REPORT ON WAVE AND TIDE MEASUREMENTS
Compiled by Uwe Dornbusch. Edited by Cherith Moses
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Aims ..................................................................................................................................1
Summary...........................................................................................................................1
Introduction .......................................................................................................................1
Site selection.....................................................................................................................1
Wave recorder settings .....................................................................................................2
Results ..............................................................................................................................2
6.1
Water levels ...............................................................................................................3
6.2
Wave heights .............................................................................................................4
7 Discussion.........................................................................................................................5
8 Outlook for Phase 2 of BAR ..............................................................................................5
1 Aims
The aim of installing two wave recorders in the intertidal zone was to obtain continuous near
shore wave and water level data to be used to correlate with beach survey data and to verify
offshore-nearshore wave transformation formulae. The water level information is used to
measure water level set up directly on type-sites and to calibrate surge models that will be
used in Phase 2.
2 Summary
Two Dobie wave recorders have been installed permanently in the nearshore zone, one at
Telscombe (October 2003) and one at Cooden Bay (February 2004). Both recorders have
been working continuously. Data gaps that exist relate either to premature battery failure or
difficulty in accessing the instruments due to environmental conditions. Maximum wave
heights recorded are 2.1m at Cooden and 2.2m at Telscombe during periods when offshore
wave heights were in excess of 6m at the Rustington buoy illustrating the wave dissipating
function of the sandy foreshore at Cooden and the shore platform at Telscombe.
3 Introduction
Two Dobie wave recorders have been installed permanently on the English side of the BAR
coast. They measure every 30min for the duration of ~7min the water level using a pressure
transducer. The onboard computer calculates from these measurements the average water
level and wave parameters like significant wave height, main spectral wave length, wave
steepness etc.
Deployment of the first recorder took place in October 2003 (Telscombe), of the second in
February 2004 (Cooden). Both recorders have been out uninterrupted except for occasional
battery problems or inaccessibility due to unfavourable water levels.
4 Site selection
The site at Telscombe was selected to be representative for long stretches of the frontage
from Brighton to Newhaven, encompassing three of the type-site beaches (Saltdean,
Telscombe and Newhaven) that were to be surveyed regularly. Deployment took place in an
BAR Phase I, February 2003 – January 2005
Science Report: Report on waves and tides
artificially dug hole to ensure the save anchorage of the recorder. The coordinates for the
pressure transducer opening are x = 539146.931, y = 101391.797 and z = -1.482m OD
(Figure 1). The vertical position is approximately at mean low water neaps which allows for
an access-window of a few hours during spring tides even under surge and storm conditions.
The recorder is 39m seawards of the beach toe and 80m seawards of the cliffs backing the
beach.
Figure 1: Position of the BAR wave recorders at Telscombe and Cooden in relation to
other wave recorders along the East Sussex frontage between Brighton and the Rother
mouth (Coastal Observatory, Soverign Light Vessel, CEFAS).
The site in Pevensey Bay was chosen as there are already 2 offshore buoys in the bay
(Figure 1) so that together with the Cooden recorder they form an almost linear array that will
allow the determination of shoaling effects as the waves move inshore. Due to problems of
securing the recorder in the intertidal zone of the extensive sandy foreshore in Pevensey Bay
the recorder had to be attached to a groyne. The terminal groyne at Cooden was the most
suitable because it is in a good state of repair, reaches furthest seaward and is easily
accessible from a road. The coordinates of the pressure transducer opening are x =
571224.380, y = 106372.617 and z = -1.397m OD. The recorder is secured to the end on the
terminal groyne and is ~40m seawards of the beach toe. The end of the groyne is usually
surrounded by water which, it was hoped, would prevent access by the public. However, as
this water is usually connected to a runnel landward of a sand ridge, the altitudinal and
positional fluctuation of the ridge has caused, in two instances, a delay in downloading data
as the water level in the runnel had not receded sufficiently to allow access to the recorder.
5 Wave recorder settings
The DOBIE reorders are set to record every 30minutes for 2048 bursts at a sample interval
of 0.2hz (ie for almost 7 minutes). The periods during which data has been recorded are
shown in Figure 2.
6 Results
Due to the time necessary for the preparation of reports at the end of phase 1 of BAR, data
analysis only occurred up to mid-November 2004 though the data collection is still
continuing.
BAR Phase I, February 2003 – January 2005
Science Report: Report on waves and tides
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Telscombe
Cooden
Oct- Nov- Dec- Jan- Feb- Mar- Apr- May- Jun03
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03
04
04
04
04
04
04
Jul- Aug- Sep- Oct- Nov- Dec04
04
04
04
04
04
Figure 2: Periods for which data has been recorded. Gaps in recording are due to battery
problems, temporary inaccessibility or maintenance.
6.1 Water levels
The only official tide gauge in the proximity is situated at the Newhaven harbour entrance
which is at a distance of 6km to the site at Telscombe and 32km to the site at Cooden. Water
levels are calculated for every 30 minutes for the DOBIE recorders and every 15 minutes at
the Newhaven tide gauge.
As expected due to the close proximity of the recorders with the tide gauge, the tidal pattern
is almost identical (Figure 3) with regard to heights and timing. Small differences in the timing
would not be distinguishable due to the sampling interval of 30min for the Dobies and 15min
for the Newhaven tide gauge. Figure 4 shows the difference in the water level of the
recorders that coincided with the highest water level at Newhaven. On average, the high tide
is 13cm higher at Telscombe and 42cm higher at Cooden. Variations around the average are
due to atmospheric pressure variations (Figure 4), i.e. high atmospheric pressure increases
the pressure on the sensor leading to the recording of higher water levels and therefore
increased difference between the recorder and the tide gauge. These variations are only
recorded with the pressure transducers of the recorders but not with the tide gauge.
5
Newhaven
Telscombe
Cooden
4
Water level (mOD)
3
2
1
0
-1
-2
-3
3/1
3/0
40
3/1
:00
3/
3/1 04 6:0
3/0
41 0
3/1
2 :0
3/0
0
41
8
3/1
4/0 :0 0
40
3/1
:00
4/
3/1 04 6:0
4/0
41 0
3/1
2 :0
4/0
0
41
8 :0
3/1
0
5/0
3/1 4 0:00
5/0
46
3/1
:00
5/0
3/1 4 12 :0
5/0
41 0
8 :0
3/1
0
6/0
3/1 4 0:00
6/0
46
3/1
:00
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41
2 :0
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8
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40
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:00
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2 :0
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8 :0
3/1
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3/1
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00
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41 0
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:0 0
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3/2 4 0:00
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6:0
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:00
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3/2 04 6:0
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4/0
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:00
-4
Figure 3: Example of tidal data from Newhaven, Telscombe and Cooden. Heights are
converted to a common datum of mOD.
BAR Phase I, February 2003 – January 2005
Science Report: Report on waves and tides
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1040
0.6
1030
0.4
1020
0.2
1010
0
1000
Telscombe - Newhaven
-0.2
Cooden - Newhaven
990
Atmospheric pressure (mbar)
Height (m)
0.8
SLP
-0.4
980
1/14/04 0:00 3/4/04 0:00 4/23/04 0:00 6/12/04 0:00 8/1/04 0:00 9/20/04 0:00
Figure 4: Example for the difference in the high tide level between the DOBIE recorders
and the Newhaven tide gauge. Also shown is sea level pressure (SLP) as recorded at the
Greenwich Light Ship.
6.2 Wave heights
Maximum wave heights recorded by the recorders are between 1.5 and 2.2m with higher
wave heights generally recorded at Telscombe rather than Cooden (Figure 5). However,
these maximum wave heights have most likely been in the form of breaking waves so that
much of the wave energy has been spent before impacting on the beach.
Frequency of occurence (%)
80
70
60
50
Cooden
40
Telscombe
30
20
10
0
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
2.4
Wave height (m)
Figure 5: Frequency distribution of the highest waves during every tide. Differences
between Cooden and Telscombe can be attributed to the different period of time during
which the recorders operated.
BAR Phase I, February 2003 – January 2005
Science Report: Report on waves and tides
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7 Discussion
The wave recorders provide a near continuous record of nearshore wave conditions over a
long time period. Maximum wave heights appear to be somewhat less than half the offshore
wave heights but from observations in the field the highest waves are breaking waves
indicating that wave attenuation has under storm conditions occurred to a large degree even
seaward of the wave recorders. From first qualitative comparisons with changes in beach
morphology it would appear as if the maximum wave heights have less influence on the
geomorphology of the beach than medium size waves that actually break onto the beach.
8 Outlook for Phase 2 of BAR
Apart from continuous data collection during Phase 2 detailed analysis of the data will be
carried out. This will include:
• analysis of the wave types associated with the higher waves (i.e. these are likely to
be breaking waves) to assess whether the highest or maybe the intermediate waves
will have the largest impact on the beach.
• corrections of water levels based on atmospheric pressure to identify wave set up
and local surge components
• comparison with measured wave run-up heights to feed into wave run-up models
• analysis to model the inshore wave heights based on the waves and winds measured
offshore.
BAR Phase I, February 2003 – January 2005
Science Report: Report on waves and tides
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