Effects of gas generation on the density and erosion of sediments from theGrand River

Measurements were made of the erosion rates and bulk properties of sediment s from eleven sites in the Grand River in Michigan. High concentrations of gas, up to 5% by volume, were present in the sediments; this significantly affected the sediment bulk densities and erosion rates. Sediments from one of these sites were reconstructed (thoroughly mixed and then allowed to con solidate) in the laboratory where the time-dependent effects of gas generat ion on the densities and erosion rates of these sediments were quantified. In consolidation studies, the density was a function of depth, time, and te mperature; it increased because of pore waters moving up and out of the sol id-water matrix and decreased because of gas generation and movement. At hi gher temperatures, the rate of gas generation and the gas volume were great er and the bulk density was lower than at the lower temperatures. For the s ame bulk properties, temperatures,and shear stresses, erosion rates for the field and reconstructed sediments were the same. For reconstructed sedimen ts at approximately 20 degrees C, the effects of gas were to decrease the s ediment densities by up to 10%, to increase the erosion rates by as much as a factor of sixty, and to decrease the critical shear stress for erosion b y as much as a factor of twenty compared to sediments with no gas present. At lower temperatures, these effects decreased significantly.