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ORDINANCE N0. 91-38 <br />Longer duration of bankfull flow than that which occurred under <br />predevelopment conditions, can have disastrous effects on the <br />stream channel and resulting sediment pollution. Since channels <br />generally size themselves for about a one and one-half to a two- <br />year frequency storm runoff event, the one-year storm was chosen as <br />the control storm to prevent significant channel erosion. <br />The "routing problem"'is the problem of channel flow <br />downstream from numerous smaller watersheds. As runoff from the <br />smaller. watersheds are altered, the effect on channel flow <br />somewhere downstream where all of the watersheds come together, can <br />be favorable or adverse. Ideally, the entire large watershed should <br />be hydraulically planned and the level of control of each sub- <br />watershed should be based on the accumulative effects downstream. <br />Since experience has proven the near impossibility, politically, of <br />realizing that degree of organizational control, the one-year <br />outlet rate is further justified in order to minimize adverse <br />effects on the "routing problem" when compensation is necessary to <br />control increased flow volumes. <br />Many people suppose that runoff peaks and volumes do not <br />increase from the large or infrequent storm because the ground is <br />saturated by these storms and most of the rainfall will run off <br />whether the land is used for forest land, cropland, or urban <br />purposes. Urbanization can, in fact, cause an increase in both <br />peak and volume of runoff. Most of the increase may be in the <br />6