Louisiana Coastal Protection and Restoration Authority

Investigation of Sediment Infilling of Channel Bars in the Lower Mississippi

Observations and Recommendations for Monitoring and Modeling

Completed May 2015

The Challenge

The Water Institute was tasked by the Coastal Protection and Restoration Authority (CPRA) to identify, investigate, and summarize the physical processes that control borrow pit infilling in riverine and coastal environments. Dredging in borrow areas located in sand bars and off-shore bed deposits serve as a ready source of aggregate for coastal engineering and restoration projects. Sediment transport processes potentially recharge these dredged areas with new material allowing repeated use and recovery of any associated degraded ecosystem functions. However, very little is known about process controls that influence the recharge rate or the sediment caliber that will most likely/efficiently infill the dredged area.

The Approach

In order to address this uncertainty and develop a framework to optimally schedule and monitor dredging in coastal borrow areas, the Institute investigated borrow pit infilling by analyzing dredging data, morpho-dynamic modeling of borrow pit evolution, and the collection of observational data (hydrodynamics and sediment transport) in active borrow pits.

Initial research results showed that while the geometry of the dredged borrow pit plays a role in the sediment recharge rate (e.g., deeper holes fill faster than longer holes), the local geomorphic character and sediment transport regime of the surrounding area also serve as a significant control. For example, the recharge rate tends to exponentially decrease as the pit depth decreases; moreover, large river flows can lead to regional bed scour and pause or temporarily reverse sediment infilling.

Field-based and numerical observations showed that the complex geometry of the borrow pit lying on the channel or bay bed can significantly alter the local flow and sediment transport fields by creating coherent turbulent structures. These turbulent structures alter regional sediment deposition rates below that expected by traditional sediment transport models that do not fully resolve complex flow dynamics.