Offshore sandbank morphodynamics modelling with sea level rise By G. Jacoub 1* , P.K. Stansby 1 , J-M. Hervouet 2 , C. Villaret 2 and M. Benoit 2 1 Tyndall Centre, School of Mechanical, Aerospace and Civil Engineering, University of Manchester, UK 2 EDF R&D, Laboratoire National d'Hydraulique et Environnement (LNHE), 6 Quai Watier, 78401 Chatou, France * Corresponding author (George.Jacoub@manchester.ac.uk , Tel. +44 161 306 5797, Fax. /3255) Abstract Coastal erosion and flooding are largely determined by extreme wave action and nearshore wave heights are controlled through energy dissipation on offshore sandbanks. East Anglia is the area of study here. The finite-element codes for tidal flows (TELEMAC-2D), for wave propagation (TOMAWAC) and for morphodynamics (SISYPHE), are used for this study. Preliminary results are presented to show the effect of waves, currents and sea level rise (SLR) on morphodynamics for a period of one year with an initial bathymetry of the present day. Offshore wave climate is driven by onshore wind data from 2002, as an example. Annual residual sediment fluxes are computed to show typical sediment pathways for the domain. Alongshore sediment transport due to waves, from the CERC formula, is qualitatively similar to that computed. 1. Introduction A coastal domain with complex offshore sandbanks off the east coast of East Anglia, UK, is studied, covering an area with length about 100 km in the longitudinal direction and about 75 km in the latitudinal direction. This region includes the Scroby sandbank which is known to be quite mobile. The impact of sea level rise (SLR) on sandbank movement can manifest itself in three ways: 1) sandbank bathymetry may be lowered due to an increase in the water depth and wave height; 2) sandbank bathymetry may be raised due to deposition; or 3) sandbank bathymetry may be unchanged. This depends on the wave, current and sediment conditions. To predict and understand these processes, it is desirable to develop a numerical model system incorporating all the relevant physical processes. The paper presents an application of the finite-element TELEMAC system (Hervouet, 2005 and 2007). The wave propagation module, TOMAWAC, the tidal module, TELEMAC-2D, and the sediment transport module, SISYPHE, were applied in a coupled way to calculate the sediment transport rates. TELEMAC-2D solves the shallow water equations including wave-induced radiation stresses, obtained from TOMAWAC. TOMAWAC (Benoit et al., 1996 and Benoit, 2005) is a third generation spectral wave model which accounts for the wave generation by wind, refraction, shoaling, nonlinear wave-wave interactions and energy dissipation due to white-capping, bottom friction and depth-induced breaking. SISYPHE (Villaret, 2004) calculates the sediment transport rates with options of various formulae; the Bijker formula is used accounting for combined current and wave action, following Villaret and Davies (2004). TOMAWAC was applied first to calculate the time-varying wave conditions where the average water level changes hourly. Then, TELEMAC coupled with SISYPHE was applied to estimate the sediment transport rates and the morphological bed evolution. The numerical simulations were performed for one physical year to gain understanding of the relevant processes, their interactions and the average or residual sediment pathways in the domain. 2. Site and Data Description The computational domain, shown in Fig 1a, extends from 52° 13’ 20‘‘N to 53° 13’ 20‘‘ N with length of about 100 km in the longitudinal direction and from 1° 0’ 00‘‘ E to 2° 10’ 00‘‘ E with length of about 75 km in the latitudinal direction. The TELEMAC system was applied to estimate the morphological changes, particularly around the Scroby sandbank, without and with sea level rise (SLR) of 0.6 m (Shennan & Horton, 2002) which is an expected value within 100 years, using the bathymetry for 2002. The boundary conditions for current and wave variables were taken at the outer boundary points which were considered to be in offshore deep water.