Updated for the 2024 manual

Fast reference for stormwater design

Daily workflow

Start with the areas. Size the BMP. Then write.

The same roof and hard-surface numbers drive the minimum requirements, the downspout trench, and the draft. You do not type the flow into a second calculator.

2024 SWMMWWMR1–MR9DispersionInfiltration

2,000

sf of new plus replaced hard surface, or 7,000 sf of disturbance, brings MR1–5.

2024 manual

5,000

sf of new plus replaced hard surface brings MR1–9. The same number of pollution-generating hard surface brings treatment.

2024 manual

10,000

sf of effective impervious area in the TDA is the usual flow-control trip, unless the receiving water is exempt.

2024 manual

Roof

Downspout dispersion and infiltration

T5.10B is 10 feet of trench per 700 sf of roof, and never more than 50 feet on one trench. T5.10A lengths follow the soil row, 20 to 190 feet per 1,000 sf.

  • 25 ft of vegetated flow path past a trench.
  • 50 ft before a slope over 15%.
  • Silt and clay cannot take an infiltration trench.

Lot

Full dispersion

BMP T5.30 wants preserved native area at least 6.5 times the hard surface draining to it, and a 100-foot flow path. Short of that, only 10% of the lot can be fully dispersed.

  • Do not count a wetland as the dispersion area.
  • Roofs still need a T5.10B device into that area.
  • Model the rest of the hard surface.

Pipe

Conveyance, not a pond

A capacity letter is Manning and the 100-year, 15-minute flow. The pipe card is the full-flow capacity. WWHM still has to supply the flow.

  • Name the as-built you scaled.
  • Use the plat’s impervious cap if it has one.
  • One sentence the reviewer can stamp against.