Laserfiche WebLink
t)-7 <br />W' <br />I <br />tf- <br />To: <br />From: <br />Date: <br />Re: <br />Memorandum <br />Minnehaha Creek Watershed Board of Managers <br />Eric Evenson, Administrator <br />Mike Panzer, P.E. <br />District Engineer <br />Tuesday, November 21, 2000 <br />Permit Application No. 00-356 <br />Brenshell Homes <br />Wildhurst Trail and Tonkaview Lane, Orono <br />L Purpose <br />The subject permit was tabled at the November 16, 2000 meeting because <br />several nearby residents appeared at the meeting to voice concerns. One of the <br />concerns was the potential adverse impact the project may have on existing <br />drainage and flooding. My understanding of the flooding problems center along <br />Wildhurst Trail, where the street intersects and crosses two natural drainage <br />paths (North Ravine and East Ravine). I base this on a site inspection, concerns <br />voiced at the November 16*^ meeting and correspondence to the City of Orono <br />on behalf of Mr. Gary Welsh. The purpose of this memorandum is to address the <br />potential runoff Impact the project will have on existing flooding problems at <br />these locations. <br />IL Potential Impact on Downstream Flooding <br />In both cases, the ravines collect runoff from the project area and areas located <br />off the project property (See attached site plan prepared by Coffin & Gronberg, <br />Inc.), the grades of the ravines are relatively steep. Runoff conveyances <br />across, under and along Wildhurst Trail appear to potentially he inadequate and <br />inefficient for the conveyance of flow given the existing topography and <br />residential development in the area downstream of the project site, the subject <br />project could exacerbate these existing conditions if it resulted in higher flow <br />rates or shortened “times of concentration” (y of flow. <br />Generally, the Coffin & C»onD)/rg, Inc. design calculations (modified TR-55 <br />method) and an indepen em? Hydro-Cad model prepared by Landform <br />Engineering Company snow mat flow rates will be remain unchanged for smaller <br />runoff events and will be significantly reduced for more severe events. <br />(Note: The Coffin Gronberg, Inc. summary (attached) addresses runoff from the <br />project property. The Landform Engineering Company model addresses runoff <br />from the project property and off-site areas as well.) <br />III.The Project Exceeds the Requirements of Rule N <br />Based upon my review of the application, as submitted, the project exceeds the