Drain Tile Design for Stormwater Basin

Title: Engineer Needed — Drain Tile Design for Stormwater Basin (Minnesota) The Situation We are a 56-home HOA with a private dry detention basin (Outlot E, approx. 0.5 acre) that has never drained since it was built in 2001 to 2002. This approach has already been discussed with the City of Brooklyn Park and the West Mississippi Watershed Management Commission, and does not require permit submittal or approval from either. Essentially we have a bathtub with the plug in. We need drain tile or an underdrain designed to drain it. Why it doesn't drain The basin floor was built flat at elevation 841.0 ft, identical to the outlet invert. Both inlets sit slightly below the outlet (elevation 840.94 and 840.97 versus 841.0). Water entering at the inlets has no gravity path to the outlet once it's in the center of the basin. The existing outlet (SK1, 12-inch RCP skimmer) drains within one to two days near the outlet itself, but the center of the basin stays wet. 2019 site visit (American Engineering Testing): On November 13, 2019, AET performed two hand auger borings in the basin to a depth of 4.5 feet. Soils at both locations were topsoil and organic black material throughout. AET concluded the pond's soils were not suitable for infiltrating water, and that the soils between the inlet and outlet appeared unsuitable for proper sloping and drainage into the outlet. May 2026 dredge: We removed roughly 12 to 18 inches of accumulated sediment and organic material and restored the basin to its 841.0 design elevation. Standing water persisted in the center afterward. June 2026 follow-up dig: A month later, with the basin still holding water, our excavator and the City's civil engineer dug down an additional 4 feet into the center of the basin at several locations. They still encountered organic material, silt, and very fine sand throughout, confirming there is no infiltratable soil even several feet below the post-dredge grade. Grading limitation: The center of the basin is unsuitable muck to significant depth, confirmed by both AET's 2019 borings and the June 2026 follow-up dig. It cannot be graded or compacted to create positive slope. It must be removed and replaced with clean granular material wherever the design requires stable grade or bedding. Vegetation and sediment history: For roughly 23 years no vegetation was removed from the basin; cattails cut in 2017 were left to decompose in place, adding organic material to the basin floor annually. Neither inlet has sediment pretreatment, so street sediment has washed in unchecked the entire time. Scope Design a drain tile or underdrain system that collects water from the basin floor, bypasses the existing outlet structure and its catch basin entirely, and runs by gravity to a downstream storm sewer manhole. The existing outlet stays in place, functioning only as high water overflow, no longer the primary drain. Two options to evaluate: Option 1 (preferred): Excavate only deep enough to bury the pipe. Pipe must still be bedded and surrounded in clean drain rock or aggregate with filter fabric or sock, backfilled with clean sand to grade. The existing muck cannot be reused as backfill or bedding. Our concern is whether this leaves residual standing water above the pipe. We need confirmation this achieves drawdown in 5 days or less. If not, tell us. Option 2: Remove approximately 2 additional feet of material beyond Option 1 for faster drawdown, replaced with the same clean rock and sand system, if needed to hit the 5-day target. We are leaning toward Option 1 on cost, but want the engineering to confirm it will actually dry out in time. Requirements Plans must be biddable and buildable by an excavation contractor. Pipe sizing, slope, invert elevations, aggregate envelope, filter sock or fabric, and cleanout locations. Clogging is the failure mode we most need designed against. Low maintenance is a hard requirement. Our board turns over regularly and has no ongoing technical continuity. Design should be consistent with MPCA Minnesota Stormwater Manual and Brooklyn Park City Code Chapter 153 standards. Engineer of record must be a Professional Engineer licensed in the State of Minnesota. We will provide the original 2001 stormwater management plans and calculations, MFRA record drawings, the 2019 AET site visit report, and City and watershed correspondence. In your bid, please include: relevant underdrain design experience, your Minnesota PE license number, your fixed fee and timeline, and whether your fee assumes we provide an existing conditions survey.