Abstract
The Authie estuary is characterized by an important southern sand spit and a northern shoreline subject to strong erosion due to the meandering of the coastal river. In order to reduce this erosion, a new soft coastal defence, namely the sand net device (SND), has been implemented inside the Authie estuary. It consists of several nets assembled in an inverted V creating a porous structure and thus trapping sand as shoreline protection. However up to now, little proof has been provided on the explicit influence of this SND on the hydrodynamic pattern and associated morphodynamics. In this paper, field surveys of morphological developments combined with numerical modelling (Telemac-2D/3D) analyze the influence of the SND into flow pattern and morphodynamics. In situ monitoring clearly points out sedimentation around the SND and a deepening of the main channel. Modelling results show that, without SND, erosion is observed around its location. With a SND implemented, the velocity has been reduced and created a deviation in its direction by a circulation around the SND location. The impact area of the structure is around 500 m in both directions, upstream and downstream part.
Highlights
Estuary mouths generally feature complex interactions between waves, tides, waveinduced currents that vary with tidal range and fluvial discharge
Estuary mouths are often characterized by sand accumulation in the form of sand bars or of a sand spit that develops across the estuary [2,3]
This estuarine mouth is largely blocked by the prominent sand spit and intertidal/subtidal platform, and this has forced the main channel to migrate along the northern shore and adjacent shoreline [4]
Summary
Estuary mouths generally feature complex interactions between waves, tides, waveinduced currents that vary with tidal range and fluvial discharge. The Authie mouth consists of an important south sand spit prograded seawards and northward and a north bank subject to strong erosion, and a large zone of saltmarshes in the middle part of the estuary. This estuarine mouth is largely blocked by the prominent sand spit and intertidal/subtidal platform, and this has forced the main channel to migrate along the northern shore and adjacent shoreline [4]. In order to prevent such events, a new soft coastal defence system has been implemented to reduce erosion inside the Authie estuary It consists of a porous groyne which aims to trap sediment inside net layers and to create a sandy deposit as shoreline protection. A 2D large scale model of the estuary serves to analyse the morphodynamic evolution and the impact on flow circulation with or without the SND in order to highlight its effects
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