Laserfiche WebLink
In this analysis, ldestmoreland Cannon was subdivided into taro <br />• sub-basins at the 7400-foot elevation contour, since the watershed is <br />essentially undisturbed above this elevation. Storm runoff was determineC <br />fer this "upper basin", and a potential "inflow hydrograph" to the mining <br />area ("lower basin") was derived using guidelines recommended by the USSR <br />(1973). <br />This hydrograph r-or the PttTS is shown in Figure 4. k step by step <br />sur„nary of the calculations made in developing this hydrograph is presented <br />in r,ppendix II. The calculations follo•,a an outline format that should be <br />easily understood by one who is familiar with standard engineering practice <br />in flood hydrology. The Pp;TS hydrograph is based on an antecedent moisture <br />condition II, Curve t:umber 75, and the point rainfall of Fieure 2. Of the <br />7.S-inch rainfall, 4.6 inches irould result as surface runoff. It would <br />• produce a peak floo-r of approximately 1150 cubic reet per second, which <br />would occur within one hour after initial rainfall. The entire even; wo ,'d <br />be over in less than 12 hours, and would produce a volume of about 39 <br />acre-feet. <br />Obviously, if this head of water mere to occur, and alloyed to f]ow <br />uncontrolled into the present mining operation, tha result ~•;ould be severe <br />damage to prir,;ary structures, and perhaps loss of life, let alone the <br />environmental damage that would result from severe :hannel erosion and <br />movement of debris from the mine site. • <br />Flood Control <br />Complete alteration of the original channel and the presence of <br />primary Structures, roads, etc. makes control of floods of the magnitude <br />• discussed above virtually impossible in Lo:•;er l;estroreland Canyon. Given <br />the advanced staee of development, there appears to be only tyro alterr:atives <br />with respect to the flood potential in Ewstmorelar.d Canyon: <br />