Dredging as a Geomorphic Process

Abstract:

Sediment mobilization by dredging is an important but often overlooked geomorphic process in many modern, heavily engineered river deltas. In the Lowermost Mississippi River (LMR) the mass of sand removed from the channel near the river’s mouth is roughly equal to that which passes Baton Rouge in suspension at River Kilometer 372. Much of this sand is also dredged as it transits the reach near Baton Rouge through fifteen meander crossings that are maintained for navigation. In this upstream reach the dredged sand is dumped back into the thalweg and transported downstream by the river before resettling. A complete understanding of sediment transport in the LMR is not possible without a full accounting of dredging activity and operational decision-making in both reaches. We mine Corps of Engineers records of dredged sediment volume and character in the two reaches and relate them to gauge estimates of water and suspended sediment discharge. We find that yearly total suspended sand transport is the best predictor of dredging activity in both the upstream and downstream reaches (r= 0.8, 0.7, respectively), and that although dredging activity in both reaches is correlated with suspended sand transport the peak dredging years do not align well, indicating that the geomorphic drivers, timing, or operational responses differ between the two reaches. This work represents a unique effort to include dredging as an essential component of the sediment budget in the LMR and to explain local dredging operational strategy as consequences of the river’s hydrology and geomorphology.