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Project 1—Upper Catchment Management PP g A quick evaluation of topographic data suggests that the majority of the eroded drainageway's j catchment (local watershed) collects in the large,grassed "bowl" on the site, as well as another depressional area to the west,which consists of a smaller grassed depression and potentially a forested wetland. Project 1 entails re-routing the large bowl runoff(which is already collected in a drainage pipe) into the smaller grassed depression to the west. This can be done by digging a new drainage path from the large bowl outlet pipe into the smaller grassed depression,lining it with filter fabric,and overlaying it with small rip-rap (4"-6" diameter). The existing channel should be abandoned (filled in) to prevent runoff from short-circuiting the depression. The smaller grassed j depression should be excavated into a flat-bottomed basin (essentially a rain garden),with a cut of 0.5 to 1.5 feet deep (with the deeper cut on the northern and western edges of the depression). Cut soil should be used to construct a berm (1 to 2 feet tall) along the southeastern edge of the basin to retain runoff in the basin. The basin bottom should be,at a minimum,rototilled to provide greater infiltration. Soil amendments such as sand and compost would also provide greater infiltration, which will help reduce the time standing water sits in the basin. Optional perforated drain tile (like the one already located in this area) could be installed to expedite drainage of the basin. Native seed, plants,and erosion control blanket should be installed in the basin to stabilize soils and to expedite j water removal. The constructed berm should be compacted, seeded with native plants,and covered with erosion control blanket. A v-notch weir should be installed along the berm at the point where water will leave the basin and flow into the existing drainageway. The weir can be constructed from pressure-treated lumber (allowing for easy modifications later),and should be installed such that it is j well keyed-in to the berm and ground beneath. The bottom of the v-notch should be approximately 0.5 feet above the bottom of the basin. Filter fabric,overlain with rip rap (>W' diameter), should be j installed at the outlet so that water leaving the basin via the v-notch weir does not erode the drainageway. I It may be advantageous to raise slightly(0.5 to 1 foot) the pipe inlet located in the potential forested wetland (downstream from the constructed basin/weir) so that this wooded area would pond more water before it was able to flow into the pipe,under the footbridge,and into the well-defined drainageway. Mounting a flexible plastic section to this pipe inlet and mounting it to a vertical post would allow for variable water level control in this area. Important Notes: Jurisdictional wetlands may exist in the area (e.g.,the potential forested wetland); this can only be determined by a qualified wetland delineator/regulator. Placing fill in a wetland requires a permit. Construction of a berm and placement of rip rap in a jurisdictional wetland would be considered fill,but this project may fall within the"de mirnmis" exemption of the Minnesota Wetland Conservation Act. This issue requires additional investigation,and it would be advantageous to see if the Minnehaha Creek Watershed District is available to assist in this determination and assessment of other permit requirements. Also, survey data from a previous Camp Teko project identifies buried utility lines in the vicinity of this project. Gopher State One Call (and potentially a private utility locator) should be contacted prior to any digging to mark any j and all underground utilities prior to any excavation occurring. Regular inspections should be made after the basin/berm/weir and flexible pipe inlet are installed to ensure that no new erosion/sedimentation problems are occurring and that impoundment levels and drainage time are performing acceptably. I I 080529:070908 2 of 4 Camp Teko Drainageway Retrofit—Conceptual Design I I I