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08/22/95 TUE 09:42 FAX WOODWARD CLYDE 0006 Drainage and Flood Control District stated that this was not a problem in areas where there were large land owners. Developers with large amounts of land were not as constrained by space and they were willing to build regional facilities on their sites. In return, the jurisdiction reimbursed them for the difference between the cost of the regional facility, and the cost of a facility that would have been required to control • stormwater from the landowners property only. Another issue associated with the siting of regional facilities is timing. Regional facilities should be sited and constructed prior to development in a watershed. However, the exact locations and schedules for private development are usually not well known in advance. This would not be an issue for regional facilities which are constructed to address existing problems as opposed to addressing problems associated with expected future development. 3.2 Effectiveness of Facilities for Flood Control On -site Facilities: The advantages and disadvantages associated with on -site facilities for flood control depend on the type of facility used (e.g., detention facilities or infiltration type facilities). Traditionally, on -site detention basins have been used for flood control. However, many recent studies have stated that, if not properly located in a watershed, on- site detention basins can do more harm than good. On -site detention basins are typically designed to reduce post - development peak flow rates to pre - development peak flow rates. Although peak flow rates are reduced, runoff volumes are not. The total volume of discharge from a detention basin is the same as its inflow, the only difference is that the discharge has a lower peak and a longer duration trailing flow (see Figure 2). Depending on the timing of the detained flow, storm enoff�fr from rates are greater than pre - developed watershed may actually combine to produce peak peaks. On -site detention facilities can work, but generally they are only successful if they are carefully designed and located considering all other drainage areas in the watershed. In other words, a watershed perspective or regional perspective should be used to site and design on -site detention facilities. Even when on -site detention facilities are properly located and designed to reduce peak flow rates, some have found that the longer- duration discharges of reduced flow rates can sometimes cause as much or more erosion and damage as the shorter - duration high peaks. On -site infiltration facilities (e.g., retention basins, infiltration trenches, dry wells, reduction of impervious surfaces) are also used for flood control. These types of facilities capture uprmwater and store it until it can seep into the ground. These facil_tLies are most effective when they are located in upstream areas of the watershed where soils are well - drained and where runoff volumes are still manageable for infiltration. These facilities most closely simulate natural conditions, as rainfall provides occurred. groundwater ot only recharge by more closely following the path it took before develop groundwater recharge facilitated, but impacts associated with increased runoff volumes (e.g., erosion, scour, reduced channel capacity) can often be reduced and/or eliminated. When used on -site, these facilities control runoff at its source, therefore reducing impacts 5