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I I 1-9 STREAMFLOW SENSITIVITIES Development. The most obvious increase 111 runoff flo",'s is due to urban- ization. The development process in which substantial fractions of impervious surface area are introduced, the remaining pervious areas become less pervious due to lItightll lawn covered are.a, and where the distances to collecting, fast-moving streams (i .e., guttel-s, ditches and storm sewers) are decreased causes well-documented effects on the flood hydrology of an area. The net effect is an increase in peak Flows, faster repsonding streams and in general longer high flow per'iods. The best example of future flow increase is the area of Lena Gulch above Maple Grove Reservoir comprised basically of Basin 6, but also Basins 3, 4 and 5. The lOa-year frequency flow, under existing channel condi- tions, but future basin development, incn.ases about 40 percent. The general implication is that on-site detention ponding should be required in areas above Maple Grove Reservoir. Such detention should be implemented to detain runoff for a relatively long duration in order to partially relieve Maple Grove Reservoir from a compounded ptoblem due to new development. Long Duration On-Site Detention. The use of on-site detention of runoff from new deve lopment can be requ i red by County Commi so' i oners in un i ncor- porated areas according to state law. These measures should be taken in all areas along Lena Gulch, not only to prevent flash flows to and along the Gulch, but to aid in pollution abatement. The general design constraints used in the design of detentio~ faci litie. refer primarily to peak discharges. The most common constraint i. to de'- sign facilities that limit the peak runoff flow rate from the new to the historical rate for the same lOa-year rainfall e.vent:. However, the law specifically refers to I imiting of runoff to the. historical volume which is more involved as far as a design constraint. The problem on Lena Gulch is that the entities should ask new developers to evaluate regu- lation of the total volume of runoff from the development in areas that are tributary to Maple Grove Reservoir and Magic Mountain Dam No. 1 on Jackson Gulch. The reason for limiting the total runoff volume is to meter the increase in runoff into Lena Gulch at a time when the reservoir water sur- faces downstream have started to drop. This will have to be evaluated on a case by case basis, but the general concept will be to examine the historical hydrograph and to attempt to de- sign facilities that limit the peak runoff rate and to extend the falling limb of the hydrograph over a period of time on the order of 12 to 16 hours compared to the normal 3 to 4 hours as shown in Figure 111-6. This process wi 11 help with the spillway capacity problems that exist in the basin.