Bio-geomorphological interactions in shoreface nourishments

A study by Janssen and Mulder (2004) showed that large amounts of the tube building polychaete Lanice conchilega are present in the surfzone at Castricum and Egmond. This worm is known to influence near bed hydrodynamics by forming dense tube fields (Friedrichs et al., 2000). Shoreface nourishments are carried out in the same area. The main purpose of this study is to determine how these tube fields influence the morphology of a coast with and without a nourishment. This is done using a Delft3D computer model of a two barred beach based on the coast of Egmond. Ten different scenarios are modeled for a 1 year period in which the presence of the nourishment, L. conchilega tube density and extinction time are varied. The presence of L. conchilega reduces cross-shore and longshore sediment transport and stabilizes the coastline. Without a nourishment present this results in a slower migration of the sand bars. When a nourishment is present L. conchilega reduces its diffusion. This prevents erosion of the coastline and extends the lifetime of the nourishment.