The floods cause overtopping of natural as well as artificial banks of the streams.
The flood is caused by run off from rainfall and/ or melting snow, too large to be confined in the banks of streams.
When banks are overtopped the flood water spreads over the adjoining area and causes heavy damage to crops and property.
It is not possible to prevent occurrence of floods but it is possible to prevent or reduce the damage due to floods by controlling the floods.
This flood control or flood management may be defined as the prevention or reduction of the flood damage.
In India about 40 million hectares of area has been identified as flood prone.
Out of this about 12 million hectares have been provided with flood protections measures by March 1982. Govt. of India set up National Flood Commission in July 1976.
The main function of this Commission is to study in depth the effectiveness of the works so far undertaken and recommend measures to be adopted.
Classification of Methods for Flood Content
The loom flood control is also sometimes referred as flood management.
According to National Flood Commission 1976 the broad classification of method of flood control are as follows.
1. Methods by which attempt is made to modify the flood.
2. Methods by which attempt is made to modify the susceptibility of flood damage.
3. Methods by attempt is made to modify the loss burden.
4. Bearing the loss.
The National Flood Commission has given a very broad Table which gives very elaborate classification of the method of flood control but of this table, the methods of flood control commonly adopt has been listed below.
Reducing the peak flood flow by constructing reservoirs.
Diverting the flood waters through flood ways or bypasses.
Confining the flood flow within predetermine channel by levees, flood walls.
Reducing of peak stage by a closed conduit improving outflow channels.
Design Flood and its Estimation
The design flood is the flood adopt for the design of a flood control project.
This flood may either be the maximum probable flood or the standard project flood or a flood correspond to some desire frequency of occurrence depending upon the degree of protection that should be provide by the flood control project.
The maximum probable flood is the flood which may be expect from the most severe combination.
It is estimate from the maximum probable storm.
Flood Control Reservoirs
The main purpose of flood control reservoir is to temporarily or permanently store a portion of the flood flow so as to lower the flood peak at the point to be protect.
This is achieved by discharging all reservoir inflow until the outflow reaches the safe capacity of the d/s channel.
All flow above this rate is stored until inflow drops below the safe d/c channel capacity.
The stored water thereafter is released to recover storage capacity for the next flood.
Since the reservoir is locate immediately U/S from the point to be protect, the hydrograph at that point is same as that released at the dam, and the peak has been reduce.
The most effective location of reservoir for flood control of area is immediately U/S from the area.
Such a reservoir would often be located in a broad flood plain and very large area of valuable land would be submerged in the reservoir.
Besides it will also need a very long dam.
On the other hand the sites farther U/S require smaller dams and less valuable land but are less effective in dropping flood peaks.
The loss is the effectiveness results from the influence of channel storage and also from the lack of control over the local inflow between the reservoir and the area to be protect.
Normally, at least one-third of the total discharge area should be under reservoir control for effective flood reduction.
Economic analysis generally favours the U/S site despite its lesser effectiveness.
Moreover, it is often prefer to establish several small reservoirs rather than having a single large reservoir.
The possible drop is peak flow by reservoir operation increases reservoir capacity increases,
since a greater portion of the flood water store.
Diverting the Flood Waters through Flood Ways
Flood ways are the large depressions into which a part of flood water is divert through natural or artificial channels.
The divert flood water remains temporarily store during the rising flood.
Once the flood recedes in the river, the water store in this depressions is allow to flow back to the river.
The flood ways thus serve two purposes firstly they create large, shallow reservoirs which store a part of the flood water and hence decrease the flow in the main channel below the diversion.
Secondly, they provide an additional outlet for water from U/S, thus increasing velocity and decreasing stage for some distance above the point of diversion.
Confining Flood Flow between Levees and Flood Walls
A levee is an earth dyke, while a flood wall is usually made of masonry or concrete.
Levees and flood walls are the oldest and most widely used methods of protecting land from flood water.
These are nothing but longitudinal dams aligned roughly parallel to the river.
They serve as artificial high banks of the river and thus prevent the river water from spilling over during floods.
The flood water remains confine between the levees or flood walls and is made to flow down the river without causing any damage to the adjoining area.
The Water conservation measures help to conserve or store water in the soil and thus reduce surface run-off and the flood flows.
Following conservation measures can be used.
1. use contour farming
2. create farm ponds to retain the flow of small creeks for irrigation
3. use of cover crops is field to avoid bare followed field during non-growing season.
However all these measure of water conservation are quite useful for reduction of soil erosion and preservation of soil-moisture, they are invariably use and any flood reduction taking place from these is an incidental benefit.
Flood proofing of specific properties.Isolated units of high value may sometimes have to be individually flood proof.
The property unit may be encircled by a high levee of as flood wall.
Flood plain management.
The flood plain of a river is formed by deposition of sediment.
Since the flood plains are nearly level they prove good sites for cities.
This results is the occupancy of the flood plains.
This however requires large annual expenditure for flood protection and such expenditure continues to increase unless a systematic approach to flood plain management is adopted.
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