Design Methodology and Data Improvements to
Facilitate Regional Gravel Beach Management
Uwe Dornbusch, Environment Agency, Worthing, UK
Andy Bradbury, New Forest District Council, UK
Bryan Curtis, Worthing Borough Council, UK
Alec Dane, Canterbury City Council, Canterbury, UK
Alastair Pitcher, Environment Agency, Worthing, UK
Andrea Polidoro, HR Wallingford, Wallingford, UK
Summary
The paper details the approaches taken and preliminary results achieved to improve beach design
and understanding of risk in a transparent way across the mixed beach frontages in Southeast
England. This includes a regional approach to joint probability analysis, an improved wave run-up
formula, regional sediment budget analysis and the potential for seaward transport of beach sediment
to aid beach management across Operating Authorities using a wider sediment cell approach.
Introduction
Currently Beach Management in Southeast England is carried out individually by each local Risk
Management Authority (Maritime Local Authorities, Environment Agency, private owners) broadly in
line with recommendations in the Beach Management Manual (Rogers et al., 2010). This individual
approach means that design is based on individual studies using different datasets and methods and
generally do not take up- and down-drift frontages and their issues into account. The former makes it
virtually impossible to compare risk and standard of protection between locations and has probably led
to some frontages being either over- or underdesigned. This leads to uncertainty over scale and
frequency of future management interventions and thus funding requirements. Not taking into account
up- and down drift frontages has over the years led to problems in some locations resulting not from
too little sediment – which is usually the problem addressed through beach management – but from
too much sediment. Beach sediment has often travelled long distances and recycling it back by the
usual means of land-based transport to areas of sediment deficit is neither economical nor
environmentally sustainable. These and the wider problems of beach management have been
previously investigated and potential solutions outlined (Dornbusch et al., 2011).
Beaches along the south-east coast of England (Figure 1) are almost exclusively barrier or fringing
beaches consisting of mixed sediment (sand filling interstitial spaces between gravel and pebbles with
a D50 of ~16mm for all sediment >2mm (Dornbusch et al., 2005; Dornbusch, 2005)). These sit on a
sand or Chalk rock platform with the intersection between the two (ie the beach toe) at ~0 to -2mOD.
Only one beach (Seaford) has a beach toe in the subtidal at ~-4 to -5mOD.
Wind waves approach predominantly from south-westerly directions, In addition swell waves penetrate
into the English Channel up to about Beachy Head (Bradbury et al., 2007) leading often to wave
conditions with a broad, bi-modal or multi-modal spectrum. Annual maximum Hs in the nearshore
(10mCD) is in the order of ~3.5 to ~4.5m depending on exposure.
The whole coast is macro-tidal with a spring tidal range increasing from ~4m in the west to ~6m in the
east of the study area. Due to the nature of the beaches, beach management and the sediment
budget refer to the mixed sediment subset of the total sediment system. It is therefore easier to
accomplish than the regional sediment management discussed by Morang et al. (2012) or smaller
scale sediment budgets to include the subtidal as discussed by Dolphin et al. (2012).
Figure 1: Frontages with shingle beach management activities in Southeast England.
Improvements to the Knowledge Base for Beach Management
For a transparent and regional approach to beach management, beach design needs to use common
methodology and data. This relates to the input conditions that drive beach change in the form of wave
and water level parameters, the resulting impact on beaches under storm conditions, in particular in
relation to run-up and overtopping, and the longer term movement of sediment not only through
individual schemes and management units but the larger sediment cell. To rebalance this natural
shingle movement, beach recycling is carried out on most frontages, but usually over short distances
of up to ~10km. For a regional approach, other means of transport over longer distances are required
and potential options need to be explored. Figure 2 illustrates schematically the base data going into
these knowledge improvement areas and how they link to Beach Management Plans (BMPs).
Wave and Water Level Conditions (Joint Probability)
Design of any coastal intervention is controlled by waves and water levels affecting a site. In the past,
these have been derived from a range of sources usually at a local scale. Especially for water levels, a
range of regional and sub-regional studies have been used (e.g. JBA 2001; JBA 2004) produced at
different times and with different base data and methodologies. This results in varying assumptions on
water level extremes even over short distances along the coast. In 2011, a national dataset (McMillan
et al., 2011) has been produced that provides water level extremes based on the same data and
methodology on a national scale with a data point every 5km along the coast. For the first time, an
uncertainty band ranging from ±0.1m to ±0.3m is attached to each water level extreme return period
(for Southeast England and return periods up to 1 in 200) so that uncertainties can be better explored.
Similar to water levels, wave extremes were derived from different datasets using different
methodologies, but almost exclusively relying on hindcast wave data from wind fields and some further
transformation to a point closer inshore. Since 2003, an increasingly dense network of wave buoys
has been deployed by the Strategic Regional Coastal Monitoring Programme (CCO, 2008). Parts of
this network have reached a level of record length that allows the calculation of return periods for
extreme waves up to ~1 in 100. Extrapolated wave extremes change with record length (for example
the 1 in 10 year return period Hs for Rustington (near Worthing) using a four year dataset is 4.1m
while for an eight year dataset it is 4.8m) so that the methodology employed for their calculation needs
to be transparent and replicable. This will ensure that updates using longer records can be used
confidently in the following processing steps.
Joint probabilities are assessed in a pragmatic and transparent way from single extremes distributions
using Hawkes (2005). The spreadsheet approach also allows for quick sensitivity analysis, for
example by exploring the uncertainty bands for the extreme water levels included in McMillan et al.
(2011) or by using a range of dependency values around those suggested in Hawkes (2005) for the
different locations along the coast. It is easily repeatable and therefore suited for incorporating
updated single extremes. The basic output is in the form of joint probability tables and curves as
illustrated in Figure 3.
Figure 2: Overview of the four knowledge base improvements (bold boxes) discussed
in the paper and how they link to data at the input side and Beach Management
Plans at the output. Arrows should read “informs / contributes to” unless otherwise
stated.
Joint exceedence curves for Worthing
Hs (significant wave height in m)
5.00
4.50
Return
period
(years)
4.00
1
2
3.50
5
10
3.00
20
50
2.50
100
200
2.00
2.5
3
3.5
H (water level in mOD)
4
Figure 3: Joint return probability curve for Worthing at ~10m water depth. Data
source: Rustington Wave buoy 2003 – 2011 and McMillan et al. (2011) point 2285.
Run-up Formula Improvement on Mixed Beaches
A critical point in the assessment of the protection provided by the beach is when water starts to runup to the pre-existing crest level. From this point onward, overwashing can lead to the crest being
build up or lowered (Bradbury and Powell, 1992) with potential for breaching in the case of low lying
land behind the beach or erosion where there is higher ground behind. While there is great uncertainty
as to which of the two scenarios will develop with the speed of waterlevel increase, overwashing
duration and general beach morphology having an impact (Orford and Anthony, 2011), there is no
formula or tool that can even determine the run-up elevation on shingle beaches with any degree of
skill. A major part of the knowledge improvement is therefore the development of an improved run-up
formula for the gravel dominated beaches in Southeast England based on field data. Details of this
work can be found in Polidoro et al. (2013, this issue), but are summarised below.
Ru Measured (m)
Over 250 measurements of run-up elevations under a wide range of tide and wave conditions have
been collected between October 2010 and November 2012 on Worthing beach (Figure 1). In addition,
wave run-up elevations have been extracted from survey data and other evidence to obtain another
200 data points from seven additional beaches in Southeast England. Using these run-up data
together with actual wave and water level data collected, allowed for an evaluation and improvement
of published run-up formulae (van der Meer and Janssen, 1994). These improvements are
documented in Figure 4.
Ru2% Prediction (m)
a
7.0
R u Mesured (m)
Ru Measured
(m)
6.0
5.0
4.0
3.0
2.0
1.0
1.0
2.0
3.0
4.0
R u2% Prediction (m)
Ru2% Prediction
(m)
5.0
6.0
7.0
b
Figure 4: Comparison between measured and predicted run-up elevations for
Worthing beach, a) Van der Meer & Janssen (1994), b) Polidoro et al. (2013).
Following the initial formula improvement using the extensive Worthing dataset, the new formula was
used on the other seven beaches to assess how location specific these improvements were. Figure 5
shows the Worthing data together with the data for four other beaches with the most measurements. It
is apparent that the formula developed for Worthing can be applied without modifications to other
beaches in Southeast England. The poor fit for the low run-up elevations at Cuckmere Haven are most
likely due to uncertainty in identifying features of low magnitude and assigning them to a particular
event. For the high run-up elevations, the data for Cuckmere Haven fit in with those for the other sites.
7.0
Worthing
Newhaven
6.0
Cuckmere
Pevensey
Ru Measured (m)
5.0
Saltdean
4.0
3.0
2.0
1.0
1.0
2.0
3.0
4.0
Ru2% Prediction (m)
5.0
6.0
7.0
Figure 5: Comparison between measured and predicted run-up elevations for
Worthing beach and four other beaches in Southeast England.
Beach Design
Using the output from the joint probability assessment as input into the run-up tool, run-up, and thus
one of the critical beach design parameters, can be assessed on a probability basis as shown in the
example in Figure 6, which is still based on the run-up formula of van der Meer & Janssen (1994) and
not on Polidoro et al. (2013). Not surprisingly, wave run-up in a depth limited environment is
dependent on water level but also on wave period. Because the joint probability assessment is carried
out based on different directional sectors and sensitivity tests using different dependency values and
wave periods, wave run-up elevations can be assessed in a probabilistic way.
Run‐up height (mOD)
5.5
5.4
2
5.3
5
5.2
20
5.1
100
5
200
4.9
2 (Tp+1)
4.8
5 (Tp+1)
4.7
20 (Tp+1)
4.6
100 (Tp+1)
4.5
200 (Tp+1)
3.60
3.80
4.00
4.20
Water depth at beach toe (m)
4.40
Figure 6: Crest height design criteria (run-up) based on joint probabilities for a range
of 1 in x scenarios. All examples are run with
5.68 .
except for the bold
series where 1s was added to Tp for illustration purposes.
Shingle Sediment Budget
Due to the dominant south-westerly waves in the English Channel, net longshore transport is towards
the east / northeast except for the north Kent coast, where it is in a westerly direction (Figure 7).
Sediment cell boundaries (Cooper and Pontee, 2006; Motyka and Brampton, 1993) are therefore
dominated by a few headlands and a small number of rivers. While the headlands do not necessarily
form a complete transport barrier, the river mouths, together with their enforcement through harbour
arms and other man-made structures like the Marina at Brighton, form almost complete barriers to
longshore transport for mixed sediment. However, this natural transport is in two locations (Shoreham
and Sovereign Harbour) at least partly replaced by harbour bypassing using road lorries. The existing
barriers and overall transport paths allow for the identification of functional sub-regional sediment
budget cells.
R
H
Brighton
Marina
Shoreham
E
Sovereign
Harbour
Figure 7: Sub-regional sediment cells and their boundaries considered in this project
and general long-term net longshore transport directions. E = Eastbourne, H =
Hastings, R = River Rother
3.5
9.5
3.0
9.0
2.5
8.5
2.0
8.0
1.5
7.5
1.0
7.0
BULVERHYTHE
0.5
6.5
Total
0.0
1860
1880
1900
1920
1940
1960
1980
2000
PEVENSEY BAY
1,000,000m³
1,000,000m³
A transparent methodology was created for using Regional Coastal Monitoring data to calculate
‘operational sediment budgets’ (Morang et al., 2012), which identify areas of erosion, accretion and
transport, providing both total volumes and rates of change including variability between years. This
information was taken from surface surveys and generated Digital Elevation Models (DEM) of the
relevant areas and was supplemented with intervention data on recharge, recycling and bypassing of
longshore transport barriers. For some frontages, where large-scale changes in shingle beach
volumes occurred in historic times, an analysis of historic maps was carried out. Using mapped lines
of MHW, MLW and the beach toe (identifiable as a change of map signature) together with present
day information about crest height, geometric relationships and beach slopes, simplified DEMs were
created for maps from the 1870s, 1890s, 1910s and 1930s from which total volumes could be
estimated and compared with those of the past decade.
EASTBOURNE
HASTINGS
BEXHILL ON SEA
SOVEREIGN HARBOUR
6.0
2020
Figure 8: Long- and short-term sediment volume changes between Eastbourne and
Hastings. Total volume (right hand y-axis) excludes Bexhill on Sea for historic times.
An example using the cell from Eastbourne to Hastings (27km long, Figure 7) is shown in Figure 8. It
can be seen that during historic times total volume across this cell has remained stable and that
individual frontages have remained largely stable from the 1890s to the 1910s. Between the 1910s
and the 1930s, the westernmost sections (Eastbourne and Sovereign Harbour) have lost material
which moved eastwards, showing accretion at Pevensey and Hastings in particular, owing to the
capture of material against the harbour arm that was build around the last turn of the century.
Comparing historical volume with the present day, only Sovereign Harbour shows a reduction; all
other frontages contain either as much material (Bulverhythe) or have increased. The maximum
increase has occurred at Hastings (~1.5mio m³) with a total increase in the order of 3mio m³. Though
no detailed analysis has been carried out, this increase includes the 1mio m³ at Bexhill, which were
not included in the historic volumes, and, in the 15 years before the first survey in 2003, ~1mio m³ was
placed on Eastbourne and Pevensey. In addition, on Seaford beach to the west of the study area,
~1.7mio m³ were placed in the 1980s in response to the beach having lowered since the start of the
century. This recharge brought the beach to approximately the extent as seen in ~1910 so that it can
be reasonably assumed that >1mio m³ left the frontage between the historic period and recent
surveys, thus accounting roughly for the 3mio m³ increase.
3.5
9.5
PEVENSEY BAY
3.0
9.0
EASTBOURNE
2.5
8.5
2.0
8.0
1.5
7.5
1.0
7.0
0.5
6.5
0.0
2003
2005
2007
2009
6.0
2011
1,000,000m³
1,000,000m³
While the comparison over more than a century provides a perspective of the magnitude of change
that has occurred, this generally took place under much different management regimes.For informing
operational decisions, the focus needs to be on changes over the past decade (Figure 9). Over the
last eight years, total volume has remained stable with a sudden increase between 2010 and 2011.
This increase is due to the recharge at Eastbourne in the Spring of 2011 in the order of 200,000m³,
because of the steady decline up to 2010. This decline at Eastbourne is due to longshore transport
leading to a build-up of material against the harbour arm at Sovereign Harbour from where beach
material is moved around the harbour by lorry and deposited on Pevensey. In addition to this gain
from bypassing, annual recharge is placed at the southern end of Pevensey Bay to a combined annual
volume of ~25,000 to 31,000m³. Over the eight years shown, this should have led to an increase in the
volume at Pevensey in the order of 200,000 to 250,000m³. Yet, Pevensey Bay does not show such an
increase but a stable volume with a subtle decrease. As there is no terminal structure at the end of
Pevensey Bay and the management contract with Pevensey Coastal Defence Ltd stipulates that
material needs to move downdrift, this material has moved on. Out of the remaining downdrift
frontages, only Bexhill shows a steady gain while the others have remained stable or accumulated due
to localised recharge (20,000m³ at Bulverhythe in 2005). The total gain at Bexhill is 150,000m³ and
therefore beach material must already be exported to downdrift of Hastings.
HASTINGS
BEXHILL ON SEA
SOVEREIGN HARBOUR
BULVERHYTHE
Total
Figure 9: Changes in total beach volume over eight years. Total volume uses the right
hand y-axis.
The annually averaged shingle sediment budget based on the last eight years shown in Figure 10
highlights how sediment passes through the cell from Eastbourne to the River Rother, and also which
sections take out or add to this drift. The Pevensey polygon can be used as an example to explain the
volumes: 8300m³ are bypassed from the Eastbourne frontage around Sovereign Harbour and
19,900m³ are placed from offshore recharge. Natural losses from shingle attrition and losses from
recharge and recycling activity amount to 6,300m³ (losses from recharge refer to the removal of
volumes of sand and finer material that are in excess of that to fill the intersticial voids of the gravel
usually as a consequence of the dredged material containing more fines that the host material).
Overall, the beach volume as surveyed is declining each year by 6,400m³, so that as a result on
average 28,300m³ are passed into the Bexhill frontage. At Bexhill, nearly two thirds of this updrift input
are retained and only 10,000m³ passed on.
Recycling
in
Recycling
out
Polygon volume
change
Recharge
in
Losses
Transfer to
next polygon
Fairlight
Bulverhythe
Pevensey
Figure 10: Annually averaged shingle sediment budget (in m³) for the cell Eastbourne
to River Rother. Figures in the polygons represent the volume change within each
polygon and transfers into and out of each cell.
G
Gain
G
Gain
Gain
Loss
Figure 11: Change of beach volume in space (x-axis represents 50m segments) and
time (y-axis) for a ~5km long frontage against the volumes measured in 2003. Bold
line separates areas of gain and loss as indicated. A volume change of 1000m³
corresponds to a blanket elevation change of ~0.4m in each 50m segment.
While the larger scale sediment budget needs to take uncertainties in association with survey errors
into account (Dornbusch, 2010), a more powerful way of documenting changes is on a local scale.
Figure 11 shows spatio-temporal changes for the ~5km long frontage at Bexhill over the period 2003
to 2011 at a spatial resolution alongshore of 50m, based on annual surveys carried out in the summer.
This high-resolution data is the basis for the analysis presented earlier. The frontage is characterised
by a dense timber groyne field. Figure 11 shows how from 2003 the western half of the frontage has
gained beach material either straight from the start or following a short-lived reduction in volumes and
how the eastern half (east of polygon number [PN] 62) has first experienced beach lowering, which
stopped and changed to accretion. This switch-over in the eastern part started in 2008 at PN 65 to 70
and in 2009 and 2010 from PN 85. The general picture is one of volume gain through time progressing
from west to east. As groyne bays become saturated, more sediment can move eastwards resulting in
downdrift groyne bays filling up too. There are some local anomalies to this pattern, in particular
around PN 16, suggesting that this area was already at capacity in 2003. It also needs to be borne in
mind that some reprofiling has been taking place along this frontage. Nevertheless, at this detailed
scale the timing and spatial extent of sediment movement can be clearly documented.
Sediment Redistribution (Long-Haul Recycling Feasibility)
As the example for the cell Eastbourne to River Rother (Figure 10) has shown, natural shingle beach
sediment transport can cover >40km. The other sediment cells shown in Figure 7 show similar
movements over comparable distances.
Balancing the natural movement through recycling from downdrift to updrift is presently carried out by
land-based plant using either lorries or dump trucks. While the former generally carry less than dump
trucks, they can cover longer distances but also impact on the local transport network along the coast.
Dump trucks, on the other hand, require enough uninterrupted beach width to travel on and create
more disruption to beach users with associated health and safety concerns. For both modes of
transport, cost generally increases with transport distance (Figure 12) but is still only a fraction of the
cost for new material from offshore dredge sites, which is approximately ~£25 per m³ in Southeast
England, depending on volume, source and deposition areas.
Initial discussions with contractors have highlighted the feasibility of using a marine transport pathway.
As most of the managed shingle beaches are fronted by a low angle (1° to 2° on average) intertidal
sand platform, beaching a flat-top barge on platform at high tide, filling the barge over a low tide cycle,
towing it to a different location between two high tides and then beaching and unloading it is a
potential alternative that would allow recycling over large distances. Costs have been estimated to be
between that for new offshore dredged material and that for land-based transport over comparatively
much shorter distances.
Cost per m³ (£)
10
8
1,000m³ 30,000m³ 60,000m³
6
4
Dump truck
Road lorry
2
0
0
10
Distance (km)
20
30
Figure 12: Transport costs for beach material depending on distance, transport mode
and overall volume based on projects carried out over the last few years in Southeast
England.
Conclusion
There is a strong need for a co-ordinated programme of beach management activities that ensures
existing shingle resources are used effectively over the wider coastline. Reduced intervention will
generate operational cost savings and benefit the maintenance and improvement of vegetated shingle
habitats. Improved understanding of risk through overwashing and overtopping will also lead to better
warning for coastal flood events.
Targeted improvements to the knowledge base for beach management have been carried out to
enhance transparency in decision making, essential for carrying out beach management across
boundaries of individual Operating Authorities. They were designed with future updates in
understanding and data in mind, leading to a modular structure. However, implementation will depend
on the co-operation of all parties involved (including private land owners) and the engagement of the
wider public, in particular in relation to sediment movement and risk assessment.
Acknowledgements
The authors would like to thank Jonathan Clarke (Canterbury City Council), Travis Mason (Channel
Coastal Observatory), Gary Page (Mackley), John Tharme (Team Van Ord), Ian Thomas (Pevensey
Coastal Defence) together with many people from the coastal Operating Authorities in the Southeast
for their contribution to this project.
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