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<br />quite hydraulically rough and velocities and flow generally are not excess- <br />ive. Further attenuation of the flow occurs because runoff is detained <br />where it falls by surface vegetation and natural swales. <br /> <br />Effect of Urbanization <br />What happens when the watershed is urbanized? A great deal of the <br />surface that previously was vegetation and natural ground becomes impervious <br />by the addition of roofs. streets, driveways, walkways. and parking lots. <br />The effects of this imperviousness are numerous. The precipitation cannot as <br />readily infiltrate through an ,impervious surface so the volume of runoff <br />increases. The surface is much smoother and thus hydraulically more efficient <br />so that runoff occurs faster. In addi.tion. less natural storage is operable <br />which further increases the rate of runoff. The collector channels replace <br />the natural channels with pipes that convey flow efficiently. The net <br />effect on a watershed that has considerable urban development as compared <br />to its natural state, is that of an increased volume of runoff and increased <br />speed of runoff. This r~sults in generally higher peak flows. <br />There is another important effect that can be referred to as the <br />"system effect" or "timing effect." ' The flow at a concentration point in <br />the system at an instant in time is the sum of the flow from the many <br />components in the system. A watershed that has been converted from natural <br />to an urban stilte Can have its Iltiming" at concentration points Significantly <br />altered. The system effect at any point can result in either an increase <br />or decrease in the flow rates depending on the relative timing effects. <br />A comprehensive table depicting the hydrologic effects associated <br />with urbanization is contained in (3) and is as follows: <br />8 <br />