Survey analysis for Newhaven
(this report has been written right up to the end of January and remains unfinished)
Though some surveys at Newhaven beach had been carried out in Phase 1, useful surveys
have only been carried out since April 2005. Earlier surveys have to be disregarded because
they either used profile lines that did not represent the curvature of the beach surface or
because the survey did not reach sufficiently far towards low water so that significant portions
of the beach remained un-surveyed.
The main problem posed for surveys at Newhaven beach is the fact that the beach toe along
the eastern part reaches to below LAT. Surveys therefore can only be carried out at low water
spring using waist-high waders. The subtidal profile – when it was surveyed – shows a step
which, together with even light wave activity makes surveying the beach toe rather difficult at
all times other than equinoxial tides and/or calm sea.
Introduction
Newhaven beach has formed since the mid 1880s when Newhaven breakwater was
constructed through intercepting material travelling eastwards along the coast. From
calculations using maps carried out during Phase it would appear as if the beach volume has
remained stable since the middle of the last century, however, even the maps show that the
beach orientation seems to have varied and it was the aim of BAR to survey the beach and to
be able to link the changes of beach distribution in the longshore direction to the wave
environment.
Method
The survey methodology used has been the same as outlined in Phase 1 reports. The
accuracy of the survey method could be verified using a ‘dynamic control’ area. During each
survey, the line between concrete slabs on the car park behind the beach was followed,
sometimes twice (Figure 1). The range in the elevation is ~0.06m (see also Figure 2) with a
distinctly higher area between 50 and 70m. The average of all measurements across the line
during each survey shows variations less than ±0.01m (Figure 2) illustrating the accuracy of
the survey set-up using the GPS on a wheel and recording in autotopo mode.
Dornbusch: Beach behaviour at Saltdean: longshore transport
6.37
03/12/2006
09/09/2006
09/09/2006 return
6.36
Elevation (m OD)
6.35
6.34
6.33
6.32
6.31
6.3
6.29
0
20
40
60
80
100
Distance w est to east (m)
Figure 1: Location and elevation of ‘dynamic control’ area at the back of Newhaven
Westbeach. Graph shows point elevations for surveys separated by several months and by
~20 minutes.
2
Elevation (m OD)
Dornbusch: Beach behaviour at Saltdean: longshore transport
6.42
6.41
6.4
6.39
6.38
6.37
6.36
6.35
6.34
6.33
6.32
6.31
6.3
6.29
6.28
6.27
10-05
Mean
Max
Min
11-05
12-05
01-06
02-06
03-06
04-06
05-06
06-06
07-06
08-06
09-06
10-06
11-06
Time
Figure 2: Mean, maximum and minimum values of the points recorded during each pass
along the line shown in Figure 1.
Results
Newhaven Westbeach
Figure 3 shows for 25 surveys the average elevation above the minimum surface. The
maximum elevation variation is 9cm. This is twice the range found at Saltdean. However,
contrary to Saltdean, surveys of Newhaven beach did not always have the same extent due to
the problems surveying the beach toe in the eastern part. This is illustrated by comparing the
elevation with the maximum depths achieved during the surveys. Low elevations coincide with
surveys that terminated above ~-3.5m. Those surveys that have been carried out to below –
3.5m have a more comparable range of ~5cm, confirming results from the much smaller beach
at Saltdean that if the whole beach can be surveyed total volume variations can be calculated
with an accuracy corresponding to a surface elevation change of ~±0.02m.
1.150
0.0
elevation
-0.5
depth of survey
1.125
Mean elevation (m)
-1.5
1.100
-2.0
-2.5
1.075
-3.0
-3.5
1.050
Max. survey depth (m OD)
-1.0
-4.0
-4.5
1.025
04-05
-5.0
06-05
08-05 10-05
12-05
02-06
04-06 06-06
08-06
10-06
12-06
Time (MM-YY)
Figure 3: Times series plot of the average beach elevation above the minimum surface and
the maximum depth achieved during each survey.
3
Dornbusch: Beach behaviour at Saltdean: longshore transport
Figure 4 shows the configuration of the beach, the areas that have undergone change over
the period of the surveys and also the range recorded. The maximum range with over 7m
(almost the change between the beach toe and beach crest) has occurred next to the
breakwater in cell 0. A second maximum of just over 5m has occurred under the cliffs towards
the western end in cell 13. A notable minimum in the range of changes (only up to 2m) can be
seen ~150m west of the breakwater across the beach in cell 3. The higher values towards the
beach toe along the eastern half relates to the problems in surveying the beach toe mentioned
above. The surface grids used are composites consisting of the surveyed surface plus a
subsurface that is based on the lowest measurements. Where the beach toe could not be
surveyed, the beach surface drops vertically down to this subsurface, creating an artificially
high range.
Figure 4: Overview over the area that has changed over the period of the surveys showing
the elevation range recorded. Also shown are the beach cells 0-15 used in the later
analysis.
Figure 5 shows the changes in beach volume above the minimum surface for each beach cell
for each survey between 4th August 2003 and 3rd December 2006. The two extreme position
during this period can be found on 3rd October 2005 and 3rd December 2006. The first shows
the beach with its most eastward sediment distribution while the second shows the lowest
beach volumes in the eastern part. The largest sediment movement event took place between
14th November and 5th December 2005 when cell 0 lost 11,000m³ and cell 1 6,000m³ (Figure
6, see also BAR Newsletter No 3)
4
Dornbusch: Beach behaviour at Saltdean: longshore transport
150000
145000
4-Aug-03
135000
20-Sep-04
130000
13-Jan-05
125000
7-Apr-05
120000
12-Apr-05
115000
6-May-05
110000
26-May-05
105000
6-Jun-05
100000
7-Jul-05
95000
5-Aug-05
90000
23-Aug-05
85000
6-Sep-05
80000
75000
70000
65000
14
13
12
11
10
9
8
7
6
5
4
3
2
1
Cell volume (m³)
140000
3-Oct-05
16-Oct-05
14-Nov-05
5-Dec-05
60000
13-Dec-05
55000
5-Jan-06
50000
2-Feb-06
45000
2-Mar-06
40000
30-Mar-06
35000
28-Apr-06
30000
26-May-06
25000
25-Jun-06
20000
27-Jul-06
15000
9-Sep-06
10000
7-Oct-06
5000
7-Nov-06
0
3-Dec-06
0
Beach cell
Figure 5: Volumes above the minimum surface in each beach cell. Subsequent surveys are
offset by 5000m³ with level of the line in the legend corresponding to 0m³ for each survey.
Figure 6: Maximum change event between 14th November and 5th December 2005 totaling
material loss of 17,000m³ from cells 0 and 1.
5
Dornbusch: Beach behaviour at Saltdean: longshore transport
Sand in the intertidal harbour basin
Since August 2005, two profile lines of >140m length have been surveyed across the intertidal
sands of the harbour basin. The first profile runs from the bottom of the western access steps
(Figure 7) in the direction of the lighthouse at the end of the harbour arm. At the seaward end
the profile is set off towards the east by ~30m (30 paces) and the return is made in the
direction of the eastern access steps.
Figure 7a shows the typical distribution of sand in the harbour with the highest levels reaches
west of the steps. Due to the limited time available, only the two profile lines were surveyed
subsequently (Figure 7b). The elevation range of the profiles is <0.2m for most of the profile
but increases towards the seawall for the last 40m, reaching almost 1m at the bottom of the
western access steps.
Figure 8 shows the profile evolution through time. The main changes happened around the
middle profile between 75 and 100m where the convex profile from the autumn of 2005 has
been replaced by a very straight profile for almost the entire period surveyed in 2006.
However, more pronounced are the changes in the upper ~25m of the profile, especially
between October 2005 and February 2006 (unfortunately, no profiles were recorded between
October and January).
a
b
Figure 7: Location of sand surface at Newhaven. a) Surface based on survey on 23-082005 and showing points from survey on 03-12-2006 for reference. b) Elevation range on
all surveys showing points from survey on 03-12-2006 for reference. Thin line in the middle
is the profile line used for Figure 8.
6
Dornbusch: Beach behaviour at Saltdean: longshore transport
3.6
3.2
03-12-06
07-10-06
27-07-06
26-05-06
30-03-06
05-01-06
03-10-05
23-08-05
Elevation (m OD)
2.8
2.4
2.0
1.6
1.2
0.8
07-11-06
09-09-06
25-06-06
28-04-06
02-02-06
16-10-05
06-09-05
0.4
0.0
-0.4
-0.8
-1.2
-1.6
-2.0
-2.4
-2.8
0
25
50
75
100
125
150
175
200
225
250
Distance from seaw all (m )
Figure 8: Stacked inertidal profiles of the sand surface in the Newhaven harbour basin.
Profiles are 0.2m offset from each other.
It is increasingly acknowledged that intertidal sand levels in front of seawalls show significant
elevation changes during one tidal cycle with little change observed at subsequent low tide
surveys. The data collected from the sand surveys cannot contribute to this discussion but
shows that the volume of sand in the harbour basin can be relatively stable over several
months but than changes quite substantially from one month to another (Figure 9). However,
the fact that the data is based only on narrow cross profile band rather than the whole basin
should restrict this interpretation because it is possible that sand was only redistributed within
the basin rather then that material actually entered or left the basin.
35
Area under profile (m²)
30
25
20
15
10
5
0
04-05
06-05
08-05
10-05
12-05
02-06
04-06
06-06
08-06
10-06
12-06
Time (MM-YY)
Figure 9: Area under each profile shown in Figure 8 between the seawall and 136m (limit
of shortest profile) in relation to the lower boundary of the profile sweep zone.
30-01-2007 Uwe Dornbusch
7