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<br /> <br />the rainfall depth for size of drainage area if needed, 4) adjust for partial to annual <br />series (if required), 5) compute incremental rainfall amounts, and 6) arrange the storm <br />rainfall increments in time. Each of these steps is described in the following <br />paragraphs. <br /> <br />3.1. STORM DURATION. Before constructing any hypothetical event <br />(including the SPS and PMS), one must estimate two storm parameters: total duration <br />and time interval for each rainfall increment. Both parameters must reflect the type <br />and size of the drainage areas being examined, the type of basin features one intends <br />to analyze, and the location of these features. The total duration of the hypothetical <br />storm is directly related to the time of concentration of the watershed (the travel time <br />from the upper portions of the watershed to the most downstream point of interest). <br />For example, if the estimated travel time is 14 hours (determined from actual records <br />or by computation) from the watershed boundary to the lower limits of the study area, <br />the storm duration must be at least 14 hours and preferably more. For most <br />applications, the duration would be set to an even day (24 hours). Since a storm <br />duration of less than 14 hours would not allow all portions of the drainage basin to <br />contribute direct runoff to the outlet simultaneously during the course of the storm, <br />the peak discharge at the basin outlet would not be reflective of the rainfall event if <br />the storm duration were made less than 14 hours. Runoff from the lower portions of <br />the basin from, say, a 6-hour event would have left the basin before the inflow from <br />the upper portion reached the outlet. Therefore, a minimum storm duration should be <br />selected at least equal to, and preferably well in excess of, the estimated travel time <br />(time of concentration) at the downstream-most point of interest. This selected <br />duration should be increased considerably if total volume of runoff as well as peak <br />discharge is of importance in the study. Drainage basins having an unusually large <br />amount of flood plain storage (wide flood plains and/or large areas of swamps) may <br />require a' storm of longer-duration to capture the attenuation effect of these large <br />natural storage areas. Reservoir studies require long-duration events for full <br />assessment of the reservoir flood storage needed. Therefore, a maximum storm <br />duration of 10 days may be used even if the travel time to the reservoir site is only 14 <br />hours. Total storm duration is normally taken as some increment of a 24-hour day (3, <br />4, 6, or 12 hours). or a multiple of a day (1 to 10 days). <br /> <br />3.2. TIME INTERVAL. Once the storm duration has been established, <br />the time interval for subdivision of the total storm must be selected. The time interval <br />must be small enough to accurately define the flood hydrograph (especially the peak); <br />however, too small an interval will result in excess computations by the individual or <br />the computer. The time interval will generally be established by the fastest peaking <br />subarea of the overall basin model for which the peak discharge is required, i.e., for <br />later use in developing water surface profiles, to evaluate the effects of a flood control <br />component, etc. The time interval must be small enough to define the rising limb and <br />peak for the hydrograph for this subarea. It has been found from past experience that <br />a time interval that gives at least 3 points on the rising limb of the hydrograph prior <br /> <br />7-13 <br /> <br /> <br />