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<br />OJ1337 <br /> <br />Chapter I I <br />TECHNICAL ELEMEIH OF THE DRAINAGE MANAGEMENT PROGRAM <br />Techniques for Evaluating Changes in Hydrologic Response <br />"eview of past work - Scatistical analysis of historic runoff <br />records is used to determine the expected frequency of various discharge <br />events. The runoff data is "fitted" to a predetermined frequency dis- <br />tribution ana peak discharges of any frequency are estimated from the <br />parameters of the "fitted" distribution. This type of analysis has been <br />presen teu by a number of authors (24, !J9). It works well for rural <br />areas that have experienced 1 ittle physiographic change throughout the <br />period of recorded runoff. <br />Unfortunately, small basins in urban areas generally do not have <br />long records of homogeneous runoff data. Some kind of model or repre- <br />sentation of the rainfall-runoff process must be used to estimate ex- <br />pected uischarge events. The model takes the place of historic runoff <br />oata and must be capable of accurately representing the rainfall-runoff <br />process for various rainfall events. Extensive work has been done in <br />developing tnese urban rainfall-runoff models (46, 92, 93). The <br />expressions were developed to estimate the design discharge for urban <br />flood control structures and are generally limited to predicting only <br />the peak runoff rate in a totally urban environment. <br />The 1 imitations of these "rural" and "urban" hydrologic techniques <br />restrict their use. urainage and flood control management of an ur- <br />baniLing basin requires an estima"ce of the growth-induced changes to all <br />aspects of runoff response (peak discharge, volume of discharge, and <br />time to peak). These changes are illustrated in Figure 11-1 and <br />