Laserfiche WebLink
<br />used. although soil moisture indexes would be useful where such measurements <br />are avallable. <br />Selection of ground conditions for the various types of hypothetical <br />floods is discussed in the appropriate chapters below. <br /> <br />Section 2.09. Runoff cOlll\lutations <br /> <br />Runoff is computed at regular intervals that are short enough to de- <br />fine the shape of the flood hydrograph adequately. These intervals can <br />range from a few minutes in very SDIall areas to a full day in very large <br />areas. A good guide is generally that the tabulation interval should be <br />about 1/5 to 1/3 of the time of concentration or of the unit-hydrograph <br />lag for the drainage area. <br />For each computation interval. rainfall that occurs is added to <br />snowmelt. Infiltration (and any other) loss is then subtracted. The <br />remaining quantity, designated as rainfall and snowmelt excess, is trans- <br />lated in time and transformed for storage detention effects by various <br />means. Most commonly, the unit hydrograph method, described in Volume <br />4, is used. Computation of rainfall and snowmelt losses and runoff in <br />accordance with these principles can be accompliShed using the computer <br />program described in Appendix 1. <br />It is often necessary to divide a drainage basin into sub-basins in <br />order to compute runoff from units where the areal patterns of preci- <br />pitation and other factors are relatively constant for different flood <br />events. Also. design considerations may require subdivision of a drain- <br />age basin in order to obtain flood estimates for various specific loca- <br />tions. When this is necessary, runoff computed for each sub-basin is <br />routed through downstream channels and reservoirs, and combined with <br />other hydrographs at downstream locations of interest. Stream system <br />computation and routing techniques are discussed in the following sec- <br />tions. <br /> <br />2-15 <br />