20 Transport and Sediment Dynamics in Lowland Rivers

Tuesday, 29 March 2011
3:00 PM - 5:30 PM
D.H. Holmes A & B

MYRTLE GROVE DELTA BUILDING DIVERSION PROJECT

Paper number: 20-10
Program: Theme Sessions
Time: 5:15 PM – 5:30 PM

Authors

Abstract

Field data collection and extensive numerical modeling is being conducted in the lower Mississippi River in the region of a meander bend at Myrtle Grove, LA (river km 96 above Head of Passes) in support of a proposed large water and sediment diversion (1,130 – 2,830cms) for coastal wetland restoration. Field studies were conducted on October 2008, April and May 2009 and 2010, at discharges ranging from 11,000-25,000 cms. Analysis of the field data supported by extensive numerical modeling focused on complex spatial variation of suspended sediment. The analysis included comparison with control sites further downstream and USGS monitoring stations slightly upstream.

Suspended loads and grain size character measured by ADCP (velocities and backscatter), isokinetic point sampler (P-63), and optical sensors (LISST, OBS, transmissometer) indicate that during the rising-to-high discharge phase (at discharges above ~19,000 cms)sand lifting off from the energetic mid-depth face of the lateral bar upriver of the bend augments that carried from further upriver This excess suspended sand is advected around the bend before concentrations are reduced to background levels over the lateral bar downstream of the bend. Both 1D (HEC-RAS and HEC6-T) and 3D (Flow3D) numerical hydrodynamic and sediment transport modeling were used to simulate this process and the large-scale eddy present in the bend that generates upriver transport along the inside of the meander bend at all observed discharges. The models were able to capture the overall flow and suspended sediment dynamics and spatial variations of this complex site. The preliminary results suggest that meander bends might be an appropriate site for sediment diversions that draw near-surface water