Date of Award
1968
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Geology
First Advisor
L. Clayton
Abstract
The effects of' groundwater seepage on the competence, bed rough ness, and water surface slope of small streams were studied in two laboratory fumes. The larger fume is 20 feet long, 3t feet wide, and 18 inches deep; the smaller flume is 12 feet long, 6 inches wide, and 10 inches deep. Both flumes are equipped so that groundwater seepage through the stream bed can be controlled and monitored.
In the absence of bedforms, upward (positive) seepage has little effect on stream competence or transport rate, even when quicksand conditions are reached. The decrease in effective grain density brought about by uptward seepage might possibly be compensated entirely or in part by a decrease in surface drag and an increase in form drag on individual grains on the stream bed.
In the presence of bedforms such as ripples or dunes, however, seepage appears to have an inverse relationship to steepness of bedform faces (angle of repose), bottom roughness and turbulence, and transport rate. The inverse relationship of seepage to transport rate under these conditions is caused by steepening of bedform faces under conditions of downward seepage which results in greater bottom roughness and turbulence than in runs without seepage. Conversely, upward seepage tends to decrease the steepness of the bedforms, thereby decreasing bottom roughness, turbulence, and transport rate. The angle of repose on bedforms was increased to an average of about 43 degrees under conditions of downward seepage in much a hydraulic seepage gradient of -1.4 was present, as compared to an average angle of repose of about 33 degrees without seepage. A hydraulic seepage gradient of about +0.8 decreased the average angle of repose to about 27 degrees.
With no seepage and with upward seepage, relative roughness of the stream bed in the 13.rger flume ranged from 0.56 to 0.83 (average value of 0.62) whereas with downward seepage it increased to 1.05. This increase in bed roughness, however, did not increase flow resistance sufficiently to cause an increase in the slope of the water surface (measured to within 0.005 ft/ft).
When bedforms are absent, downward {negative) seepage appears to have little effect on competence or transport rate if clear water is present in the channel. If the water is sufficiently turbid, however, the surface o:t the stream bed becomes clogged by the infiltration of the turbid water. This clogging, or mid-seal effect, decreases the permeability of the stream bed until the sediment underlying the uppermost layer of the bed becomes unsaturated. When this occurs, the weight of the water in the channel must be supported. by the grains lying within the mid-seal layer. The affective density of the grains may be increased several hundred times, resulting in a cessation of bedload transport and an increase in compressive strength of the stream bed. Effective grain density under conditions varies inversely with grain diameter and directly with the depth of water 1n the channel. A mud seal can develop even under conditions of high-regime flow with bedload transport occurring in the form of a moving carpet. If punctured, the seal, is self-healing, provided sufficient suspended sediment is present. It seems likely that natural mud seals may be present on the bed or many irrigation canals and ephemeral steams such as arroyos and alluvial fan streams.
Recommended Citation
Harrison, Samuel S., "The effects of groundwater seepage on stream regimen : a laboratory study" (1968). Theses and Dissertations. 127.
https://commons.und.edu/theses/127