10/25/1999 Council Packet (2) - Laserfiche WebLink

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• 10/18/1999 14:11 651 - 766 -4196 Compensatory Floodplaln Storage Volume RICE CREEK W DIST PAGE 03 As mentioned earlier in this memorandum, compensatory floodplain storage volume totaling 640 cubic yards was required as part of the RCWD permits. The following water elevations were used in my analysis of as -built floodplain storage volumes:, • Otter Lake Normal Water Level (NWL) 910.3 (Bald Eagle Lake tailwater elevation controls) • Otter Lake Ordinary High Water Elevation (OHW) 911.5 (Per Tom Hovey, DNR) • Otter Lake 100 -year Flood Level (HWL) 912.9 (Highest recorded water level — 1986) RCWD policies specify that compensatory storage volume must be provided in an amount equal to the floodplain fill quantity and must be excavated at an elevation between the impacted basin's NWL and HWL. Additionally, storage cannot be "double- counted" for portions of the compensatory storage basin below the localized or regional NWL (run -out elevation). In this case, the 640 cubic yards of floodplain storage were to be provided between Otter Lake's NWL of 910.3 and HWL of 912.9. Robert Adamson calculated 650± cubic yards between io elevation 910.5 and 912.8 in his September 15, 1999 letter to the RCWD. The calculated volume appears to be in error. • The volume calculation is incorrect because portions of the 650± cubic yards provided by the existing pond are below the pond run -out elevation of approximately 911.5. Therefore, compensatory floodplain storage is valid only between the pond's run -out elevation of 911.5 and Otter Lake's HWL of 912.9. The compensatory storage volume calculations need to be revised to reflect these requirements. Site Drainage Issues As mentioned earlier in this memorandum, another goal of drainage easements is to ensure preservation of adequate drainage routes to avoid future alterations which may excessively impound stormwater runoff on properties upstream of final or intermediate receiving waterbodies. Based upon my site inspection, the hockey rink does not appear to singularly pose a significant drainage hindrance to the Schwartz property to the west. A small swale has been constructed to convey water northeast from a low area along the Melton/Schwartz property line to the stormwater pond. However, it appears this swale was graded with insufficient positive gradient to the pond and may actually back up runoff onto the Schwartz property. The current survey does not adequately identify the swale and centerline flow elevations. -2-