The Bioretention Cell That Never Drains
A 2,400-sf bioretention cell receives runoff from 0.9 ac of parking lot. It was sized for a 1-in. water quality volume of about 3,270 cf with a maximum ponding depth of 12 in. After value engineering, the underdrain was deleted. The planting soil is a specified sandy loam, but the underlying native soil tested at 0.08 in./hr on two field infiltrometer tests, and the seasonal high water table was found 18 in. below the basin bottom. The state stormwater manual requires the cell to draw down within 48 hours. Since April the cell has held standing water continuously, and residents have complained about mosquitoes. The owner's engineer proposes "adding a surface layer of sand."
1. Show with numbers that the cell cannot meet the 48-hour drawdown, and identify the controlling constraint.
Read the worked answer
Run the drawdown arithmetic.
- Standing water depth = 12 in.
- Native infiltration rate = 0.08 in./hr
- Time to drain by infiltration alone = 12 / 0.08 = 150 hours = 6.25 days
That is already 3.1 times the 48-hour requirement before any clogging, sediment loading, or seasonal effects - and infiltration BMPs are expected to lose capacity over time, not gain it.
Then add the water table. Seasonal high groundwater is 18 in. below the basin bottom. As water infiltrates, the mounded water table rises to meet the bottom; once it does, the hydraulic gradient approaches zero and infiltration essentially stops. The effective storage above a shallow water table is far less than the design volume, and in a wet season it can be near zero. That is why the standing water is continuous from April onward rather than intermittent.
So what is the controlling constraint? Not the surface area, not the planting soil, and not the 3,270 cf sizing - it is the vertical separation to groundwater plus a native rate roughly an order of magnitude below what an infiltration credit requires. No amount of surface sand fixes a limiting layer 3 ft below grade.
What "add sand" actually does: it slightly improves the media interface and hides the problem for one season. It does not change the receiving soil, and it adds cost with no credit.
2. Design the fix. What are the options, and how do you size the preferred one?
Read the worked answer
Option A - restore the underdrain (preferred).
- 4-in. perforated PVC in a 12-in. gravel trench beneath the cell, wrapped or with a graded filter at the soil interface, sloped to daylight or to a discharge point, with cleanouts at the head and at intervals of roughly 100 ft if no straight daylight is available.
- Internal water storage: raise the underdrain outlet with an upturned elbow or orifice set ~9 in. above the bottom so the bottom 9 in. stays saturated for denitrification and the top 3 in. drains.
- Capacity check: volume to be drained = 2,400 sf x 1 ft = 2,400 cf. Over 48 hours that is 50 cf/hr, about 0.014 cfs. A 4-in. underdrain even at a modest slope passes far more than that; the pipe is not the constraint. The constraint is the soil-to-gravel interface, which is why the graded filter or filter fabric specification matters more than the pipe diameter. Confirm tailwater at the outfall cannot back the system up.
Option B - reduce ponding depth and increase surface area. Dropping to 6 in. of ponding halves the drain time but also halves storage; the cell would then be undersized for the 1-in. water quality volume unless the footprint grows, which on this site means taking parking spaces.
Option C - amend the media with a higher-conductivity engineered soil. Helps the media, does not help the native soil or the water table.
Also required: verify the planting soil elevation relative to the SHGW (do not plant upland species into permanently saturated media), and get a permit modification if the approved plan shows infiltration credit that the site cannot deliver.
3. What does the value-engineering decision to delete the underdrain mean for the stormwater permit, and what do you tell the owner?
Read the worked answer
Start with the credit. Most manuals grant infiltration BMPs a volume-reduction credit that a filter-only BMP with an underdrain does not receive. If the approved plan claimed that credit, deleting the underdrain removed the basis for the credit while the plan still claimed it. That is a compliance problem, not just a performance problem: the site may need compensating storage, an alternative BMP, or a new submittal and review cycle.
Document the chain. The underdrain was deleted in value engineering; no design professional evaluated the consequence; the as-built no longer matches the approved plan. Say this plainly and in writing to the owner and to the reviewing agency if the approved drawings still show infiltration.
What the owner needs to hear:
- The cell fails its 48-hour requirement by a factor of about 3 on native soil alone, and the shallow water table makes it worse, so this is not a maintenance issue - it is a design omission.
- Restoring the underdrain is the least-cost path, but it changes the BMP classification and may require a permit modification and replacement of lost volume elsewhere on the site.
- Costs of the delay and the re-review belong to the decision to value-engineer a drainage element without an engineering review.
Prevention on the next project: list underdrains, separators, and any element carrying a stormwater credit as non-substitutable in the specifications, so a substitution request triggers a design review rather than a purchase order.