Morphodynamics of the erosional phase of crevasse-splay evolution and implications for river sediment diversion function

Despite being a primarily depositional landform, a crevasse splay experiences an initial evolutionary phase that is primarily erosional as sediment-laden river water spills from a main river channel and incises a new route through the river banks and levee into an interdistributary basin or floodplain. This phase sets the dimensions and the conveyance properties of the crevasse, which, in turn, influences the continued expansion or closure of the crevasse channel. However, little is known about the controlling morphodynamics or how the erosional processes transition to depositional processes during this phase. The objective of this study is to investigate these phenomena at the West Bay sediment diversion (Louisiana, USA) using coupled field observations and numerical modeling. The West Bay diversion was cut into a lower Mississippi River levee to mimic the function of a crevasse-splay, i.e., to divert river water and sediment to an adjacent receiving basin for land-building purposes. Bathymetric measurements show that the diversion channel experienced significant natural morphologic evolution during the initial decade (2004-2014). Hydrodynamic and sediment transport modeling suggests that this evolution initially increased the discharge of flow and sediment through the crevasse as the channel became wider and deeper and altered its orientation relative to the main river flow direction. After 5 years, the model results predict that further evolution led to monotonically reduced diversion discharges. During this time, natural and engineered sediment deposition in the receiving basin decreased predicted basin-flow velocities and promoted a backwater effect that reduced the sediment transport capacity of the diversion channel. Observations during the final 2 years show that much of the initial erosion around the diversion had abated indicating that diversion morphology may have stabilized. A modeling sensitivity analysis confirmed that the observed changes to channel geometry and orientation likely promoted flows of water and sediment through the diversion while increases in basin-bed elevation would have had a contrary effect. The morphodynamic evolution of the West Bay diversion documented in this study presents a model indicative of the erosional phase of crevasse-splay evolution in a deltaic distributary fluvial network. Study results offer an analogue on how an engineered river sediment diversion constructed for coastal restoration may function during its first years of operation and suggest that the desired land-building processes may take time to become established. (C) 2016 Elsevier B.V. All rights reserved.