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SENT BY: RELi ANCE R. E. SERVICES; 3-16-98 2 : 56PM (612) 338-8971--1 6124730510:# 3/ 4 March 16, 1998 Page2 Existing C<indifonu; For the cbaina.ge analysis, MFRA visited the site1 took photographs and reviewed information frotn your surveyor and landscape archjtect. 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 of Bobn's Point Road aJso 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 calcuJations and proposed depression schematics prepared 'With this analysis wtte bat\ed on prvviding a small iow a.re-a to re'tain the e1tcess runoff from hard cover exeeeding the allowable amount. The proposed depres:s.ion i$ shown on the attached figure. The ciepression will retain runoff from most stonn events and will tie drained b:y infiltration into the ground. The Hennepin County Soils Surv~y shows the surface soils being Haydr::n lo.:llll which have moderate infiltration cates. ln larger storm events~ or if the depress:ion is at capacity, it would overflow east towards the lake-Since thc:re an: no standard requirement$ for si.tlng a depression in the case of a variance request, !\·1FRA reviewed runoff volumes from various stonn events for the excess hard cover and prepared a schematic that fits the existing lot and proposed building configuration. Th.e calculations assumed that the, depression would be dry prior to rainfall events and be available to receive and store runoff. Retention volumee. for the excess hard cover for various storm events are shown on the following table: Storm Event l O year-24 hc.m.r 50 year-24 hour 100 year-24 hour Precipitation (in) 4,2 5.3 6.0 Excess Runo:ffVolume (c.f.)* 542 684 775 --.Excess runoff volume equals 1,549 s.f. roultipli~d by precipitation in feet The attached figure shows the proposed depression havmg a volume of 717 c.f. (26.6 c.y.) b~tween eleva.tiOI1.$ 931 and 933. 1bat volume would retain runoff for stomt events in excess of the 50 yew;-event. The depi-essi.uu would be created by excavating ii basin to a. deplJ:1. of approxfrx:i.a~ly 3 f'eet and the construction of a small berm to retain runoff, &!other location for the depression 1 and perhaps simpler and less intrusive for tbe property, would be near the south east comer of the lot in an area more dist.ant from the proposed house .and lower in elev_ation. However that location was not further pursued since we understand that water storage areas ar~ not allowed within 75 feet of the lake by city code.