#14 - Upper 33rd Conditional Use Permit

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Upper 33rd Street Townhouse Development SUMMARY OF STORMWATER RUNOFF Introduction: This project will consist of the construction of a new 4-unit townhouse, two shared driveways, and associated stormwater utilities. Stormwater calculations were modeled using HydroCAD with Atlas 14 24-hour rainfall data. Existing Conditions: The existing site is an open grass residential lot. Runoff is generally from south to north across the site. To the north of the site is a grass lot. Per the LIDAR contours shown on MnTopo, it appears the grass lot to the north may be landlocked, as it is unclear where this lot ultimately drains. The existing conditions were modeled as >75% Grass Cover, Type D soils, with a Curve Number of 80. There are no existing impervious surfaces. Proposed Conditions: The runoff from the proposed site will be routed to an underground pipe infiltration system. The pipe infiltration system will outlet into the storm sewer in 33rd St. Per Valley Branch Watershed District (VBWD) standards, the runoff rate in the proposed conditions must not exceed the runoff rate in the existing conditions for the 2-, 10-, and 100-year 24-hour storms, as well as the 7.2-inch 100-year 10-day snow melt event, for all points where discharge leaves the site. The existing and proposed peak runoff rates leaving the site are listed in the table below. Existing peak runoff rates (in cubic feet per second): Area 2-year 10-year 100-year 100-yr, 10-day Runoff North 0.57 1.17 2.59 0.19 Runoff South 0.00 0.00 0.00 0.00 Total Runoff 0.57 1.17 2.59 0.19 Proposed peak runoff rates (in cubic feet per second): Area 2-year 10-year 100-year 100-yr, 10-day Runoff North 0.00 0.00 0.00 0.00 Runoff South 0.56 0.90 2.51 0.19 Total Runoff 0.56 0.90 2.51 0.19 Since all of the existing runoff is directed towards the north, and the proposed underground system is discharging into the storm sewer in 33rd St, the runoff rates to the south are increasing. However, the total rates leaving the site have been reduced. We feel this is a better design than sending the proposed drainage to the north into the neighboring yard, where it can potentially be directed to a landlocked area.