Storm Impacts on Gravel Beaches: Application of the Argus
Video-System
Amaia Ruiz de Alegria1, Gerd Masselink1, Kenneth Kingston2, Jon Williams1
1
School of Geography, University of Plymouth, Plymouth,
PL4 8AA, UK Tel: 0044 (0)17522 33077 (amaia.alegria@plymouth.ac.uk)
2
School of Earth, Ocean & Environmental Sciences, University of Plymouth, Plymouth, PL4 8AA, UK
Introduction
More than 1,000 km of the English and Welsh coastline is fronted by beaches composed
almost entirely of gravel (d50>2mm). A significant proportion of these beaches are barrier
beaches and act as natural coastal defences protecting backshore low-lying regions from
flooding during extreme events.
Most gravel beaches are experiencing management problems, either related to
flooding/breaching, or due to littoral drift-induced coastal erosion. These problems occur
mainly during extreme water levels and storms, and are likely to be aggravated in the future by
the projected increase in the frequency of extreme water levels.
Slapton Sands is a transgressive retreating swash-aligned gravel barrier, located in Devon, UK
(Fig. 1). This barrier is approximately 5 km long and 100-140 m wide, located between two
cliff-outcrops within Start Bay. The beach is a continuous barrier fronting a freshwater lagoon,
and is devoid of coastal structures. Mean sediment size varies considerably according to
location, but is approximately 6mm. It is a ‘pure’ gravel beach, meaning that interstitial sands
are effectively absent, and there is no sandy low tide terrace. The beach is slightly graded
alongshore, becoming coarser to the south. Beach gradients are high, typically tan β 0.1 – 0.2.
Inshore hydrodynamics are dominated by SW and NE swells, which have undergone
considerable refraction and attenuation by rocky headlands and subtidal banks. The beach is
characterised by medium-to-high energy wave conditions (mean Hs  1 m; storm Hs  3 m) and
a mean spring tide range of 4.3 m. The beach is highly dynamic and significant damage have
ocurred during recent storm events due to flooding and overtopping processes.
Fig. 1 –Aerial overview of Slapton Sands from Strete Gate (left)
and Torcross (right), (May, 2007).
Objectives
The present project aims to address the gap in current knowledge and modelling capability by
undertaking a comprehensive investigation into the storm response and medium-term stability
at Slapton beach. This approach demands new observations and methodology in order to
predict the morphological behaviour of the barrier. Limited morphodynamic data has been
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collected on gravel beaches due to the logistical challenges they present. Moreover, the effect
of storms on gravel beaches and an ability to make quantitative predictions of these impacts,
remains very limited.
Methods
The current research will combine field surveying and remote sensing techniques in order to
develop and evaluate a new numerical modelling system for gravel beaches.
Beach surveys – An extensive beach survey program has already begun and will be conducted
over a 3-year period. Intertidal surveys will be carried out fortnightly along the entire beach
using real-time-kinematic GPS. These periodic measurements will be complemented by 10
‘event-driven’ cross shore surveys before, during and after major storms. Profile and planform
changes in the intertidal volume derived from the beach surveys and the ARGUS video images
(see below) will be used to compute cross-shore and longshore sediment transport rates
(averaged over the medium-term). These transport rates will be related to offshore wave and
tide conditions measured using a local directional wave buoy managed by the Coastal Channel
Observatory (CCO).
ARGUS monitoring – A 3-camera ARGUS video monitoring
station has been installed at Slapton Sands (Fig. 2), and has
been operational since December 2005. Beach profiles and
morphological change will be derived from the ARGUS
images (Fig. 3), revealing storm response. Additionally
recent advances in ARGUS technology will be used in order
to extract hydrodynamic data from the images, including
wave angles, surface current velocities and run-up motion, to
build a picture of the local hydrodynamic conditions over the
period of the project. This methodology enables, for the first
time, the collection of hydrodynamic data on gravel beaches
during storms, when in-situ instrumentation is unlikely to
survive. A particularly exciting prospect is the recording of
beach response to extreme storm activity (overtopping,
overwashing, or breaching of the barrier).
Fig. 2 _ The ARGUS camera
system at Slapton Sands
Fig. 3 _ Rectified and merged ARGUS image of the northern part of Slapton Sands. The length of the beach
in the image is c. 2 km.
Numerical modelling – Cross-shore and alongshore sediment transport rates derived from the
beach surveys and ARGUS imagery will be parameterised using a behavioural-type sediment
transport model similar to a ‘shape function’ (which has been used successfully for simulating
bar and berm formation on fine-grained beaches). This will use a simple mathematical
expression to describe the observed pattern of the sediment flux across a profile, which is
appropriately scaled and advected across the nearshore zone to account for changing input
wave energy and tidal elevation. A coastal area model will be built up from a series of profile
lines along which cross-shore and alongshore sediment transport rates are described using such
an approach, allowing the resulting 3D morphological change in the area to be determined.
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