Methods of river training with sketches

River training works are structures built on or along a river to guide and confine the flow, control its erosion and deposition, and keep it in a stable course so that bridges, headworks, towns and farmland are protected. Rivers of the Nepal Terai (Koshi, Narayani, Bagmati, Karnali) are steep, braided and carry heavy bed load from the Churia hills, so they need training works.

Objectives

• Prevent floods and spilling over the banks.
• Protect banks from erosion and the river from shifting its course (flanking a bridge or barrage).
• Give a steady, axial approach flow to a structure.
• Control silt entry and maintain a navigable or regular channel.

Methods normally adopted in the Terai

1. Guide bunds (guide banks) are embankments built on both sides of the river upstream (and a short length downstream) of a bridge or barrage, to guide flow centrally through the opening.

 flow --->      ____ guide bund (curved head)
 ______________/    \______
                   waterway W      = bridge/barrage
 ______________    ______/
               \____/

2. Spurs (groynes) project from the bank into the river to push the current away from the bank, create slack water and cause silt to settle between them. They protect concave banks.

  bank ======+==+==+==+====
             |  |  |  |      spurs (L)
             v  v  v  v      current deflected away

3. Marginal (flood) embankments are earth levees along the river, set back from the bank, which confine the flood within the river corridor.

4. Bank protection (revetment / pitching) covers the bank with stone pitching, concrete blocks or gabion mattresses over a filter, with a launching apron at the toe.

5. Cut-offs are artificial channels across the neck of a meander that straighten the river and lower flood levels.

6. Check dams / bed sills and gabion crate walls control bed degradation in steep tributaries.

7. Bio-engineering (vegetative spurs, bamboo piles, planting of grass and trees) is used as a cheap supplement along banks.

8. Pitched islands and training walls are used where the river must be held in one channel.

Selection depends on river slope, bed material, flood discharge and cost; in practice a combination (embankment, spurs and gabion revetment) is used.