A drainage ditch is an open channel that carries surface water away from buildings, gardens and tracks, and it needs only labour and shovels. Dig an interceptor ditch along the uphill side of the settlement, collection ditches between the buildings and an outfall ditch to a safe discharge point, and slope every ditch continuously toward its outlet at no less than 0.5 percent, which is a 5 cm drop in every 10 m.
Part of Flood Control and Drainage
A drainage ditch is a shallow open trench, graded so water flows along it to an outlet. It is the backbone of any settlement drainage system.
Three Kinds Of Ditch Do Three Jobs
An interceptor ditch runs along the uphill side of the settlement and catches water before it reaches the buildings. A collection ditch runs through the settlement to gather water from between the buildings. An outfall ditch carries the combined water away to a safe discharge point, which may be a stream, a river or an infiltration area.
A drainage system that has all three works as a whole. An interceptor with no outfall floods, and collection ditches with no interceptor above them are overwhelmed by the water arriving from the hill.
What You Need
- A map of where water flows — see Water Flow Mapping.
- A level and pegs — to set the grade. A ditch dug by eye often runs backward over a rise.
- Shovels and a mattock — or any digging tool.
- Flat stones — to line the bed where the soil is loose or the slope is steep.
- An outlet — see Safe Outlets.
Method: The Short Version
Step 1: Choose the route. Follow the map. Place the interceptor uphill of the buildings and join the collection ditches to it, then take the outfall to the outlet. Ditch Layout covers the routing.
Step 2: Set the grade. Every ditch must slope continuously toward its outlet. This page uses a minimum of 0.5 percent, a 5 cm drop in every 10 m, for settlement ditches, and up to 2 percent for ditches left unlined. Other pages in this wiki quote different floors for road ditches, so check which kind you are digging. For a gentle first ditch, 0.5 percent is the figure to start with.
Step 3: Dig to size. For a settlement of 20-30 people on 1-2 hectares, the interceptor is 50 cm wide and 40 cm deep, with a trapezoidal section. Collection ditches are 30 cm wide and 30 cm deep. The outfall is the same as the interceptor or larger. Ditch Construction explains the sizing and the arithmetic.
Step 4: Line where needed. In sandy or loose soil, line the bottom with flat stones. In stable clay, unlined is fine. Where the slope exceeds 2 percent, line with stone or concrete so the ditch does not cut into a gully.
Step 5: Check it carries water. Pour a bucket in at the top. The water should run to the outlet without stopping. A pool means a low spot to fix.
Common Mistakes
- No outlet — a ditch with nowhere to go floods the place you wanted to dry.
- Flat or backward sections — water stalls and silts the ditch. Use a level, not an eye.
- Too steep and unlined — fast water cuts the ditch deeper and washes the banks away.
- Discharging onto a neighbour — this moves the flood to someone else and causes conflict. Route to a natural waterway or an infiltration area.
- Not keeping the ditch clear — a ditch full of leaves and silt is a dam. Clear it before the rainy season.
Safety
Water in a ditch can move fast and deep
A ditch running full in a storm carries enough water to knock a person off their feet, since about 15 cm of fast-flowing water can do that, and a ditch leading into a culvert pulls anything in toward the opening. Stay out of ditches while they are running. Clear them before or after storms, not during.
What’s Next
- Learn next: Ditch Layout — route the three kinds of ditch
- Learn next: Culverts — carry a ditch under a track
- Combine with: Drainage and Ditching — the road-focused treatment of the same subject