Methods of flood control

Flood control aims to reduce flood damage by storing, diverting, confining or reducing the flood flow. The structural methods are as follows.

1. Storage reservoirs

A dam stores flood water and releases it gradually, cutting the peak flow downstream. It is the most effective method but also the most costly. In Nepal, large storage projects of this kind have been proposed on the Koshi and Karnali river systems.

2. Detention basins and retarding basins

Low-lying areas or small ponds hold flood water temporarily.

3. Embankments (levees or dykes)

Earth banks built along the river confine the flood and prevent spilling. The flood level rises between the embankments, so they need adequate free-board and must be raised periodically.

   ____ levee               levee ____
  /    \_______ river _______/    \
 /  flood plain protected          \

4. Channel improvement

Straightening, widening, deepening, desilting or removing obstacles increases the carrying capacity and lowers flood levels.

5. Cut-offs

A shortcut across a meander neck steepens the slope and lowers flood levels upstream.

6. Diversion and flood-ways

Excess flood water is diverted to a side channel, lake or another basin through a spillway or gated weir.

7. Bank protection and river training

Spurs, revetments and guide bunds protect the banks and hold the river in its course.

8. Watershed management

Afforestation, check dams and terracing in the catchment reduce run-off and sediment.

9. Flood proofing and warning

Raised platforms, flood warning systems and flood-plain zoning reduce damage to people and property (these are non-structural, but are used together with the above).