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CIVIL METHODS, INC• 763.210.5713 1 info@civilmethods.com <br /> Consulting Engineers 1551 Livingston Avenue, Suite 104 <br /> West St. Paul, MN 55118 <br /> August 1, 2014 <br /> Mr. Bob Wallace <br /> Mom's Landscaping & Design <br /> 12276 Johnson Memorial Parkway <br /> Shakopee, MN 55379 <br /> RE: Culvert Design—Buhman Property, 1560 Tanglewood Road, Orono, MN <br /> Dear Bob: <br /> As requested, we have analyzed the Buhman property ditch crossing in an effort to adequately size a <br /> culvert meeting industry design standards. The ditch watershed has been delineated using available <br /> LiDAR contour information, and the SCS TR-20 runoff methodology was used with the HydroCAD <br /> modeling to determine flowrates and size the culvert. In addition, we have provided an erosion and <br /> sediment control plan designed to minimize the potential for sediment transport to the ditch and <br /> downstream waters (as required per the MCWD Waterbody Crossings and Structures Rule. <br /> As per the Rule,two alternatives have been evaluated including a "no-build" alternative, as well as a <br /> "full bridge" alternative. The no-build alternative would not allow for safe access to the new recreation <br /> area, which is unacceptable. Constructing a full bridge overpass would minimize the impact to the ditch, <br /> but would add excessive cost to the project for minimal benefit. Therefore, the culvert option has been <br /> selected as a way to convey runoff, minimize site impact, and meet budgetary constraints. <br /> The ditch receives runoff from a 15.1 acre watershed (see attached figure), consisting entirely of <br /> natural/woodland area. The soils at the site are a mixture of Hydrologic Soil Group (HSG)Type B and <br /> Type C, ranging from moderate to limited infiltration capacity. A 10-year rainfall event of 4.25 inches <br /> (NOAH Atlas 14) over the watershed yields a flowrate of 7.3 cfs, which can be adequately conveyed by <br /> an 18" HDPE, smooth-interior, dual-wall pipe placed at a 1.0%slope. The full-flow capacity of the pipe is <br /> 12.5 cfs, and during an extreme event the crossing will overtop. <br /> Silt fence, biorolls, and erosion control blanket will be used to stabilize the site during and after <br /> construction. Grades have been minimized to reduce the potential for future erosion, with the steepest <br /> sideslope at a 4:1 (H:V)grade. The site should be vegetated as soon as possible after grading operations <br /> are completed. <br /> Please contact me with any questions at your convenience. <br /> Sincerely, <br /> David Poggi, PE <br /> Principal Engineer <br /> RECEIVED <br /> AUG 2 8 2014 Page 1 of 1 <br /> CITY OF ORONO <br />