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HighPointe Crossing Development Stormwater Review <br />Page: 2 <br /> <br /> <br />2. Volume Control and Water Quality Treatment: The Valley Branch Watershed requires volume <br />control and water quality treatment for 1.3 inches of runoff from the sum of new and fully <br />reconstructed impervious surfaces (1.3 inches over 14.6 acres = 1.58 acre-feet). <br />o Two proposed wet sediment (NURP) ponds will provide water quality treatment for the <br />development along with a proposed stormwater reuse system. J.R. Hill, Inc. submitted <br />Valley Branch Stormwater Reuse Calculator results indicating 87.4% reduction in <br />average annual runoff volume. It appears that the computed 87.4% reduction in <br />average annual runoff volume attributed to the reuse system is for the watershed area <br />(51.97 acres @ 25.9% impervious) tributary to the stormwater harvest and irrigation <br />system rather than for the full site (78.8 acres @ 18.5% impervious). Please compute <br />the reduction in runoff volume for the full site and please also submit the MIDS <br />calculator analysis referenced in the VBWD Reuse Summary Table. <br />o If meeting the total volume control requirement is infeasible, please document the <br />reasoning and analyze the flexible treatment options (FTOs) outlined in the Valley <br />Branch Watershed rules (for example FTO1: retain 0.55 inches of runoff from <br />impervious and 75% TP removal). <br />3. Stormwater Reuse Pond: The MPCA Stormwater Manual lists a number of design <br />considerations for multipurpose stormwater reuse ponds. Please provide information regarding <br />the following: <br />o Does a drawdown limit need to be set…to maintain sufficient water levels for pond <br />aesthetics? <br />o Will riprap or other measures be used to address potential erosion from pond inlets <br />under low pond water levels? <br />o <br />4. Pond 2P: Please label the location and elevation of the EOF on Sheet 3.24. <br />5. Pond 2AP: Please show pond 2AP on the plan sheets 3.20 and 3.24 so the plans match the <br />HydroCAD model. Please also label the NWL, HWL and EOF elevations of the pond. <br />6. Pond Liner: Valley Branch Watershed Rule 2 Standard 10.D requires: “Impermeable liners shall <br />be used at ponds used as reservoirs to draw irrigation water.” Please show the pond liner in the <br />plans or explain why a liner is not feasible. <br />7. Storm Drain Design: The 27" RC Pipe segment between 123 and 122 is at 8% grade with design <br />velocity of 22 ft./s.--exceeding the 15 ft./s. maximum city standard. Please reduce the design <br />pipe velocity by adjusting grades or explain why adjustments are not feasible. <br />8. Swale 2% Grade: Sheet 3.22 shows swales along Lots 1 through 10 of Block 1 and Sheet 3.23 <br />shows swales along lots 10 through 20 of Block 4, and Sheet 3.24 shows swales along Lots 1 <br />through 6 of Block 2 and Lots 1 through 3 of Block 3. that appear to have grades less than the <br />city design standard of 2%. Please revise the grades or explain why this is not feasible. <br />9. SWPPP: The erosion control sheets and SWPPP were reviewed using a checklist from the MPCA. <br />The SWPPP might be improved with the following: <br />o Identify those responsible for long-term operation and maintenance of the permanent <br />stormwater treatment systems.