Hydraulic response and morphological evolution at a lower Mississippi River channel bar after sand mining
Wednesday, 17 December 2014
Sand mining in alluvial rivers by hydraulic or bucket dredge causes a significant disturbance within the geomorphic processes controlling river form and function. While the reach-scale impacts of dredging associated with a general decline in sediment availability (e.g., channel incision) are well documented, the effects of the borrow pit on the local flow and sediment transport field are not well understood. These local effects are important because they control the post-dredge evolution of the borrow pit, setting the lifespan of the pit as well as affecting channel morphology on a reach-scale. This study documents the observed morphological evolution of a large (1.46 x 106 m3) borrow pit located on a lateral sandbar in the lower Mississippi River channel that was mined for coastal restoration purposes using a time series of multibeam bathymetric surveys. Observations show that within the 2.5 year study period, 53 % of the initial pit volume infilled with sediment, decreasing pit depth by 0.88 m/yr on average. To explore the possible controls of the observed pit infilling, a calibrated 3D hydraulic model (Delft3D) was used to simulate flow and sediment transport within the affected river reach. Results indicate that the observed infilling rates were closely related to predicted sediment supply rates and borrow pit geometry. The pit geometry (pit depth, length) influenced the predicted magnitude of the bed stress within the pit relative to its initial pre-dredged bed stress value, i.e., the bed stress reduction ratio (R*), a metric that was correlated with the magnitude and spatial distribution of infilling. A simple 1D model was derived using sediment supply and R* to simulate temporal and spatial patterns of pit infilling. This model was able to closely approximate the cumulative amount of observed infilling during the study period and reproduce realistic longitudinal infilling patterns. Additional infilling model experiments show that, for a borrow pit of a set volume located within a lower Mississippi river channel bar, creating deeper rather than longer borrow pits can reduce the time required to infill with sand by an order of magnitude. Study results provide insight into the resilience of alluvial river channels after a disturbance and the sustainability of sand mining as a sediment source for coastal restoration.