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property drains into the school district property. The combination of on~site run-off, off- site run-off, and unique hydrogeologic nature of the site presents many challenges with regards to drainage. The following is a summary of major issues: Storm Water Quantity Strictly speaking, the development site is landlocked, since it does not have any natural drainage out-fall. Roseville's stormwater management plan suggests that drainage systems for landlocked parcels be designed to handle a 7,2" run-off event. This very strict criteria can be somewhat reduced to take into consideration unique site characteristics such as infiltration, For non-landlocked parcels, stormwater systems are to be designed for a 100- year rain event. Considering existing infiltration rates and the proposed installation of a pumping station adjacent to Wetland Basin 5, it is reasonable to relax the suggested storage requirements for landlocked parcels. The actual storage requirement should be between that required for a standard 100 year rain event and the 7.2" landlocked standard. A review of this issue is currently being done on behalf of the City by SEH, the local consulting engineering firm that prepared Roseville's Stormwater Management Plan. Historically, stormwater discharged into the existing wetlands has quickly percolated into the ground. Wetland Basin 6 adjacent to Dale Street fills rapidly after each rain. However, it has been observed that three feet of ponding in this wetland drains completely within four days-very strong evidence of rapid infiltration. When designing a stormwater system, however, the engineering community generally ignores ground infiltration, This somewhat conservative approach recognizes that future infiltration could be reduced by silt deposited from developed urban areas. Discounting infiltration, a great deal of on-site storage is required to handle design events. Based on the information received so far, the school district proposes to utilize a combination of new NURP ponds and existing wetland basins to control run-off. NURP ponds are stormwater treatment basins designed according to national standards. These ponds utilize a permanent pool of water to remove pollutants from stormwater. Generally speaking, the entire site eventually drains into the low spot at Wetland Basin 5. A new NURP pond will be constructed within the proposed park to provide flood storage, As currently designed, a majority of the park would be flooded during a 100-year event. To drain this pond, it is proposed to construct a stormwater pumping station adjacent to Wetland Basin 5. This pumping station would discharge to Lake Owasso. The location and design of this out-fall has not yet been determined and is being discussed with the developer. Stormwater Quality As previously discussed, a large percentage of the stormwater to be handled on this site is generated off-site. The majority oftms water enters Wetland Basins 3 and 6 directly by existing storm sewers. The majority of stormwater collected on-site is proposed to be treated in new NURP ponds prior to discharge into existing wetlands. All site drainage will flow through the NURP basin to be constructed adjacent to Wetland Basin 5 in the proposed park. Thus, all stormwater, both existing and new, will receive some treatment prior to discharge to Lake Owasso, It should be noted that the quantity and frequency of 10