Your flood risk depends on the whole upstream catchment, not on the rain at your site: a small stream can send far more water in a storm than it carries in a normal week. The rational method estimates the peak as Q = 0.278 x C x i x A, with flow in m3/s when rainfall intensity i is in mm/h and catchment area A is in km2, and the carrier rule is to build living structures at least 3 meters above the highest visible flood evidence and at least 50 meters from the main channel.
Reading a site’s position in its catchment, estimating peak storm runoff, and placing buildings above and away from flood paths.
Floods Come From Upstream, Often Under a Clear Sky
A watershed (catchment) is all the land that drains past a given point. Every drop that falls on it and does not soak in or evaporate runs through your site. So the question is not “how much rain falls here” but “how much land drains through here, how steep is it, and how fast does it shed water”. Steep, bare, small catchments produce short violent floods within minutes of heavy rain; large flat ones rise slowly and stay up for days. A storm 20 km upstream can send a flood past a site where it has not rained at all.
What You Need
- A map or sketch of the catchment — ridgelines make the boundary; see Contour Mapping.
- Local rainfall knowledge — the heaviest rain anyone remembers, in millimeters per hour, is your i.
- Eyes — the evidence the carrier lists: debris lines on tree trunks, flat silty areas near rivers, and terrace levels along a valley.
Method: Estimate the Peak and Read the Marks
Step 1: Outline the catchment Walk or map from your site up to the ridges on both sides. The ridgeline from one side of the site around to the other encloses the catchment. Estimate its area A in square kilometers (1 km2 = 1,000,000 m2, a square 1 km on a side).
Step 2: Choose a runoff fraction C C is the share of rain that runs off instead of soaking in: near 0 on sand and dense forest, higher on clay, bare rock and slopes. These are judgments, not tabulated facts; pick a range and compute both ends.
Step 3: Choose a storm intensity i Use the hardest rain you have experienced or been told of, in mm/h.
Step 4: Compute Q = 0.278 x C x i x A is the rational method, a standard first estimate for small catchments.
Step 5: Compare with the marks Debris lines, flat silty ground and terraces show where water has actually gone. Where marks and the computed flow disagree, trust the marks.
Worked Example
Assumed inputs, for illustration only: A = 5 km2, C = 0.4, i = 50 mm/h.
- Q = 0.278 x 0.4 x 50 x 5 = 27.8 m3/s.
- The normal-day stream in River & Stream Assessment carries 0.85 m3/s; 27.8 / 0.85 is about 33 times as much.
- So a stream whose low-flow measurement looks gentle can still carry roughly thirty times its normal volume in a storm. Gauging a stream in fair weather says nothing about its flood.
Where the Carrier Rule Puts You
The carrier says: build living structures at least 3 meters above the highest visible flood evidence, and at least 50 meters from the main water channel. Flat, silty valley floors are floodplains: fertile for farming, dangerous for buildings. Terraces along a valley mark flood levels from past centuries. Choose a convex slope or terrace above them, but not at the base of a cliff or steep slope, where rockfall is the carrier’s warning.
Common Mistakes
- Using the site’s own rain — the catchment, not the site, sets the flood.
- Trusting a calm year — a record of one dry decade says little about the largest event.
- Dams and debris — a log jam or landslide upstream can hold water back and release it at once.
Safety
Never camp or park in a dry channel or on the lowest ground
Flash floods arrive without warning on dry riverbeds and valley floors. If water is rising or the stream turns muddy or carries debris, move to high ground immediately instead of trying to cross.
What’s Next
- Learn next: Flood Planning — protecting structures and crossings
- Combine with: Slope & Drainage Assessment — keeping rain off the building itself