Sediment Management in the Mississippi River

Abstract

The Mississippi River is one of the most engineered rivers in the world. Levees, bank revetments and training dikes ensure navigability and safety against ?ooding. Construction of dams in the upstream branches led to a decay in sediment supply of 70% during the last 100 years. Due to man-made constrictions, more sediment is spilled out in the Gulf of Mexico. In this Master Thesis, the Lower Mississippi River is subject to morphodynamic modeling with Delft3D to ?nd answers for the question, if there is a feasible way to restore the deltaic wetlands that su?er from high land loss rates due to land subsidence and sea-level rise without putting navigation at stake. In delta restoration plannings, sediment diversions play an important role as they convey sediment laden water to the marsh areas and, this way, largely contribute to new land-building. Several approaches are tested with the created model to understand the e?ect of altered ?ow and sediment discharge distributions on the morphology of the main channel. For a good approximation and description of the physical processes, the model comprises important factors such as dredging activities, diversion sites, salt water intrusion, and quasi-steady discharge periods. Several scenarios were tested, e.g. di?erent operation modes of planned sediment diversions and discharge distributions at Old River Control Structure. From the model outcome the conclusions can be drawn that a combination of a pulsed operation of ORCS and large-scale diversions as proposed by Parker et al. seems to be the most e?ective solution for wetland restoration in themodel domain as it only leads to similar deposition rates upstream of Venice, only slightly higher dredging volumes in the downstream reach and high sediment diversion, whereas the implementation of multiple diversions seems tbe more feasible, as they are already considered in the CWPPRA Masterplan. Moreover, it was found that additional groin ?elds can help to ensure navigability downstream of sediment diversions and thus mitigate negative morphological e?ects in the main channel. In the end, it must be emphasized that the application of additional diversions can stabilize local delta regions, but without higher sediment supply from upstream, the diverted amount of sediment is not su?cient for wetland restoration on a larger scale.