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Rate Control The StormTech system will provide sufficient volume to provide live storage for runoff generated on site, and will limit the rate of discharge to less than existing conditions for the Type II 24-hour, 2-year and 100-year rainfall events. An outlet structure will be used to control the discharge rate, which will reduce runoff rates for both of the design storms through the use of a 15" orifice and 15" outlet pipe. The results of the runoff rate comparison are listed in the following tabie, and further details can be found in the attached HydroCAD calculations. Maximum Runoff Rate Qmax Icfs) Storm Event Existing Propos 2-Year 14.33 0.09 100-Year 30.61 5.17 II. Water Quality & Infiltration One of the benefits of the StormTech system is its dual ability to act as an underground storage and infiltration system. Per prior experience in Rice Creek Wateershed District, it was determined that if the system could provide sufficient dead volume capable of retaining and infiltrating the 2.5" rainfall event, the system would meet both the infiltration and water quality requirements set by the district. The volume of runoff from the 2.5" event over the drainage area for the StormTech system is 0.75 acre-feet, or 32,670 cubic feet. Through use of the volume of the voids in the angular stone and part of the volume of the chambers, a total volume of 33,490 cubic feet is provided below the outlet pipe, meeting the storage requirement. Per prior experience in Roseville, the soils typically have, in general, low permeabilities, and would not be considered ideal for infiltration. Previously we have estimated the infiltration rate of these soils to be between .0006 and .003 feet per day, recommending use of a value near the lower end of this spectrum. This value is well below the lowest infiltration rate specified by the district to be used for Type D soils, and as a result, a layer of coarse aggregate base with draintile is to be installed beneath the crushed angular stone of the StormTech system in order to ensure that the system drains. If we conservatively assume that this coarse aggregate has an infiltration rate of 0.25 in/hr, the full volume of 32,670 cf of runoff spread out over the 24,000 square foot footprint of the system will drain in approximately 65 hours. During this time, settlement of suspended particles in the water will occur, and the system wilf in essence act as though it were an above ground pond. Additional water quality measures will be used at the entrance points to the system, and include the use of sump catch basins and isolator rows. The isolator row is simply a row of the standard StormTech chambers wrapped in a geotextile, with access manholes on either end. This feature removes much of the TSS prior to the water entering the main chambers, and simplifies maintenance, as all of the sediment is in one easy to access location. Further detail on these features can be found on the StormTech details sheet, and in the StormTech design manual.