Sand size variability inside the hopper of a trailing suction dredger in regards to beach nourishment projects

Abstract

Amongst the design parameters for a beach nourishment project, sand size must be highlighted. Sand size is critical not only to compare the suitability of a borrow sand to substitute the eroded native sand, but also to identify the new equilibrium beach profile and to calculate the sediment volume necessary to achieve the projected berm or beach width. Some researchers have already taken into account the phenomena that change the theoretical volume of sand needed for a beach nourishment project (CUR, 1987; CEM 2002) such as the methodology for the hopper measurements taken on board of the dredger (Muñoz-Perez et al., 2003) or the sand porosity variability (Roman-Sierra et al., 2014). Nevertheless, no investigation has been carried out about the variability of sand size inside the hopper of a trailing suction dredger till nowadays. There are several reasons to justify this possible heterogeneity of the granulometric parameters inside the hopper. Firstly, there is the intrinsic variability of the sediment characteristics at the sea bottom. On top of that, most of the vessels which dredge nowadays are of the Trailing Suction Hopper Dredgers (TSHD) type and this means that these vessels do not dredge in a stationary way, i.e. at a certain location and vicinity, but along a big stretch of the bottom. Moreover, the fall speed of sand depends directly on the grain diameter (for a given density); i.e. the larger the grain sizes the faster the grain deposits and so, probably, the coarser grains would be at the bottom of the hopper and the finer grains would be at the top. Finally, it is noteworthy a word about the fragments of shells or bioclasts: because of their flat shape, these pieces linger more than the silica grains to decant and, therefore, their percentage will most likely be larger at the surface of the hopper.