FLOOD05148 - Laserfiche WebLink

Home Browse Search

DRAINAGE CRITERIA MANUAL RUNOFF When the cumulative amount in Col. 4 equals the total shown at the bottom of . the column, the depression storage is fully used and all remaining values are zero. Note that the last value of depression storage will usually be less than Col. 2 minus Col. 3 and will be determined by finding the difference between the maximum amount and the cumulative amount of infiltration through the previous time periods. Col. 5 Effective precipitation for the pervious area is Col. 2 minus Columns 3 and 4. Use positive values only. Col. 6 Col. 5 times the (decimal) percent of the pervious area gives the area-weighted depth of water that will run Dff in each time increment for the pervious area. Impervious Area, Columns 7 - 10 Col. 7 Enter the total assumed impervious depression storage determined from Table 2-1 at the bottom of Col. 7. The impervious depression storage in Col. 7 is then either the amount of precipitation in Col. 2 or the amount available as determined by deducting the total accumulated amount from the total assumed . value shown at the bottom of Col. 7. When the total assumed amount is fully used, all remaining values are zero. Col. 8 After all of the impervious storage has been .filled, an impervious loss will still occur. To account for this, Col. 8 is computed by taking 5 percent of Col. 2 less Col. 7. Col. 9 Effective precipitation for the impervious area is Col. 2 less Columns 7 and 8. Col. 10 Col. 9 times the (decimal) percent of impervious area gives the area-weighted depth of water that will run off in each time increment for the pervi ous area. Col. 11 Add Col. 10 and Col. 6 to obtain the average effective rainfall. This is the "design effective rainfall" that will be applied to the unit hydrograph, Chapter 4, to obtain the design storm runoff hydrograph. . 5-1-84