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1 Q = c1A. <br />' Q is the peak discharge in inches per acre per hour [which is approximately equal to <br />cubic feet per second (cfs), the conversion factor (1 cfs = 1.008 acre ~ in/hr) is considered <br />to be included in the runoff coefficient (Chow and others, 1988)]; I is the rainfall <br />' intensity, expressed in inches per hour, and A is the area in acres. Cis the dimensionless <br />runoff coefficient which is dependent on relief or slope, soil types, vegetation cover, and <br />surface storage. Runoff is also dependent on antecedent moisture conditions in the soils, <br />' a parameter that is not commonly measured. <br />The rainfall intensity, I, may be selected in accordance with local regulatory <br />' requirements, on the basis of reasonable design considerations, or more directly by <br />calculating the cumulative t~ value in minutes and arriving at a design storm intensity <br />associated with the t~ value on the basis ofintensity-duration-frequency precipitation data <br />provided by on site measurements or regional data from the National Atmospheric and <br />Oceanic Administration (NOAA). <br />The rational method is based on the following assumptions: <br />1. Peak discharge is presumed to occur at the time of concentration, t~, when all <br />' portions of a given watershed or subbasin are contributing flow to a designated <br />point of discharge such as an inlet of a drainage structure. The time of <br />concentration is assumed to equal the storm duration. The time of concentration is <br />' measured as the time it takes for a concentrated flow from the most remote point <br />in the subbasin to reach the designated outlet discharge point. The time of <br />' concentration for a watershed may consist of overland flow travel time and the <br />flow time from upstream drainage structures or discharge points connected to the <br />outer discharge point of interest. <br />' 2. Rainfall intensity is constant throughout the storm duration and uniform <br />throughout the subbasin. <br />These assumptions are likely to be more valid for small basins with short storm events. <br />The rational method is particularly applicable at Logan Wash Mine because it solves for <br />' peak discharge in a relatively small watershed where storage is not a design requirement <br />for the project. <br />' The following sections address the derivation of each component of the rational method <br />equation for the Logan Wash Mine area. <br />2.1 Subbasin Areas and Conceptual Stormwater Routing <br />' The conceptual stormwater routing plan was dictated by topography at the site as <br />indicated on the U.S. Geological Survey 7.5 minute quadrangle and observations made in <br />the field. The total area that potentially contributes runoff to the mine site is shown in <br />Western Water 8 Land Inc. 2 <br />