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I I I !I I I I I I I I I I I I I I I I -3- The only land available for developing the required amount of stormwater storage was an area adjacent to the existing channel and Englewood High School at the upstream City limit. The site was occupied by the school baseball field, wood/steel bleachers, a school maintenance yard, a basketball court and a playground, Two private residences abutted the site, and an existing irrigation ditch pipeline traversed the area, An off-channel, excavated storage concept was proposed to the School Board. The facility would be multi-purpose, providing athletic fields during nonflood periods and flood storage during major storm events. Numerous meetings were held with representatives of the City and the School Board at which alternative grading plans were reviewed_ The facility was optimized to maximize flood storage and provide two regulation soccer fields in addition to a baseball field, Data from soil borings at the storage site indicated that the proposed elevation of portions of the athletic fields was below groundwater levels. Further, the groundwater levels were closely related to the surface water levels in the adjacent Little Dry Creek channel. It was determined that an extensive underdrain system would be necessary to maintain dry conditions for the athletic fields during non- flood periods, The system was designed to convey subsurface flows to the northwest corner of the site, where a belew-grade pump station pumps the flow to the creek, Manholes were designed for subdrain pipeline junctions to provide good maintenance access to the system_ The underdrain system connects to one of two wetwells at the pump station, where two 5-horsepower submersible pumps direct the water to the creek via a 16-inch ductile-iron pipeline_ A second wetwell houses a 14-horsepower pump that provides surcharge capacity for the subdrain pumps and provides additional capacity to the gravity outlet for removal of stored flood waters or dewatering when creek levels are high, A free-draining sand layer was chosen as the ideal surface for the athletic fields to provide good drainage and stability, A layer of sand 2-feet thick was chosen to provide a good hydraulic connection to the subsurface drainage system, Fortunately, excavation for the storage facility provided enough clean sandy material to form the lower 12-inch layer. Imported clean sand was placed in the upper 12-inch layer. For the athletic field surface, a special sand-grown sod was specified and installed. As Figure 2 illustrates, a combination drop structure and constriction structure in the creek channel provides the hydraulic control by which the upstream water surface is raised and flow is directed over a 320-foot long grassed side channel spillway into the storage area. To determine the hydraulic characteristics of the side channel spillway for the full range of design flows, backwater calculations were first performed for the improved channel downstream of the constriction structure to determine