07-09-2003 Council Agenda - Laserfiche WebLink

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indicated that the 100 yr. flood elevation is 862.5 and historically the lake has been as high as 863.0. From this analysis it appears the existing ditch has sufficient capacity to convey the drainage without changing the predeveloped conditions. The existing ditch does have vegetation growth and variations in crossection, which may result in somewhat less capacity than the maximum calculated volume. However, it appears that there is positive drainage and any detention that occurs as a result will eventually drain to Kohlman Lake. Maplewood Pond The records and topography reviewed indicate that the storm water pond located near the Maplewood /Little Canada border was built by constructing an earthen berm within the wetland. No excavation was done within the pond area; therefore it can be assumed that the lowest elevation within the pond has remained the elevation of the original wetland. When originally constructed, storm water runoff from the north was allowed to enter into the north end of the pond at approximately an elevation of 864 as determined from the Markhurd aerial topo of March 1981. However, when the Beam Lakeside Addition was constructed (after 1981) the north end of the pond was closed and a drainage ditch constructed along the west side of the pond to direct drainage from the north around the pond to Kohlman Lake. It was likely that the pond was closed on the north end to provide enough storage within the treatment pond to handle the additional storm water from the new development without flooding more area to the northwest of the pond. To verify this the subcatchment area, approximately 79 acres, which drains to the detention pond was analyzed. The pond water level is controlled by a 12" culvert located on the south side of the pond, which discharges to Kohlman Lake. The invert of this outlet is 860.4, which is the normal water level of the pond. The elevation at the top of the berm varies but is roughly 867. The storm water runoff rates shown were obtained from the overall subcatchment area and are a general calculation which does not detail the entire storm sewer piping network. The calculations were also based on Kohlman Lake being at a normal water level of 859.0. The flows from this area and resulting peak pond elevations are summarized in the table below. STORM FREQUENCY (years) RUNOFF (cfs) PEAK POND ELEVATION 1 20 861.2 5 61 862.7 10 85 863.6 25 113 864.7 100 164 866.9 The 100 -yr high water level (HWL) of he pond was calculated to be 866.9 resulting in a freeboard of 0.1 feet. While this is short of the recommended 1.0 -foot of freeboard it appears the pond will not overtop during a 100 -yr event. During the 100 -yr storm, the pond will discharge the detained storm water over more than a 96 hour period with the bulk of the discharge taking as long as 48 hours to exit the pond. The pond treatment with these long detention times is well within accepted standards. In addition the geometry of the pond is well suited to provide acceptable treatment with the inlet and outlet situated opposite each other to prevent short- circuiting. With all these observations, it appears the existing pond provides adequate treatment and detention of storm water. C: \WINDOWS \Temporary Internet Files \OLKFIA3 \Sculley Drainage Findings.doc 4