Fluvial Erosion Rate of Cohesive Streambanks Is Directly Related to the Difference in Soil and Water Temperatures

Abstract

Despite decades of research in the field of cohesive soil scour, a major challenge in water resource engineering is an understanding of the fundamental processes governing the erosion of cohesive streambank soils. Given that cohesive soil erodibility is affected by many factors simultaneously, it is necessary to study these factors independently to obtain insights into the erosion process. Three natural soils with different mineralogies were chosen for this study: montmorillonite-dominated fat clay, vermiculitedominated lean clay, and kaolinite- and illite-dominated silty sand. The soils were remolded at maximum dry densities and optimum moisture contents and subjected to 15-min erosion tests in a laboratory flume. Erosion tests were performed at water temperatures of 15 and 25°C and corresponding soil temperatures of 0, 15, and 25°C, and 15, 25, and 40°C. Test results show that, irrespective of soil type, erosion rate increased with an increase in water temperature but decreased with an increase in soil temperature. When soil and water temperatures were equal, there was no significant change in erosion rate (α = 0.05). Further analyses showed that, irrespective of soil type, erosion rate was a function of the difference in soil and water temperatures and not either temperature alone, indicating that the important thermal factor in the erosion process was the difference in soil and water temperatures. These results show the importance of accounting for soil and water temperatures in erosion studies and suggest that the use of stormwater control measures to control runoff temperatures may be necessary to combat streambank degradation resulting from urbanization.