Storm water management assessment memorandum

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TECHNICAL MEMORANDUM HDR ENG/NEER/NG, INC. POND CONDITION A site visit was conducted by HDR to assess the existing condition of the Orono storm water detention pond on October 19, 2005. The purpose of the pond site visit was to inspect the pond condition, assess its current performance, and perform level surveying to determine the approximate depth of the sediments deposited on the pond bottom. Photographs taken during the site visit are included in the Appendix of this report. Existing Vegetation Almost the entire periphery of the Orono Pond was covered by medium size vegetation, except the northeast side bank of the pond (area around/near inlet). However, no vegetation was observed near the center of the pond. The pond periphery (except the northeast corner) supports a medium population of reeds and sedges, which are extended into the pond. The assessment team was able to walk through the vegetation without any problem. Overall, the vegetation supported by the pond is normal and desirable for the pond operation. The northeast side bank of the pond lacks a vegetative buffer strip and is experiencing erosion. The balance of the pond surface is open water. Runoff Sources The drainage area of the Orono wet detention pond consists of the paved parking lot, truck station building, roads, landscape areas, and storage facility. The visual inspection of the Hennepin County Orono wet detention pond revealed the pond site is moderately clean and no floating debris was observed in the pond. However, some garbage/trash (bottles, cans, etc.) were noticed at the side banks and near the outlet structure of the pond. Inlet Structure The inlet structure of the Orono wet detention pond appeared to be in good shape. The inlet culvert/pipe discharging the runoff into the pond had no visible problems. However, no grate was observed over the inlet culvert/pipe. Moreover, no sediment deposition in the inlet pipe was observed during the inspection visit. Rock riprap was provided around inlet structure and at the foot of the pipe to prevent erosion. It was noticed that the side bank near the inlet of the Orono pond has experienced erosion problems. Since no vegetation protection is available in that part of the pond, the soil has eroded away and some cracks in the side bank were observed. Outlet Structure The outlet structure at the Orono wet detention pond was located and visually inspected. The metal standpipe installed at the outlet structure had no visible problems. Rock riprap was placed around the outlet structure and the bottom of the pipe to prevent erosion. Sediment Elevation Survey The purpose of the elevation survey was to take several measurements as close to the pond bottom as possible to try to ascertain the amount of sediment deposited in the pond. The Orono pond had vegetation around the perimeter and open water for most of the pond surface area. Depth measurements were taken at points in the pond that could be safely accessed by the survey team. The invert of the downstream end of the outlet structure was at an elevation of 934.31 ft per the design drawings and was used as a reference elevation. Elevation measurements were taken at several locations within the pond (Table 1). 11/4/2005 HDR PRo.IECT#32560 4 HENNEPIN COUNTY,MINNESOTA ORONO TRUCK STATION FACILITY STORM WATER POND EVALUAT/ON