Small-site stormwater calc sheet (free)
- Site and new impervious area
- Design storm and rainfall depth
- Soils and infiltration rate
- Runoff volume and peak flow
- Drywell, trench or pond size
- Overflow route
Homeowners, builders and designers on small projects
Where is the project?
Free runoff, detention and drywell sizing sheet for a small project, with Atlas 14 rainfall and mapped soils filled in. Your town's engineer reviews it.
- Free to read
- No sign-up until the draft
- Hands off to the right door
ePlan fills in
- Design rainfall
- Soil group
- Roof area
- Slope
How it works
Small-site stormwater calc sheet, from one answer
- Pin the site and enter the new roof and paving.
- ePlan fills Atlas 14 rainfall and mapped soils. A field infiltration test and your town's engineer confirm the size.
- Read the free sheet. Disturbing an acre or more? Start a SWPPP draft.
What you get
Small-site stormwater calc sheet and 1 more document
The check is free. The paperwork after it is drafted at the SWPPP door.
Construction SWPPP
- Site and receiving water
- Erosion and sediment controls
- Inspections
From public data
What ePlan fills in, and from where
- Design rainfall depthsNOAA Atlas 14 (Precipitation Frequency Data Server)
- Soil group and drainage classUSDA NRCS Web Soil Survey (SSURGO)
- SlopeUSGS 3D Elevation Program
- Existing roof areaMicrosoft US Building Footprints
1 acre
of disturbance is where towns under a small MS4 permit must control a project's runoff after construction
Source: 40 CFR 122.34(b)(5), eCFR
Ready for the paperwork?
Disturbing an acre or more? Answer four questions and get a site-specific SWPPP draft with BMPs, an inspection form and NOI notes. Free to read.
Questions
Stormwater Calculator: what people ask
Where does the design rainfall come from?
NOAA's Precipitation Frequency Data Server delivers NOAA Atlas 14 precipitation frequency estimates for any point you pick, by storm duration and return period.
When must a town control runoff after construction?
Towns under a small MS4 permit must have a program for new development and redevelopment that disturbs 1 acre or more, including smaller projects in a larger common plan, that discharges into the town's storm sewer.
Who maintains the drywell or pond?
Towns under a small MS4 permit must ensure adequate long-term operation and maintenance of these controls, so expect the town to ask who will maintain yours.
The rules this door follows
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Stormwater Calculator: check it free
Answer it and ePlan opens the SWPPP door with your answer filled in.