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Title: Geotechnical aspects of rock erosion in emergency spillway channels. Report 1
Authors: Texas A & M University.
University of Southern Mississippi. Department of Geology.
Repair, Evaluation, Maintenance, and Rehabilitation Research Program.
Cameron, Christopher P.
Cato, Kerry Don, 1959-
McAneny, Colin C.
May, James H.
Keywords: Erosion
Hydraulic structures
Soil mechanics
Repair, Evaluation, Maintenance, and Rehabilitation Research Program (U.S.)
Issue Date: Aug-1986
Publisher: Geotechnical Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Description: Technical Report
Abstract: The problem of rock erosion in unlined emergency spillway channels is described in this first-of-a series report. Recent CE and non-CE case histories which illustrate the potentially hazardous impacts of excessive erosion in discharge channels are described as are the various factors controlling erosion and other responses to emergency spillway overflow. Research programs designed to improve geotechnical capabilities with respect to selection of cost-effective preventive and remedial measures in discharge channels where the risk of excessive erosion appears high are also discussed. Response to emergency spillway overflow is controlled by a variety of hydraulic and geologic factors including flood frequency and magnitude, engineering design, discharge channel gradient(s), discontinuity of earth materials, and erodibility of earth materials. A major controlling factor of erosion in spillway discharge channels lined by sedimentary strata appears to be the interrelated effect of stratigraphic discontinuity and channel gradient change(s). These factors combine to initiate and control headward migration of knickpoints, where resistant sedimentary layers are undercut by scouring of softer, underlying strata. In terms of erodability of earth materials, the scale of the hydraulic forces generated during emergency spillway overflows suggests that rippability and lithostratigraphic discontinuity may serve as a good point of departure in describing the relative resistance to erosion of rocks lining discharge channels. Responses to emergency spillway flow include channel floor and bank erosion, sediment transport and deposition, and overbank flooding. Erosion of the material underlying unlined channels is the most serious of spillway flow impacts, since channel floor degradation can undermine spillway structures and threaten reservoir integrity. Responses to emergency spillway overflow are not limited to the immediate area of the dam however. Spillway overflow can act to cause stream thresholds (which limit change on the system) to be exceeded in the main channel into which spillway overflow exits and can influence or induce changes for significant distances downstream. Several case histories provide ample evidence that knickpoint migration and headcutting can be initiated at a point considerably downstream from a control structure. Sediment deposition can build bars and deltas in spillway discharge channels, at the exit channel - main channel confluence, and in downstream reaches of the main channel. Deposition in the main channel may impede passage of reservoir overflow and, by deflecting flow into the channel banks, cause irregular channel widening. This impact could conceivably initiate or accelerate erosion of streambanks and levees, impact navigation, endanger ecological balances, and increase the danger of overbank flooding. Research is in progress to determine the quantitative effects of stratigraphic variation on erosion rates in sedimentary rocks using simulated earth materials and designed stratigraphic variability in a self-contained, recirculating and tilting hydraulic flume. Adjunct research is in progress to generate predictive erosion indices on a comparative site-specific level and to more adequately address the downstream impacts of spillway overflow. Results of this research as well as recommendations in the realm of preventive and remedial measures will be promulgated by future reports in this series. NOTE: This file is large. Allow your browser several minutes to download the file.
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