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HYDROLOGY CALCULATIONS AND SIZING OF EMP PONDS AND CHANNELS <br />SEDCAD4 is a useful tool for determining the armoring requirement of channels <br />and for general estimation of the required volume for detention ponds. However, the <br />infiltration calculations performed by SEDCAD4 may underestimate the total volume of <br />excess precipitation or runoff because SEDCAD4 uses an arithmetic average for <br />combining "land use" Curve Numbers. A more conservative approach uses a form of <br />geometric averaging, whereby the higher runoff potential of less permeable areas <br />(higher Curve Numbers) is more equitably addressed for determining the required pond <br />volume. In the alternative procedure the runoff volume generated by each portion of a <br />sub - watershed with the same land -use (Curve Number) is determined, and those land - <br />use- directed volumes are summed to provide the total volume for the basin. The <br />equations are shown on Table I (provided on Drawing Sheet 2 and at the end of this <br />report). <br />Runoff estimates were calculated for two scenarios <br />• A 10 -yr /24 hr Storm during Operations to determine the size of ponds. The runoff <br />volume from the storm was doubled (after routing effects) to determine the required <br />volume of the storm water detention ponds. <br />• A 100 - year /24 -hr Storm to determine the design flow capacity of channels and <br />spillways. <br />The results of the calculations are summarized on the Table at the bottom of Drawing <br />Sheet 2 (Drawing No. CCV10 -SA2. The table lists the following results for each sub - <br />watershed basin: <br />o the basin area <br />o the calculated potential SCS retention (S) and excess precipitation (Q) for three land <br />use categories <br />o the arithmetic average Curve Number (used in SEDCAD4) <br />o the total basin runoff using the geometric average model <br />o the design criteria for each EMP Pond <br />o the minimum pond volume (generally twice the 10- yr /24 -hr volume) <br />o The ratio of pond size compared to the basin area, based on the geometric <br />averaging infiltration model. <br />Client: CC &V Gold Mining Co. 2/2/12 <br />Title: Storm Water Management, MLE2 Page 4 <br />