Last reviewed 1 Oct 2026 · 7 min read
Design of open drains
Basic hydraulic design
Open drains are designed as open-channel flow using Manning's formula:
| Symbol | Meaning |
|---|---|
| design discharge (m³/s) | |
| flow area (m²) | |
| velocity (m/s) | |
| Manning's roughness coefficient | |
| hydraulic radius (m) | |
| bed slope |
Roughness coefficients (indicative): well-maintained earth drains about 0.025–0.030; drains with some weeds 0.035–0.040; weedy or neglected 0.045–0.060; concrete-lined 0.013–0.017. Use a higher (a conservative value) for design because weeds grow in drains.
Cross-section
A trapezoidal section is the usual shape:
where = bottom width, = depth of flow, = side slope (horizontal : 1 vertical).
- Side slopes: from about 1:1 in stable clay to 2:1–3:1 in sandy or loose soils, and flatter where farm machines cross the drain or where grass must be mown.
- Best hydraulic section (maximum discharge for a given area): a half-hexagon for a trapezoid, with ; in practice depth and width are fixed by ease of excavation and maintenance (a width to depth ratio of 2–4 for small drains).
- Freeboard: 0.15–0.30 m above the design water level.
- Spoil banks are kept back from the drain edge (the berm, 1–3 m) to prevent slips and to allow runoff to enter.
Velocity limits
| Condition | Velocity |
|---|---|
| Minimum (non-silting) velocity | about 0.3–0.6 m/s (higher where silt load is heavy) |
| Maximum (non-scouring) velocity | about 0.5–0.8 m/s in sandy loam, 0.7–1.0 m/s in loam and clay loam, 1.0–1.5 m/s in stiff clay (more with grass lining) |
If the natural grade gives a velocity above the limit, drop structures are used to break the grade; if below, the slope or cross-section is adjusted.
Longitudinal profile
- The drain's water level must be below the field's drain outlets (pipe inverts) by at least 0.15–0.30 m at the design flow.
- Bed gradient follows the ground slope as far as possible, with a minimum about 0.02–0.05 % for flat lands; very flat land needs pumping.
- Junctions are made with the tributary entering at an angle (about 30–45° downstream) with bed levels matched to avoid back-up and erosion.
Design discharge m³/s; bed slope ; side slope ; ; flow depth m.
Try m: m²; m; m.
m/s; m³/s — slightly below 0.60, so increase to about 1.4 m ( m², m, m, m/s, m³/s ✓.) The velocity (0.37 m/s) is above the minimum non-silting limit.