04-27-2022 Council Packet - Laserfiche WebLink

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April 2014  Surface water flow direction and storm water pipe water flow direction shall be indicated on the pre and post project watershed maps.  Minimum recommended watershed map scale is 1” =100’. Project applications submitted with smaller scales (e.g., 1”=500’) may be rejected and returned to the applicant. The same would apply for project watershed maps that do not include topographic contours or basic land use information such as the location of buildings, pavement and “green space”. Watershed maps submitted as pdf files or CAD files shall be readily printable at scales that allow for good readability.  Pre and post project watershed maps shall be clearly linked to the drainage models such that the names of the sub-watersheds, ponds and drainage structures are the same in the models as shown on the watershed maps. In addition, watershed and sub-watershed boundaries shall be clearly shown.  Submission of the actual pre and post project HydroCAD or XPSWMM models is required: pdf copies of the drainage model simulations are unacceptable. In the event that the models cannot be transferred readily by electronic mail or electronic repository site, a hardcopy CD shall be provided.  Curve numbers shall be determined per NRCS methodology and should be modified as needed based upon detailed knowledge of soil type and specific conditions on site. HydroCAD modeling software includes NRCS guidance for determining curve numbers based upon land use and condition.  Time of concentration (Tc) computations and assumptions that in MnDOT’s assessment clearly overestimate or underestimate this critical runoff parameter will be rejected. Two common assumptions that lead to overestimating Tc include: using the “Lag/CN” method to determine peak runoff from watersheds that have a relatively long and/or diverse flow path, and assuming that sheet flow occurs for a distance exceeding100 feet. Conversely, pre-project Tc shall not be underestimated to offset post project increases in peak discharge.  Available freeboard for existing and proposed treatment ponds shall be shown on the watershed maps as well as the normal and 100-year high water levels. All proposed pond treatment systems along MnDOT right-of-way shall have a minimum freeboard of 2.0 feet between the road surface and the proposed 100-year HWL.  Infiltration basins, filtration basins and ponds adjacent to MnDOT right-of-way shall be designed to provide at least 2 feet of elevation difference between the 100-year HWL and the crest of the basin berm. The berm crest shall be at least 5 feet wide. The emergency overflow shall be lined from crest to toe of slope with Turf Reinforcement mat or Category 6 or 7 Erosion Control Blanket.