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Map water flow, catchments and wet areas

Find where water comes from, where it goes and where it collects, so you protect building sites and septic areas and capture water where it is useful.

How

1. On the contour map, draw arrows perpendicular to contours to show surface flow paths, and outline each small catchment that drains across your land.
2. Walk the land during and just after heavy rain. Mark ponding, springs, seeps, gullies and where water enters from neighbouring land.
3. Mark any mapped flood zone and the edges of streams and drainage channels.
4. England example: the GOV.UK long term flood risk service maps rivers and the sea, surface water and reservoirs.
5. Estimate runoff volumes from each catchment with the runoff calculator in this module, using a runoff coefficient that suits the surface.
6. Flag low areas and drainage lines where runoff converges: avoid these for septic fields and buildings.
7. Flag high, dry points that suit tanks (gravity feed) and wells (upslope of contamination sources).

Safety: Do not enter fast-flowing water or flooded channels to make observations.

Do these first

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Sources

1. Site and Soil Evaluation for Onsite Wastewater Systems (MF2645). Kansas State University Research and Extension, accessed 2026-11
2. Flood Maps. FEMA, accessed 2026-11
3. Protecting Your Private Well. Mississippi State University Extension, accessed 2026-11
4. Design Manual 4A-5: Using the Rational Method to Determine Peak Flow. Iowa Department of Transportation, accessed 2026-11
5. Low-Volume Roads Engineering Best Management Practices, Chapter 5: Hydrologic Analysis. USDA Forest Service, accessed 2026-11
6. Check the long term flood risk for an area in England. GOV.UK (Environment Agency), accessed 2026-11

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