel at the point where it presently intersects with the seabed any wider.
Work started on the channel deepening program in October 2005 and was completed in March 2006
with an estimated 1.9Mm3 of material removed as part of the capital dredging campaign of which
approximately 55% was taken from the Swash Channel.
As part of the planning process the predicted changes to the tidal dynamics were assessed by means of
a numerical modeling study undertaken by PHC’s consultants HR Wallingford Ltd.
In particular the effect of dredging on tidal propagation within the harbour was assessed by outputting
water levels and tidal currents at eleven strategic locations within the Harbour at every model
timestep; a procedure designed to ensure that any small changes to the tidal phasing would be
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
resolved. This analysis predicted that, although high water levels would be little changed, generally
Spring tide low water levels would be lowered by up to 20mm.
As regards the impact on tidal currents within Poole Harbour the following conclusions were drawn
based on the modeling undertaken as part of the Environmental Impact Assessment:
• The Spring tide current directions would be little altered (relative to the baseline conditions)
by the modified depth, although changes to the current speed could be anticipated. In most
areas the change to the current speed was however suggested to be small in comparison with
the peak speeds flood/ebb;
• Within Poole Harbour itself the effect of the deepening would be to introduce a tendency to
draw more water through the approach channel, this was suggested to have the effect of
reducing currents in adjacent areas. This is particularly evident on the flood tide, where
currents in North Channel and also the Middle Mud area (between the north side of Brownsea
Island and the approach channel) are reduced;
• Although more water would be drawn into the channel as a result of the deepening (so that
the channel conveys a greater discharge) would not necessarily imply faster currents in the
channel, because of the additional cross sectional area of the channel;
• Currents in the deepened Turning Basin were expected to be generally reduced.
The changes to current speeds on peak ebb and peak flood resulting from the proposed deepening are
illustrated in Figure 1.3.
Outside the Harbour the general effect of the deepening was suggested to result in the entrainment of
the flow within the navigation channel, which would tend to reduce the currents in East Looe Channel
and over Hook Sand. There were also predicted increases in current speed in the outer part of the
Swash Channel, although nearer to the entrance of Poole Harbour the deeper water model predictions
indicated that this would give rise to reduced currents.
At the Harbour entrance, the removal of up to 4m of seabed on the south side of the channel was
predicted to result in the greatest change in current speed with an increase of up to 0.35m/s (on the
flood tide) just north of South Haven Point. At this location there were also predicted changes to the
current direction.
A comparison of the post-dredged scenario with the baseline data using 3D modeling techniques was
also undertaken which identified that changes in the flow field at the Harbour entrance colud be
anticipated, mainly associated with the nature of an eddy in the lee of South Haven Point which is
present on the ebb tide. This was suggested to result in stronger currents closer to the coast on the ebb
tide, whereas the flood tide would be little altered.
This prediction is illustrated by Figure 1.4 which shows the velocity through a section running from
Shell Bay towards the north east. In this figure the velocity perpendicular to the section is plotted with
positive values representing flow into the harbour and negative values representing flow out of the
harbour. The figures have a vertical exaggeration factor of ten, as indicated by the horizontal and
vertical scales. The asymmetry of the flood and ebb current is apparent, as is the presence of the eddy
which forms during the ebb (positive speeds at the south (left) part of the section). The change in
cross-section due to the dredging can also be seen, as well as its impact on flow.
Report No. 06/09/10047/1/JAT
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Figure 1.3 Predicted Changes in the Spring peak Flood and Ebb currents following channel deepening a)
ebb tide b) flood tide. After Simpson M, White N & M Dearnaley (2004)
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Compass Hydrographic Services Ltd
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Figure 1.4 Predicted 3D currents through the harbour entrance, before and after channel deepening,
After Simpson M, White N & M Dearnaley (2004).
During the flood, the flow speeds are decreased due to the larger cross-sectional area following
dredging. During the ebb, the eddy flow is locally intensified, resulting in stronger northward currents
close to the coast although there is little change to the current speeds in the main channel.
The effect of the removal by dredging of the bathymetric feature known as Chapman’s Peak was also
investigated. Chapman’s Peak lies on the Northern side of the entrance channel and historically has
been an area of net accumulation despite it’s repeated removal by previous dredging campaigns.
Analysis of the residual flow pattern identified that near to the bed there is convergence of the
residual flows which is consistent with the formation and recovery of Chapman’s Peak seen from
earlier dredging campaigns. The modeling studies concluded that the removal of Chapman’s Peak as
part of the channel deepening would not significantly alter the residual flows near the bed with the
area continuing to behave as a zone of convergence and leading to the conclusion that if it were to be
removed re-accretion would occur which would require future maintenance.
These predictions are illustrated in Figure 1.5 which shows the near bed residual current before and
after the removal of Chapman’s Peak. On the left hand side the vectors show the condition before the
removal of Chapman’s Peak whilst on the other side the near bed residual vectors before and after the
removal are shown. Various small differences are evident in the immediate area of Chapman’s Peak
but both before and after the removal of this feature the area in which the Peak is located is shown to
be an area of converging near bed currents and therefore an area of potential deposition.
Report No. 06/09/10047/1/JAT
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Figure 1.5 Near bed residual currents in the vicinity of the harbour entrance before and after the removal
of the Chapman’s Peak bathymetric feature. After Simpson M, White N & M Dearnaley (2004)
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2.0
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
METHODOLOGY
Two periods of data collection were undertaken at the entrance to Poole Harbour in order to provide
pre and post dredge comparison for both Spring and Neap tides of the flows through the harbour. The
first baseline study being undertaken in April 2004 with a second campaign undertaken in July and
September 2006 following completion of the Channel Deepening works.
In addition to providing a baseline for flows in the entrance of the harbour data from the first
campaign was additionally used to calibrate and validate the numerical model developed by HR
Wallingford and used in the Environmental Assessment for the channel deepening, HR Wallingford
(2004).
In both cases the flows were measured using a Vessel Mounted 600 kHz Acoustic Doppler Current
Profiler system mounted on the PHC workboat ‘Rough Rider’ with a sequence of survey lines
spanning the entrance run over a period of 13 hours on a Spring and Neap tide.
Figure 2.1 shows the survey lines run during the two survey campaigns whilst tables 2.1 & 2.2 detail
the timings of the surveys together with the ranges of the tides on both flood and ebb for the tides
surveyed.
Figure 2.1 Survey area showing the positions of the transects analysed within the present study
Of the 5 survey lines run only the main transects (denoted as 1, 2 & 3) have been considered for
analysis within the present study.
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 2.1 Details of the Survey Data Collected on each Survey
Year
2004
2004
2006
2006
Survey
Spring
Neap
Neap
Spring
Min DateTime
08/04/2004 06:28
13/04/2004 06:14
21/07/2006 05:07
12/09/2006 05:48
Max DateTime
08/04/2004 19:02
13/04/2004 18:30
21/07/2006 18:03
12/09/2006 18:34
No Ensembles
8142
7435
7319
7778
Missing Ensembles
6
27
0
29
Table 2.2 Details of the Tides Measured by Each Survey
Year
2004
2004
2006
2006
Survey
Spring
Neap
Neap
Spring
HW Time
08/04/2004 10:45
13/04/2004 19:45
21/07/2006 09:45
12/09/2006 11:35
LW Time
08/04/2004 18:15
13/04/2004 10:45
21/07/2006 12:55
12/09/2006 06:55
Flood Range
2.03
1.00
0.79
1.72
Ebb Range
1.97
0.76
0.69
1.52
The start and end co-ordinates (OSGB Eastings and Northings) of these transect lines extracted from
the main ADCP database for each survey campaign are detailed in Table 2.3.
Table 2.3 Start and End Co-ordinates of the Transects Extracted from the ADCP Database for
Each of the Four Surveys.
Line
SOLE
SOLN
SOL
Radius
EOLE
EOLN
EOL
Radius
Transect
Length (m)
1
2
3
4
5
404087.0
403634.3
403214.6
403800.5
403384.3
86800.4
87105.5
87595.3
86663.5
87069.2
75
50
50
50
75
403889.3
403462.0
403395.5
403637.7
403220.2
86605.3
86938.1
87470.5
87129.8
87598.0
75
50
50
25
25
277.8
240.3
219.8
493.9
553.7
2.1
Transect
Bearing (º
G)
225.4
225.8
124.6
340.8
342.8
FIELD SURVEY
The Acoustic Doppler Current Profiler or ADCP is a highly flexible Doppler sonar widely used in
current profiling applications. The ADCP can be deployed either as moored instrument or vessel
mounted, the latter application additionally requiring measurement of the movement of the vessel over
the ground at each measurement interval the vector components of which are then subtracted from the
measured flow speeds derived from the water column echoes to leave the speed of water motion itself.
In it’s normal mode of operation the Vessel Mounted utilises a second bottom tracking pulse to
determine the translation of the survey vessel over the ground. However due to the high velocities
prevailing in the entrance channel it was anticipated that moving bed conditions would prevail on
Spring tides at the times of maximum flood and ebb current. Moving bed conditions arise from the
mobilisation of bed sediment due to excedence of their threshold of motion which can cause the
ADCP’s bottom tracking to be misled resulting in an incorrect estimation of the survey vessel’s
course and speed over the ground and hence erroneous estimates of the underlying flow.
In order to overcome this limitation a high accuracy Real Time Kinematic GPS/Glonas base station
was deployed at a known benchmark located on the roof of the Haven Hotel at Sandbanks to provide
differential corrections to the receiver installed onboard the survey vessel thereby allowing high
accuracy GPS (<0.05m) positioning to be used in place of the conventional bottom tracking as the
method used to determine the corrections for vessel motion to be applied to each ADCP profile.
In the present study an RDI 614.4 kHz ADCP (Serial No 1352) was deployed downward looking over
the starboard side of the survey vessel and positioned immediately astern of the cabin (approximately
amidships) to provide profile information on flow velocity.
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Measurement of the Changes in Flow
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The instrument was operated using the RDI WinRiver v01.06.02 acquisition software which
controlled the ADCP and took in data from the RTKGPS Receiver and the external heading sensor.
This was backed up by a second survey computer running the survey software package and which was
configured to log the ADCP, GPS and Compass data streams together with data from a conventional
Trimble 212 DGPS receiver and provide a navigational display to the helmsman.
Corrections for instrument pitch, heave and roll were applied using data recorded from ADCP's
internal sensors and the data converted from instrument co-ordinates to true earth co-ordinates using
input from a Fluxgate Compass.
In order to achieve this transformation of velocities from instrument co-ordinates to true earth coordinates the orientations of the lubber lines of both the ADCP's internal compass (beam 3) and the
external compass must be known. In accordance with standard CHS survey protocols these were
aligned parallel to the centre line of the vessel and facing forward, however in practice it is found that
this alignment is difficult to achieve beyond a precision of +/- 3 degrees. The compass was calibrated
daily to map the vessels deviation field by driving the vessel through a series of circles in an area of
relatively quiescent currents and comparing the bottom track with the GPS track with the alignment of
the compass adjusted within the acquisition software until the two tracks matched.
The length of time required by the ADCP to range gate the backscattered signals into discrete bins of
the specified length was based on the speed of sound computed by the ADCP using a fixed value of
salinity, the transducer depth and the measured temperature.
For both sets of surveys the instrument configuration utilised a 10 ping ensemble average with 0.5m
bins with the data acquired using a Mode 1 ping, the time between pings being set to 0.2 seconds.
This resulted in a velocity profile being acquired every 2 seconds as the vessel moved through the
survey area.
2.2
DATA PROCESSING
The RDI proprietary binary data files recorded during each of the four surveys were imported into the
ADCP Processing Suite developed by Compass Data Systems Ltd/SeaZone Solutions Ltd.
As a result of the likelihood of moving bed conditions prevailing it was decided to use a GPS
determined bottom tracking rather than the more conventional acoustic bottom tracking method
whereby the ADCP determines its course and speed directly using a second acoustic ‘bottom tracking
pulse’.
Using the GPS method values of vessel course and speed over the ground was derived from the RTK
GPS position fixes recorded by the ADCP acquisition software using a ∆X/∆Y by ∆t approach. The
values of vessel course and speed determined being removed from the velocities recorded by the
ADCP to leave only the currents prevailing.
The GPS data was also used to locate the position of each ensemble in space with the position fixes
converted from the WGS84 format recorded by the ADCP acquisition software to UK Ordnance
Survey Eastings and Northings using the OSTN97 conversion algorithms included in the processing
software.
During import, those cells known to be intersecting or below the sea bed were removed and each data
cell filtered using a quality parameter derived from the RDI Data Quality Flags, which takes into
account the percentage of good acoustic returns as well as other QA information provided by the
ADCP for each current profile (ensemble).
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Where the RDI quality parameter fell below a value of 98%, the data cell was marked bad and not
used in subsequent data processing. Filters were applied to remove all ensembles where more that
25% of the data cells were found to be bad, and finally a visual check made for spikes and anomalies.
ADCP Parameters were also averaged through the instruments water column, with vector averaged
current speeds and directions derived from the results.
As part of the processing procedure observed tidal elevation data was also imported from the local
gauge operated by the PHC and the tidal elevation data cross-linked to the main ADCP data set
allowing each measurement to be assigned a value in terms of it’s time relative to local HW and LW
as well as providing a record of the height of the water surface at each measurement point relative to a
recognised vertical datum, in the case of this project ODN.
Inclusion of this information within the dataset enables appropriate analysis of the tidal regimes and
provides for longer-term analysis by facilitating comparison with past and future datasets.
Transects lines were defined in space by specifying the coordinates of the start and end of the line
together with a radius within which data would be extracted in order to allow a degree of tolerance for
the cross track errors.
The measured data falling within the specified corridor was extracted from the main ADCP database
grouped as passes (each pass constituting an individual journey along the transect line) and presented
in time order starting with the first pass of the day.
From these data colour flood fill plots of velocity, direction and the individual velocity components
(U,V& W) together with the Along and Across transect flow components were created for the valid
ADCP profiles1 from each pass.
Unfortunately although the four surveys were undertaken on tides of similar range inspection of the
data revealed that there were significant differences in the shapes of the tidal curves for each tide, as
shown in figure 2.2.
These distortions in the shape of the tidal curves with respect to the mean tidal curves are typical of
this area where the amplitude of M2 is small and were most prevalent on Neaps but were also present
on Spring tides in the form of variations in the height and timing of the HW stands and act make it
difficult to undertake a direct comparison of the data in any form since slight differences in the shapes
of the tidal curves (stands of water level) and in the timing of each pass relative to local high or low
water can result in very different tidal streams being measured making the comparison of the two time
series problematic. For this reason the transect data was resampled to form a transect point timeseries.
A transect point may be thought of as a point in space which is visited as repeatedly over the course of
the survey such that a time-series of the prevailing currents can be built by averaging the data from
each pass through the point into a single value of speed and direction. Transect points can also be
specified in terms of their depth range i.e,., such that they only contain data from the first 5 or last 10
measurement bins, making it possible to compare the flows prevailing at different depths.
Since the survey vessel cannot visit all points defined within the survey area at the same time but
rather as a sequence it is inevitable that there will be differences in the timing of the passes made
through each point both within each survey and between surveys. For this reason passes are
1
A valid ADCP profile is defined as those measurement bins lying between the first bin at 0.35m range and the
last valid bin, which we define as the measurement bin lying at range of less than 94% of the distance between
the instrument and the sea bed.
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
referenced in terms of time relative to a fixed point in the tidal curve such as the time of high or low
water as determined from the tide gauge data incorporated into the data set.
Unlike other ports around the UK the complexity of the tidal curves for Poole means that HW does
not provide a particularly good tidal reference and hence all data has been referenced to the timing of
low water recorded at the local tide gauge. This is consistent with the approach adopted by the UK
Hydrographic Office (UKHO) for the mean Spring and Neap tidal curves reported in the Admiralty
Tide Tables .
The time-series data extracted at each transect point was interpolated using a spline interpolator and
resampled every 10 minutes in order to build a continuous time series of currents for comparison
purposes. Figure 2.3 shows the results of one such curve fitting exercise with the pass and depth
averaged values of the horizontal current components ( U & V) shown by the circles and the fitted
curve by the solid line.
Poole Harbour Tides
1.00
Tidal Elevation (mAODN)
0.50
0.00
-0.50
-1.00
-1.50
Neap 2004
Neap 2006
Spring 2004
Spring 2006
Figure 2.2 Predicted Tidal Curves for Poole Harbour Entrance for the Duration of each Survey Period for
the 4 Tides Surveyed
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Figure 2.3 Transect Point A Depth Averaged Time Series Spring Tide 2006 Observed and Fitted Time
Series.
In this manner it has been possible to resample each survey onto a common timebase referenced to the
timing of local low water allowing a more meaningful comparison to be made between individual
surveys.
In order to overcome the variations in the shape of the tidal curves between the two spring tides or
two neap tides surveyed the interpolated data from at each transect point has also been scaled in terms
of the intra-tidal range variation through comparison with a theoretical mean spring or neap tide
constructed from the range factors detailed in the Admiralty Tide Tables (UKHO (2006). Figure 2.4
shows one such example of this range correction method applied to a Spring tide data set.
Report No. 06/09/10047/1/JAT
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Compass Hydrographic Services Ltd
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Figure 2.4 Transect Point A Depth Averaged Time Series Spring Tide 2006 Showing Range Correction
Applied
This analytical approach is based on the method used to scale tidal diamonds between tides of
different ranges but in the present application does not simply consider the overall range of the tide
but also the shape of the tidal curve by incorporating the tidal factor for each tidal hour. This tidal
factor is used to scale the observed tide to that of a mean Neap or Spring tide with the same scaling
then applied to the U and V current time-series to derive a range corrected current time-series.
In the absence of a longer term data set of elevations and currents which could be harmonically
analysed to determine the relationship between water levels and the resultant tidal streams this is
considered the most viable way to account for the variations in the tidal curves of the data sets
available.
It should be noted however that a degree of caution must be applied to the interpretation of this data
since the flows through the channel entrance are also influenced by non-tidal forces such as wind and
barometric pressure effects which cannot be accounted for and hence compensated in the analysis
reported here.
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3.0
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
RESULTS
3.1
Transect Lines
Appendix A details the plots derived from the individual passes made along each of the three main
transect lines. Data are presented as flood fill plots of the current speed and direction across the
transect as a function of distance from the transect origin and depth below the water surface.
The appendices are arranged in tide and then time order with Appendix A1 showing the data for the
Spring 2004 data A2 the Spring 2006 data and so on.
The tables presented in this section provide and inventory of the data collected with tables 3.1 to 3.6
listing the timings of the passes made on each transect line for the Neap surveys whilst tables 3.7 to
3.12 list the data equivalent for the Spring surveys.
All times are GMT with current speeds given in m/s and directions quoted as degrees magnetic
detailing the direction towards which the current is flowing.and throughout the remainder of this
report the 2004 data (Springs & Neaps) is taken as the background data with changes in the flow
expressed relative to their 2004 values.
Table 3.1 Neap Survey 2004 Transect Line 1
Year
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
Line
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Pass
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Start Time
13-Apr-04 06:15:40
13-Apr-04 06:48:03
13-Apr-04 07:18:48
13-Apr-04 07:51:22
13-Apr-04 08:23:37
13-Apr-04 08:53:59
13-Apr-04 09:25:24
13-Apr-04 09:55:31
13-Apr-04 10:24:29
13-Apr-04 10:56:23
13-Apr-04 11:27:25
13-Apr-04 11:58:06
13-Apr-04 12:32:39
13-Apr-04 13:09:39
13-Apr-04 13:38:15
13-Apr-04 15:17:20
13-Apr-04 15:47:06
13-Apr-04 16:20:50
13-Apr-04 16:56:16
13-Apr-04 17:27:20
13-Apr-04 17:58:43
Report No. 06/09/10047/1/JAT
End Time
13-Apr-04 06:18:18
13-Apr-04 06:50:37
13-Apr-04 07:21:43
13-Apr-04 07:53:44
13-Apr-04 08:26:04
13-Apr-04 08:56:17
13-Apr-04 09:28:03
13-Apr-04 09:58:09
13-Apr-04 10:27:20
13-Apr-04 10:58:50
13-Apr-04 11:29:39
13-Apr-04 12:00:41
13-Apr-04 12:35:58
13-Apr-04 13:11:50
13-Apr-04 13:40:25
13-Apr-04 15:19:47
13-Apr-04 15:49:33
13-Apr-04 16:23:29
13-Apr-04 16:59:11
13-Apr-04 17:30:19
13-Apr-04 18:01:22
16
Tidal Elevation (mAODN)
0.25
0.25
0.24
0.17
0.03
-0.14
-0.30
-0.41
-0.49
-0.50
-0.42
-0.29
-0.16
-0.06
-0.04
-0.06
0.00
0.08
0.18
0.24
0.30
LW+/-4.47
-3.93
-3.41
-2.87
-2.34
-1.83
-1.30
-0.80
-0.32
0.21
0.73
1.24
1.82
2.43
2.91
4.56
5.06
5.62
6.21
-6.27
-5.75
Compass Hydrographic Services Ltd
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.2 Neap Survey 2004 Transect Line 2
Year
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
Line
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Pass
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Start Time
13/04/2004 06:25
13/04/2004 06:57
13/04/2004 07:29
13/04/2004 08:00
13/04/2004 08:32
13/04/2004 09:03
13/04/2004 09:34
13/04/2004 10:04
13/04/2004 10:33
13/04/2004 11:05
13/04/2004 11:36
13/04/2004 12:06
13/04/2004 12:43
13/04/2004 13:17
13/04/2004 13:46
13/04/2004 15:25
13/04/2004 15:55
13/04/2004 16:29
13/04/2004 17:05
13/04/2004 17:37
13/04/2004 18:07
End Time
13/04/2004 06:27
13/04/2004 06:59
13/04/2004 07:31
13/04/2004 08:02
13/04/2004 08:34
13/04/2004 09:06
13/04/2004 09:36
13/04/2004 10:06
13/04/2004 10:35
13/04/2004 11:07
13/04/2004 11:38
13/04/2004 12:09
13/04/2004 12:46
13/04/2004 13:19
13/04/2004 13:48
13/04/2004 15:27
13/04/2004 15:57
13/04/2004 16:31
13/04/2004 17:08
13/04/2004 17:40
13/04/2004 18:10
Tidal Elevation (mAODN)
0.25
0.25
0.22
0.14
-0.02
-0.19
-0.33
-0.45
-0.50
-0.48
-0.38
-0.26
-0.12
-0.05
-0.04
-0.05
0.01
0.11
0.20
0.26
0.32
LW+/-4.31
-3.78
-3.24
-2.72
-2.19
-1.67
-1.15
-0.66
-0.17
0.35
0.87
1.39
2.00
2.56
3.05
4.69
5.19
5.76
6.37
-6.10
-5.59
Tidal Elevation (mAODN)
0.25
0.25
0.20
0.10
-0.07
-0.23
-0.37
-0.47
-0.51
-0.46
-0.35
-0.22
-0.09
-0.04
-0.04
-0.03
0.03
0.14
0.22
0.27
0.34
LW+/-4.13
-3.62
-3.08
-2.58
-2.03
-1.52
-1.01
-0.52
-0.02
0.51
1.01
1.53
2.18
2.71
3.22
4.84
5.33
5.96
6.51
-5.95
-5.43
Table 3.3 Neap Survey 2004 Transect Line 3
Year
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
Line
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
Pass
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Start Time
13/04/2004 06:35
13/04/2004 07:06
13/04/2004 07:39
13/04/2004 08:09
13/04/2004 08:42
13/04/2004 09:12
13/04/2004 09:43
13/04/2004 10:12
13/04/2004 10:43
13/04/2004 11:14
13/04/2004 11:44
13/04/2004 12:16
13/04/2004 12:54
13/04/2004 13:26
13/04/2004 13:56
13/04/2004 15:34
13/04/2004 16:04
13/04/2004 16:41
13/04/2004 17:14
13/04/2004 17:47
13/04/2004 18:18
Report No. 06/09/10047/1/JAT
End Time
13/04/2004 06:38
13/04/2004 07:08
13/04/2004 07:41
13/04/2004 08:10
13/04/2004 08:43
13/04/2004 09:14
13/04/2004 09:45
13/04/2004 10:14
13/04/2004 10:45
13/04/2004 11:16
13/04/2004 11:46
13/04/2004 12:17
13/04/2004 12:56
13/04/2004 13:28
13/04/2004 13:59
13/04/2004 15:36
13/04/2004 16:05
13/04/2004 16:43
13/04/2004 17:16
13/04/2004 17:49
13/04/2004 18:20
17
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.4 Neap Survey 2006 Transect Line 1
Year
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
Line
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Pass
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Start Time
21/07/2006 05:08
21/07/2006 06:03
21/07/2006 06:33
21/07/2006 07:02
21/07/2006 07:33
21/07/2006 08:05
21/07/2006 08:34
21/07/2006 09:04
21/07/2006 09:32
21/07/2006 10:03
21/07/2006 10:32
21/07/2006 11:08
21/07/2006 11:39
21/07/2006 12:14
21/07/2006 13:00
21/07/2006 13:26
21/07/2006 13:54
21/07/2006 14:24
21/07/2006 14:52
21/07/2006 15:25
21/07/2006 15:51
21/07/2006 16:17
21/07/2006 16:47
21/07/2006 17:16
21/07/2006 17:43
End Time
21/07/2006 05:11
21/07/2006 06:05
21/07/2006 06:35
21/07/2006 07:05
21/07/2006 07:35
21/07/2006 08:07
21/07/2006 08:37
21/07/2006 09:06
21/07/2006 09:34
21/07/2006 10:06
21/07/2006 10:35
21/07/2006 11:10
21/07/2006 11:42
21/07/2006 12:16
21/07/2006 13:02
21/07/2006 13:28
21/07/2006 13:57
21/07/2006 14:27
21/07/2006 14:55
21/07/2006 15:28
21/07/2006 15:53
21/07/2006 16:20
21/07/2006 16:49
21/07/2006 17:18
21/07/2006 17:44
Tidal Elevation (mAODN)
0.18
0.26
0.30
0.31
0.31
0.31
0.34
0.40
0.44
0.45
0.37
0.18
0.01
-0.14
-0.24
-0.21
-0.10
0.06
0.21
0.32
0.37
0.39
0.40
0.40
0.38
LW+/4.58
5.49
5.99
6.48
-5.34
-4.81
-4.32
-3.83
-3.36
-2.84
-2.35
-1.76
-1.23
-0.66
0.11
0.54
1.01
1.52
1.98
2.53
2.95
3.40
3.90
4.37
4.82
Tidal Elevation (mAODN)
0.19
0.27
0.31
0.31
0.31
0.32
0.36
0.42
0.45
0.42
0.31
0.13
-0.03
-0.18
-0.24
-0.18
-0.06
0.11
0.25
0.34
0.39
0.40
0.41
0.40
0.38
LW+/4.74
5.64
6.16
6.64
-5.18
-4.65
-4.16
-3.68
-3.21
-2.68
-2.16
-1.61
-1.08
-0.45
0.25
0.69
1.15
1.64
2.11
2.66
3.10
3.55
4.04
4.53
4.96
Table 3.5 Neap Survey 2006 Transect Line 2
Year
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
Line
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Pass
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Start Time
21/07/2006 05:18
21/07/2006 06:12
21/07/2006 06:43
21/07/2006 07:12
21/07/2006 07:43
21/07/2006 08:14
21/07/2006 08:44
21/07/2006 09:13
21/07/2006 09:41
21/07/2006 10:12
21/07/2006 10:44
21/07/2006 11:17
21/07/2006 11:48
21/07/2006 12:26
21/07/2006 13:08
21/07/2006 13:35
21/07/2006 14:02
21/07/2006 14:32
21/07/2006 15:00
21/07/2006 15:33
21/07/2006 15:59
21/07/2006 16:26
21/07/2006 16:56
21/07/2006 17:25
21/07/2006 17:51
Report No. 06/09/10047/1/JAT
End Time
21/07/2006 05:20
21/07/2006 06:14
21/07/2006 06:45
21/07/2006 07:14
21/07/2006 07:45
21/07/2006 08:17
21/07/2006 08:46
21/07/2006 09:15
21/07/2006 09:43
21/07/2006 10:15
21/07/2006 10:46
21/07/2006 11:19
21/07/2006 11:51
21/07/2006 12:29
21/07/2006 13:10
21/07/2006 13:37
21/07/2006 14:05
21/07/2006 14:34
21/07/2006 15:02
21/07/2006 15:35
21/07/2006 16:01
21/07/2006 16:29
21/07/2006 16:58
21/07/2006 17:27
21/07/2006 17:54
18
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.6 Neap Survey 2006 Transect Line 3
Year
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
Line
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
Pass
1
2
3
4
5
6
7
8
10
11
12
13
14
15
16
17
18
20
21
22
23
24
25
26
Start Time
21/07/2006 05:27
21/07/2006 06:21
21/07/2006 06:52
21/07/2006 07:23
21/07/2006 07:52
21/07/2006 08:23
21/07/2006 08:53
21/07/2006 09:21
21/07/2006 09:54
21/07/2006 10:22
21/07/2006 10:53
21/07/2006 11:27
21/07/2006 12:03
21/07/2006 12:36
21/07/2006 13:17
21/07/2006 13:43
21/07/2006 14:12
21/07/2006 14:40
21/07/2006 15:40
21/07/2006 16:07
21/07/2006 16:35
21/07/2006 17:04
21/07/2006 17:34
21/07/2006 18:01
Report No. 06/09/10047/1/JAT
End Time
21/07/2006 05:29
21/07/2006 06:23
21/07/2006 06:54
21/07/2006 07:24
21/07/2006 07:54
21/07/2006 08:25
21/07/2006 08:55
21/07/2006 09:23
21/07/2006 09:55
21/07/2006 10:24
21/07/2006 10:54
21/07/2006 11:28
21/07/2006 12:05
21/07/2006 12:38
21/07/2006 13:18
21/07/2006 13:45
21/07/2006 14:14
21/07/2006 14:43
21/07/2006 15:42
21/07/2006 16:09
21/07/2006 16:37
21/07/2006 17:06
21/07/2006 17:35
21/07/2006 18:02
19
Tidal Elevation (mAODN)
0.20
0.29
0.31
0.31
0.31
0.33
0.38
0.43
0.45
0.40
0.27
0.08
-0.10
-0.20
-0.23
-0.15
0.00
0.15
0.35
0.39
0.40
0.40
0.39
0.39
LW+/4.89
5.80
6.31
6.81
-5.02
-4.50
-4.00
-3.54
-3.00
-2.52
-2.01
-1.45
-0.84
-0.29
0.38
0.82
1.30
1.79
2.78
3.23
3.69
4.17
4.67
5.12
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.7 Spring Survey 2004 Transect Line 1
Year
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
Line
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Pass
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Start Time
08/04/2004 06:28
08/04/2004 06:56
08/04/2004 07:25
08/04/2004 07:53
08/04/2004 08:18
08/04/2004 08:49
08/04/2004 09:18
08/04/2004 09:47
08/04/2004 10:15
08/04/2004 10:46
08/04/2004 11:17
08/04/2004 11:45
08/04/2004 12:13
08/04/2004 12:43
08/04/2004 13:09
08/04/2004 13:38
08/04/2004 14:08
08/04/2004 14:36
08/04/2004 15:00
08/04/2004 15:25
08/04/2004 15:52
08/04/2004 16:20
08/04/2004 16:54
08/04/2004 17:26
08/04/2004 17:52
08/04/2004 18:18
08/04/2004 18:46
Report No. 06/09/10047/1/JAT
End Time
08/04/2004 06:30
08/04/2004 06:59
08/04/2004 07:27
08/04/2004 07:55
08/04/2004 08:20
08/04/2004 08:50
08/04/2004 09:20
08/04/2004 09:49
08/04/2004 10:17
08/04/2004 10:48
08/04/2004 11:19
08/04/2004 11:48
08/04/2004 12:16
08/04/2004 12:46
08/04/2004 13:11
08/04/2004 13:40
08/04/2004 14:10
08/04/2004 14:38
08/04/2004 15:02
08/04/2004 15:28
08/04/2004 15:55
08/04/2004 16:24
08/04/2004 16:56
08/04/2004 17:28
08/04/2004 17:55
08/04/2004 18:21
08/04/2004 18:48
20
Tidal Elevation (mAODN)
-0.94
-0.55
-0.16
0.16
0.36
0.54
0.67
0.78
0.86
0.90
0.87
0.77
0.61
0.39
0.20
0.07
0.15
0.31
0.38
0.38
0.25
-0.01
-0.38
-0.73
-0.96
-1.07
-0.91
LW+/0.58
1.05
1.52
1.99
2.40
2.92
3.41
3.89
4.36
4.88
5.38
5.87
6.34
-5.50
-5.07
-4.60
-4.09
-3.63
-3.22
-2.80
-2.35
-1.88
-1.32
-0.79
-0.35
0.08
0.55
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.8 Spring Survey 2004 Transect Line 2
Year
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
Line
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Pass
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Start Time
08/04/2004 06:35
08/04/2004 07:02
08/04/2004 07:29
08/04/2004 07:59
08/04/2004 08:24
08/04/2004 08:55
08/04/2004 09:25
08/04/2004 09:54
08/04/2004 10:23
08/04/2004 10:55
08/04/2004 11:25
08/04/2004 11:54
08/04/2004 12:23
08/04/2004 12:52
08/04/2004 13:18
08/04/2004 13:45
08/04/2004 14:16
08/04/2004 14:42
08/04/2004 15:07
08/04/2004 15:34
08/04/2004 16:01
08/04/2004 16:33
08/04/2004 17:05
08/04/2004 17:35
08/04/2004 18:00
08/04/2004 18:26
08/04/2004 18:52
Report No. 06/09/10047/1/JAT
End Time
08/04/2004 06:36
08/04/2004 07:05
08/04/2004 07:33
08/04/2004 08:01
08/04/2004 08:26
08/04/2004 08:57
08/04/2004 09:27
08/04/2004 09:56
08/04/2004 10:26
08/04/2004 10:57
08/04/2004 11:27
08/04/2004 11:56
08/04/2004 12:25
08/04/2004 12:54
08/04/2004 13:20
08/04/2004 13:47
08/04/2004 14:18
08/04/2004 14:44
08/04/2004 15:09
08/04/2004 15:35
08/04/2004 16:03
08/04/2004 16:36
08/04/2004 17:07
08/04/2004 17:37
08/04/2004 18:02
08/04/2004 18:28
08/04/2004 18:55
21
Tidal Elevation (mAODN)
-0.87
-0.45
-0.09
0.21
0.40
0.56
0.69
0.81
0.87
0.90
0.86
0.73
0.54
0.33
0.15
0.06
0.19
0.33
0.39
0.35
0.18
-0.14
-0.50
-0.82
-1.02
-1.06
-0.84
LW+/0.68
1.15
1.61
2.09
2.51
3.02
3.52
4.01
4.50
5.02
5.52
6.01
6.49
-5.36
-4.92
-4.47
-3.96
-3.52
-3.10
-2.67
-2.21
-1.67
-1.15
-0.64
-0.22
0.21
0.64
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.9 Spring Survey 2004 Transect Line 3
Year
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
2004
Line
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
Pass
1
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Start Time
08/04/2004 06:41
08/04/2004 07:37
08/04/2004 08:06
08/04/2004 08:31
08/04/2004 09:01
08/04/2004 09:33
08/04/2004 10:02
08/04/2004 10:33
08/04/2004 11:04
08/04/2004 11:34
08/04/2004 12:03
08/04/2004 12:32
08/04/2004 13:00
08/04/2004 13:28
08/04/2004 13:54
08/04/2004 14:23
08/04/2004 14:49
08/04/2004 15:15
08/04/2004 15:42
08/04/2004 16:10
08/04/2004 16:45
08/04/2004 17:13
08/04/2004 17:43
08/04/2004 18:08
08/04/2004 18:34
08/04/2004 18:59
Report No. 06/09/10047/1/JAT
End Time
08/04/2004 06:43
08/04/2004 07:40
08/04/2004 08:08
08/04/2004 08:32
08/04/2004 09:03
08/04/2004 09:35
08/04/2004 10:04
08/04/2004 10:36
08/04/2004 11:06
08/04/2004 11:36
08/04/2004 12:05
08/04/2004 12:34
08/04/2004 13:02
08/04/2004 13:29
08/04/2004 13:54
08/04/2004 14:25
08/04/2004 14:51
08/04/2004 15:16
08/04/2004 15:43
08/04/2004 16:11
08/04/2004 16:46
08/04/2004 17:15
08/04/2004 17:45
08/04/2004 18:09
08/04/2004 18:36
08/04/2004 19:02
22
Tidal Elevation (mAODN)
-0.78
0.00
0.28
0.45
0.60
0.73
0.83
0.89
0.89
0.83
0.68
0.47
0.27
0.11
0.07
0.24
0.35
0.39
0.31
0.10
-0.27
-0.59
-0.89
-1.05
-1.02
-0.73
LW+/0.79
1.73
2.20
2.62
3.13
3.66
4.14
4.67
5.17
5.67
6.15
6.64
7.11
-4.77
-4.34
-3.84
-3.41
-2.98
-2.53
-2.06
-1.49
-1.01
-0.51
-0.10
0.34
0.77
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.10 Spring Survey 2006 Transect Line 1
Year
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
Line
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Pass
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Start Time
12/09/2006 05:50
12/09/2006 06:36
12/09/2006 07:21
12/09/2006 07:52
12/09/2006 08:26
12/09/2006 09:04
12/09/2006 09:32
12/09/2006 10:11
12/09/2006 10:46
12/09/2006 11:13
12/09/2006 11:39
12/09/2006 12:07
12/09/2006 12:16
12/09/2006 12:44
12/09/2006 13:10
12/09/2006 13:42
12/09/2006 14:12
12/09/2006 14:46
12/09/2006 15:26
12/09/2006 16:02
12/09/2006 16:36
12/09/2006 16:41
12/09/2006 17:11
12/09/2006 17:42
12/09/2006 18:12
Report No. 06/09/10047/1/JAT
End Time
12/09/2006 05:53
12/09/2006 06:38
12/09/2006 07:23
12/09/2006 07:54
12/09/2006 08:30
12/09/2006 09:06
12/09/2006 09:35
12/09/2006 10:13
12/09/2006 10:48
12/09/2006 11:15
12/09/2006 11:41
12/09/2006 12:09
12/09/2006 12:19
12/09/2006 12:47
12/09/2006 13:13
12/09/2006 13:45
12/09/2006 14:16
12/09/2006 14:48
12/09/2006 15:29
12/09/2006 16:04
12/09/2006 16:38
12/09/2006 16:43
12/09/2006 17:14
12/09/2006 17:45
12/09/2006 18:14
23
Tidal Elevation (mAODN)
-0.51
-0.90
-0.77
-0.40
0.00
0.31
0.47
0.63
0.73
0.77
0.79
0.78
0.76
0.69
0.58
0.40
0.26
0.29
0.54
0.66
0.60
0.58
0.32
0.01
-0.30
LW+/-1.05
-0.29
0.46
0.97
1.56
2.17
2.65
3.29
3.88
4.32
4.75
5.23
5.38
5.85
6.28
-5.51
-5.01
-4.46
-3.78
-3.19
-2.63
-2.54
-2.03
-1.52
-1.02
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.11 Spring Survey 2006 Transect Line 2
Year
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
Line
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Pass
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
Start Time
12/09/2006 05:59
12/09/2006 06:47
12/09/2006 07:12
12/09/2006 07:40
12/09/2006 08:14
12/09/2006 08:52
12/09/2006 09:23
12/09/2006 10:02
12/09/2006 10:39
12/09/2006 11:04
12/09/2006 11:31
12/09/2006 11:58
12/09/2006 12:26
12/09/2006 12:54
12/09/2006 13:19
12/09/2006 13:51
12/09/2006 14:23
12/09/2006 14:54
12/09/2006 15:16
12/09/2006 15:51
12/09/2006 16:27
12/09/2006 16:51
12/09/2006 17:23
12/09/2006 17:53
12/09/2006 18:20
Report No. 06/09/10047/1/JAT
End Time
12/09/2006 06:02
12/09/2006 06:50
12/09/2006 07:14
12/09/2006 07:43
12/09/2006 08:18
12/09/2006 08:55
12/09/2006 09:26
12/09/2006 10:04
12/09/2006 10:41
12/09/2006 11:06
12/09/2006 11:33
12/09/2006 12:00
12/09/2006 12:28
12/09/2006 12:56
12/09/2006 13:21
12/09/2006 13:53
12/09/2006 14:26
12/09/2006 14:56
12/09/2006 15:18
12/09/2006 15:53
12/09/2006 16:30
12/09/2006 16:54
12/09/2006 17:25
12/09/2006 17:56
12/09/2006 18:24
24
Tidal Elevation (mAODN)
-0.61
-0.92
-0.85
-0.54
-0.13
0.23
0.43
0.59
0.71
0.77
0.79
0.79
0.75
0.65
0.53
0.36
0.24
0.34
0.48
0.63
0.64
0.51
0.21
-0.11
-0.39
LW+/-0.89
-0.10
0.30
0.78
1.36
1.99
2.50
3.14
3.75
4.17
4.62
5.08
5.53
6.01
6.42
-5.37
-4.83
-4.33
-3.96
-3.37
-2.77
-2.37
-1.84
-1.34
-0.87
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.12 Data Inventory Spring Survey 2006 Transect Line 3
Year
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
2006
Line
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
2006.3
Pass
1
2
3
4
6
8
10
12
14
16
18
19
20
21
22
23
24
25
26
28
30
31
32
33
34
Start Time
12/09/2006 06:10
12/09/2006 07:00
12/09/2006 07:04
12/09/2006 07:30
12/09/2006 08:01
12/09/2006 08:39
12/09/2006 09:13
12/09/2006 09:52
12/09/2006 10:29
12/09/2006 10:55
12/09/2006 11:23
12/09/2006 11:51
12/09/2006 12:34
12/09/2006 13:02
12/09/2006 13:28
12/09/2006 14:01
12/09/2006 14:32
12/09/2006 15:01
12/09/2006 15:06
12/09/2006 15:41
12/09/2006 16:19
12/09/2006 17:01
12/09/2006 17:35
12/09/2006 18:05
12/09/2006 18:32
End Time
12/09/2006 06:11
12/09/2006 07:02
12/09/2006 07:06
12/09/2006 07:31
12/09/2006 08:02
12/09/2006 08:41
12/09/2006 09:15
12/09/2006 09:54
12/09/2006 10:30
12/09/2006 10:57
12/09/2006 11:24
12/09/2006 11:52
12/09/2006 12:36
12/09/2006 13:04
12/09/2006 13:29
12/09/2006 14:02
12/09/2006 14:34
12/09/2006 15:03
12/09/2006 15:07
12/09/2006 15:42
12/09/2006 16:21
12/09/2006 17:03
12/09/2006 17:35
12/09/2006 18:05
12/09/2006 18:33
Tidal Elevation (mAODN)
-0.70
-0.91
-0.90
-0.68
-0.30
0.11
0.37
0.56
0.68
0.75
0.78
0.79
0.72
0.62
0.49
0.31
0.25
0.39
0.42
0.59
0.66
0.42
0.09
-0.22
-0.48
LW+/-0.73
0.11
0.18
0.60
1.12
1.76
2.33
2.97
3.59
4.03
4.49
4.95
5.67
6.14
6.56
-5.22
-4.69
-4.21
-4.14
-3.55
-2.91
-2.21
-1.66
-1.16
-0.70
An analysis of the pass data for each survey for each of the principal transect lines has been made to
find the times of peak flood and ebb currents based on the maxima and minima of the across transect
velocity components the results of which are presented in Tables 3.13 to 3.16 taking as a reference the
times of High and Low Water.
Table 3.13 Timing with Respect to HW of Peak Flood Currents on Each Transect Line
Transect
1
2
3
Spring_2004
Time
HW+/08/04/2004
16:22
5.62
08/04/2004
16:34
5.83
08/04/2004
16:46
6.02
Spring_2006
Time
HW+/12/09/2006
17:43
6.15
12/09/2006
17:24
5.82
12/09/2006
18:33
6.98
Neap_2004
Time
HW+/13/04/2004
08:55
2.84
13/04/2004
09:04
3.00
13/04/2004
08:43
2.64
Neap_2006
Time
HW+/21/07/2006
11:08
1.40
21/07/2006
11:18
1.56
21/07/2006
12:04
2.33
Table 3.14 Timing with Respect to LW of Peak Flood Currents on Each Transect Line
Transect
1
2
3
Spring_2004
Time
LW+/08/04/2004
07:27
1.54
08/04/2004
07:30
1.60
08/04/2004
07:38
1.72
Report No. 06/09/10047/1/JAT
Spring_2006
Time
LW+/12/09/2006
08:27
1.55
12/09/2006
08:15
1.34
12/09/2006
08:41
1.78
25
Neap_2004
Time
LW+/13/04/2004
12:34
1.82
13/04/2004
12:06
1.35
13/04/2004
12:17
1.54
Neap_2006
Time
LW+/21/07/2006
14:26
1.52
21/07/2006
14:33
1.64
21/07/2006
14:41
1.78
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.15 Timing with Respect to HW of Peak Ebb Currents on Each Transect Line
Transect
1
2
3
Spring_2004
Time
HW+/08/04/2004
07:27
-3.30
08/04/2004
07:30
-3.24
08/04/2004
07:38
-3.11
Spring_2006
Time
HW+/12/09/2006
08:27
-3.12
12/09/2006
08:15
-3.33
12/09/2006
08:41
-2.89
Neap_2004
Time
HW+/13/04/2004
12:34
6.49
13/04/2004
12:06
6.02
13/04/2004
12:17
6.21
Neap_2006
Time
HW+/21/07/2006
14:26
4.69
21/07/2006
14:33
4.80
21/07/2006
14:41
4.94
Table 3.16 Timing with Respect to LW of Peak Ebb Currents on Each Transect Line
Transect
1
2
3
Spring_2004
Time
LW+/08/04/2004
16:22
-1.88
08/04/2004
16:34
-1.67
08/04/2004
16:46
-1.48
Spring_2006
Time
LW+/12/09/2006
17:43
-1.52
12/09/2006
17:24
-1.84
12/09/2006
18:33
-0.69
Neap_2004
Time
LW+/13/04/2004
08:55
-1.82
13/04/2004
09:04
-1.67
13/04/2004
08:43
-2.02
Neap_2006
Time
LW+/21/07/2006
11:08
-1.77
21/07/2006
11:18
-1.61
21/07/2006
12:04
-0.84
Using these tabulated data it is possible to see that the timing of peak flood and ebb is highly variable
when referenced to the time of HW especially on Neaps. However if the time of local LW is taken as
the reference the timing is more regular for both Spring and Neap tides with peak flood currents
occurring between +1.3 to +2.0 hours after LW and the peak ebb currents occurring 0.7 to1.9 hours
before LW the exact timing depending on the location. The innermost transect (Line 3) generally
experiences the ebb maxima before and the flood maxima after the neighbouring transects as would
be expected.
The actual magnitudes of the peak across transect current component recorded on each transect line
are shown in tables 3.17 and 3.18.
Plots of the individual passes acquired at the times of peak flood and ebb flows for each of the four
tides surveyed for the three main transects are presented in Section 4.1.
Table 3.17 Peak Ebb Currents Values on Each Transect Line
Transect
1
2
3
Spring_2004
Across
Time
(m/s)
08/04/2004
16:22
1.94
08/04/2004
16:34
2.15
08/04/2004
16:46
1.58
Spring_2006
Across
Time
(m/s)
12/09/2006
17:43
2.07
12/09/2006
17:24
1.75
12/09/2006
18:33
1.48
Neap_2004
Time
13/04/2004
08:55
13/04/2004
09:04
13/04/2004
08:43
Across
(m/s)
1.50
1.57
0.41
Neap_2006
Across
Time
(m/s)
21/07/2006
11:08
1.22
21/07/2006
11:18
1.14
21/07/2006
12:04
1.40
Table 3.18 Peak Flood Current Values on Each Transect Line
Transect
1
2
3
Spring_2004
Across
Time
(m/s)
08/04/2004
07:27
-2.18
08/04/2004
07:30
-2.13
08/04/2004
07:38
-1.43
Spring_2006
Across
Time
(m/s)
12/09/2006
08:27
-1.44
12/09/2006
08:15
-2.04
12/09/2006
08:41
-1.86
Neap_2004
Time
13/04/2004
12:34
13/04/2004
12:06
13/04/2004
12:17
Across
(m/s)
-0.31
-0.85
-1.26
Neap_2006
Across
Time
(m/s)
21/07/2006
14:26
-0.88
21/07/2006
14:33
-1.05
21/07/2006
14:41
-0.98
From these data is evident that although the tidal ranges of the tides surveyed were similar the
magnitude of the measured currents at this location does not simply scale linearly with tidal range nor
can simply one tide be taken from the other in order to assess the effects the channel deepening
campaign has had on the flows.
Report No. 06/09/10047/1/JAT
26
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
For this reason a more complex approach to the analysis of the form outlined in the Methodology has
been adopted the results of which are presented in the following section.
3.2
Transect Points
In order to describe the variation of currents over time a depth averaged range corrected time-series
was constructed from a single transect point at the mid point of the each of the main ADCP transects
using the methodology described in Section 2.2 of this report.
For the purposes of the comparison we define Transect Point A as being located at the mid point of
Transect Line 1 ie outside the harbour entrance, with B at the mid point of Transect Line 2 within the
entrance itself and with C at the midpoint of the line running between Sandbanks and Brownsea
Island.
Along with the Depth Averaged current time series Near Surface and Near Bed time series were also
computed for each Transect point covering in the case of the latter time series those bins within 2.5m
of the surface and 2.5m of the bed in order to consider the variations in the current with depth.
Results from these transect point extractions are presented in Appendix B of this report with
Appendix B1 detailing the spatial variations of flow across the harbour through comparison of the
Depth Averaged, Near Bed and Near Surface time-series obtained at each of the 3 points from each
Survey whilst Appendix B2 is focussed in the variation of the streams with respect to depth with the
Depth Averaged, Near Surface and Near Bed time-series plotted for each transect point survey by
survey.
A further comparison of these time-series has been made for the Pre and Post Dredging surveys for
the Neap and Spring tides by calculating a pseudo depth averaged tidal diamond for each time series.
From this data set, which has been subjected to both temporal and spatial vector averaging, the
velocity difference between the Pre and Post Dredge surveys has been calculated for each tidal hour
has been calculated. These data are presented in the Tables (3.19-.3.24).
Table 3.19 Transect Point A Spring Tides Depth Averaged Range Corrected Velocity
Ebb
Flood
LWTime
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
Spring_2004
Speed m/s
0.350
1.023
0.320
0.195
1.455
1.501
0.502
1.275
1.449
1.027
0.702
0.347
0.691
Spring_2004
Dir (ºM)
146.7
106.6
317.3
285.0
118.4
114.5
078.0
312.0
291.0
287.3
283.5
273.7
112.9
Spring_2006
Speed m/s
0.768
0.626
0.474
0.479
1.533
1.591
0.541
1.071
1.064
0.700
0.351
0.109
0.598
Spring_2006
Dir (ºM)
272.7
132.6
289.2
273.7
147.2
138.1
119.3
314.1
304.1
307.2
320.9
011.1
148.8
Speed
Difference2
(m/s)
-0.418
0.397
-0.154
-0.284
-0.078
-0.090
-0.040
0.204
0.384
0.327
0.352
0.239
0.093
2
Speed Difference is defined as the velocity at the specified tidal hour determined from the 2004 survey minus
the equivalent velocity from the 2006 survey. Thus where the difference is negative the post dredge data shows
lower velocities to prevail.
Report No. 06/09/10047/1/JAT
27
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.20 Transect Point B Spring Tides Depth Averaged Range Corrected Velocity
Ebb
Flood
LWTime
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
Spring_2004
Speed m/s
0.350
0.766
0.421
0.336
1.193
1.567
0.451
1.544
1.815
1.318
0.843
0.426
0.538
Spring_2004
Dir (ºM)
146.7
119.9
336.1
298.6
135.1
134.9
092.0
330.8
310.7
301.2
293.8
282.8
134.5
Spring_2006
Speed m/s
0.768
0.456
0.895
0.699
1.339
1.444
0.423
1.525
1.617
1.044
0.575
0.118
0.579
Spring_2006
Dir (ºM)
272.7
123.3
290.5
277.8
138.5
135.3
119.6
307.0
294.9
299.3
308.7
338.7
121.5
Speed
Difference2
(m/s)
-0.418
0.309
-0.474
-0.363
-0.146
0.123
0.028
0.019
0.198
0.274
0.268
0.308
-0.041
Table 3.21Transect Point C Spring Tides Depth Averaged Range Corrected Velocity
Ebb
Flood
LWTime
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
Spring_2004
Speed m/s
0.350
0.487
0.554
0.272
1.077
1.241
0.224
1.229
0.984
0.451
0.268
0.048
0.479
Spring_2004
Dir (ºM)
146.7
170.5
035.9
030.5
188.3
186.7
121.8
024.3
008.5
006.9
006.7
208.5
185.3
Spring_2006
Speed m/s
0.768
0.459
0.382
0.532
1.322
1.509
0.384
1.006
0.777
0.385
0.256
0.191
0.380
Spring_2006
Dir (ºM)
272.7
197.4
354.0
259.8
199.2
194.4
175.6
004.6
347.1
350.5
321.4
229.6
182.0
Speed
Difference2
(m/s)
-0.418
0.027
0.172
-0.260
-0.245
-0.267
-0.160
0.223
0.207
0.066
0.012
-0.143
0.099
Consideration of the Spring tide data set for all three transect points suggests that generally for the
ebbing tide the flow speeds are lower on the post dredge surveys than on their pre-dredge equivalent
but show more positive values on the flooding tide suggesting a slight increase in flow speeds
following dredging of the channel.
However it is noteworthy that whilst most of the speed differences are relatively small <0.25 m/s
there are times when even after the process of range correction has been applied that the velocity
differences attain a values of twice this amount. Most of this variation reflects the differences in the
shape of the tidal curve between the pre and post dredge surveys and is particularly evident around
the HW period (LW-6 and LW +6) as a result of the stands of water level previously described and
which can result in flows being totally reversed from one tide to another.
Report No. 06/09/10047/1/JAT
28
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.22 Transect Point A Neap Tides Depth Averaged Range Corrected Velocity
Ebb
Flood
LWTime
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
Spring_2004
Speed m/s
0.350
0.337
0.209
0.661
1.054
0.939
0.485
0.327
0.150
0.552
0.620
0.362
0.165
Spring_2004
Dir (ºM)
146.7
153.1
141.8
132.9
128.0
119.8
085.8
068.7
025.8
136.3
164.8
125.6
153.7
Spring_2006
Speed m/s
0.768
0.099
0.242
0.386
0.879
0.820
0.329
0.408
0.596
0.539
0.549
0.353
0.290
Spring_2006
Dir (ºM)
272.7
198.1
258.9
203.9
162.9
135.9
137.5
283.7
278.1
243.8
218.1
200.0
246.1
Speed
Difference2
(m/s)
-0.418
0.237
-0.033
0.276
0.175
0.119
0.156
-0.081
-0.446
0.013
0.071
0.009
-0.125
Table 3.23 Transect Point B Neap Tides Depth Averaged Range Corrected Velocity
Ebb
Flood
LWTime
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
Spring_2004
Speed m/s
0.199
0.067
0.143
0.507
1.031
1.049
0.466
0.320
0.380
0.345
0.405
0.306
0.287
Report No. 06/09/10047/1/JAT
Spring_2004
Dir (ºM)
129.9
335.2
141.3
135.2
133.9
125.8
086.0
047.7
022.4
130.5
122.1
099.2
103.1
29
Spring_2006
Speed m/s
0.209
0.102
0.315
0.371
0.801
0.868
0.284
0.604
0.874
0.615
0.455
0.431
0.303
Spring_2006
Dir (ºM)
248.6
212.5
269.9
214.7
150.6
137.4
156.5
294.2
286.8
250.5
222.4
204.2
253.1
Speed
Difference2
(m/s)
-0.009
-0.035
-0.172
0.136
0.230
0.181
0.182
-0.283
-0.494
-0.270
-0.050
-0.125
-0.016
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.24 Transect Point C Neap Tides Depth Averaged Range Corrected Velocity
Ebb
Flood
LWTime
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
Spring_2004
Speed m/s
0.993
0.610
0.491
0.384
0.315
0.469
0.892
1.299
1.204
0.851
0.616
0.999
0.867
Spring_2004
Dir (ºM)
043.6
048.4
042.3
061.1
098.7
062.6
045.2
043.5
039.9
048.8
036.1
041.1
037.4
Spring_2006
Speed m/s
1.482
0.067
0.187
0.231
0.798
0.800
0.265
0.630
0.485
0.138
0.315
0.216
0.072
Spring_2006
Dir (ºM)
315.7
095.7
070.9
188.5
197.3
189.5
173.1
044.0
011.6
201.6
200.0
175.7
038.5
Speed
Difference2
(m/s)
-0.489
0.544
0.304
0.153
-0.483
-0.331
0.627
0.670
0.719
0.713
0.300
0.783
0.795
Differences in the calculated speed differences for the Neap tide data set show again that despite the
attempts made to correct for the highly distorted tidal curve prevalent on the Neap tides differences
between the 2004 and 2006 data still remain which are somewhat larger than are seen in the Spring
tide especially on the innermost transect line.
Using the tidal diamond time-series a statistical summary for each Transect Point has been computed
comparing the speeds measured at each transect point on the various tides surveyed the results of
which are presented in Tables 3.25 to 3.27 . These summaries also split the data into flood and ebb
based on the time relative to local LW such that a comparison between the magnitude of flood and
ebb currents can be made.
Table 3.25 Transect Point A - All Tides
Spring_2004
Speed m/s
1.50
0.20
0.83
0.48
Spring_2006
Speed m/s
1.59
0.11
0.76
0.44
Neap_2004
Speed m/s
1.05
0.15
0.48
0.28
Neap_2006
Speed m/s
0.88
0.10
0.48
0.24
Max Flood
Min Flood
Mean Flood
Std Dev Flood
1.50
0.20
0.76
0.56
1.59
0.47
0.86
0.49
1.05
0.21
0.58
0.32
0.88
0.10
0.50
0.31
Max Ebb
Min Ebb
Mean Ebb
Std Dev Ebb
1.45
0.35
0.86
0.41
1.07
0.11
0.63
0.35
0.62
0.15
0.38
0.18
0.60
0.29
0.44
0.12
All Tidal Hours
Max
Min
Mean
Stdev
Report No. 06/09/10047/1/JAT
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Table 3.26 Transect Point B – All Tides
Spring_2004
Speed m/s
1.81
0.34
0.89
0.53
Spring_2006
Speed m/s
1.62
0.12
0.88
0.48
Neap_2004
Speed m/s
1.05
0.07
0.42
0.30
Neap_2006
Speed m/s
0.87
0.10
0.48
0.25
Max Flood
Min Flood
Mean Flood
Std Dev Flood
1.57
0.34
0.73
0.48
1.44
0.42
0.86
0.40
1.05
0.07
0.49
0.41
0.87
0.10
0.42
0.30
Max Ebb
Min Ebb
Mean Ebb
Std Dev Ebb
1.81
0.43
0.99
0.57
1.62
0.12
0.84
0.57
0.47
0.29
0.36
0.06
0.87
0.28
0.51
0.21
Spring_2004
Speed m/s
1.24
0.05
0.59
0.40
Spring_2006
Speed m/s
1.51
0.19
0.64
0.41
Neap_2004
Speed m/s
1.30
0.31
0.77
0.31
Neap_2006
Speed m/s
1.48
0.07
0.44
0.40
Max Flood
Min Flood
Mean Flood
Std Dev Flood
1.24
0.22
0.60
0.40
1.51
0.38
0.77
0.47
0.99
0.31
0.59
0.26
1.48
0.07
0.55
0.51
Max Ebb
Min Ebb
Mean Ebb
Std Dev Ebb
1.23
0.05
0.53
0.43
1.01
0.19
0.48
0.30
1.30
0.62
0.96
0.23
0.63
0.07
0.30
0.20
All Tidal Hours
Max
Min
Mean
Stdev
Table 3.27 Transect Point C – All Tides
All Tidal Hours
Max
Min
Mean
Stdev
Report No. 06/09/10047/1/JAT
31
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
4.0
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
DISCUSSION OF RESULTS
4.1 Transect Lines
Three principal transect lines were extracted from the data sets collected on the Spring and Neap tide
surveys undertaken prior to and following the channel deepening works. Of these transects lines 1 is
located immediately outside the harbour entrance showing flows in the Swash Channel with line 2
lying just inside the entrance itself whilst line 3 lies between Brownsea Island and the Sandbanks spit
spanning Middle Ship Channel.
Flow speed and direction data has been plotted in terms of the distance from the transect origin which
in the case of transects 1 and 2 is the northern side of the channel whilst the origin of transect 3 is on
the western side of the channel.
Transect Line 1
Figures 4.1 to 4.4 show the cross sectional variations of current speed and direction on transect line 1,
the outermost of the three main transects at the times of peak flood and ebb for the 4 tides surveyed.
The line runs approximately NE-SW spanning the Swash Channel and just entering Shell Bay at it’s
northernmost limit.
Flows in this region are predicted by the modelling studies carried out as part of the Environmental
Impact Assessment to generally decrease by order 0.10 m/s on Spring tides on both flood and ebb as a
result of the widening and deepening of the channel. A small zone of increased currents is however
indicated in the centre of the channel on both flood and ebb in which the flows may increase by order
0.20 m/s of the flood and 0.15m/s on the ebb, these values being less than 10% of the peak flood and
ebb values.
The 3 dimensional model developed by HR Wallingford for the entrance channel also suggests that in
this area flows on the flooding tide would reduce following dredging as a result of the larger cross
sectional area through which the water can move. On the ebb tide however the 3d model predicted
little change in the magnitude of the velocities within the centre of the channel but shows the eddy
formed within the lee of the entrance located within Shell Bay to intensify resulting in increased
northerly velocities close to the southern margin.
Comparing the two Neap tide data sets for this line at the times of peak flood and ebb flow (as shown
in Figures 4.1 and 4.2) clearly shows the dominance of the ebb flows from the harbour with the
currents on the ebb attaining around 1.5 m/s compared with flood tide peak velocities of order 1.0
m/s. The stronger currents on the 2006 flood tide and 2004 ebb tide are also evident principally
resulting from differences in ranges and shape of the tidal curves on the two tides surveyed.
Differences between the flows on these two tides are notable in the more stratified nature of the flows
on the background survey (2004) data set with a more westerly flow at depth and on the northern
boundary of the channel with a more north-westerly flow at the surface on the southern side of the
channel. This structure is less clear in the 2006 data although there is some indication that this
structure may persist.
More obvious however is the region of apparent separation of the flows on the North and South banks
by the region of southerly flowing water approximately coincident with the channel centreline. This
flow region is associated with missing ADCP data suggesting that this part of the transect corresponds
to a zone of high shear since the ADCP is unable to resolve flow components under these conditions.
This is arises from the fact that the ADCP can resolve one horizontal and one vertical flow component
from each beam pair and must assume that the flow seen by each beam pair is homogenous. Since the
cells centres in each beam pair are separated by a distance of 7m at a depth of 10m below the surface
regions which are highly in-homogenous will inevitably result in the ADCP failing to estimate flows.
Report No. 06/09/10047/1/JAT
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
This same structure is evident on both the 2006 and 2004 Neap ebb tides and can also be discerned in
the flood data evident as a zone of darker blue colouring, close to the channel centreline in both flood
tide data sets.
The two Spring tide survey data sets are presented in Figures 4.3 and 4.4 respectively and show flows
of order 2.0 m/s to be attained on both flood and ebb at this location.
In terms of the variation of the flow with respect to the cross section again the pattern observed on the
Neap tides of more westerly flow on the northern bank and at depth with a more northerly westerly
flow on the southern margin and at the surface is still evident. Superimposed on this directional
variation is a clear variation in the distribution of velocity across the section with the strongest flows
on the flooding tide being found on the southern side of the main channel whilst on the ebbing tide the
strongest flow seems to favour the northern side of the deep water channel.
In assessing the differences pre and post dredge it is apparent that, at the times of peak flow, lower
velocities are seen on the Spring Flood in the 2006 data than are shown by the pre-dredge 2004 data.
This pattern fits well with the predictions made by the 3d modelling studies carried out as part of the
initial studies which suggest a reduction in the flood current speed of order 10%. However it is also
evident that the flow structure is also different with the stronger flows located only in the surface
layers close to the southern shore rather than extending across the section as seen in the 2004 data.
The two transects thus represent somewhat different discharge regimes and illustrate clearly the
problems involved in comparing these data sets.
Straightforward comparison of the flow velocity would indicate a reduction in the flood velocities as a
result of the channel deepening programme to be of the order 20%. This is somewhat higher than
predicted by the modelling leading to the suggestion that the differences seen in the pre and post
dredge surveys are unlikely to be solely the result of the channel deepening exercise but to also to
include variations induced other factors
These factors are suggested to include;
a) deviations from the planned survey line as a result of the difficulties associated in
navigating the vessel in the strong currents and with a high volume of small craft traffic (as
evidenced by differences in the bathymetric profiles from pass to pass)
.
b) variations in the timing of the survey passes within the tidal cycle (which can differ by
order 20 minutes)
c) variations in the nature of the tidal curves of the tides measured on each survey (identical
tides having a repeat frequency of approximately 19 years)
d) the impact of any residual currents arising from wind driven circulations in Poole Bay itself
or from the discharge of fresh water into the harbour .
Report No. 06/09/10047/1/JAT
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Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Curre nt Speed
Curre nt Spee d
Mask
Speed
0
2
3
3
4
4
1.5
1.5
1
8
9
10
Depth (m BSL)
7
Current Speed (m/s)
6
6
7
8
1
9
10
Current Speed (m/s)
5
5
11
11
0.5
12
12
13
13
0.5
14
14
15
15
0
50
100
150
Distance Along Transect (m)
200
0
250
50
Pass 013
100
150
Distance Along Transect (m)
200
250
Pas s 006
Curre nt Dire ction
Curre nt Direction
Mask
Direction
360
0
1
Mask
Direction
360
0
1
2
2
3
3
300
4
5
5
7
180
8
9
10
120
Depth (m BSL)
240
6
Current Direction (°)
4
7
8
10
11
12
60
14
15
15
0
200
120
60
13
14
100
150
Distance Along Transect (m)
180
9
12
50
240
6
11
13
300
0
250
50
Pass 013
Current Direction (°)
Depth (m BSL)
2
1
2
Depth (m BSL)
Mask
Speed
0
2
1
100
150
Distance Along Transect (m)
200
250
Pas s 006
Figure 4.1 Neap Tide 2004 Transect Line 1: a) Peak Flood Currents Pass 13 13/04/2004 12:34 L+1.82
b) Peak Ebb Currents Pass 6 13/04/2004 08:55 LW -1.83
Curre nt Speed
Curre nt Spee d
Mask
Speed
0
1
2
1
2
2
3
3
1.5
4
1.5
1
7
8
Depth (m BSL)
6
5
6
1
7
8
Current Speed (m/s)
4
5
Current Speed (m/s)
Depth (m BSL)
Mask
Speed
0
2
9
9
0.5
0.5
10
10
11
11
12
0
50
100
150
Distance Along Transect (m)
200
50
Pass 018
100
150
Distance Along Transect (m)
200
250
Pas s 012
Curre nt Dire ction
Curre nt Direction
Mask
Direction
360
0
1
Mask
Direction
360
0
1
2
2
300
3
300
3
4
180
7
8
120
240
Depth (m BSL)
6
5
6
180
7
8
120
9
9
10
60
60
10
11
11
12
0
50
100
150
Distance Along Transect (m)
200
0
12
250
50
Pass 018
100
150
Distance Along Transect (m)
200
250
Pas s 012
Figure 4.2 Neap Tide 2006 Transect Line 1: a) Peak Flood Currents Pass 18 21/07/2006 14:26 LW+1.52
b) Peak Ebb Currents Pass 12 21/07/2006 11:09 LW -1.76
Report No. 06/09/10047/1/JAT
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Compass Hydrographic Services Ltd
Current Direction (°)
4
240
5
Current Direction (°)
Depth (m BSL)
0
12
250
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Curre nt Speed
Curre nt Spee d
Mask
Speed
0
Mask
Speed
0
2
1
2
1
2
2
3
3
4
1.5
1
9
10
11
6
7
1
8
9
Current Speed (m/s)
8
Depth (m BSL)
7
Current Speed (m/s)
5
6
Depth (m BSL)
4
1.5
5
10
12
0.5
0.5
11
13
12
14
13
15
16
14
0
50
100
150
Distance Along Transect (m)
200
0
250
50
Pass 003
100
150
Distance Along Transect (m)
200
250
Pas s 022
Curre nt Dire ction
Curre nt Direction
Mask
Direction
360
0
1
Mask
Direction
360
0
1
2
2
3
3
300
300
4
8
180
9
10
11
120
Depth (m BSL)
240
7
Current Direction (°)
Depth (m BSL)
6
5
240
6
7
180
8
9
120
10
12
Current Direction (°)
4
5
11
13
12
60
14
60
13
15
16
14
0
50
100
150
Distance Along Transect (m)
200
0
250
50
Pass 003
100
150
Distance Along Transect (m)
200
250
Pas s 022
Figure 4.3 Spring Tide 2004 Transect Line 1: a) Peak Flood Currents Pass 3 08/04/2004 07:26 LW+1.52
b) Peak Ebb Currents Pass 22 08/04/2004 16:22
LW-1.88
Current Speed
Current Speed
Mask
Speed
2
0
1
1
2
2
3
3
1.5
4
1.5
1
7
8
Depth (m BSL)
6
6
1
7
8
9
0.5
10
5
0.5
9
10
11
11
12
0
50
100
150
Distance Along Transect (m)
200
0
250
50
Pass 005
100
150
Distance Along Transect (m)
200
250
Pass 024
Current Direction
Current Direction
Mask
Direction
360
0
1
Mask
Direction
360
0
1
2
2
300
3
300
180
7
8
120
9
4
Depth (m BSL)
6
Current Direction (°)
240
5
240
5
6
180
7
120
8
9
10
60
60
10
11
11
12
0
50
100
150
Distance Along Transect (m)
200
0
250
50
Pass 005
100
150
Distance Along Transect (m)
200
250
Pass 024
Figure 4.4 Spring Tide 2006 Transect Line 1: a) Peak Flood Currents Pass 512/09/2006 08:28 LW+1.56
b) Peak Ebb Currents Pass 24 12/09/2006 17:43LW -1.52
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Compass Hydrographic Services Ltd
Current Direction (°)
3
4
Depth (m BSL)
Current Speed (m/s)
4
5
Current Speed (m/s)
Depth (m BSL)
Mask
Speed
2
0
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Transect Line 2
Transect line 2 spans the harbour entrance proper and is situated slightly west of the Ferry starting
close to the bathymetric feature known as Chapman’s Peak on the Northern side of the channel and
running on a bearing of 226˚(Grid) across the main channel.
In this region the modelling studies carried out suggest that on the both flood and ebb tides the
enhancement of velocity due to the channel deepening would be small, of order 5cm/s, with the area
showing increased ebb tidal flows being restricted to the vicinity of South Haven Point where the ebb
current was predicted to increase by as much as 0.3 m/s locally.
The feature evident in the bathymetry extending out from the northern margin of the channel in the
2004 surveys is the feature known as Chapman’s Peak, a feature formed by the accumulation gravely
sands in an area of convergence of the residual flows within the harbour entrance. This feature has
been removed by dredging in previous channel deepening campaigns and was again removed in the
2006 campaign and accounts for the change in bathymetry seen in the 2006 surveys.
As can be seen from the figures (4.5 to 4.8) flows in this region exceed 2 m/s on both the Spring
Flood and Ebb although as at Transect Lines 1 and 3 an asymmetry is evident in the magnitude of the
flood and ebb currents with the ebb currents being the stronger.
Figure 4.5 shows the conditions prevailing in the main channel 1.4 hours after LW on the pre-dredge
Neap Tide surveys revealing a complex flow structure with a central core of water still exiting the
harbour bounded on it’s northern and southern margins by a flow of water into the harbour. Flows at
this time are however <0.5m/s throughout most of the cross section although the flow on the northern
side of the channel is slightly stronger approaching 1 m/s.
The corresponding ebb tide does not show the same degree of complexity with ebb currents reaching
1.5 m/s with a south easterly direction on the northern margin whilst flow speeds of order 1.0 m/s are
present on the southern bank with a slightly more southerly set.
The Neap data set for the post dredge survey is much simpler on the flood with the more westerly
flows evident on the southern side of the channel on the flood tide with slightly weaker currents with
the stronger more northerly westerly flows located on the Sandbanks side of the channel, the ebb tide
pattern being similar with an region of weaker flows apparent on the southern side of the channel.
Peak flows are however stronger than on the flood tide at ∼1.3 m/s but are slightly lower than the
2004 peak ebb values.
Consideration of the peak Spring tide flows, flood and ebb (LW+1.6 & LW-1.7), shows an intense
band of strong flows, order 2 m/s, to extend pretty much across channel and throughout the depth
profile on the flood tide in 2004 survey data, although an area of weaker flow is evident in the
uppermost 5m of the water column on the southern margin. On the ebb tide the strongest flows (again
order 2 m/s) are slightly more restricted spatially forming over the northern slope of the channel wall
with a much more pronounced area of slower flowing water to the south the interface between these
water masses being denoted by an area of strong shear where the ADCP failed to derive velocity
estimates. Directionally there is little lateral variation across the section on either the flood or ebb
flows at this time.
Contrasting this against the 2006 data for LW-1.8 and LW+1.4 suggests that following dredging the
region of strongest flood currents to be much more diffuse than was seen in the 2004 data forming at
the top of the slope of the northern channel wall. This finding is exactly line with the predictions
made by the 3d model which also suggested an overall reduction in velocity on the flood. The region
of weaker flow on the southern margin is still evident and is again associated with areas of high
turbulence where the ADCP cannot fails to derive velocity estimates. There is also once again
evidence of more westerly flows at depth and on the southern side of the channel such as was seen in
the Neap tide data although this may be an artefact of other factors.
Report No. 06/09/10047/1/JAT
36
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
As with the flood tide the flow structure seen in the peak ebb flows in the 2006 data is also more
diffuse and tends to indicate slightly lower velocities of the ebb.
The region of fastest flowing water again lies almost entirely on the northern channel wall and again
has a more diffuse appearance than was shown by the pre dredge flows. As the survey vessel tracked
across the transect a point was reached approximately halfway down the channel wall where the
turbulent nature of the flow was too intense to for the ADCP to derive reliable velocity estimates.
This zone of ‘no data return’ is more extensive than was seen in the 2004 data suggesting that
although the flows may be slightly lower they may be more turbulence as a result of a roughening of
the bed by the dredging process.
Report No. 06/09/10047/1/JAT
37
Compass Hydrographic Services Ltd
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Current Speed
0
Current Speed
0
Mask
Speed
2
1
2
2
3
1.5
6
1
7
8
9
10
1.5
4
5
Depth (m BSL)
6
1
7
8
9
10
0.5
11
0.5
11
12
12
13
13
0
0
50
100
150
Distance Along Transect (m)
200
0
20
Pass 012
40
60
80
100
120
140
160
Distance Along Transect (m)
180
200
220
Pass 006
Current Direction
0
Current Direction
0
Mask
Direction
360
1
Mask
Direction
360
1
2
2
300
3
300
3
4
4
180
7
8
9
120
240
5
Depth (m BSL)
6
Current Direction (°)
240
5
6
180
7
8
9
10
120
Current Direction (°)
Depth (m BSL)
5
Current Speed (m/s)
4
Current Speed (m/s)
3
Depth (m BSL)
Mask
Speed
2
1
10
11
11
60
12
60
12
13
13
0
0
50
100
150
Distance Along Transect (m)
200
0
20
Pass 012
40
60
80
100
120
140
160
Distance Along Transect (m)
180
200
220
Pass 006
Figure 4.5 Neap Tide 2004 Transect Line 2: a) Peak Flood Currents Pass 12 13/04/2004 12:08 LW+1.39
b) Peak Ebb Currents Pass 6 13/04/2004 09:05 LW-1.67
Current Speed
0
Current Speed
0
Mask
Speed
2
1
2
2
3
3
1.5
7
1
8
9
1.5
5
Depth (m BSL)
6
4
Current Speed (m/s)
6
7
1
8
9
10
10
0.5
11
0.5
11
12
12
13
13
14
0
50
100
150
Distance Along Transect (m)
0
14
200
50
Pass 019
100
150
Distance Along Transect (m)
200
Pass 013
Current Direction
0
Current Direction
0
Mask
Direction
360
1
Mask
Direction
360
1
2
2
3
300
300
3
4
4
7
180
8
9
120
10
5
Depth (m BSL)
240
6
Current Direction (°)
5
240
6
7
180
8
9
120
10
11
11
60
12
60
12
13
13
14
0
50
100
150
Distance Along Transect (m)
0
14
200
50
Pass 019
100
150
Distance Along Transect (m)
200
Pass 013
Figure 4.6 Neap Tide 2006 Transect Line 2: a) Peak Flood Currents Pass 19 21/07/2006 14:33 LW +1.64
b) Peak Ebb Currents Pass 13 21/07/2006 11:18 LW-1.61
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Compass Hydrographic Services Ltd
Current Direction (°)
Depth (m BSL)
5
Current Speed (m/s)
4
Depth (m BSL)
Mask
Speed
2
1
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Current Speed
Current Speed
Mask
Speed
2
0
1
1
2
2
3
3
7
1
8
9
Depth (m BSL)
6
1.5
4
Current Speed (m/s)
5
6
1
7
8
9
10
0.5
0.5
10
11
11
12
12
13
0
50
100
150
Distance Along Transect (m)
0
13
200
50
Pass 003
100
150
Distance Along Transect (m)
200
Pass 022
Current Direction
Current Direction
Mask
Direction
360
0
1
Mask
Direction
360
0
1
2
2
300
3
300
3
4
6
7
180
8
9
120
Depth (m BSL)
240
5
Current Direction (°)
4
240
5
6
180
7
8
120
9
10
Current Direction (°)
Depth (m BSL)
5
Current Speed (m/s)
1.5
4
Depth (m BSL)
Mask
Speed
2
0
10
11
60
60
11
12
12
13
0
50
100
150
Distance Along Transect (m)
0
13
200
50
Pass 003
100
150
Distance Along Transect (m)
200
Pass 022
Figure 4.7 Spring Tide 2004 Transect Line 2: a) Peak Flood Currents Pass 3 08/04/2004 07:31 LW+1.61
b) Peak Ebb Currents Pass 22 08/04/2004 16:35 LW -1.67
Current Speed
Current Speed
Mask
Speed
2
0
1
1
2
2
3
3
4
4
1.5
1
8
9
6
7
1
8
9
Current Speed (m/s)
7
Depth (m BSL)
6
Current Speed (m/s)
10
10
0.5
11
12
13
13
14
14
0
50
100
150
Distance Along Transect (m)
0.5
11
12
200
0
20
Pass 005
40
60
80
100
120
140
160
Distance Along Transect (m)
180
200
220
Pass 023
Current Direction
Current Direction
Mask
Direction
360
0
1
Mask
Direction
360
0
1
2
2
3
300
3
4
5
5
7
180
8
9
120
10
Depth (m BSL)
240
6
Current Direction (°)
4
300
240
6
7
180
8
9
120
10
11
11
12
12
60
13
13
14
14
0
50
100
150
Distance Along Transect (m)
200
60
0
20
Pass 005
40
60
80
100
120
140
160
Distance Along Transect (m)
180
200
220
Pass 023
Figure 4.8 Spring Tide 2006 Transect Line 2: a) Peak Flood Currents Pass 5 12/09/2006 08:16 LW+1.36
b) Peak Ebb Currents Pass 23 12/09/2006 17:24 LW-1.84
Transect Line 3
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Compass Hydrographic Services Ltd
Current Direction (°)
Depth (m BSL)
1.5
5
5
Depth (m BSL)
Mask
Speed
2
0
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Transect line 3 covers the cross section inside the main harbour between Brownsea Island and the
Sandbanks Spit to the north of the North Haven Beacon.
Model predictions of change in flow speed induced by the dredging of the channel suggest that in this
region the changes will be slight (<5cm/s) on both flood and ebb.
Flows in this area are generally slower than the flows within the main harbour entrance and the area is
known to be influenced by a large eddy feature which forms in the lee of the Sandbanks Spit and
which is clearly visible in the flow data in the 2004 Spring Ebb and 2006 Spring Flood as well as in
the Neap Ebb tide data set in the 2006.
The passes made at the times of peak flood and ebb currents are presented in Figures 4.9 and 4.10 for
the Neap tides and 4.11 and 4.12 for the Spring Tide data sets whilst the complete sequence of plots
for this data set is presented in Appendices A1.3, A2.3, A3.3 and A4.3
Flows on the 2004 Neap surveys show peak flood to attain 1.3m/s during the flood but only 0.5m/s on
the ebb whilst the 2006 data set shows flows of order 1.0m/s to be attained on both flood and ebb
Comparative values for the Spring tide yield values of ∼1.6m/s (flood and ebb) for the 2004 but lower
values (order 1.2 m/s) evident in the data sets 2006 with a lower overall discharge.
Inspection of the spatial variation in flow at the times of strongest currents (flood/ebb) suggests that
on the flooding tide flows are stronger on the Brownsea (western) side of the channel in the deeper
water whilst on the ebb the flows appear to be stronger on the eastern side. The exception being the
2004 Neap data set which shows very little variation in flow speed across the section on either the
flood or ebb. Generally the flow field is fairly uniform throughout the water column, although during
smaller tides there is evidence of a reduction in flow speed towards the bed
The strong feature seen in the 2006 Spring flood data cannot be attributed as a spike in the data since
there is no evidence of enhanced backscatter on this pass such as would occur if the ADCP passed
through a ship wake nor is there any indication that the data is invalid such as would be indicated by
the ADCP’s internal quality indices (error velocity, percent good returned echoes, correlation
magnitude , number of three beam solutions etc).
The above is also true for the feature evident in the 2006 Neap ebb tide plot, although in this case a
slight drop in data quality is evident.
Whilst there is no evidence to suggest these features are not real in the latter case the feature could be
associated with a zone of strong lateral shear.
All of the features described are consistent with the combination of the tidal streams with the residual
flows shown by the numerical modelling undertaken by HR Wallingford and which is presented in
Figure 1.5 which shows the residual current direction at the bed to change direction across the width
of the channel from northerly on the western side to southerly on the east.
This would account for the strongest velocities being seen on the western side of the transect on the
flood and the eastern side on the ebb as well as the possibility of zones of high lateral current shear
which would impact on the physics of the Doppler Profiler measurement principal causing data loss.
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Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Current Speed
Current Speed
Mask
Speed
2
0
1
1
2
2
3
3
1.5
4
8
9
10
5
6
1
7
8
9
0.5
Current Speed (m/s)
1
7
Depth (m BSL)
6
Current Speed (m/s)
0.5
10
11
11
12
13
0
50
100
150
Distance Along Transect (m)
12
0
200
50
Pass 012
100
150
Distance Along Transect (m)
200
Pass 005
Current Direction
Current Direction
Mask
Direction
360
0
1
Mask
Direction
360
0
1
2
2
300
3
300
3
4
4
180
7
8
120
9
240
Depth (m BSL)
6
Current Direction (°)
240
5
5
6
180
7
8
120
Current Direction (°)
Depth (m BSL)
1.5
4
5
Depth (m BSL)
Mask
Speed
2
0
9
10
11
10
60
12
60
11
13
0
50
100
150
Distance Along Transect (m)
12
0
200
50
Pass 012
100
150
Distance Along Transect (m)
200
Pass 005
Figure 4.9 Neap Tide 2004 Transect Line 3: a) Peak Flood Currents Pass 12 13/04/2004 12:17 LW+1.53
b) Peak Ebb Currents Pass 5 13/04/2004 08:43 LW -2.03
Current Speed
Current Speed
Mask
Speed
2
0
1
1
2
2
3
1.5
6
1
7
8
1.5
4
Depth (m BSL)
5
Current Speed (m/s)
4
5
1
6
7
Current Speed (m/s)
3
Depth (m BSL)
Mask
Speed
2
0
8
9
0.5
10
9
11
10
12
11
0
50
100
150
Distance Along Transect (m)
0.5
200
0
40
Pass 020
60
80
100
120
140
160
Distance Along Transect (m)
180
200
Pass 014
Current Direction
Current Direction
Mask
Direction
360
0
1
Mask
Direction
360
0
1
2
2
300
3
300
3
180
7
8
120
4
240
5
180
6
7
9
8
10
9
120
60
60
11
10
12
11
0
50
100
150
Distance Along Transect (m)
200
0
40
Pass 020
60
80
100
120
140
160
Distance Along Transect (m)
180
200
Pass 014
Figure 4.10 Neap Tide 2006 Transect Line 3: a) Peak Flood Currents Pass 20 21/07/2006 14:42 LW+1.79
b) Peak Ebb Currents Pass 14 21/07/2006 12:04 LW-0.84
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Current Direction (°)
6
Depth (m BSL)
240
5
Current Direction (°)
Depth (m BSL)
4
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Current Speed
Current Speed
Mask
Speed
2
0
1
Mask
Speed
2
0
1
2
2
3
1
7
8
9
5
1
6
7
Current Speed (m/s)
6
1.5
4
Depth (m BSL)
5
Current Speed (m/s)
4
Depth (m BSL)
3
1.5
8
0.5
10
0.5
9
11
10
12
11
0
13
50
100
150
Distance Along Transect (m)
200
0
20
Pass 003
40
60
80
100
120
140
Distance Along Transect (m)
160
180
200
Pass 022
Current Direction
Current Direction
Mask
Direction
360
0
1
Mask
Direction
360
0
1
2
2
300
3
300
3
180
7
8
120
9
4
240
5
180
6
7
120
8
Current Direction (°)
6
Depth (m BSL)
240
5
Current Direction (°)
Depth (m BSL)
4
10
9
60
11
60
10
12
11
0
13
50
100
150
Distance Along Transect (m)
200
0
20
Pass 003
40
60
80
100
120
140
Distance Along Transect (m)
160
180
200
Pass 022
Figure 4.11 Spring Tide 2004 Transect Line 3: a) Peak Flood Currents Pass 3 08/04/2004 07:38 LW+1.73
b) Peak Ebb Currents Pass 22 08/04/2004 16:45 LW-1.49
Current Speed
Current Speed
Mask
Speed
2
0
1
1
2
2
3
1.5
6
1
7
8
Depth (m BSL)
5
5
1
6
7
8
0.5
9
1.5
4
Current Speed (m/s)
0.5
9
10
10
11
0
40
60
80
100
120
140
Distance Along Transect (m)
160
180
0
11
200
80
Pass 008
100
120
140
160
Distance Along Transect (m)
180
200
Pass 034
Current Direction
Current Direction
Mask
Direction
360
0
1
Mask
Direction
360
0
1
2
2
300
3
300
3
6
180
7
8
120
4
Depth (m BSL)
240
5
Current Direction (°)
4
240
5
180
6
7
120
8
9
9
60
10
60
10
11
0
40
60
80
100
120
140
Distance Along Transect (m)
160
180
0
11
200
80
Pass 008
100
120
140
160
Distance Along Transect (m)
180
200
Pass 034
Figure 4.12 Spring Tide 2006 Transect Line 3: a) Peak Flood Currents Pass 8 12/09/2006 08:40 LW+1.76
b) Peak Ebb Currents Pass 34 12/09/2006 18:33 LW-0.70
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Current Direction (°)
Depth (m BSL)
4
Current Speed (m/s)
3
Depth (m BSL)
Mask
Speed
2
0
Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
From this analysis it is clear that whilst the transect data highly valuable in that it provides a snapshot
of the spatial variation of flows in the channel factors such as the variation of the ranges and the
shapes of the tidal curves between the individual tides as well as the relative timing of the passes
along the transect makes it difficult to make a direct comparison of the data.
This can be shown by Figure 4.13 which compares the tide curves for the 4 survey data sets.
Poole Harbour Observed Tides - Neap Surveys
1
Tidal Elevation (mAODN)
0.5
0
-0.5
-1
-1.5
-6.5
-5.5
-4.5
-3.5
-2.5
-1.5
-0.5
0.5
1.5
2.5
3.5
4.5
5.5
6.5
3.5
4.5
5.5
6.5
LW+/Neap 2004
Neap 2006
Poole Harbour Observed Tides - Spring Surveys
1.00
Tidal Elevation (mAODN)
0.50
0.00
-0.50
-1.00
-1.50
-6.5
-5.5
-4.5
-3.5
-2.5
-1.5
-0.5
0.5
1.5
2.5
LW +/Spring 2004
Spring 2006
Figure 4.13 Observed tidal curves for the Spring and Neap tide surveys
undertaken pre & post dredging showing the relative timings of passes made on
transect line 1.
On the positive side the figure clearly demonstrates that whilst the surveys provide a good coverage of
the tidal cycle in terms of the timing of passes made across the transect line and are indeed
comparable from the survey to survey
However it is clear that although both Spring and Neap tides pre and post dredging have similar
ranges the actual tidal curves are quite different. For example the gradient of the flooding tide of the
2006 survey is somewhat steeper than in the pre dredge surveys.
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Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Similarly there are pronounced differences between the two ebb tides in the Spring tide data sets
although the flood tides are more comparable.
Also evident is the fact that successive tides are quite different especially on the Neaps and thus the
question as to whether the measured data actually represents a true picture of a single tidal cycle arise.
For example on the Spring tide survey for 2006 measured ebb tide is smaller in range than the
corresponding flood tide.
Such variations have implications on the magnitude of the flows flood and ebb since the flow
responds principally to the regional gradient of sea level.
Since the return period of the tides is order 19 years it is difficult in practice to find tides which are
exactly identical although, in retrospect, it may have been possible to find tides which were closer in
character to a mean Spring and Neap or were just more comparable. However within the operational
constraints of the study these data must form the basis of the comparison.
In attempting to use a more quantitative methodology with which to analyse the data available an
approach of calculating a pseudo-tidal diamond has been adopted which encompassed the corrections
for deviations in the range of the measured tide from that of a mean Spring or a mean Neap tide.
The following sections compares and contrasts these data and attempts to draw conclusions as to the
impact that the channel deepening has had on each of the transects for Spring and Neap Tides.
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Poole Harbour Commissioners
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
4.2 Transect Point Time Series
The depth averaged time series constructed from the data falling within the three transect points is
presented in Figure 4.14 to 4.16 for both Spring and Neap tides pre and post dredge.
To produce these plots the data collected from each in the field has been vector averaged over the
vertical to form depth average velocity values and then averaged spatially again using vector
averaging, such that all data falling within a circle of 50m radius has been averaged into a single value
of U and V for each pass (Pass Averaging).
The resultant U & V velocity component data have been fitted with a splined curve and scaled on the
basis of the tidal range at each time interval within the tidal cycle to an equivalent mean Spring or
Neap tide before calculating the current speed and direction.
Using this approach it is anticipated that the time-series are as comparable as possible with all
temporal and range artefacts removed from the data. It is also important to understand that the
spatially averaging applied will have taken out much of the spatial variability seen in the transect data
previously described in section 4.1.
Even after this process it is evident that the time-series for the two Neap tides are vastly different
however in all cases the Spring tides are much closer.
Transect Point A
Considering the Neap tide data for Transect Point A it is apparent that in the 2004 survey that the
asymmetry between the flood and ebb was much exaggerated such that the flows appeared to ebb
throughout most of the tidal cycle. The data for the post dredge Neap surveys on the other hand
showing much more distinct flood and ebb phases within the tide.
The apparent dominance of seaward flows in the 2004 Neap surveys masks the finding detailed in
Section 4.1 in that there exist definite zones of flow across the section seen in the transect data such
that the tide can be flooding and ebbing at the same time on different parts of the cross section. The
directional sensitivity of this behaviour will be lost in the spatial averaging process resulting in the
velocities in opposing directions cancelling out such that the vector average will reflect the dominant
flow direction and magnitude.
Considering the current speed time series and using the time with respect to low water as a guide two
peaks are evident on the Neap tide the first occurring 2 hours before LW corresponding to the ebb
flow with a second lower peak occurring anywhere between 1 and 4 hours after LW corresponding to
the flooding phase.
The two speed time-series show, in both cases, the ebb maxima to be almost twice that of the flood
maxima whilst on both flood and ebb tides the 2004 data shows the currents to be comparable to that
seen in the post dredge surveys.
The pre and post dredge Spring tide time-series shown in figure 4.14b are much more coherent in both
their overall pattern and in terms of the comparative behaviour.
The Spring tide time-series suggest that at this point currents flow into the harbour between LW-4.5
and LW-2.5 with the main flood occurring between LW+0.5 and LW+5.5 with the flood tide velocity
reaching it’s maximum around 2 hours after LW.
Two phases of currents leaving the harbour are observed the first between LW-2.5 and LW when
currents are at their maximum with a second phase occurring between LW +5.5 and LW-4.5.
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Transect Point A
2.00
360
1.80
315
1.60
270
225
1.20
1.00
180
0.80
135
Current Direction (deg M)
Current Speed (m/s)
1.40
0.60
090
0.40
045
0.20
0.00
-8.0
000
-6.0
-4.0
-2.0
0.0
2.0
4.0
Hours +/- LW
Ebb
Neap 2004 - Speed
Neap 2006 - Speed
6.0
8.0
Flood
Neap 2004 - Direction
Neap 2006 - Direction
Transect Point A
2.00
360
1.80
315
1.60
270
225
1.20
1.00
180
0.80
135
Current Direction (deg M)
Current Speed (m/s)
1.40
0.60
090
0.40
045
0.20
0.00
-6.0
000
-4.0
-2.0
Ebb
Spring 2004 - Speed
0.0
2.0
4.0
Hours +/- LW
Spring 2006 - Speed
6.0
8.0
Flood
Spring 2004 - Direction
Spring 2006 - Direction
Figure 4.14 Range Corrected Depth Averaged Current Speed and Direction Time Series – Transect Point
A; a) Neap Tides b) Spring Tides
This phasing of the flows into/out of the harbour relates to the formation of the stands of water level
evident in the tidal curves.
In terms of the magnitudes of the currents pre and post dredging the ebb currents during the main
period of ebb flows are slightly increased with respect to their 2004 values, order <0.05 m/s (3%) but
are reduced during the second period of weaker flows (order 0.30 m/s -15%).
On the flood tide following dredging the strength of the peak flood currents are also reduced (order
(0.2m/s - 26%) in comparison to the pre dredge conditions although the earlier period of flooding
currents is slightly elevated order (0.2 m/s -12%).
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Transect Point B
For the point lying at the centre of Transect Line 2 just inside of the harbour entrance again the data
for the two Neap tides are widely different with ebb tide conditions in the prevailing between LW-3
and LW pre dredge surveys when flows reached their maximum of 1.2 m/s with no obvious flood tide
maxima, although as with the 2006 data set clear peaks are observed at LW+2 and LW-1
corresponding to the flood and ebb tide maxima respectively the ebb tide maxima being lower in the
2006 data.
The Spring tide pattern is again similar to that seen at Point A with 2 phases of flood and ebb the
flood tide peak velocity occurring 2 hours after LW with the Ebb maxima occurring between LW-1
and LW-2.
Transect Point B
2.00
360
1.80
315
1.60
270
225
1.20
1.00
180
0.80
135
Current Direction (deg M)
Current Speed (m/s)
1.40
0.60
090
0.40
045
0.20
0.00
-8.0
000
-6.0
-4.0
-2.0
0.0
2.0
4.0
Hours +/- LW
Ebb
Neap 2004 - Speed
Neap 2006 - Speed
6.0
8.0
Flood
Neap 2004 - Direction
Neap 2006 - Direction
Transect Point B
2.00
360
1.80
315
1.60
270
225
1.20
1.00
180
0.80
135
Current Direction (deg M)
Current Speed (m/s)
1.40
0.60
090
0.40
045
0.20
0.00
-6.0
000
-4.0
-2.0
Ebb
Spring 2004 - Speed
0.0
2.0
4.0
Hours +/- LW
Spring 2006 - Speed
6.0
8.0
Flood
Spring 2004 - Direction
Spring 2006 - Direction
Figure 4.15 Range Corrected Depth Averaged Current Speed and Direction Time Series – Transect Point
B; a) Neap Tides b) Spring Tides
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
At this point the Spring tide data show both ebb peaks and the main flood peak show the velocities to
have reduced following channel deepening with only the first phase of flooding at around LW-3.5 to
be elevated.
The actual magnitudes of the velocity differences being 0.25m in the case of the peak ebb currents
and <0.05m/s in the case of the flood corresponding to a reduction of 14% and 3% of their 2004
values respectively.
Interestingly at this point it appears that on the Spring tides the flood and ebb currents are much more
similar in magnitude although this may be an artefact of the averaging process.
Transect Point C
The data for the transect point C, which is located within the Harbour proper, shows on the predredge
Neap survey flows to reach a maximum value of approximately 1.4 m/s with a North Easterly set
around LW+1, i.e. flooding, with no corresponding ebb peak evident in the data.
On the 2006 survey however peaks in the velocity time-series are visible at LW-4.0, LW-2.0 LW+1.0
and LW+4.0 the peaks at LW-4 and LW+1 corresponding to northerly setting currents (flooding
tides).
Of these peaks only the peak at HW-2.0 and LW+1.0 are significant with the flood maxima reaching
0.75m/s whilst the main ebb appears somewhat higher at around 0.90 m/s again indicating a strong
asymmetry between flood and ebb.
The corresponding Spring data set shows again 4 peaks (2 flood 2 ebb) with the peak flood occurring
approximately 1 hour after LW and the main main peak occurring around LW-2. On the 2004 Spring
survey the flood and ebb peaks are similar in magnitude reaching 1.40 m/s but on the 2006 Spring
show the more typical Ebb/Flood Asymmetry with the flood currents reaching 1.15 m/s and ebb
currents the 1.60m/s.
The 2nd phase of ebbing is of similar magnitude on both tides whilst the smaller 1st flood peak which
occurs around LW-4 shows stronger currents to prevail in the predredge surveys data set.
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Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
Transect Point C
2.00
360
1.80
315
1.60
270
225
1.20
1.00
180
0.80
135
Current Direction (deg M)
Current Speed (m/s)
1.40
0.60
090
0.40
045
0.20
0.00
-8.0
000
-6.0
-4.0
-2.0
0.0
2.0
4.0
Hours +/- LW
Ebb
Neap 2004 - Speed
Neap 2006 - Speed
6.0
8.0
Flood
Neap 2004 - Direction
Neap 2006 - Direction
Transect Point C
2.00
360
1.80
315
1.60
270
225
1.20
1.00
180
0.80
135
Current Direction (deg M)
Current Speed (m/s)
1.40
0.60
090
0.40
045
0.20
0.00
-6.0
000
-4.0
-2.0
Ebb
Spring 2004 - Speed
0.0
2.0
4.0
Hours +/- LW
Spring 2006 - Speed
6.0
8.0
Flood
Spring 2004 - Direction
Spring 2006 - Direction
Figure 4.16 Range Corrected Depth Averaged Current Speed and Direction Time Series – Transect Point
C; a) Neap Tides b) Spring Tides
In order to summarise these data tables 4.1 to 4.3 show the differences in current speed (m/s) pre and
post dredging calculated from the range corrected tidal diamonds calculated at each point for the
Spring and Neap tides and presented in Tables 3.19- 3.24.
These tables details the statistics for the entire data set data but also with the data split into nominal
flood and ebb values based on their time relative to local LW (Ebb Corresponding to LW-6 to LW
and Flood LW to LW+6.0).
In calculating these tidal diamonds the data from the range corrected time-series has been further
averaged such that values of Speed and Direction corresponding to LW actually represents a vector
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Measurement of the Changes in Flow
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average of all the velocity measurements falling between LW-0.5 and LW+0.5 in a manner similar to
that for conventional tidal diamonds.
The tables thus show the differences in current speed at each site between the 2004 and 2006 data as
specified by the pre-dredge or background values minus the equivalent velocity from the post dredge
(2006) survey. Thus where the difference is negative the post dredge data shows lower velocities to
prevail as a possible result of the deepening of the entrance channel.
Table 4.1 Speed Difference Pre-Post Channel Deepening Springs
Transect Point A
Transect Point B
All Tidal Hours
Spring Tide
Spring Tide
0.40
0.31
Max
-0.42
-0.47
Min
0.07
0.01
Mean
0.27
0.28
Stdev
Transect Point C
Spring Tide
0.22
-0.42
-0.05
0.21
Max Ebb
Min Ebb
Mean Ebb
Std Dev Ebb
0.40
-0.42
-0.10
0.25
0.31
-0.47
-0.13
0.30
0.17
-0.42
-0.16
0.20
Max Flood
Min Flood
Mean Flood
Std Dev Flood
0.38
-0.04
0.22
0.15
0.31
-0.04
0.15
0.14
0.22
-0.16
0.04
0.15
Table 4.2 Speed Difference Pre-Post Channel Deepening Neap Tides
Transect Point A
Transect Point B
All Tidal Hours
Neap Tide
Neap Tide
0.28
0.23
Max
-0.45
-0.49
Min
0.00
-0.06
Mean
0.22
0.21
Stdev
Transect Point C
Neap Tide
0.80
-0.49
0.33
0.48
Max Ebb
Min Ebb
Mean Ebb
Std Dev Ebb
0.28
-0.42
0.07
0.24
0.23
-0.17
0.07
0.15
0.63
-0.49
0.05
0.48
Max Flood
Min Flood
Mean Flood
Std Dev Flood
0.16
-0.45
-0.06
0.19
0.18
-0.49
-0.15
0.22
0.80
0.30
0.66
0.17
The apparent large differences in the Neap tide data reflect the wide differences in the shapes of the
two Neap tidal curves and thus little inference can be made as to how flow has changed in anything
other than a semi-quantitative manner.
Comparing the Spring tide data does however suggest that overall the change in velocities as a result
of the channel deepening has been negligible with average values of the Spring tide speed differences
of <0.1 m/s (0.2 of a knot).
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Splitting this into nominal ebb and flood values based on the time relative to LW suggests that at all
three sites the strength of flows on the rising tide may have slightly increased with respect to their
2004 values whilst the strength of the flows on the falling tide has reduced.
On the rising tide the change is greatest outside the harbour whilst on the falling tide it is within the
harbour that the greatest change was recorded.
It should be noted however that due to complexity of the tidal curve at Poole splitting the data into
rising and falling tides is something of an oversimplification since the two periods actually include
phases of flow both into and out of the harbour due the presence of stands of sea level in the tidal
curves.
Calculating the statistics of the current speeds based on subdividing the data based on the actual
direction of flow (positive or negative with respect to the across transect flow component) yields the
following statistics for the three transect points.
Table 4.3 Speed Difference Pre-Post Channel Deepening Spring Tides Subdivided Based on the Direction
of Flow Across the Transect
Transect Point C
Transect Point A
Transect Point B
Spring Tide
Spring Tide
Spring Tide
Difference between Max Ebb Tidal Currents
Difference between Min Ebb Tidal Currents
Difference between Mean Ebb Tidal Currents
0.09
-0.24
-0.09
-0.12
0.07
0.04
0.27
0.14
0.14
Difference between Max Flood Tidal Currents
Difference between Min Flood Tidal Currents
Difference between Mean Flood Tidal Currents
-0.38
0.16
-0.06
-0.20
-0.22
-0.05
0.25
-0.01
0.04
Table 4.4 Speed Difference Pre-Post Channel Deepening Neap Tides Subdivided Based on the Direction of
Flow Across the Transect
Transect Point A
Transect Point B
Transect Point C
Neap Tide
Neap Tide
Neap Tide
Difference between Max Ebb Tidal Currents
Difference between Min Ebb Tidal Currents
Difference between Mean Ebb Tidal Currents
Difference between Max Flood Tidal Currents
Difference between Min Flood Tidal Currents
Difference between Mean Flood Tidal Currents
-0.17
0.18
0.20
-0.18
0.14
0.20
0.18
-0.20
-0.11
-
-
-
The Spring tide results thus show that the maximum difference pre and post channel deepening to be
of the order of 0.4m/s lower on the flooding tide than the background survey values with largest
outside the harbour whilst on the ebb the peak difference was at most 0.3m/s this occurring inside the
harbour on Transect Line 3. On average however the differences suggest that on the flood the flow
inside the harbour is now slightly lower than prior to dredging whilst seaward going currents are now
lower outside the harbour but inside may be slightly higher on average. What is however clear is that
the changes seen in the data set are small when compared with the overall flow velocities in the
channel which can exceed 2.0 m/s on Spring tides
As the differences quoted in Tables 4.3 and 4.4 refer to all phases of the flood and ebb tides not just
the times of peak currents they are considered to represent a more realistic subdivision of the flows
over the tidal cycle as a whole
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In Summary; from the evidence presented here it is difficult to build a clear picture of how the flow
has been altered by the dredging of the channel. This is partly because the changes themselves are
small but more because, even though the tides surveyed were of similar overall range, the shapes of
the curves and hence the regional gradients of sea level were quite different resulting very different
patterns of flow through the entrance channel.
Taking the picture revealed by this analysis as a whole, however, it is clear that little useful
information on the changes seen following channel deepening can be gleaned from the Neap tide data
set. However on Springs it is suggested that the results obtained from the 2006 transect data at the
times of peak velocities show on the ebb tide the flows to be slightly weaker and more diffuse than
were observed in the pre dredge data set whilst the flood tide currents are reduced in line with model
predictions as a result of the larger cross section offered by the deepened channel.
4.3 Recommendations
The present study has revealed the problems associated with attempting to extract comparable data
pertinent to variations in current regime prevailing in an area of complex tidal dynamics from what
are effectively a series of three dimensional shapshots of the flows.
As the flow regime is closely linked with the regional gradients of sea level future studies would
benefit from a longer term set of current and tidal elevation observations adjacent to the entrance
through deployment of a recording current meter or through access to model based time-series of
these parameters.
Using such a data set would allow the linkage between tidal elevation and the resulting flows to be
better defined with the result that an improved approach to range correction strategy could be
developed. This would facilitate variations in current velocity arsing directly from variations in the
range of the tides to be more accurately defined.
One possible approach would be to partition flows and elevations into their harmonic constituents
recombining them into a time-series for the surveyed tides and using this to as a guide to adjust the
measured values of velocity. This would require as a minimum a time-series of 29 days duration but
would provide the ability to quantify the relative proportions of the signal contributed by each of the
principal tidal constituents at each tidal hour making it possible to provide a better comparison of the
transect data.
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5.0 CONCLUSIONS
5.1
An analysis has been made of the flows prevailing within the entrance to Poole Harbour pre
and post dredging following a major capital dredging program to deepen the approach channel
to the port of Poole.
5.2
Surveys were undertaken on both Spring and Neap Tides pre and post construction using a
vessel mounted Acoustic Doppler Current Profiler with bottom tracking correction determined
using RTK GPS positioning data.
5.3
Both Spring and Neap tide surveys were undertaken on tides of similar range however in all
cases there were however significant differences in the shapes of the tidal curves for each tide
5.4
These distortions in the shape of the tidal curves with respect to the mean tidal curve are
typical of the area and are most prevalent on Neaps but are also present on Spring tides and
make it difficult to undertake a direct comparison of the data in any form since slight
differences in the shapes of the tidal curves (stands of water level) and in the timing of each
pass relative to local high or low water can result in very different tidal streams being measured
making comparison of two time series problematic.
5.5
In order to overcome these limitations a comparison of transect points as well as the flows
across the measured transect line at the time of peak flood/ebbhas been made.
5.6
A transect point may be thought of as a point in space which is visited as repeatedly over the
course of the survey such that a time-series of the prevailing currents can be built by averaging
the data from each pass through the point into a single value of speed and direction. Each pass
through the point being referenced to a fixed point in the tidal curve in this case local low water
as determined from the tide gauge data incorporated into the data set.
5.7
In order to construct comparable time-series which account for the variations in the shape of
the tidal curves on the survey days the data extracted at each transect point has been
interpolated using a spline interpolator and resampled every 10 minutes and scaled in terms of
the intra-tidal range variation through comparison with a theoretical mean spring or neap tide
constructed from the range factors detailed in the Admiralty Tide Tables
5.8
It should however be that a degree of caution must be applied to the interpretation of this data
since the analysis ignores the effects of non-tidal forces such as wind and barometric pressure
effects which cannot be accounted for and hence compensated.
5.9
Comparing the two Neap tide data sets for the transect line located outside the harbour clearly
shows the dominance of the ebb flows from the harbour with the currents on the ebb attaining
around 1.5 m/s compared with flood tide peak velocities of order 1.0 m/s whilst both Spring
tide data sets show flows of order 2.0 m/s to be attained on both flood and ebb.
5.10
The Spring tide time-series suggest that water flows into the harbour between LW-4.5 and
LW-2.5 with the main flood occurring between LW+0.5 and LW+5.5 with the flood tide
velocity reaching it’s maximum around 2 hours after LW.
5.11
Two phases of currents leaving the harbour are also observed the first between LW-2.5 and
LW when currents are at their maximum with a second phase occurring between LW +5.5 and
LW-4.5.
5.12
Comparing these data pre and post dredge suggest that velocities during the main period of
ebb flows are slightly increased with respect to their 2004 values, order <0.05 m/s (3%) but are
reduced during the second period of weaker flows (order 0.30 m/s -15%).
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5.13
On the flood tide following dredging the strength of the peak flood currents are also reduced
(order (0.2m/s - 26%) in comparison to the pre dredge conditions although the earlier period of
flooding currents is slightly elevated order (0.2 m/s -12%).
5.14
The second transect just inside the entrance shows flows to exceed 2 m/s on both the Spring
Flood and Ebb with an asymmetry is evident in the magnitude of the flood and ebb currents
with the ebb currents being the stronger.
5.15
As at the transect point 1 outside the harbour the two Neap tides show widely different
behaviours with ebb tide conditions in the prevailing between LW-3 and LW during the pre
dredge surveys when flows reached their maximum of 1.2 m/s with no obvious flood tide
maxima. The 2006 data set on the other hand shows peaks at LW+2 and LW-1 corresponding to
the flood and ebb tide maxima respectively.
5.16
The Spring tide pattern for transect point 2 shows 2 phases of flood and ebb with the flood
tide peak velocity occurring 2 hours after LW with the Ebb maxima occurring between LW-1
and LW-2.
5.17
At this point the Spring tide data show both ebb peaks and the main flood peak to have
reduced following channel deepening with only the first phase of flooding at around LW-3.5 to
be elevated with respect to the 2004 values.
5.18
The magnitudes of these velocity differences observed between the pre and post dredge
surveys is 0.25m in the case of the peak ebb currents and <0.05m/s in the case of the flood
corresponding to a reduction of 14% and 3% of their 2004 values respectively.
5.19
Data for the transect line located inside the main harbour between Brownsea Island and the
Sandbanks Spit to the north of the North Haven Beacon shows flows to be generally slower
than the flows within the main harbour entrance and influenced by a large eddy feature which
forms in the lee of the Sandbanks Spit. In this region relatively changes of velocity <0.05 m/s
have been predicted in response to channel deepening.
5.20
At this site the 2006 data set shows peak values of flood currents to have a maxima of
0.75m/s whilst the main ebb current on Neaps attain 0.90 m/s. On the Neap tide surveyed in
2004 however only a single maximum with a flood orientation is visible with a peak reaching
1.40 m/s.
5.21
The corresponding Spring data set again 4 exhibits peaks (2 flood 2 ebb) with the peak flood
occurring approximately 1 hour after LW with the main peak occurring around LW-2. On the
2004 Spring survey the flood and ebb peaks are similar in magnitude reaching 1.40 m/s but on
the 2006 Spring show the more typical Ebb/Flood Asymmetry with the flood currents reaching
1.15 m/s and ebb currents the 1.60m/s.
5.22
Even after scaling the data to an equivalent mean neap tide it is apparent that the two Neap
tides surveyed remain too different in character to be able to draw any meaningful conclusions
from. The Spring tide data sets are however more comparable and since they provide the
strongest current velocities are the most interesting and important in terms their effects and the
ability of the hydrodynamic models used to predict the changes.
5.23
Comparison of the Spring tide data pre and post deepening of the basis of tidal diamonds
computed from range corrected time-series for a transect point on each transect line suggests
that overall the change in velocities as a result of the channel deepening has been negligible
with average values of Spring tide speed differences being <0.1 m/s (0.2 of a knot).
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5.24
Subdividing this further into nominal ebb and flood values based on the time relative to LW
suggests that at all three sites the strength of flows on the rising tide may have slightly
increased with respect to their 2004 values whilst the strength of the flows on the falling tide
has reduced.
5.25
On the rising tide the change is greatest outside the harbour whilst on the falling tide it is
within the harbour that the greatest change was observed.
5.26
As both the rising and falling tides include phases of flow both into and out of the harbour
due the presence of stands of sea level in the tidal curves a more accurate picture can be gained
by subdividing the data based on the actual direction of flow (positive or negative with respect
to the across transect flow component) These differences thus refer to all phases of the flood
and ebb tides not just the times of peak currents.
5.27
Presenting the data in this manner shows that following channel deepening the biggest change
observed was in the Spring flood tidal streams which were at most 0.4m/s lower on the flooding
tide following dredging with largest differences found outside the harbour.
5.28
On the ebbing tide the largest difference observed was an increase of 0.3m/s this occurring
inside the harbour on Transect Line 3.
5.29
On average however the differences between the 2006 and 2004 data sets are less than 0.5
knots with the flood flow now being slightly lower at two of the sites (B & C) than was the case
prior to dredging whilst the ebb currents are now lower outside the harbour but inside may have
been slightly increased.
5.30
Since it is can be seen from the time-series that these differences largely represent variations
of flow behaviour during the stands of sea level the implication is that overall the differences
in the magnitude of the currents following the capital dredging program are small compared to
strong currents which prevail at the times of peak flood and ebb.
.
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6.0 REFERENCES
Dyrynda, P, (2003)
Marine ecology of Poole Harbour, http://www.swan.ac.uk/biodiv/poole)
Gordon R.L. (1996)
Acoustic Doppler Current Profiler Principles of Operation: A Practical Primer.2nd Edition. Publ. RD
Instruments, San Diego. .
HR Wallingford (2004)
Poole Harbour Approach Channel Deepening - Hydrodynamic and sedimentation studies. EX4945Poole Harbour studies rev3-0.doc.
Simpson M, White N and M Dearnaley (2004)
Poole Harbour Approach Channel Deepening and Beneficial Use of Dredged Material: Environmental
Statement. Royal Posford Haskoning Document Ref 9P0171/R/mas/Exet
UKHO (2006).
Admiralty Tide Tables Volume 1 United Kingdom and Ireland (including European Channel Ports)
NP201-06
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7.0
Measurement of the Changes in Flow
Regime at Poole Harbour Entrance
AUDIT TRAIL
Title: Measurement of the Changes in Flow Regime at Poole Harbour Entrance Following
Channel Deepening
Report No:
Job No:
Client Name:
Client Contact:
06/09/10047/1
131084
Poole Harbour Commissioners
Mr S Pearce
Project Manager
Dr Jonathan Taylor
CHS
Field Survey
Dr Jonathan Taylor
CHS
Report written by
Dr Jonathan Taylor
CHS
Data analysis by
Dr Jonathan Taylor
CHS
Mr Mark Jonas
SeaZone
Dr Jonathan Taylor
CHS
Report Authorised by
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