Bioretention on Clay with a High Water Table
An infill site of 2.1 acres must meet a local standard requiring retention of the first 1.0 in of rainfall over all new impervious area, 80 percent TSS removal, and a 50-ft undisturbed buffer along a mapped stream. The program is 1.6 acres of impervious cover, 0.3 acre of usable green space, and 0.2 acre inside the buffer. Borings show 18 in of sandy loam over stiff silty clay, and a falling-head test gives 0.08 in/hr. Seasonal high water is 30 in below grade in April and 60 in below grade in October. The site drains to the stream through an existing swale that also carries flow from 12 acres off site. The client wants bioretention because it is the cheapest item in the opinion of probable cost, and the permit submittal is due in three weeks.
1. Compute the required water quality volume and test whether surface bioretention can meet it on this site.
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Water quality volume: 1.0 in over 1.6 ac of impervious = 1.6 x 43,560 x (1/12) = 5,808 cf, or 0.133 ac-ft.
Now test bioretention. The measured rate of 0.08 in/hr is far below the 0.5 in/hr that most manuals set as the floor for infiltration credit, so the cell would act as a filter with an underdrain, not a volume-reduction practice; the full 5,808 cf must be stored, not infiltrated. Even at a 12-in ponding depth the cell needs about 5,800 sf of surface, which is 44 percent of the 0.3-ac green space. And the required section, 12 in of media plus 12 in of ponding plus freeboard, puts the media bottom below the April high water at 30 in, forcing a liner and underdrain and eliminating the last of any credit.
It does not work. The cost opinion is also wrong: a lined, underdrained cell with an inlet structure, an overflow, and maintenance access is not the cheapest item on the list. Reject bioretention and document why; the borings and the seasonal high water are the reason.
2. Given that bioretention fails, describe a defensible alternative and the geotechnical checks that govern it.
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Hold the volume in an underground vault or arch-chamber system beneath the parking, sized for 5,808 cf plus sediment storage, typically 10 percent, and build a treatment train: a forebay or hydrodynamic separator ahead of the vault to reach 80 percent TSS removal, then the vault.
Two geotechnical checks drive the design. The excavation bottom must sit above the April high water at 30 in, or the vault must be designed as a buoyant structure with a ballast slab, anchored footings, and a waterproof liner; if the bottom goes below 30 in, also check the October level and the uplift factor with the vault empty. Keep the vault outside the 50-ft buffer, and route the emergency overflow so a clogged vault does not flood the building.
Confirm that maintenance access, a 30-in manhole at each end and a vacuum truck, actually works within the parking layout. If cost is the driver, the impervious reduction used to fix the detention problem also cuts this retention volume by the same percentage.
3. The swale carries 12 acres of off-site flow. How does that affect the design?
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The swale carries 12 acres of off-site flow, and that water is not yours to detain or to block. Two consequences follow.
First, the retention standard applies only to the new impervious area, so do not size the vault for the off-site contribution, and do not route the off-site flow through the vault, or sediment will fill it and the vault will be blamed for downstream flooding.
Second, the plan must preserve a continuous, capacity-checked path for the 12-acre flow. Either the existing swale stays open and undisturbed, or it is replaced by a pipe or lined channel with equal or greater capacity and a recorded easement. Check that the vault, forebay, and parking grades do not raise the swale's hydraulic grade and that finished grade does not push water onto the neighbor. Because the swale is a tributary to the mapped stream, the 50-ft buffer probably applies to it, which may rule out a forebay in that zone. Show the off-site flow path on the grading plan; it is the item most often missed in permit review.