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Transfer of solutes into and out of streambeds

Elliott, Alexander H. (1990) Transfer of solutes into and out of streambeds. W. M. Keck Laboratory of Hydraulics and Water Resources Report, 52. California Institute of Technology , Pasadena, CA. (Unpublished)

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Laboratory experiments were conducted to determine the mechanisms and rates of bed/stream exchange of non-reactive solutes for beds of medium and fine sand. Experiments were conducted under steady flow conditions with and without sediment transport in a recirculating flume. Flat beds and beds covered with ripples or triangular bedforms were studied. The net mass exchange was determined by measuring concentration changes in the main flow. The penetration of the solute (fluorescent dye) into the bed was also observed visually. Two key exchange mechanisms, 'pumping' and 'turnover', were identified. Pumping is the movement of pore water into and out of the bed due to flows induced by pressure variations over bedforms (ripples and dunes). Turnover occurs as moving bedforms trap and release interstitial fluid. Predictive models based on the details of the exchange processes were developed. A residence time distribution approach was used. The models do not require calibration. Appropriate scaling variables were identified. With stationary bedforms the exchange is strongly influenced by pumping. The predictions of net mass exchange based on models of pumping with periodic bedforms show good agreement with the measured exchange in the initial stages of the experiments (hours to days). The models under-predict the exchange later in the experiments. The deviation is associated with the large-scale and somewhat random features in the penetration of the dye cloud. Such features are expected to influence the net exchange for large time in natural streams. When the bedforms move slowly in relation to the characteristic pore water velocity, turnover can be neglected and pumping dominates. A model based on a random distribution of bedform sizes provides a good prediction of the mass exchange with slowly-moving bedforms. With rapidly-moving bedforms, turnover dominates the exchange at the start of the experiments, when the solute penetration is limited to the maximum bedform scour depth. The scour depth can be predicted well. Later the depth of penetration is greater than the scour depth and the model predictions.

Item Type:Report or Paper (Technical Report)
Additional Information:© 1990 Alexander Hewgill Elliott All Rights Reserved. W. M. Keck Laboratory of Hydraulics and Water Resources. California Institute of Technology. Notice: This is the final report to the U.S. Geological Survey for Grant No. 1408-0001-61488. Grantee: California Institute of Technology. Principal Investigators: Norman H. Brooks, James J. Morgan, Robert C. Y. Koh Title of proposal: Contaminant Exchange Processes Between Water and Sediment Beds in Rivers Project period: July 15, 1987-January 14, 1990. This report has also been submitted to the California Institute of Technology by the author as a thesis in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Environmental Engineering Science.
Group:W. M. Keck Laboratory of Hydraulics and Water Resources
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Series Name:W. M. Keck Laboratory of Hydraulics and Water Resources Report
Issue or Number:52
Record Number:CaltechKHR:KH-R-52
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ID Code:25969
Deposited By: Imported from CaltechKHR
Deposited On:30 Apr 2004
Last Modified:03 Oct 2019 03:10

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