Layout planning of surface drainage system

The layout of a surface drainage system is planned so that every field outlet discharges by gravity into a drain, and the water moves from small drains to larger ones until it reaches the natural outlet.

Components

1. Field drains (laterals): shallow drains at the lower side of the fields, which receive water from the bunded fields' outlets.
2. Collector drains: receive water from several field drains; run across or along the ground slope.
3. Main (outfall) drain: receives the flow of the collectors and delivers it to a natural drain/river.
4. Outlet: structure at the junction with the river with a flap gate or sill to prevent backflow.
5. Structures: culverts at crossings of roads/canals, drops where the slope is steep.

        irrigation canal ======================
        | field | field | field |  <- bunded fields
  ------+-------+-------+-------+----- field drains
        |       |       |       |
        v       v       v       v
  ===========================>  collector drain
                |
                v  main drain
              ~~~~~~~~~~~~~~~~~~~ natural river

Planning principles

• Drains follow the lowest lines of the land (natural depressions), and are kept parallel to the contours for the field drains and along the slope for collectors.
• The irrigation canal runs on the ridge, the drains in the valleys; drain and canal do not run side by side.
• The spacing of the field drains depends on the size of fields and the land slope (typically 100–300 m).
• Water level in each drain is at least 0.3 m below the outlets it receives.
• Minimum number of crossings and a short route to the outfall; the layout should avoid cutting through villages and roads.