Lay out a ditch system from the outlet upward: fix the height of the discharge point first, then work uphill adding 5 cm of height for every 10 m of ditch, which is the 0.5 percent minimum gradient, and peg the route with a level. Put the interceptor ditch uphill of the buildings, run collection ditches through the settlement into it, and join branches to the main ditch at an angle that points downstream so water does not slow at the junction.
Part of Flood Control and Drainage
Ditch layout is the plan view of a drainage system: where each ditch runs, how it joins the others, and the height of every point along it.
Work Backwards From The Outlet
A ditch that starts at the top and hopes to reach the outlet often arrives above it, below it, or not at all. Design from the other end. The outlet’s height fixes the lowest point of the whole system, and every ditch above it rises with the distance from that point.
What You Need
- The flow map — see Water Flow Mapping.
- A water level or A-frame — see Levels and Plumbs for how to read heights.
- A measuring cord and pegs.
- A chosen outlet — see Safe Outlets.
Method: Peg The Layout
Step 1: Fix the outlet height. Peg the discharge point and call its ground height zero.
Step 2: Peg the main outfall. Walk uphill along the proposed route, setting a peg every 10 m. At each peg, the ditch bed must be higher than the previous peg by at least 5 cm, which is 0.5 percent. On a slope steeper than the ditch can safely follow, angle the route across the slope instead of straight down it so the ditch stays near the intended gradient.
Step 3: Place the interceptor. Run it along the uphill side of the settlement, perpendicular to the flow lines you mapped, with its own fall toward the outfall. It must be far enough above the buildings that its bank does not undermine them.
Step 4: Add collection ditches. Run these between buildings and join them into the interceptor or outfall. Each must fall toward its junction.
Step 5: Check junctions. Join a branch at an acute angle pointing downstream. A branch entering square to the flow checks the speed of the main ditch and drops silt.
Step 6: Mark crossings. Wherever a ditch meets a track, mark a culvert. See Culverts.
Worked Example: How Much Fall A Route Needs
An outfall of 120 m must reach the stream at the outlet.
- Minimum fall at 0.5 percent: 120 m x 0.005 = 0.60 m.
- At 2 percent, the steepest slope for an unlined ditch in this wiki: 120 m x 0.02 = 2.4 m.
So the ground at the top of the outfall must be at least 0.60 m above the outlet. If the ground is only 0.30 m higher, the ditch will fall at 0.30 / 120 = 0.25 percent, which is under the minimum, and it must either be shortened, deepened toward the outlet, or replaced with an infiltration outlet. If the ground is 3 m higher, the 2.4 m limit is passed, and the route must be lengthened by angling across the slope or lined with stone.
Common Mistakes
- Designing from the top — the route arrives at the wrong height. Work from the outlet.
- Running a ditch straight down a steep slope — it scours to a gully. Angle across the slope.
- Square junctions — silt drops and the junction blocks.
- Dead ends — a collection ditch that ends against a building floods that building.
- Putting the interceptor too close to buildings — a bank beside a wall seeps into it.
Safety
Do not peg a layout from inside running water
Layout is best done on dry ground, but the flow lines are easiest to see in rain. About 15 cm of fast-flowing water can knock an adult off their feet, and an undercut bank can give way under your weight. Mark the route from the bank, work in pairs, and set pegs after the flow has dropped.
What’s Next
- Learn next: Ditch Construction — dig the layout to size
- Combine with: Surveying — more careful ways to carry a height across a site