Sorted bedforms developed on sandy deposits derived from small ephemeral streams (Catalan continental shelf)

Abstract

The morphology and sedimentological characteristics of sorted bedforms identified in the Catalan continental shelf (NW Mediterranean Sea) have been characterized using multibeam echosounder data and sediment samples collected in 2013 within the FORMED project. Bathymetric data was compared with previous data gathered in 2004 within the ESPACE project to assess decadal stability of these bedforms. The sorted bedforms were observed on the inner shelf off Lloret and Tossa de Mar, at water depths of 10 to 40 m (Fig. 1). They exhibit elongated shapes and are oriented nearly perpendicular to the shoreline. The sorted bedforms display lateral symmetry in bathymetric relief and backscatter, with high backscatter corresponding to coarse sand (median size of 0.55-0.96 mm) centred on the bathymetric depression, and low backscatter corresponding to well-sorted fine to medium sand (median sized of 0.22-0.35 mm) on the crest. They appear associated with elongated finer-grained deposits fed by short, ephemeral streams that provide the required substratum for sorted bedform development. The local contribution of well-sorted fine sand by these ephemeral streams over the coarse sand domain of the infralittoral prograding wedge contributes to the bed sediment heterogeneity (mixture of sediment), which is further reorganized to form sorted bedforms. At a decadal time scale, the morphological evolution of these bedforms indicates that they are persistent features, showing small changes in their boundaries over a decadal timescale