Optimal conditions for floodplain sedimentation and land aggradation in river deltas

Abstract

River levee breaches can result in crevasse splay formation and floodplain sedimentation. As such, crevasse splays are a potentially effective mechanism for land aggradation of deltas facing relative sea-level rise. Exploring crevasse splays in the Rhine and Mississippi river deltas, however, we find a large variety of splay lifetimes, volumes and surface areas, and consequently, floodplain sedimentation efficiencies. Here we present a field- and modelling study where we explore conditions that lead to effective and efficient floodplain sedimentation for land rise. We use idealized Delft3D simulations of a river and an adjacent floodplain to explore the effects of floodplain geometry, floodplain size and sedimentology. From observations we find a positive relation between floodplain extent and crevasse length and, consequently, crevasse splay area. We find that crevasse splays seem to be limited in size to about 10% of their floodplain area. Peat rich floodplains can further reduce crevasse splay width. The size-limiting role of the floodplain is confirmed in our Delft3D simulations. These findings may have important implications for ongoing or planned sediment diversion projects.