Design criteria of surface drainage

The design of surface drains is based on the following criteria.

1. Design rainfall: the maximum 3-day rainfall of 5–10 years return period (taken as higher for main drains and outfalls, e.g. 10–25 years), based on frequency analysis and growth factors.
2. Crop tolerance: the permissible depth and duration of flooding of the crop. For rice: a maximum of 300–400 mm for 1 day and 200–250 mm for 2–3 days; the initial depth in the field is 40–50 mm.
3. Drainage rate (coefficient): found in mm/day or l/s/ha such that the depth–duration limits are not exceeded; ET and percolation are generally neglected.
4. Design discharge: $Q = D \times A$ for the area served.
5. Hydraulic design: Manning's formula $Q=\frac{1}{n}AR^{2/3}S^{1/2}$ with $n$ = 0.025–0.035 for earthen drains.
6. Velocity limits: non-silting (about 0.3 m/s) and non-scouring (0.6–0.9 m/s depending on the soil).
7. Section: trapezoidal; side slopes 1:1 to 2:1 as per the soil; economical bed width/depth ratio; free board 0.15–0.3 m.
8. Bed slope: following the natural ground slope (generally 1 in 500 to 1 in 2000 in the Terai).
9. Water level: the full-supply level of the drain is at least 0.3 m below the field level (or the outlets) so that the field drains freely.
10. Outlet: the lowest point of the system must be higher than the high flood level of the receiving river, or a flap gate is provided to prevent backflow.
11. Provision of structures (culverts, drops, check structures) and access for maintenance.