Interaction of Clay Wash Load With Gravel Beds

Abstract

This study focuses on the interaction of wash load particles with gravel bed rivers. The effects of excess fine sediment loading to streams on general water quality, contaminant transport, and benthic organism mortality has been well examined. A fundamental assumption in fluvial geomorphology and river engineering is that wash load particles ($d<63mu m$) do not deposit to stream beds, but are instead transported downstream until they deposit in reservoirs or estuaries. The goal of this study is to determine if wash load sized particles can deposit to gravel beds, where within the bed substrate deposition occurs, under what hydraulic conditions it occurs, and how the composition of the bed affects the spatial and temporal deposition pattern. Further, this study attempts to quantify the mass flux of wash load to the bed based on a simple mass conservation model using the aforementioned conditions as model parameters. This was accomplished through a series of experiments in which a mixture of pure kaolinite clay was allowed to deposit at constant shear over an acrylic, gravel, or sand-gravel mixture. Discharge was then increased to determine the effects of increased bed shear stress on deposited material and further wash load interaction with the bed.

Results indicate that wash load will deposit to acrylic, gravel, and sand-gravel beds during conditions where no bedload movement is occurring. Bed composition is the primary factor controlling the mass flux of wash load from the water column to the bed. Deposition on acrylic beds forms clay ripples which translate downstream, while deposition in porous beds occurs primarily within the bed substrate. Shear stress also affects mass flux and the magnitude of its effects are related to the bed composition. Discharge increases below the threshold of bedload movement only cause large scale entrainment of deposited particles over non-porous beds. Periods of higher discharge over porous beds result in continued deposition within the bed substrates.

This research enhances not only our knowledge of sediment processes within fluvial systems, but also allows for the quantification of the wash load portion of those processes given minimal initial condition information. The model developed here may be used within larger hydrologic models when examining contaminant spills or mass loading of stream networks with wash load to estimate the mass deposition to the bed. Instances where wash load is contaminated the mass of contaminated sediment retained by the bed is of great importance to local communities given a reliance of residents on that water source for water, livelihood, and recreation.