Caswell Bay study July 2014

City and County of Swansea
Caswell Bay Beach Survey Report
KPAL Report No: 160725
14 July 2014
(brynmill
Kenneth Pye Associates Ltd.
Scientific Research, Consultancy and Investigations
City and County of Swansea
Caswell Bay Beach Survey Report
Professor Kenneth Pye ScD PhD MA CGeol FGS
Dr. Simon J. Blott PhD MRes BSc FGS
KPAL Report No. 160725
Report history
Version 1.0
Version 2.0
Draft
Revised
11 July 2014
14 July 2014
Kenneth Pye Associates Ltd
Research, Consultancy and Investigations
Blythe Valley Innovation Centre
Central Boulevard
Blythe Valley Park
SOLIHULL
B90 8AJ
United Kingdom
Telephone: + 44 (0)121 506 9067
E-mail: [email protected]
website: www.kpal.co.uk
2
Contents
Page
Summary
4
1.0
Report scope and purpose
5
2.0
Survey methods
5
3.0
Survey Results
6
4.0
Effects of storms on beach levels
7
5.0
Conclusions
7
Tables
10
Figures
13
Appendix 1: Field photographs
25
3
Summary
Since late March 2014 there has been a significant increase in the extent of exposed rock and
cobbles / pebbles within Caswell Bay, most notably in the western part of the bay in front
Redcliffe Flats. A topographic survey carried out on 10th July 2014 has shown that beach
levels are now lower than at any time since 1999. An analysis of wind, wave and tide gauge
records for the Swansea Bay / Gower area indicates that the winter of 2013-14 was very
stormy and there were several instances when high tide levels coincided with high and steep
onshore waves. These conditions promote beach erosion and seaward movement of sand
from the intertidal beach area towards the sub-tidal zone. Gravel (cobbles and pebbles)
present below the sand are remobilized and mainly reworked towards the high water mark.
The most severe storms occurred between early January and early March but further stormy
periods occurred in April and early May. These later storms, building of the effect of storms
in the winter, exposed progressively larger areas of rock and gravel. The movement by CCS
Council of a quantity of cobbles and pebbles from the area near Surfside café to the upper
beach in the western part of Caswell Bay has had no significant effect on the general trend of
beach lowering or on the extent of rock / gravel exposure. There have been previous periods
of beach lowering in Caswell Bay followed by recovery as sand has moved back onshore
during periods of constructive swell waves. It is likely that this process will occur again,
although fully recovery of the beach may take several years.
It is considered that there would be no benefit in removing the cobbles / pebbles now exposed
on the upper beach in the western part of the bay. Such action would lower the beach level
and further cobbles / pebbles are likely to become exposed as sand is winnowed away.
Removal of the cobbles / pebble beach cusps would also remove the wave protection afforded
to the strip of sand in front of the upper storm beach.
4
1.0
Report scope and purpose
This report summarises the results of a RTK GPS ground survey of the beach at Caswell Bay
undertaken on 10th July 2014. The purpose of the survey was to determine the extent of
beach lowering since the last survey in 2013 and to assess the origin of gravel accumulations
which are presently exposed in a number of parts of the Bay.
The winter of 2013-14 was the stormiest experienced in South Wales for more than 30 years
and resulted in widespread beach erosion. In Caswell Bay serious damage was caused to the
Surfside Café in the eastern part of the Bay and to the wall behind the upper storm beach in
the western (Redcliffe) part of the bay. A significant accumulation of cobbles and pebbles
formed in front of the promenade in the eastern part of the bay and at the end of the winter
(March 2014) these were moved by City and County of Swansea Council and placed at the
bottom of the upper beach in the western (Redcliffe) side of the bay, following the
assumption that they had been moved from this area by wave action during the winter. Since
that time the beach levels within the bay have fallen further, exposing additional areas of rock
and cobbles / pebbles.
2.0
Survey methods
A ground-based RTK GPS topographic survey was undertaken using Leica Smart-Rover
equipment around the time of low tide (10.30 - 13.00 hrs) on 10th July 2014 (see Table 1).
The tide was a medium tide with low water mid-way between mean lower water neap and
mean low water spring tide level.
A base station was established above a stud bench mark on the promenade which is used as
the starting point for Profile P111 surveyed each year as part of the Swansea Bay and
Carmarthen Bay Coastal Engineering Group’s strategic coastal monitoring programme. Eight
new profiles across the beach were surveyed (labelled A to H in Figure 1), together with
strategic profile P111. The boundaries of features of interest (e.g. rock exposures and cobble /
pebble accumulations) were also mapped using the survey equipment. Average horizontal
and vertical errors for the survey are summarised in Table 2.
5
The topographic profiles were compared with historical profile data for strategic profile P111
and with profiles extracted from a LiDAR digital elevation model (DEM) based on a survey
flown in 2006. Historical aerial and ground photographs taken between the 1890s and
summer 2013 were also examined to provide additional information about the previous
extent of sand and rock / cobble/ pebbles exposures.
3.0
Survey results
The distribution of rock exposures and areas of cobbles and pebbles at the time of survey is
shown in Figures 3 and 4. The appearance of the exposures is shown in the photographs in
Appendix1.
At profile A the surface level at the time of survey was 38 to 53 cm lower than in April 2006
(Figure 5). The loss of sand has resulted in the exposure of extensive areas of fixed rock,
cobbles and scattered pebbles with intervening areas of sand, now partly covered with algae.
At profile B the beach level is now 42 to 56 cm lower than in April 2006. At profile C the
level of the mid beach is now 42 to 63 cm lower than in April 2006 but the level of the upper
beach between MHWN and MHWS level is slightly higher than in 2006 due to accumulation
of gravel (Figure 6). A similar pattern of change is evident at profile D, profile E and profile
F (Figure 7). There is no detectable change in the profile levels of the upper cobble storm
beach compared with 2006, suggesting that large quantities of material were not eroded and
moved down the beach during the winter of 2013-14. A slight dip in the beach profiles
between 1.5 and 2.5 m OD shown in the surveyed profiles suggests significant erosion in this
area, and that the coarser material has moved higher up the beach profile.
At profiles G and H, located between the western and eastern parts of the Bay, the levels of
the upper and mid beach have fallen by up to 58 cm compared with 2006 but there has been
little net change at the seaward end of the surveyed profiles (at c. - 2.5 m OD) (Figure 8).
At profile P111 the upper beach level on 10 July 2104 was 49 cm lower than on 15 July 2013,
but at the end of the profile the level is essentially the same as in July 2013 (Figure 9). This is
consistent with transport of sand eroded from the upper beach in a seawards direction.
6
Although not surveyed due to tide conditions, it is likely that the area below MLWN has seen
accumulation of sand and an increase in surface levels.
4.0
Effects of storms on beach levels
During the winter period 2013-14 there were several occasions when high tides coincided
with strong waves, notably on 3rd January, 1-2 February, 3 March 2014 and 10 May 2014.
These conditions were responsible for beach lowering and seaward movement of sediment at
many locations on the coast of South Wales. Figure 10 provides a summary of wind speeds
and direction recorded at Mumbles lighthouse between December 2012 and June 2014, while
Figure 11 shows the significant wave height recorded at the Scarweather Sands wave buoy
during the same period. Of particular note with regard to the present issue is the occurrence
of a further stormy periods during April and early May 2014, the largest event being recorded
on 10 May 2014.
An aerial drone video survey carried out in early April 2014 (c. the 5th) shows that the extent
of exposed rock and pebbles in Caswell Bay was significantly less than at the time of the
survey on 10 July (Figure 12). Most of the further beach lowering in the intervening period
can be attributed to the storms in the second half of April and early May.
5.0
Conclusions
The increased exposure of gravel, cobbles and rock on both sides of Caswell Bay is due very
largely to loss of sand and a general fall of beach levels during the stormy winter of 2013-14.
Comparison of the results from the 10 July 2014 survey with those from surveys
commissioned undertaken by Swansea Bay and Carmarthen Bay Coastal Engineering Group
show that the beach is currently at its lowest level since monitoring began in 1999. However,
historical aerial and ground photographs provide evidence that there have been previous
periods when the beach levels have been lowered by storms but has subsequently recovered.
7
Although there was some landward movement of sand and gravel (cobbles and pebbles) on
the upper part of the beaches during the winter period 2013-14, sand was mainly lost in a
seawards direction and currently resides below the mean low water mark. It therefore remains
potentially available to be moved back into the intertidal area. If there are no further serious
storms in the 2014-15 winter, much of the sand is likely to return to the beach and levels will
rise, covering much of the exposed rock, cobbles and pebbles below mean high water spring
tide level. However, this process may take a number of years, depending on weather and
wave conditions.
Some of the gravel presently exposed in the mid to upper beach area in the western part of the
bay, including the material deposited there by CCS, has been reworked landwards and has
formed cusp-like features close to the normal spring tide high water mark in front of
Redcliffe flats and the lifeguard station. Gravel and sandy gravel is also present beneath the
veneer of sand which forms a zone between the gravel cusps and the base of the exposed
upper part of the storm beach.
The size grading of the gravel on both the upper storm beach and on the upper foreshore in
the western part of the bay indicates a net eastward direction of movement. It is very likely
that some material drawn down from the upper beach in the western bay during storms was
moved into the eastern part of the bay during the winter of 2013-14. However, before
construction of the promenade (opened in its present form in 1987) a cobble / pebble storm
beach also existed in the eastern part of the bay. Some material of this size grade is still
present beneath the surface sand cover and is available to be reworked landwards towards the
promenade during storms.
The placement of cobbles and pebbles on the upper beach in the western part of the bay by
CCS in March 2014 is very unlikely to have played any role in causing a further fall in beach
levels during April and May 2014; these further changes can be attributed to continuing
periods of storms and high steepness waves.
Much of the rock and cobble material now exposed in the western part of the bay has been
present beneath the sand cover for many years. The majority of the rocks and cobbles now
exposed are much larger than the material deposited on the upper beach by CCS Council and
cannot represent a re-distribution of this material. The increase in rock and cobble exposure
8
seen since April 2014 would have occurred with or without the removal and placement of
material by the Council.
There would be no benefit in removing the cobbles / pebbles now exposed on the upper beach
in the western part of the bay. The effect of such action would be to lower the beach level and
increase the local wave energy; further cobbles / pebbles present at depth would become
exposed as sand is winnowed away. Removal of the cobbles / pebbles would also remove the
protection afforded to the strip of sand in front of the upper storm beach.
9
Tables
10
Table 1: Job Summary
KPAL Job No:
100714
Report Date:
11/07/2014
Client:
City and County of Swansea Council
Job Title:
Caswell Bay Topographic Survey
Survey conducted:
10th July 2014
Instruments used:
Leica ATX1230GG SmartRover mounted on GLS30 pole (2 m)
Leica GX1230 RTK base station and AS10 antenna mounted on
GST20-9 tripod
Leica RX1250TC and RX1210T Field Controllers
Pacific Crest ADL Vantage radio transceiver (430-470 MHz)
No. of data points:
485
RTK Control Station: Metal stud in sea defence, situated behind bench, used as the profile
marker for Swansea Bay and Carmarthen Bay Coastal Engineering
Group Strategic Profile P111 (BM1):
Easting: 259333.425 m
Northing: 187680.712 m
Height: 7.822 m OD
Fixed profiles:
Strategic Profile 111 surveyed and compared with existing
31/03/1999 and 15/07/2013. Chainages along the profile line were
interpolated at positions on a theoretical straight-line between the
zero and end points of the profile to ensure comparability. Eight
additional profiles were surveyed and compared directly with
elevations extracted from a 2 metre resolution digital elevation
model of the beach surveyed by the Environment Agency using
airborne LiDAR flown on 16/04/2006.
Survey undertaken by: S.J. Blott, K. Pye
11
Table 2. Average instantaneous error reported by the Leica instrument for all 485 data points
1-D (height) quality control
2-D (position) quality control
Average
13.2 mm
7.9 mm
St Dev
6.9 mm
4.4 mm
12
Figures
13
Figure 1. Locations of data points (black dots) and cross-profiles (blue lines), overlaid on aerial photographs
flown 12 July 2013.
14
Figure 2. Locations of data points (black dots) and cross-profiles (blue lines), overlaid on a digital elevation
model of Caswell Bay beach, flown by the Environment Agency on 16th April 2006 using airborne LiDAR.
15
Figure 3. Distribution of exposed rock / cobbles and pebbles in the eastern part of Caswell Bay at the
time of survey on 10th July 2014, superimposed on an aerial photograph data 12th July 2013.
16
Figure 4. Distribution of exposed rock / cobbles and pebbles in the western part of Caswell Bay at the
time of survey on 10th July 2014, superimposed on an aerial photograph data 12th July 2013.
17
9
MHWS
(a) Profile A
8
MSL
MLWS
7
16/04/2006 (LiDAR)
10/07/2014 (sand)
6
10/07/2014 (cobbles/boulders/rock)
Elevation (m OD)
5
4
beach level lowered
by 53 cm between
16/04/2006 and
10/07/2014
beach level lowered
by 38 cm between
16/04/2006 and
10/07/2014
3
2
1
0
-1
-2
-3
-4
-20
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Distance along profile (metres)
9
MHWS
(b) Profile B
8
MSL
MLWS
7
16/04/2006 (LiDAR)
10/07/2014 (sand)
6
10/07/2014 (cobbles/boulders/rock)
rock armour
Elevation (m OD)
5
4
beach level lowered
by 56 cm between
16/04/2006 and
10/07/2014
beach level lowered
by 42 cm between
16/04/2006 and
10/07/2014
3
2
1
0
-1
-2
-3
-4
-20
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Distance along profile (metres)
Figure 5. Change in beach levels at profiles A and B between April 2006 and July 2014.
18
9
8
MHWS
(c) Profile C
MSL
MLWS
cobble upper beach,
not changed since
16/04/2006
7
6
16/04/2006 (LiDAR)
10/07/2014 (sand)
10/07/2014 (cobbles/boulders/rock)
Elevation (m OD)
5
4
beach level lowered
by 63 cm between
16/04/2006 and
10/07/2014
beach level lowered
by 42 cm between
16/04/2006 and
10/07/2014
3
2
1
0
-1
old pipe exposed,
previously buried
by sand
-2
-3
-4
-20
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Distance along profile (metres)
9
MSL
cobble upper beach,
not changed since
16/04/2006
7
MLWS
16/04/2006 (LiDAR)
10/07/2014 (sand)
6
upper beach level increased
by 40 cm between 16/04/2006
and 10/07/2014
5
Elevation (m OD)
MHWS
(d) Profile D
8
10/07/2014 (cobbles/boulders/rock)
4
beach level lowered
by 58 cm between
16/04/2006 and
10/07/2014
3
2
1
beach level lowered
by 45 cm between
16/04/2006 and
10/07/2014
0
-1
-2
-3
-4
-20
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Distance along profile (metres)
Figure 6. Change in beach levels at profiles C and D between April 2006 and July 2014.
19
9
(e) Profile E
8
cobble upper beach,
not changed since
16/04/2006
7
MSL
MLWS
16/04/2006 (LiDAR)
10/07/2014 (sand)
6
upper beach level increased
by 20 cm between 16/04/2006
and 10/07/2014
5
Elevation (m OD)
MHWS
4
10/07/2014 (cobbles/boulders/rock)
beach level lowered
by 64 cm between
16/04/2006 and
10/07/2014
3
2
beach level lowered
by 38 cm between
16/04/2006 and
10/07/2014
1
0
-1
-2
-3
-4
-20
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Distance along profile (metres)
9
(f) Profile F
8
cobble upper beach,
not changed since
16/04/2006
7
MSL
MLWS
16/04/2006 (LiDAR)
10/07/2014 (sand)
6
upper beach level increased
by 60 cm between 16/04/2006
and 10/07/2014
5
Elevation (m OD)
MHWS
4
10/07/2014 (cobbles/boulders/rock)
beach level lowered
by 63 cm between
16/04/2006 and
10/07/2014
3
2
beach level lowered
by 38 cm between
16/04/2006 and
10/07/2014
1
0
-1
-2
little change in beach
levels at 280 m from
top of beach
-3
-4
-20
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Distance along profile (metres)
Figure 7. Change in beach levels at profiles E and F between April 2006 and July 2014.
20
9
MHWS
(g) Profile G
8
MSL
MLWS
7
5
Elevation (m OD)
16/04/2006 (LiDAR)
rock gully
6
ramp of sand built up
in rock gully at back
of beach
4
10/07/2014 (sand)
maximum beach level
lowering of 58 cm
between 16/04/2006
and 10/07/2014
3
2
1
0
-1
-2
little change in beach
levels at 270 m from
top of beach
no cobbles
present on this
profile
-3
-4
-20
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Distance along profile (metres)
9
MHWS
(h) Profile H
8
MSL
MLWS
7
16/04/2006 (LiDAR)
6
10/07/2014 (sand)
4
maximum beach level
lowering of 54 cm
between 16/04/2006
and 10/07/2014
rock
Elevation (m OD)
5
3
2
1
0
-1
-2
little change in beach
levels at 260 m from
top of beach
no cobbles
present on this
profile
-3
-4
-20
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Distance along profile (metres)
Figure 8. Change in beach levels at profiles G and H between April 2006 and July 2014.
21
9
MSL
upper beach level
increased by 49 cm
between 15/07/2013
and 10/07/2014
7
6
MHWS
(i) Profile 111
8
sea
defence
MLWS
31/03/1999
02/06/2006
15/07/2013
Elevation (m OD)
5
10/07/2014 (sand)
4
10/07/2014 (scattered cobbles/rock)
3
beach below -2.68 m
OD (380 m from the
benchmark) has
changed little since
15/07/2013
2
1
0
mid beach level
lowered by 49 cm
between 15/07/2013
and 10/07/2014
-1
-2
-3
lower beach level
lowered by 32 cm
between 15/07/2013
and 10/07/2014
-4
-20
0
20
40
60
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Distance along profile (metres)
Figure 9. Comparison of beach levels on Profile P111 between March 1999 and July 2014.
22
23
29/06/2014
15/06/2014
01/06/2014
29/06/2014
15/06/2014
01/06/2014
18/05/2014
04/05/2014
20/04/2014
06/04/2014
23/03/2014
09/03/2014
23/02/2014
09/02/2014
26/01/2014
12/01/2014
29/12/2013
15/12/2013
01/12/2013
Mean hourly wind speed (knots)
45
18/05/2014
04/05/2014
20/04/2014
06/04/2014
23/03/2014
09/03/2014
23/02/2014
09/02/2014
26/01/2014
12/01/2014
29/12/2013
15/12/2013
01/12/2013
Wind direction (degrees)
50
E
40
35
30
28
knots
25
20
15
10
11
knots
5
0
360
Winds >11 knots
N
315
Winds >28 knots
E
NW
270
W
225
SW
180
S
135
SE
90
E
45
NE
0
N
Figure 10. Mean hourly wind speed and direction recorded at Mumbles Lighthouse, December 2012 to June
2014. High wind speed event ‘E’ occurred on 10th May 2014.
7.0
Significant wave height
B
Significant wave height (m)
6.0
A
C
5.0
D
E
4.0
3.0
2.0
1.0
29/06/2014
15/06/2014
01/06/2014
18/05/2014
04/05/2014
20/04/2014
06/04/2014
23/03/2014
09/03/2014
23/02/2014
09/02/2014
26/01/2014
12/01/2014
29/12/2013
15/12/2013
01/12/2013
0.0
Figure 11. Significant wave height recorded at Scarweather Sands wave buoy, December 2012 to June 2014.
Large wave event ‘E’ occurred on 10th May 2014
Figure 12. Photograph of Caswell Bay taken. C 5th April 2014 http://www.southwaleseveningpost.co.uk/Beautiful-Caswell-Bay-seagull-s-point-view-thanks/story-20929282-detail/story.html
24
Appendix 1
Field photographs taken 10th July 2014
25
Photograph 1. Caswell Bay east. Survey bench mark station on promenade
Photograph 2. Caswell Bay east. Concrete steps and gravel / cobble accumulation in corner by the Surfside cafe
26
Photograph 3. Caswell Bay east. Upper beach in front of Surfside café with isolated patches of gravel just
above MHWM.
Photograph 4. Caswell Bay east. View from promenade towards the sea, showing exposed rocks and cobbles in
the mid to upper beach area.
27
Photograph 5. Caswell Bay east. Exposed rocks and large cobbles on the beach below the MHW line
Photograph 6. Caswell Bay east. View landwards from the mid beach towards the promenade.
28
Photograph 7. Caswell Bay west, view towards flats and Redley Cliff.
Photograph 8. Caswell Bay west. Gravel beach cusps formed by wave action near high water mark, cobble gravel upper storm beach adjacent to lifeguard station and intervening superficial sand cover.
29
Photograph 9. Caswell Bay west. Upper storm beach composed of cobbles and gravel in the central part of the
bay; note recent repairs to the wall behind the top of the beach
Photograph 10. Caswell Bay east. Exposed rocks, cobbles and gravel on the mid beach opposite the flats.
30
Blythe Valley Innovation Centre
Central Boulevard
Blythe Valley Park
SOLIHULL
B90 8AJ
United Kingdom
Telephone: 0121 506 9067
E-mail: [email protected]
website: www.kpal.co.uk
31