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Howard R, Green Company <br /> CONSULTING ENGINEERS <br />May 16, 2001 <br />File: 812970J-0240 <br /> <br />CONISULTI~IG <br /> <br />Consulting Engineers <br /> <br />Mr. Scott Harlicker <br />Senior Planner <br />City of Elk River <br />13065 Orono Parkway <br />P.O. Box 490 <br />Elk River, MN 55330-0490 <br /> <br />RE: <br /> <br />WEST OAKS DEVELOPMENT <br />DRAINAGE CALCULATIONS <br /> <br />Dear Mr. Harlicker: <br /> <br />As you are aware, we have been working with the developer's engineer for the West Oaks <br />Subdivision regarding the drainage calculations. At the hearing before the Planning <br />Commission, numerous concerns were raised by the residents in Big Lake Township to the west <br />of this proposed development regarding potential flooding if too much water was discharged to <br />the natural flow, or draining of pond areas that the residents have developed in their rear yards <br />if too much excavation was allowed in the natural flowage. We believe the drainage <br />calculations that have now been submitted eliminate these concerns while also substantially <br />meeting the City drainage requirements. We do have a few comments which are as follows: <br /> <br />1. There are two large detention ponds proposed. Both are located in the center looped road <br /> area, one below the existing wetland and one above. These two ponds provide enough <br /> storage to hold the 100-year storm from the full development of this subdivision while only <br /> discharging to the wetland at the predevelopment rate. The one thing lacking in these two <br /> ponds is neither of them appear to have sufficient storage for quality control. The ponds <br /> need to be redesigned slightly-to meet NURP standards for quality control of the runoff. <br /> <br />2. The proposed drainage plan includes replacing the existing 15-inch culvert under the old <br /> Waco Street alignment. It appears that the culvert will be replaced at the same elevation <br /> and with the same size pipe so there will be no significant change in the natural flow. <br /> <br />3. The calculations indicate that the 100-year high water level in the two ponds will provide <br /> for at least two feet of freeboard to the adjacent structures. <br /> <br />4. There appears to be a minimal amount of grading in the existing wetlands. These are <br /> Type 2 wetlands and excavation in them is allowed. However, draining them is prohibited. <br /> The developer has previously submitted hand augers which indicate where the confining <br /> layer is located. In our review of the final grading plan for this development, we will ensure <br /> that the excavation in the existing wetlands does not break through the confining area, <br /> therefore not allowing the wetlands to be drained. This will ensure that there is no adverse <br /> effect upstream relative to the draining of ponds located in the existing home rear yards. <br /> <br />C:\windows\TEMP\Ltr 05-16-01 Harlicker.doc <br /> <br /> 1326 Energy Park Drive · St. Paul, MN 55108 · 612/644-4389 fax 612/644-9446 toll free 800/888-2923 <br /> <br /> <br />