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Last modified
7/14/2009 5:02:31 PM
Creation date
6/1/2009 11:33:59 AM
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UCREFRP
UCREFRP Catalog Number
7910
Author
Bovee, K. D. and R. T. Milhous.
Title
Hydraulic Simulation In Instream Flow Studies
USFW Year
1978.
USFW - Doc Type
Theory And Techniques, Instream Flow Information Paper No. 5.
Copyright Material
NO
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An increase in the sediment load with no change in discharge will <br />also result in aggradation, but with width increasing. Land clearing and <br />gravel washing operations often stimulate aggradation, acccompanied by a <br />reduction in substrate size. Channel braiding is often symptomatic of <br />this type of aggradation. A reduction in discharge coupled with an <br />increased sediment load will amplify and aggravate the situation. <br />A reduction of sediment with no change in discharge promotes a <br />gradual removal of fines from the stream, leaving behind only large <br />substrate materials. This process is termed armoring and is very common <br />below reservoirs or stilling basins- <br /> <br />Art increase in the discharge coupled with a reduction in-sediment <br />load results in a marked increase in depth and reduction in the channel's <br />width-to-depth ratio. Armoring may be so pronounced that spawning gravels <br />are removed as in the tailrace area of many dams. If an increase in <br />discharge and reduction in sediment load occurs in braided alluvial <br />channels, the braided channel may well revert to a single channel, a <br />process which is presently occurring on the Bighorn River, Montana <br />(Montana Department of Natural Resources, 1977). <br />The preceding discussion of the processes of aggradation and <br />degradation is designed to alert the reader to the types of changes which <br />can be expected to occur in a river channel in response to an imbalance in <br />the sediment-discharge ledger. It is important that the concepts of <br />equilibrium, aggradation, and degradation are well understood; for an <br />instream flow. recommendation which is perfectly adequate from a <br />biological aspect may be completely undone by geomorphic processes which <br />were- ignoredduring the development of the recommendation. <br />. DEFINITIONS OF TERMS <br />USED IN THE ANALYSIS OF OPEN CHANNEL FLOWS <br />The terms of importance in the analysis of open channel flow are <br />defined below, and illustrated in Figure 1. <br />Width (w) - the distance across a channel at the water surface <br />measured normal to flow. <br />Depth (d) the vertical distance from a point on the bed to .the <br />water surface. <br />Thalweg depth (y) - the vertical distance of the lowest point of <br />a channel section to the water surface. Max- <br />imum depth of cross section. <br />Thalweg - the longitudinal line connecting points of minimum <br />bed elevation along the streambed. <br />Hydraulic depth (d) - equivalent to mean depth. d = A/w. <br />Stage - the elevation, or vertical distance of the water surface <br />above a datum (a plane of known or arbitrary elevation). <br />S
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