Andhra Pradesh PSC Channel Modification modification, engineering approaches and reducing vulnerability are important topics for

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Channel Modifications

Humans decide that stream should now along a specified path for such reasons as flood control. enhancement of drainage. control of erosion. increasing access to the floodplain for development, or improvement of the appearance of the channel. Such channel modifications involve measures such as the straightening the channel, deepening or widening the channel, clearing vegetation from the banks, or lining the channel with concrete. These modifications are referred to as channelization.

  • Channelization can also interfere with the natural habitat of the stream system and decrease the 8csthetic value of the stream.

  • Channelization or any other modification of a stream system changes the validity of all historic data collected over the years on that stream. Effects of Development on Flood Hazard: Whenever humans modify the landscape in any way, changes are to be expected in the way water drains from the land. Unless careful consideration is given to the possible drainage consequences, such landscape modifications can result in higher incidence of flooding. Development on floodplains should therefore be undertaken only with great care. Existing developments that have enhanced flooding problems are often costly to fix. Among the factors that enhance the flood potential are:

  • Channelization - Channelization is undertaken to reduce flood hazards. But, channelization is also undertaken to allow development on the floodplain. lf the channelization results in decreasing the cross-sectional area of the stream. as in the example above, then the same discharge that may not have produced flooding prior to channelization, may over now the banks and cause extensive flooding and channelization.

  • Subsidence - Subsidence often results in developed areas due to compaction of the sediment. both due to the increasing weight of structures and hydrocompaction associated with the lowering of the water table. Any time the elevation of in area is lowered. It becomes subject to collection of more water, and in severe cases, could drastically change the drainage pattern.

  • Storm Sewers-In order to collect run off from streets, parking lots and buildings all of which block the infiltration of water into the soi1, storm sewers are installed to provide underground drainage of the surface. While this may prevent local flooding of streets, it moves water more rapidly to the major stream systems and thus decreases the lag time and increases the peak discharge of the streams collecting the run off from the storm sewers.

  • Reduction of infiltration - Any time the surface materials of the Earth are covered with impermeable materials like concrete, asphalt, or buildings. The infiltration of water into the soil is prevented. Urbanization tends to reduce infiltration and thus water must collect in storm sewers and eventually in the main drainage systems. Thus extensive urbanization also decreases the lag time and increases the peak discharge even further. Urbanization can therefore lead to a higher incidence or flash floods. Organized Response to Flood Hazards: Response to flood hazards can be attempted in two main ways: An engineering approach. In control flooding, and a regulatory approach designed to decrease vulnerability to flooding.

Engineering Approaches

  • Channel modifications - As discussed above, channelization can enlarge cross-sectional area and thus create a situation where a higher stage is necessary before flooding. In other words by enlarging the cross-sectional area, higher discharge can be held within the channel. Channelization also increases water velocity, and thus reduces drainage time.

  • Dams: Dams can be used to bold water back 80 that discharge downstream can be regulated at a desired rate. Human constructed dams have spillways that can be opened to reduce the level of water in the reservoir behind the dam. Thus, the water level can be lowered prior to a heavy rain, and more water can be trapped in the reservoir and released later at a controlled discharge.

  • Retention ponds - Retention ponds serve a similar purpose to dams. Water can be trapped in a retention pond and then released at controlled discharge to prevent flooding downstream.

  • Levees: These are structures built along side the channel to increase the stage at which the stream floods. Some controversy has developed concerning the use of such structures.

  • Floodways - Floodways are areas that can be built to provide an outlet to a stream and allow its flood into an area that has been designated as a floodway. Floodways are areas where no construction is allowed, and where the land is used for agricultural or recreational purposes when there is no threat of a flood, but which provide an outlet for flood water during periods of high discharge.

Reduction Of Vulnerability

With a better understanding of the behavior of streams, the probability of flooding, and areas likely to be flooded during high discharge, humans can undertake measures to reduce vulnerability to flooding. Among the nonstructural measures are:

  • Floodplain zoning

  • Floodplain building codes

  • Floodplain buyout programs

  • Mortgage limitations

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