Mud and sand effects on wave propagation over the French Guiana coasts

Abstract

The dampening effect of waves by soft mud layers is observed throughout the spectrum, in laboratory (WELLS & KEMP, 1986) as on the Louisiana or Guyana coasts (WINTERWERP et al., 2007; GENSAC, 2012). Since the bi-layer theoretical approach of GADE (1958), multi parameterizations have been proposed and implemented in wave numerical models (e.g. ROGERS & HOLLAND, 2009) but many efforts of calibrations and additional works are still required to obtain realistic representations of in situ processes. The Guyanese's coast are impacted by the Amazon sediments discharge whose 20 to 30 % migrate longshore either in turbid or in mud banks forms due to the waves and current combined actions (BISCARA, 2016). These mud banks cause rapid coastline variations, leading to accretion, erosion and submersion risks. The French operational wave forecasting system at coastal scale is based on WAVEWATCH III ® (WW3, TOLMAN, 2016), using an unstructured grid that covers Guiana with a resolution of 200 m nearshore. An implementation has been realized in 2017 in the framework of the HOMONIM project (History, Observation, Modeling sea levels, joint SHOM and Meteo-France project) for the French Guiana coasts however this version didn't include the effects of the mud and sand banks on waves. In this paper, we evaluate the mud effect on the wave propagation in order to improve the future version of the operational French Guiana configuration. Numerical tests on different parameterizations are performed on a laboratory case, to assess the behaviour of WW3. A more specific application on Guiana is carried out via the creation of a seabed map (grain size) as well as a fine description of the characteristics and location of the mud banks, thanks to high resolution satellite imagery and in-situ data. 1. Introduction Guyana shoreline is characterised by muddy sedimentation fed continuously by deposits brought to the ocean by the Amazon, 800 km further south. At the mouth of the river, this intense sediment load is set in motion by the North Brazilian current and swell, and spread along the coast of Guyana during its ascent to the north. The sediments that are deposited form huge mud banks (up to 5 m thick, 10 to 60 km long, 20 to 30 km wide and 15 to 25 km apart) that migrate rapidly (1 to 5 km.y-1) in low water depth (< 20 m) causing rapid coastline morphological changes which are difficult to predict. The mud banks present on the entire coast of Guyana quickly absorb and dissipate wave energy across the full spectrum (about 70% and more, (WELLS & KEMP, 1986)) and in particular short waves to long waves as long as they pass through the sedimentary body. In the HOMONIM project, the objective is to develop a wave forecasting model in order to better anticipate flooding from the sea and to improve warning systems on French metropolitan and overseas coasts. Initial configurations have been delivered since 2014. For Guiana, a first version was produced in 2016 (V1), based on the WAVEWATCH III ® model using an unstructured grid with a resolution of 200 m nearshore and 8 km offshore. However, this version does not include the effects of sandy and mud banks or current and water level variations on the waves. The objective of this paper is therefore to evaluate the effects of seabed sedimentary characteristics on wave propagation in order to improve the future version of the operational configuration for the French Guiana coastal area. Numerical tests on different parameterizations are performed on the laboratory case of DE WIT (1995), to assess the behaviour of WW3. A more specific application on Guiana during winter storm 2016 is then carried out via the creation of a seabed map as well as a fine description of the characteristics and location of the mud banks, thanks to high resolution satellite imagery and in-situ data.