Laserfiche WebLink
• Hydrograph Response Shape. A unit hydrograph was chosen for each <br />drainage area or sub-area model to predict the runoff response. The <br />hydrograph responses available in the SEDCAD+ model are slow, medium, <br />and fast. A slow response corresponds to a forested area. or an area <br />with a number of obstructions, whereas a fast response corresponds to an <br />unvegetated or poorly protected area. For each of the areas modeled, <br />the appropriate runoff hydrograph was chosen. <br />The internal convolution increment is 0.05 hours and values are saved at <br />the user specified interval of 0.1 hours or greater. (A convolution <br />increment of 0.1 was specified for all revised area sediment ponds). It <br />should be noted a time of concentration less than 0.125 hours bypasses <br />the unit hydrograph technique and instantaneous runoff is assumed. As <br />explained in the SEDCAD+ User's Manual, page 113, the time of <br />concentration restriction of 0.125 hours is somewhat mandated due to a <br />combination of array size restrictions, minimum internal convolution <br />• interval, and the user-specified time increment for saving convoluted <br />values. <br />Drainage Basin Area. The drainage areas were determined by digitizing <br />directly from a 1°500' scale map. <br />Time of Concentration, Tc. Time of concentration was calculated by <br />using the SCS Upland Method as presented in Barfield, et al.. (1981) <br />page 100. All hydraulic lengths, drainage heights, and slopes were <br />measured directly from the 1'=500' scale map. <br />For land uses simulating flow in large gullies, diversions. small upland <br />gullies, and small streams flowing bankfull two (2) additional equations <br />were used to calculate flow velocities Dased on Manning's equation as <br />follows: <br />Land Use E nation <br />. Large gullies, diversions. and low flow streams Y ~ 3(S)'S <br />Smaii streams flowing bank full Y ~ 9(5)'5 <br />23-63 <br />