Jon J. Williams

Title: MICORE: DUNE EROSION AND OVERWASH MODEL VALIDATION WITH DATA FROM NINE EUROPEAN FIELD SITES

In this paper we present the first results of beach profile hindcasting with XBeach using recently measured coastal data acquired under storm conditions at eight European sites, including a comparison to model results obtained with off-the- shelf models. The results show consistently that the XBeach has skill in predicting the coastal profile, albeit that in most cases the erosion around the mean water line is overpredicted and the depositions at the lower beach face are overpredicted. The causes for this model effect are under active investigation but not resolved yet. Likely candidates are the modeling of onshore (asymmetry) transports which reduces the offshore transports due to undertow (currents) or the modeling of sediment motion in the swash zone.

Offshore sand bank dynamics

Abstract The present paper reports some key results from field investigations and numerical modelling studies of the tide- and wind-induced hydrodynamics and sediment dynamics of Middelkerke Bank (MB) in the southern North Sea of Europe conducted during December 1992 to March 1993. Strong surface current refraction and acceleration effects were observed over MB using the HF radar system OSCR ( O cean S urface C urrent R adar). Results suggest that OSCR data may be used remotely to monitor large-scale bathymetry in shallow coastal environments. Spatial variation in tidal propagation characteristics and modification of shoreward propagating waves was not detected at locations around MB during the experiment. Observed residual currents were found to be correlated strongly with wind speed and direction during the period 26 February to 18 March 1993. However, in low wind stress condition, a three-dimensional numerical model (3D-Bank) indicated the presence of a clockwise residual circulation of water around MB consistent with theory. Spatial and temporal variation in the average total drag coefficient ( C d ) of MB were investigated and found to correlate strongly with tidal current speed. Fluorescent sand tracers, used to monitor net sediment transport pathways, revealed a net clockwise movement of sediments around MB consistent with predictions by 3D-Bank and with theory.

Acoustic instrumentation for measuring near-bed sediment processes and hydrodynamics

Improved understanding and modelling of interactions between biological and physical processes has required progress in the understanding of sediment processes and their relationship to bedforms, hydrodynamics and benthic biology. Recently, simultaneous, co-located in situ measurements of suspended sediments, near-bed velocities and bed morphology have been made possible using new acoustic instruments, including a triple frequency acoustic backscatter system, (ABS); a uniaxial, and triple axis, coherent Doppler velocity profiler, (CDVP); a sand ripple imager (SRI); and a sand ripple profiler (SRP). Together these instruments measure the relevant hydrodynamic and sediment parameters necessary to investigate sediment processes in the bottom metre above the bed, with centimetric resolution. The range of data from such instruments is required to fully understand the related processes occurring near the seabed. The outcome of such measurements should improve our understanding of benthic processes in general.

Interactions between a benthic tripod and waves on a sandy bed

In the past the amount of local disturbance to hydrodynamic and sediment processes by instrumented benthic frames has not been investigated. Here, in a series of tests on sandy beds in a large wave flume, measurements of wave-induced flows, turbulence, bedforms and suspended sediments have been obtained from the large tripod frame STABLE and from a control site away from the influence of the frame. Present measurements show surprisingly small modifications by the frame to local hydrodynamic conditions, bed morphology and vertical profiles of suspended sediment.

New approach to measurement of sediment processes in a tidal inlet

Strong tidal currents, shallow water, waves and mobile bed sediments present in many tidal inlets prevent deployment of instrumentation using benthic frames and restrict measurements that can be obtained safely from a boat and thus data from these environments remain scarce. Recent deployments of the new POL Instrument Package (PIP) from a jack-up barge have provided a solution to this problem. Here the performance of the various sensors on the PIP is assessed critically and the degree to which near-bed processes are affected by the PIP is investigated by comparing measured and predicted flows, bedforms and rates of sediment transport.

Modelling storm responses on a high-energy coastline with XBeach

The XBeach model has been used to simulate the morphological impacts of storms on sandy and gravel beaches. Taking as a case study Rossbeigh Spit located on the high-energy coast of western Ireland, the study reported here tests the capacity of XBeach to reproduce barrier breaching during a storm in December 2008. It demonstrates that predictions of the breaching event agree reasonably well with observations. However, the main focus of the paper is to establish using the model results, site-specific critical wave and water level conditions giving rise to dune erosion, overwashing and breaching. By deriving simple-to-use expressions to define hydrodynamic thresholds the study advances the ability to predict the impacts of infrequent and rarely observed storm events and is considered to provide useful coastal management tool for assessing the vulnerability of sandy barriers to breaching high-energy during storms.

Nearshore sediment resuspension and bed morphology.

ABSTRACT Thompson, C.E.L., Kassem, H. and Williams, J., 2013. BARDEX II: Nearshore sediment resuspension and bed morphology. Sediment resuspension in the region outside the surf zone is known to contribute to the morphological response of barrier beaches to wave forcing, such as onshore bar migration processes. However, few measurements in this region exist, limiting our ability to quantify its contribution. These processes are complicated by the presence of bedforms in the nearshore, which alter the sand transport processes while modifying bed roughness in a complex feedback mechanism. The Hydralab IV funded BARDEX II experiments, which took place in the Delta Flume in 2012, were used to provide measurements of these processes in the nearshore of a sandy barrier beach (D50 = 0.42mm) under a range of wave conditions (Hs = 0.3 - 0.8 m; Tp = 4 – 12 s) and water levels, through deployment of a suite of acoustic instruments measuring flow velocity, near-bed turbulence, sediment resuspension profiles and bed morphology in cross-section and plan view. Initial findings indicate that sediment suspension in the nearshore appears to be controlled by a combination of near-bed turbulent bursting processes which results in near-instantaneous low concentration suspensions restricted to the bottom boundary layer, and vortex shedding from bedforms which results in higher concentration suspensions which are larger in scale than vertical eddy sizes, and perpetuate outside of the bottom boundary layer.

Application of an operational storm coastal impact forecasting system

The National Oceanography Centre (NOC), in collaboration with the UK Met Office, has developed an operational suite of models for the Irish Sea. This work has extended this suite to include a coastal vulnerability early warning system for the Sefton Coast in Liverpool Bay. The system is designed to forecast when the offshore hydrodynamic and wave conditions exceed thresholds for low, moderate and high hazard of storm impact and also gives an indication of the extent of dune erosion. The model simulates a 72h period (yesterday:today:tomorrow), allowing a 24h spin-up period and 48h forecast. The main idea of the system is to use forecasts for impending storms to assist with management decisions.

Hydrodynamic modelling of a dynamic inlet

This paper describes details of a 2D hydrodynamic model and its applications to a dynamic inlet. The model computes the depth-average tide and wave-driven currents, as well as wave characteristics. It forms part of a Q3D model which also includes a module to compute undertow velocity distribution through the water column at a point-in-plan in the surf zone. The 2D hydrodynamic module allows for the effects of wave diffraction and refraction, wave breaking and wave-current interaction to be included in dynamic simulations for combined tidal and wave situations. The 2D model was applied to a newly-opened dynamic inlet near Faro at southern Portugal, with conditions obtained during a four-month intensive field campaign under Project INDIA (Inlet Dynamics Initiative: Algarve) funded by EU MAST III Research Programme. The model results were compared with field data measured by a number of remote sensing instruments and showed good agreement.

Using Combined Modelling Approaches to Improve Coastal Defence Design: a case study at Hopton, UK

ABSTRACT Williams, J.J., Esteves, L.S., Conduche, T., Barber, P., Tindle, A., 2014. Using Combined Modelling Approaches to Improve Coastal Defence Design: a case study at Hopton, UK. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 018–022, ISSN 0749-0208. A storm that occurred close to the spring tidal maxima in March 2013 resulted in beach lowering and cliff recession of c. 5 m along a 110 m frontage at Hopton-on-Sea, UK and threatened the static caravan park of Bourne Leisure Ltd. This paper reports a study using XBeach and MIKE21 models to assist with the design of new coastal defences to reduce cliff and beach erosion. Two schemes are examined here: three fishtail rock groynes (Scheme 1); and ten ‘double-head’ curved rock groynes (Scheme 2). The selected design must provide acceptable level of protection and, to be granted consent, no adverse environmental impact must be demonstrated. Current practice using a single numerical model can provide a cost-effective tool for coastal defence assessments. However, the work presented here show that when good agreement between complementary models can be demonstrated, greater confidence can be given to model results. Specifically, refinements to the present scheme design were made possible by the use of the XBeach model, which allowed identification of the cross-shore limits of sediment transport in storm conditions, and by the MIKE21 model which allowed quantification of alongshore scheme impacts. Together, the model results have assisted the development of an improved final scheme design which minimizes potential environmental impacts.