Exchange of colloidal kaolinite between stream and sand bed in a laboratory flume

Abstract

Experiments were conducted in a recirculating flume to study the exchange of colloids between a stream and sand bed. Observations of the net transport of kaolinite clay from the stream to the bed were used to validate models for colloid exchange which include the hydraulics of water exchange along with the particle-specific effects of filtration and settling. The flume allowed extensive control of the stream conditions (slope, depth, velocity) and bed parameters (bed depth, bedform height, wavelength, and velocity). All experiments had steady, uniform stream flow and well-developed bedforms. To define the chemistry of the system, the composition of the flume water was controlled, the clay was reproducibly prepared, and the sand was cleaned prior to every experiment. Experiments involved observing the exchange of both a conservative lithium chloride tracer and the kaolinite tracer. Net exchange was determined by measuring the change of tracer concentration in the stream water. Vertical profiles of the tracer distribution in the bed were also measured. Column experiments were performed to measure kaolinite filtration by the bed sediment. The pH and ionic strength of both the flume and column water were controlled to vary the extent of filtration. Two mechanisms are responsible for exchange between the stream and stream bed--an advective pore-water flow driven by bedform-induced pressure variations at the surface of the bed (pumping), and the burial and release of water due to bedform motion (turnover). Pumping causes kaolinite to be carried deep in the bed where it is trapped due to filtration and settling. Turnover causes continuous mixing of the uppermost portion of the bed which hinders penetration of clay to the deeper bed. Models incorporating the relevant physical and chemical processes controlling colloid transport were developed to predict the net exchange of kaolinite between the stream and sand bed. Model parameters were nondimensionalized so the models can be applied to problems of any scale. Models for conservative solutes were developed to predict the exchange with a finite bed and exchange due to fast-moving bedforms. Models for colloid transport were developed which are based on the solute transport models, but include the impact of settling on particle flow paths and filtration along path lines. When input data were taken from laboratory experiments, the models generally predicted the flume results well with no calibration. In all flume experiments, filtration and settling of colloids in the bed were sufficiently high so that it could be assumed that all colloid which entered the bed was irreversibly trapped. Additional model simulations were performed to demonstrate the effect of major input variables on exchange. These simulations cover cases not examined in experiments and provide a sensitivity analysis for the model inputs.