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<br />oa1333 <br /> <br />10 <br /> <br />1I-2. Figure II-l illustrates the frequently observed increase in peak <br />discharge that accompanies watershed development. The development in- <br />creases the stormwater carrying capacity of upstream channels resulting <br />in a quicker and higher peak discharge rate. Figure 1I-2 illustrates <br />the hydrologic response when the development provides for natural or <br />man-made detention of stormwater. The volume of stormwater and the <br />discharge duration are increased, but the peak discharge rate remains <br />relatively unchanged. <br />Regardless of the response, an estimate of the change is required <br />to identify flood control benefits and liabilities. All aspects of the <br />response are necessary to allow flexibility in design. With all aspects <br />of the runoff response, an engineer can design storage facilities <br />(detention ponds) as well as discharge facilities (open channels, <br />culverts, pipes). Another set of predictive models and expressions, or <br />adaptations of the current ones, is required for satisfactorily repre- <br />senting urbanizing watersheds. These new models would be capable of <br />predicting the changes in peak runoff rate, volume of runoff, and time <br />distribution of runoff attributable to urban development. <br />Since the mid-19SO's a number of studies have addressed the <br />measurement of these particular changes in hydrologic response. The <br />results of these studies are compiled in Appendix A. The results are <br />expressed in common terms for easier comparison. The interpolation of <br />published expressions (equations and graphs) required by this manipu- <br />lation were confined within the limits of those expressions. Although <br />the table in Appendix A is informative, the variability of the results <br />provides little, if any, help to municipal governments attempting <br />