POLICIES ON ENGINEERING SERVICES FOR FLOOD IMPACT REDUCTION

 

Depending upon the nature of works, flood impact reduction and flood management measures may be broadly classified as under: 

• Engineering / Structural Measures

• Non-Structural Measures

• Catchment Area Treatment

 

The engineering measures for flood control which bring relief to the flood prone areas by reducing flood flows and thereby the flood levels are: 

• An artificially created reservoir behind a dam across a river 

• A natural depression suitably improved and regulated, if necessary or by diversion of a part of the peak flow to another river or basin, where such diversion  would not cause appreciable damage.

• By constructing a parallel channel bypassing a particular town/reach of the river prone to flooding. 

 

The engineering methods of flood protection, which do not reduce the flood flow but reduce spilling, are:

• Embankments which artificially raise the effective river bank and thereby prevent spilling

• Channel and drainage improvement works, which artificially reduce the flood water level so as to keep water confined within the river banks and thus prevent spilling.

 

The non-structural methods endeavour to mitigate the flood damages by:  

• Facilitating timely evacuation of the people and shifting of their movable property to safer grounds by having advance warning of incoming flood i.e. flood forecasting, flood warning in case of threatened inundation

• Discouraging creation of valuable assets/settlement of the people in the areas subject to frequent flooding i.e. enforcing flood plain zoning regulation.

 

Various engineering services that may be employed for flood impact reduction are-

• Reservoirs: Reservoirs can moderate the intensity and timing of the incoming flood. They store the water during periods of high discharges in the river and release it after the critical high flow condition is over, so as to be ready to receive the next wave. As a policy, a minimum flood cushion of 10% of the storage capacity should be provided in all new dams and, if affordable with respect to other purposes, providing even a flood cushion up to 20% could be considered. A portion of the capital cost of the reservoir should be allocated to flood control and shared by all beneficiary States.

• Detention Basins: Detention basins are usually formed by utilizing natural depressions/ swamps and lakes by improving their capacity by constructing encircling embankments and providing suitable devices for regulating the release of stored waters. Since, the land under the marshes or low depression may hardly require much compensation and rehabilitation measures, this method is relatively less expensive.

• Embankments: Embankments (including ring bunds and town protection works) confine the flood flows and prevent spilling, thereby reducing the damage. These are generally cheap, quick and most popular methods of flood protection and have been constructed extensively in the past. These are reported to have given considerable protection at comparatively low costs, particularly in the lower reaches of large rivers. In many places, embankments may be the only feasible method of preventing inundation. Some embankments have provided positive benefits by ensuring sustained protection against floods and river spills while on the other hand, some embankments have, in certain reaches of the river, aggravated the flood problem by rising river bed levels, decreasing their carrying capacity, causing drainage congestion in the countryside and distorting the levels/gradient of the outfall points.

• Channelization of Rivers: Some of the States are proposing channelization of rivers, at least in certain reaches, in the context of tackling the extensive meandering problems of the rivers, activating navigational channels and training these rivers into their original courses. While venturing to channelize rivers, thought must be given in allowing the river certain freedom to flow and right of way to pass its flood waters and silt load within its natural waterway. The dynamic nature of the rivers should be appreciated and preventive measures planned accordingly instead of pinning down the river by channelizing.

• Channel Improvement: channel improvement is the method of improving the channel by improving the hydraulic conditions of the river channels by desilting, dredging, lining etc., to enable the river to carry its discharges at lower levels or within its banks. Use of this technique has been often advocated but adopted on a very limited extent because of its high cost and other problems.

• Drainage Improvement: Surface water drainage congestion due to inadequacy of natural or artificial drainage channels to carry the storm water discharge within a reasonable period causes damages. It is often difficult to distinguish between flood and drainage congestion situations. Therefore, improvement of drainage by construction of new channels or improvement in the discharge capacity of the existing drainage system is recommended as an integral part of flood management.

• Diversion of Flood Waters: Diversion of flood waters takes a part of the flood discharge to another basin or to the same basin downstream of the problem area or to a depression where it could be stored for subsequent release. This measure can be used to manage unusual floods around cities.

• Watershed Management: The watershed management measures include developing and conserving the vegetative and soil covers and also to undertake structural works like check dams, detention basins, diversion channels, etc. In the watershed management of upper catchment, land treatment through afforestation and grass land development practices should be supplemented by structural works for retarding the water velocity and arresting silt.