Internal/external drainage; water logging; Terai surface drainage

Internal and external drainage

• Internal drainage is the system inside the command area or farm boundary: the field drains and the lateral and collector drains (open or tile) that collect excess water from the fields and carry it to the boundary of the project area.
• External drainage is the system outside the farm/project area: the main drains, outfall channels and improved natural streams that carry the collected water to the final outlet (river), together with the drains and flood protection works that deal with runoff entering from the catchment outside the project.

 field --> field drain --> collector --> main drain --> river
 |<--- internal drainage --->|<--- external drainage --->|

Causes of water logging

• Seepage from unlined canals and over-irrigation raise the water table.
• Flat land and poor natural drainage in the Terai, with blocked or obstructed drains (roads, embankments).
• Heavy monsoon rain with flooding from the rivers, on an impervious or clayey subsoil.
• Shallow impervious layer; absence of proper drains and poor maintenance.

Remedial measures

1. Prevention: lining of canals and water courses, controlled irrigation and good water management, and a cropping pattern suited to the water availability.
2. Surface drainage: improving natural drains and constructing artificial open drains.
3. Subsurface drainage: tile or pipe drains, mole drains.
4. Vertical drainage: tube wells to lower the water table (the Terai has a good aquifer) and to supply irrigation water.
5. Interceptor drains near canals, land levelling and the use of deep-rooted crops.

Method and assumptions for surface drainage design in the Terai

The method is based on the drainage rate for bunded paddy fields:

1. Take the design 3-day rainfall for a return period of 5–10 years (mean annual maximum × growth factor).
2. Assume a bunded field with an initial water depth $h_0$ = 40–50 mm and no inflow from outside.
3. Rice tolerance: a maximum depth (300–400 mm) for 1 day, and depth above a limit (200–250 mm) for 2–3 days.
4. ET and deep percolation are neglected (safe side); the rain falls uniformly over 3 days and the drainage rate is constant.
5. Find the drainage rate $D$ (mm/day, then l/s/ha) that satisfies the depth–duration limits, by trial.
6. Design discharge $Q = D \times A$; the drains are sized by Manning's formula with $n$ = 0.025–0.035, non-silting and non-scouring velocities, and a free board of 0.15–0.3 m.
7. The drains follow the natural slope and discharge to a natural drain or river at a level lower than the field outlets.