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HomeMy WebLinkAboutLtr re high water elevation �Vc�nci:A,sociatc5. Inc. I�S�ti Pioneer�reek Ltr. f.C�. 6ox 428 Maple Plain,MN 55359-0428 � ����� (612)479-4200 — � I,ix iF,121 479-4242 l � � ��._ � � ,i��, June 30, 1999 � Mr. Greg Gappa, P.E. Public Service Director City of Orono P.O. Box 66 Crystal Bay, MN 55323-0066 Re: Required Drainage Evaluation (Winton Site) � �� � �"�V�� �, ! ���.� �/i����� Wenck File No. 0905-01 ��1 V Dear Mr. Gappa: Wenck Associates,Inc. (Wenck)was retained by James McNeal Architects to determine an appropriate high water elevation(HWE) in the drainageway adjacent to the proposed building site on the above referenced site. The site is approximately 2.5 acres in size and is located south of Colin Drive and west of South Brown Road in Orono. The site has a direct drainage area of approximately 4.7 acres. The site has a total drainage area of approximately 23 acres including nearly 16 acres from the City of Long Lake(Long Lake SWMP Dec. 1998). However, storm water storage and attenuation for all but 2.9 acres of drainage, is significant with three large wetlands, extensive ditch storage and numerous culvert restrictions. The existing control for most of the tributary drainage area is a 12-inch CMP culvert under Colin Drive. The proposed drive will be constructed with a 15-inch CMP culvert and will serve as the new control for the drainage swale leading to the steep ravine that drains the site to the south. Despite a very wet spring, during a site visit on June 8, 1999 there was no standing water in the drainage swale downstream of proposed drive. The HWE for the drainage swale adjacent to the proposed new home is estimated to be 963. The HWE was estimated by calculating peak inflows to the swale from the proposed 15-inch culvert and the approximately 2.9 acre drainage area tributary to the swale. Haestad Methods, Inc. open channel flow module was then used to calculate the flow depth for the channel configuration shown on the site plan. The calculated flow depth was then rounded up to the nearest foot. (See attached flow calculations) The 963 HWE is based on the May 24, 1999 site plan and topography provided to Wenck by James McNeal Architects. Survey data was obtained by Precision Surveys, Inc. Modifications of the drainage swale, from its existing condition as depicted on the above site plan, may impact the HWE. Please call me at(612)479-4224 or Kelvin Retterath at(612) 332-4790 if you have any questions regarding our review. . Sincerely, �v 1Q�''i,a e� �^ I - � C� G WENCK ASSOC , INC. � l � �--- c�, �'� Andrew E. Syverson, .E. 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Rectangular Channel Analysis & Design Open Channel - Uniform flow Worksheet Name : Winton Site Comment : Orono drainage channel high water Solve For Depth Given Input Data: Manning' s n. . . . . . 0 .240 Channel Slope . . . . 0 . 0100 ft/ft Discharge. . . . . . . . 15 . 00 cfs Computed Results : Depth. . . . . . . . . . . . 0 . 65 ft Velocity. . . . . . . . . 0 .46 fps Flow Area. . . . . . . . 32 . 71 sf Flow Top Width. . . 50 . Q0 ft Wetted Perimeter. 51.31 ft Critical Depth. . . 0 . 14 ft Critical Slope. . . 1. 63 ft/ft Froude Number. . . . 0 . 10 (flow is Subcritical) Open Channel Flow Module, Version 2 . 01 (c) 1990 Haestad Methods, Inc . * 37 Brookside Rd * Waterbury, Ct 06708