Stability of Cohesive Sediments Subject to Pore Water and Gas Ebullition Fluxes and Effectiveness of Sand and Aquablok Caps in Reducing the Resuspension Rates.

Abstract

This study investigated resuspension of contaminated cohesive sediments subject to pore water flow and/or microbially generated gas release which may potentially enhance resuspension relative to flow generated shear stresses alone. In addition, the effectiveness of capping to control resuspension was examined. The caps considered were sand and clay (AquaBlok®) caps. The laboratory experiments were conducted in a flume with a 2 m test bed constructed of either capped or uncapped sediments that were collected from the Anacostia River pilot study site in Washington DC. Ebullition and seepage processes were simulated with air and water injection into the test bed. Suspended sediment concentrations were measured using a turbidimeter. Due to conflicting conclusions in the literature, an investigation on the stability of non-cohesive sediments (representative of sand caps) subject to injection (discharge from the bed) or suction (flow into the bed). A methodology was developed to estimate the bed shear stress from local Reynolds stress measurements to avoid difficulties in estimating shear stress by more conventional methods. A modification to the well-known Shields relations for initiation of motion was proposed to account for the effect of the seepage. When the data are presented in terms of the modified dimensionless shear stress that incorporates the effect of bed seepage, the results are consistent with the Shields curve. Experiments on cohesive sediments required the development of the entire experimental protocol; air and water flow through the sediment bed was observed to occur through discrete channels. Resuspension rates were quantified in relation to the applied shear stress and pore water or gas fluxes; both effects were shown to increase resuspension compared to baseline experiments with only applied shear stress.Additional measurements were performed with the selected caps to determine their effectiveness in reducing resuspension. The sand cap performed very well in filtering the cohesive sediment that otherwise would be resuspended. The AquaBlok® cap was determined to be highly stable under advective flow induced shear stress. However, it was observed that pressure head build-up beneath the cap either due to gas or water flux on the order of 25 cm results in cap failure.