Map where water goes by walking the whole site after the next heavy rain and marking four things on a sketch: where water collects, where it flows, where it erodes, and where it enters and leaves the settlement. Add the ridge lines that bound the area draining onto the site, measure that area, and use the map to place interceptor ditches uphill of buildings and culverts where flow paths cross tracks. A catchment of 250 m by 200 m is 5 hectares, the figure the flow calculations need.
Part of Flood Control and Drainage
Water flow mapping records how rain moves across a site: its paths, pools, scour points and boundaries. It is the survey that every ditch, culvert and berm decision rests on.
Water Follows Paths You Can See Only While It Is Moving
A dry site hides its drainage. Two weeks of dry weather leave no trace of the path a hillside sheds water down, but the first hour of a downpour shows it plainly. The map you draw in that hour is worth more than any plan made from the doorstep.
Draw it in two passes. During or just after the storm, record what the water does. A day or two later, return and note what it left behind, such as scoured gullies, silt fans and debris lines.
What You Need
- A sketch plan — paper on a board, or a scratch plan on a flat stone, with north marked and the buildings drawn roughly to scale.
- A way to measure — a rope or cord of known length, knotted every metre or every 5 m.
- A level — a water level or an A-frame, to find which way and how steeply the ground falls. See Levels and Plumbs and Surveying.
- Marker stakes or cloth strips — to flag the places you find.
Method: Map The Water In Five Passes
The sketch records four things: where water collects, where it flows, where it erodes, and where it enters and leaves.
Step 1: Mark where water collects. Note puddles, standing water, wet ground and any spot where the grass stays green in a dry week. These are low spots or places where the soil cannot take water.
Step 2: Draw the flow lines. Mark an arrow for each path water follows. Distinguish two kinds. Sheet flow is a thin film spread across a slope. Concentrated flow runs in a channel, and it is the dangerous kind because it picks up speed and cuts into the ground.
Step 3: Mark where it erodes. Record gullies, undercut banks and exposed roots. Erosion shows where the water has been strong enough to move soil, and so where protection such as Riprap or Vegetation Cover is needed first.
Step 4: Mark where it enters and leaves. Find where water arrives from outside the settlement and where it exits. A ditch with no outlet floods, so the exit points decide where your outfall must go.
Step 5: Draw the catchment. Walk or sight the ridge lines above the site. The area between them that drains toward you is the catchment. Measure its length and width and multiply.
Worked Example: Catchment Area From A Sketch
A hillside above the settlement is bounded by two ridges. Pacing against a measured rope gives 250 m along the slope and 200 m across.
- Area: 250 m x 200 m = 50,000 m2.
- One hectare is 100 m x 100 m = 10,000 m2.
- Catchment: 50,000 / 10,000 = 5 hectares.
A gradient from the level is read the same way. A 20 m run that falls 0.5 m has a slope of 0.5 / 20 = 0.025, which is 2.5 percent. The catchment area feeds the flow calculation in Culvert Sizing, and the slope tells you whether a ditch will scour (see Ditch Construction).
Common Mistakes
- Mapping only once — a single dry-day walk misses the flow paths. Map in the rain.
- Looking only at your own site — water arrives from uphill. A map that stops at the boundary misses the cause.
- Ignoring small flows — a trickle that crosses the track every storm is where the culvert goes.
- Drawing flow lines straight — water follows the ground, not the plan.
- Leaving out the exits — without a known outfall, every ditch you plan is a dead end.
Safety
Do not wade or cross flowing water to map it
About 15 cm of fast-flowing water can knock an adult off their feet, and an undercut bank can give way under your weight. Map from dry ground and high ground, work in pairs, stay out of channels and culvert inlets, and never cross flowing water to reach a feature. Come back when the flow has dropped. Do not walk open ground during lightning.
What’s Next
- Learn next: Historical Flood Marks — what past floods have left on the landscape
- Learn next: Ditch Layout — turn the map into ditch routes
- Combine with: Soil Drainage Capacity — how much of the rain the soil takes in