Home / The guide / Land and site

Plan drainage and stormwater

Design how rain will move around buildings, roads and fields without flooding, eroding or soaking foundations and septic areas. Roof water is also a resource for the water module.

How

1. Use the catchments from site-6. For each, estimate runoff volume and peak flow with the runoff calculator. The rational method is Q = C x i x A, and is meant for small catchments.
2. Pick runoff coefficients from a drainage manual. Iowa DOT values: roofs and pavement about 0.94 to 0.98, compacted gravel about 0.45 to 0.60, lawns about 0.22 to 0.45 depending on slope and storm size.
3. Divert upslope runoff around buildings and septic fields with shallow diversion ditches or berms.
4. Grade ground away from foundations and wellheads.
5. Route roof water to tanks (handoff to the water module) and send overflow to a safe, stable outlet.
6. Provide stable outlets everywhere water leaves a pipe, ditch or culvert, using rock or vegetation, so outlets do not erode.

Do these first

• site-6
• site-10

Sources

1. Protecting Your Private Well. Mississippi State University Extension, accessed 2026-11
2. Design Manual 4A-5: Using the Rational Method to Determine Peak Flow. Iowa Department of Transportation, accessed 2026-11
3. Low-Volume Roads Engineering Best Management Practices, Chapter 5: Hydrologic Analysis. USDA Forest Service, accessed 2026-11

Open in the interactive guide