Water logging remedies and surface drainage in Terai

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, 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 (Terai has a good aquifer) and to give 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 the 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 drainage is constant.
5. Find the drainage rate $D$ (mm/day, then l/s/ha) satisfying 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 free board 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.