BMP Selector

Choosing a stormwater practice is a process of elimination, not inspiration. The selector above screens sixteen practices against the conditions of a specific site — drainage area, soils, head, water table, slope, space and land use — and returns the candidates that survive, each linked to its fact sheet and design calculator. The guide below explains what is being screened and why, so the output reads as an engineering shortlist rather than a black box.

What are stormwater BMPs?

A stormwater BMP — best management practice — is any technique used to reduce the quantity or improve the quality of stormwater runoff. The term covers two distinct families. Structural BMPs are built facilities that capture and treat runoff: ponds, wetlands, infiltration trenches, sand filters, bioretention cells, swales. Non-structural BMPs are practices and behaviors that keep pollutants out of runoff in the first place: street sweeping, pollution prevention at businesses, lawn-care and pet-waste programs, and land-use decisions that limit impervious cover. Many manuals now use “stormwater treatment practice” (STP) or “stormwater control measure” (SCM) for the structural family; this selector deals with the structural practices, since those are the ones a site plan must choose among.

The same abbreviation has a second life on construction sites, where “BMPs” means temporary erosion and sediment controls — silt fence, inlet protection, stabilized entrances. That distinction gets its own section below, because mixing up the two families is one of the most common sources of confusion in plan review.

Types of stormwater BMPs

Structural practices sort into five groups, each with a shared treatment mechanism and a shared set of siting constraints:

• Ponds — basins with a permanent pool (or micropool) that treat by settling and biological uptake: the wet pond and its extended-detention variants. Need large drainage areas; deliver low-maintenance treatment at low unit cost.
• Wetlands — shallow, planted systems such as the stormwater wetland; the highest habitat value, the largest land take.
• Infiltration — the infiltration trench and infiltration basin, which return runoff to groundwater. Demand permeable A/B soils, water-table separation and disciplined pretreatment.
• Filtering systems — the sand filter family and bioretention, which strain runoff through engineered media and collect it in an underdrain. Work on any soil, fit small urban catchments, accept hotspot runoff.
• Open channels — the dry swale, wet swale and grass channel, which treat along the conveyance path.

How the selector screens practices

The selector compresses the classic step-wise screening matrices from regional design manuals into one pass. The underlying logic runs through the same factors a designer would check by hand, in the same order:

The screening output is deliberately a shortlist, not a single answer: most sites have two or three defensible options, and the final call belongs to the designer and the reviewing authority.

BMP selection criteria explained

Five physical thresholds do most of the eliminating in Screen 2:

• Drainage area. Ponds and wetlands need roughly 10–25 acres or more of contributing drainage to sustain a permanent pool or marsh; infiltration and filtering practices cap out near 2–10 acres before flows overwhelm them. Drainage area alone usually cuts the list in half.
• Soils. Infiltration practices require hydrologic soil group A or B (measured rates above about 0.5 in/hr). Practices with underdrains — sand filters, bioretention, dry swales — work on any group. Wet swales actually want the C/D soils everything else avoids.
• Head. The elevation difference available to push water through the practice. Surface sand filters and bioretention typically need about 5 feet; perimeter filters get by on 2; swales and infiltration practices on 1 to 3.
• Water table. Practices that filter or infiltrate need separation — commonly 2 to 4 feet — between the bed bottom and the seasonally high water table; ponds and wetlands can tolerate, or even exploit, groundwater contact.
• Space. Surface footprint as a share of the site: underground filters consume effectively none, swales and bioretention claim linear and landscape space, ponds and especially wetlands take whole site corners.

Feasibility at a glance: sixteen practices

The table below summarizes the thresholds the selector applies — the same values, practice by practice, that drive its output:

Cost and maintenance comparison

Once feasibility narrows the field, lifecycle realities separate the finalists. The pattern across the groups is remarkably consistent:

Two rules of thumb fall out of the table. Filters buy small-footprint, any-soil flexibility with the highest unit costs and the most demanding maintenance. Ponds buy cheap, low-effort treatment with land and drainage-area requirements — for the design consequences, see the pond design calculator.

Verifying performance after selection

Feasibility says a practice fits; it says nothing about how well it treats. Monitored removal efficiency varies widely between groups — and between studies of the same group — which is why a pollutant-driven project (a TMDL watershed, a nutrient-limited lake) should rank its shortlist against observed data rather than design intent:

The full study-level evidence behind those medians — including the spread that the medians hide — is in the pollutant removal database. As a shortcut: dry swales and infiltration practices post the highest median removals, ponds and wetlands the most consistent ones, and grass channels the lowest of the structural set.

Selection by setting

Ultra-urban sites. When the entire site is building and pavement, the surviving practices are the ones with no footprint: underground and perimeter sand filters, bioretention in planter form, and infiltration trenches in utility strips. This is the one setting where the expensive filter family wins by default.

Residential developments. Safety and appearance carry formal weight — deep open water argues for wetlands or micropool designs over full wet ponds, and visible practices need a landscaping plan to earn community acceptance. Maintenance falls to an HOA or the municipality, which favors low-burden practices.

Cold climates. Chloride from road salt passes through nearly everything, submerged inlets can freeze, and infiltration is restricted in seasonally frozen ground; designs compensate with deeper sumps, on-line pools and conservative drawdown assumptions.

Sensitive receiving waters. The downstream resource modifies the choice: discharges over aquifers restrict untreated infiltration and demand pretreatment; urban lakes and reservoirs reward the phosphorus removers (large permanent pools); shellfish and swimming waters prioritize bacteria reduction through long detention times; cold-water trout streams penalize big sun-warmed pools and favor shaded, quick-draining practices.

Construction-phase vs post-construction BMPs

“BMP” on an erosion-control plan and “BMP” on a stormwater management plan are different animals. Construction-phase BMPs are temporary measures — silt fence, sediment traps and basins, inlet protection, stabilized construction entrances, seeding and matting — installed to keep soil on site while the ground is open, and removed when vegetation establishes. Post-construction BMPs are the permanent facilities this selector screens, designed to operate for decades after buildout. The two interact in one critical way: a permanent practice that receives construction-phase sediment can be ruined before the project closes out, so infiltration and filtering practices stay offline until their drainage areas are fully stabilized, and sediment basins are converted to permanent ponds only with deliberate cleanout and re-grading.

Common selection mistakes

• Choosing the practice before checking drainage area. The wet pond sketched on the concept plan dies in review when the site delivers eight acres, not twenty-five.
• Assuming soils instead of testing them. Infiltration practices sized on mapped soil groups, without borings and field infiltration tests at the practice location, are the classic failure — historic failure rates for infiltration basins trace mostly to this shortcut.
• Ignoring head until the grading plan. A surface sand filter needs about five feet of fall; flat coastal sites simply do not have it, and discovering that late forces a redesign around a perimeter filter or wet practice.
• One practice asked to do every job. Few practices deliver recharge, water quality, channel protection and flood control alone. Treatment trains — bioretention to an ED pond, a filter to an infiltration trench — are the norm, not the exception.
• Forgetting the maintenance owner. A practice with no named, funded, legally bound maintenance party fails on schedule regardless of design quality.
• Treating screening guidance as law, or law as guidance. Some thresholds are engineering judgment; some are regulatory mandates. Knowing which is which requires the local manual, not a generic matrix.

How this selector relates to local manuals

The thresholds in this tool are the generic values used in regional design manuals; nearly every jurisdiction adjusts some of them — drainage-area minimums, recharge requirements, cold-climate modifications, accepted practice lists. The selector is a screening instrument for early site planning: use it to build the shortlist and frame the pre-application meeting, then confirm every criterion against the reviewing authority’s published manual, since the local document governs whenever the two disagree. The local review process guide describes how that conversation typically runs, and the sizing criteria guide covers the volume computations every accepted practice must then satisfy — with the dry swale and pond calculators handling the design math for the two most common outcomes.

Frequently asked questions