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I I I I I I I I I I I I I I I I I I I 18 REGIONAL DRAINAGE CONTROL Detention storage at Node 4 and upgraded crossings upstream will effectively control storm runoff under existing conditions. However, as undeveloped areas south of Iris Drive and west of the Pioneer Ditch are urbanized, the increase in peak discharge and runoff volume will have a significant adverse impact on the lower ditch even with the proposed improvements. Accordingly, as these areas are annexed, preventive controls should be placed on development such that future-condition ten-year peaks and volumes from the stippled areas in Figure 2 cannot exceed present condition peaks and volumes. Similar preventive controls should be imposed on large commercial or equivalent developments in the non-stippled areas, but with drainage design criteria con~s,~nt with those proposed by Hoskins and Associates (1957) and Leaf and RCI (1983) for service areas 6 through 11 (Figure 5). WATER QUALITY CONTROL , In addition to reducing peak flows downstream from Node 4, detention storage would be highly effective in improving water quality. Using methods proposed by EPA (1986), prel iminary estimates suggest that the long term average reduction of total suspended solids can be more than 90 percent. DISCUSSION The areas served by the Pioneer Drainage Ditch are subject to serious local storm runoff problems and regional flooding. Mitigation of both regional and local flooding is a complex, expensive, and long term problem. The proposed near-term improvements are consistent with the Master Drainage Plan developed by Leaf and RCI (1983). The near-term improvements als,o are compatible with future needs (and opportunities) for regional flood control, and allow additional development in the flood plain in the northwest part of the city.