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Long Hollow Reservoir <br />Feasibility Study <br />1. Drainage Area. The basin outline is shown on E~chibit 2.1-2. The area was calculated <br />electronically using Geographic Information System (GIS) software. <br />Z. Basin Slope (,S'). The basin slope was determined by taking the total relief in feet <br />(determined from appropriate USGS quaclrangle maps) and dividing by the length of the <br />watercourse in miles. <br />3. Length of Watercourse (L). The watercourse was digitized onto the USGS quadrangle <br />maps. The watercourse length was queried electronically using GIS. <br />4. Distance to Centroid (Lca). The basin centroid was determined digitally using GIS. <br />The distance to the centroid is the length along the watercourse from the design point to a <br />point opposite the centroid on the watercourse. <br />5. Average Weighted Manning's n(Kn). The average weighted (by stream length) <br />Manning's n for the principal watercourse in the drainage basin. A detailed site <br />assessment of basin vegetation has not been completed; therefore, an average value of <br />0.056 was used for this parameter. <br />6. Lag Time (Lg). The lag time is calculated as follows: <br />Lg = 26 * Kn * ((Lca * L) / So.s ~0.33 <br />7. Unit Duration (D). The unit rainfali duration is approximated by the lag time divided by <br />5.5. This will yield adequate definition near the peak of the hydrograph. <br />8. Lag Time Plus Semi-duration (Lg + D/2). The calculated hydrograph is expressed in <br />percent of the lag time plus the semi-duration, which is calculated as the lag time plus <br />half the unit duration. <br />9. Basin Factor. The basin factor is calculated as follows: <br />BF = L * Lca / So.s <br />991-077.120 Wright Water Engineers, Inc. Page 18 <br />April 2005 <br />