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Rick Hauser March 16, 1998 Page2 Existing Conditions For the drainage analysis, MFRA visited the site, took photographs and reviewed information from your surveyor and landscape architect. The existing site generally drains south towards the southerly lot line and east to Lake Minnetonka. There is a low area approximately 75 feet upland of the lake near the southerly property line that may hold water on the adjacent lot during rainfall events. The low area receives runoff from the Hauser lot and the lot to the south. Runoff from portions ofBohn's Point Road also reaches the low area as evidenced by minor erosion along the east side of the roadway leading into the Hauser lot. Calculations and Schematics The drainage calculations and proposed depression schematics prepared with this analysis were based on providing a small low area to retain the excess runoff from hard cover exceeding the allowable amount. The proposed depression is shown on the attached figure. The depression will retain runoff from most storm events and will be drained by infiltration into the ground. The Hennepin County Soils Survey shows the surface soils being Hayden loam which have moderate infiltration rates. In larger storm events, or if the depression is at capacity, it would overflow east towards the lake. Since there are no standard requirements for sizing a depression in the case of a variance request, MFRA reviewed runoff volumes from various storm events for the excess hard cover and prepared a schematic that fits the existing lot and proposed building configuration. The calculations assumed that the depression would be dry prior to rainfall events and be available to receive and store runoff. Retention volumes for the excess hard cover for various storm events are shown on the following table: Storm Event 10 year-24 hour 50 year-24 hour 100 year-24 hour Precipitation (in) 4.2 5.3 6.0 Excess Runoff Volume (c.f.)* 542 684 775 *Excess runoff volume equals 1,549 s.f. multiplied by precipitation in feet The attached figure shows the proposed depression having a volume of717 c.f. (26.6 c.y.) between elevations 931 and 933. That volume would retain runoff for storm events in excess of the 50 year event. The depression would be created by excavating a basin to a depth of approximately 3 feet and the construction of a small berm to retain runoff. Another location for the depression, and perhaps simpler and less intrusive for the property, would be near the south east corner of the lot in an area more distant from the proposed house and lower in elevation. However that location was not further pursued since we understand that water storage areas are not allowed within 75 feet of the lake by city code.