A simplified analytic investigation of the riverside effects of sediment diversions

Abstract : Coastal land loss is a striking reality in many coastal communities. The issue is especially acute in coastal Louisiana. Problems of land loss in coastal Louisiana have been extensively documented (Gagliano 1981; Britsch and Dunbar 1993; Boesch et al. 1994; Day et al. 2000).There are several factors that have been identified as causes of this land loss. Among others, these include the following: The rate of relative sea level rise in coastal Louisiana, which is generally faster than the rate of marsh accretion (due primarily to regional subsidence). The reduction of sediment loads in the Mississippi River. The decreased sediment supply to the wetlands due to the lack of hydraulic connection between the wetlands and the Mississippi River. The introduction of dredged canals that criss-cross the marshes and expose marsh vegetation to increased currents and salinity. Other factors, such as vegetation losses due, for example, to nutria, etc. As part of an effort to build land in coastal Louisiana, planners are considering the construction of sediment diversions. Sediment diversions typically consist of lateral water diversions, either controlled or uncontrolled, that are designed to capture and remove large quantities of sediment from a river. In coastal Louisiana, these diversions are intended to re-introduce sediment into shallow bays and degraded coastal marshes, in order to rebuild land in these systems (CPRA 2012). In order for a sediment diversion to be implemented successfully, a variety of factors need to be evaluated. A given sediment diversion must be able to divert and distribute sufficient sediment to effectively mitigate the causal land loss factors listed above. However, the diversion design is subject to several additional constraints.