Inputs
Example Data Table
| Scenario | Area (ac) | Impervious (%) | P (in) | BMP Train | Targets (TSS/TP/TN) |
|---|---|---|---|---|---|
| Small grading pad | 0.80 | 45 | 1.00 | Swale → Bioretention | 80 / 50 / 30 |
| Parking expansion | 2.50 | 85 | 1.20 | Permeable Pavement | 80 / 50 / 30 |
| Warehouse site | 6.00 | 70 | 0.90 | Separator → Wet Pond | 70 / 40 / 25 |
Formulas Used
- Runoff coefficient (screening): C = 0.05 + 0.90 × ImperviousFraction (bounded 0.05–0.99).
- Manual coefficient method: RunoffDepth = C × P.
- NRCS curve number runoff (event depth): S = (1000/CN) − 10, Ia = 0.2S, Q = (P − Ia)² / (P − Ia + S) for P > Ia.
- Runoff volume: V(ft³) = RunoffDepth(in) × Area(ac) × 3630.
- Water quality volume requirement: VWQ(ft³) = WQDepth(in) × Area(ac) × 3630.
- Peak flow (screening): Q(cfs) ≈ 1.008 × C × i(in/hr) × Area(ac).
- Pollutant load (kg) using EMC: Load = EMC(mg/L) × V(L) / 1,000,000, and V(L) = V(ft³) × 28.3168466.
- BMP train combined removal: ηcombined = 1 − Π(1 − ηstage).
How to Use This Calculator
- Enter project name, drainage area, and impervious percentage.
- Select a runoff method. Use curve number when you have soils data.
- Set design storm depth and the required water quality depth.
- Choose a BMP train. Add optional stages for treatment in series.
- Enter BMP storage, infiltration rate, and drawdown time if known.
- Adjust EMC values and reduction targets to match local criteria.
- Click Calculate Compliance to see results above the form.
- Use the download buttons to attach CSV or PDF to submittals.
Notes for Practitioners
This tool is designed for planning-level and permit support checks. BMP performance varies with design details, maintenance, media, and site constraints. Use jurisdiction-approved crediting where required.
The treated volume estimate credits storage plus a conservative drawdown infiltration assist. If you have a known BMP footprint and infiltration area, adjust storage and infiltration inputs to reflect that design basis.
Professional Guide to Stormwater BMP Compliance
1) Compliance checkpoints used in this tool
Stormwater reviews usually pair a capture volume requirement with water-quality targets. This calculator checks treated volume against the required water-quality volume and evaluates percent reductions for TSS, total phosphorus, and total nitrogen from the selected BMP train. An overall status is shown when every checkpoint passes. Where required, align stage efficiencies with jurisdiction-approved crediting tables and documented O&M expectations.
2) Selecting a runoff method
Use impervious-based screening early in design when detail is limited. Choose a manual runoff coefficient when local coefficients are specified. Select the curve number method when soils and cover are known, because it estimates event runoff depth from the design storm.
3) Water quality volume sizing
Many programs define a water-quality depth, often 0.75–1.25 inches. The calculator converts depth to volume using 1 inch over 1 acre = 3,630 ft³. Meeting or exceeding the required WQ volume satisfies the volume checkpoint for review.
4) Peak flow screening for construction changes
Peak discharge is estimated with a Rational-style screening equation: Q ≈ 1.008 × C × i × A, using intensity i (in/hr) and area A (ac). It helps compare alternatives and flag where temporary conveyance, inlet protection, or extra storage may be needed.
5) EMCs and load calculations
Event mean concentrations (EMCs) represent average untreated runoff quality for a storm. Loads are computed from EMC and runoff volume after converting 1 ft³ to about 28.3168 liters. On active construction sites, TSS EMC can increase sharply with soil exposure, so refresh inputs as stabilization and housekeeping improve.
6) BMP trains and compounded removals
Series treatment is not additive. Combined removal is calculated as 1 − Π(1 − ηstage), capturing diminishing returns after each stage. Use pretreatment when sediment is high, then provide storage or filtration to protect downstream practices.
7) Storage, infiltration, and drawdown credit
The tool credits storage plus a conservative infiltration assist based on infiltration rate and drawdown time, with bounded credit. Reviewers commonly expect drawdown within 24–48 hours. Low infiltration rates often mean you must rely more on storage and pretreatment.
8) Interpreting pass/fail and documenting results
A PASS means the configured assumptions meet the selected targets for the modeled storm. A FAIL suggests increasing storage, selecting higher-performing stages, adding pretreatment, or revising targets to match the permit basis. Use exported reports to compare options during design changes and value engineering.
FAQs
1) What is a BMP train?
A BMP train is a sequence of practices that treat runoff in stages, such as pretreatment, storage, and polishing. The calculator combines stage efficiencies using a compounded removal method rather than simple addition.
2) Does this replace a jurisdictional model or manual?
No. It is a planning and documentation aid. If your authority requires specific routing, hydrologic methods, or approved credit tables, use those requirements as the final basis and match the calculator inputs accordingly.
3) How do I choose EMC values?
Start with land-use guidance commonly used by your program, then adjust for construction conditions. Exposed soils and poor housekeeping can increase TSS substantially, while stabilized areas and effective track-out controls generally reduce EMCs.
4) Why can volume fail while pollutants pass?
Pollutant targets are percentage-based, but volume criteria are absolute. A BMP may remove a high fraction of pollutants in the treated portion yet still lack enough storage to capture the required water-quality volume.
5) What if I only have a single BMP stage?
Select one stage and leave the others as None. The calculator will evaluate performance based on that practice. Adding a pretreatment stage can improve reliability, especially where sediment loads fluctuate during grading.
6) Which inputs most affect compliance?
Drainage area, storm depth, and imperviousness drive runoff volume, while BMP storage and selected efficiencies drive treatment. EMCs and target reductions strongly influence pollutant pass/fail, so keep those aligned with your permit assumptions.
7) What should I include with a submittal?
Include the design storm basis, drainage area map, BMP sizing calculations, drawdown assumptions, and the selected performance targets. Attach the exported report and clearly note any conservatism or adjustments made to match local criteria.