Laserfiche WebLink
<br /> <br />.. <br /> <br />f\:) <br />to <br />.... <br />a> <br /> <br /> <br />CHANNEL EROSION <br /> <br /> <br />Channel erosion is gully and streambank erosion. Sheet and rill erosion is <br />driven by raindrop impact and sheet and rill flows., Channel erosion is driyen <br />by the tractive force of channelized flows. Lane (1955) proposed that eyery <br />stream seeks an equilibrium among four forces at work in all streamflow <br />situations. His equation is shown below with the symbols defined: <br /> <br />QsDllQwS <br /> <br />Q = sediment load <br />s <br /> <br />D = size of sediment particle <br /> <br />OW = amount of water <br /> <br />S = slope of stream channel <br /> <br />Simon and S~ntUrk (1976) show numerous examples of how channels adjust these <br />factors naturally if anyone or more are changed. The change can be natural <br />or man-induced. As a simple example, if you put a dam across a channel you <br />will probably remove the sediment load the channel naturally carries. Unless <br />flows are reduced or the channel boundaries are protected below the dam, the <br />"clean" water will begin to pick up sediment to replace the load it formerly <br />carried. It will do this by eroding its banks and bottom until the energy of <br />the flow has been brought back into balance. <br /> <br />(5) <br /> <br />This natural state of balance is your key to understanding why channel <br />erosion, or aggradation, occurs. Lane's equation will help you qualitatiyely <br />define what is happening in a channel system. The following discussion will <br />help you quantify the channel changes you have qualitatively defined. <br /> <br />FIELD MEASUREMENT PROCEDURES <br /> <br />There is no better way to quantify channel erosion than to measure it directly <br />in the field. A basic principle in measuring channel erosion is to survey, <br />flag, or in some way fix a "before" image of the channel you are studying. <br />This establishes a baseline condition. Changes due to channel erosion or <br />deposition can then be monitored oyer time by going back to the study area and <br />remeasuring from your fixed reference points. <br /> <br />Channel cross-sections can be surveyed and plotted on a periodic basis to <br />monitor change. Stakes or pins can be driyen into channel banks or bottom <br />flush with the surface. The amount of stake or pin exposed due to erosion or <br />the thickness of sediment above the stakes or pins is the amount of change in <br />the chan~el system between your times of observation. These procedures are <br />used in the Vigil Network program of the United States Geological Survey <br />(USGS). A description of the Vigil Network is in publications of the <br />International Association Science Hydrology Bulletins (Leopold, 1962). A more <br />accessible reference however is Leopold (1962b). William Emmett with USGS <br />Water Resources Division in Boise, Idaho, has been establishing monitoring <br />sites in the Vigil Network. <br /> <br />16 <br />