Laserfiche WebLink
<br />of 9 for an increase in imperviousness of from 1~ (rural area) to <br />35~ (completely developed urban area) for the 2-year exceedence interval <br />rainfall event, For the 50-year exceedence interval rainfall event, the <br />flow increases by a factor of 5 for the same increase in imperviousness. <br />The less frequent events are indicated to have proportionately less <br />increase in peak flow. <br />A report (5) prepared for the San Francisco Bay region indicates the <br />following effect for urbanization: <br /> <br />RATIO OF PEAK FLOW FOR STATED CONDITIONS TO NATURAL FLOW <br /> <br />Exceedence Interval <br />(years) <br />2 <br />5 <br />10 <br />25 <br />50 <br />100 <br /> <br />50% Development <br />50% Storm Sewered <br /> <br />100% Development <br />100% Storm Sewered <br /> <br />2.0 <br />1.7 <br />1.6 <br />1.5 <br />1.4 <br />1.35 <br /> <br />4.2 <br />3.5 <br />3.1 <br />2.8 <br />2.6 <br />2.5 <br /> <br />The results were obtained by regression analysis of runoff records for <br />existing conditions within the Bay area. <br />A model has been developed (6) that is proposed for use in determining <br />the drainage consequence of urbanization. This model uses the rainfall <br />runoff procedllres of effective rainfall determination, unit hydrographs <br />and storage routing. A demonstration of the model for a watershed in San <br />Diego County, Ca1iforni~ indicates the following effects of urbanization. <br />The results are for the 100-year exceedence interval storm. <br /> <br />11 <br />