Contents preface (VII) introduction 1—37

12.5.1. Levees
A levee (also known as an embankment, bund, marginal bunds or dike) is an embankment running parallel (or nearly so) to the river and is constructed to protect the area on one side of it from flooding. The method of constructing levees on one or both sides of a river to contain the flood within the leveed portion is the oldest and most commonly used method of flood control. Levees along the Nile river in Egypt were constructed prior to 600 BC. Levees have been constructed recently on many important rivers of the world such as the Ganga, the Kosi, the Mahanadi and the Gandak in India, the Yellow, the Pearl, the Yangtze and the Huai in China, the Mississippi in the USA, and the Danube and the Rhine in Europe.
The alignment of levees for a river is decided by the location of important cities, industries, and other areas along the river which need to be protected against floods. Closely-spaced levees will be very high and, hence, massive and uneconomic. Hence, levee spacing is also governed by economic considerations. Levees should be located farther apart considering: (i) the desirability of having high discharge capacity of river for a given stage, and (ii) the requirement that the entire meander belt be within the levees so that they are not strongly attacked by the river. The levees should, obviously, have the general curvature of the river so that the river does not attack the levees.

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The design of a levee is similar to that of an earth dam. It should, however, be noted that while the upstream face of an earth dam is exposed to water most of the time, that of a levee is exposed to water for a very short period during the flood season only. The top width of a levee is generally kept between 3 to 8 m or more depending upon the levee height. The levee height is decided such that the levee is able to contain a flood of a reasonable return period of, say about 500 years (5). The flood stage at any section of a river corresponding to such a flood can be obtained by routing the flood through the river. A freeboard of 1-2 m is added to the flood stage to obtain the elevation of the top of the levee. The probable settlement of levee after its construction should also be accounted for while determining the levee height. The side slopes of levees vary from 1V : 2H to 1V : 6H. In case of high levees, berms are also provided on the land-side slope.

One of the major effects on regime of river due to levee construction is the reduction in the river width and, hence, increase in velocity of flow. As such, the sediment, which would have deposited on the river bed/flood plains in the absence of levees, is now carried downstream and deposited either in an unleveed portion or in the sea. Other effects of confining the flood within levees are as follows (12):
(i) Increase in the rate of travel of flood wave in the downstream direction, (ii) Rise in the water surface elevation in the river during flood,

(iii) Reduction of storage and, hence, an increase in the maximum discharge downstream, and

(iv) Decrease in the water surface slope of the stream above the leveed portion as a result of which aggradation occurs upstream of the leveed reach.
Failure of levees can be due to one or more of the following causes (12): (i) Overtopping,

(ii) Erosion of riverside slope by river current, (iii) Caving in of the banks,

(iv) Infiltration through the foundation, (v) Infiltration through the embankment,

(vi) Leaks as a result of holes dug by rats, crabs, and white ants, or from rotten roots and cracks due to shrinkage of soil,

(vii) Loosening of the embankment by wind action on large trees planted on it, and (viii) Human action.

As such, levees are very susceptible to failure during floods. Hence, continuous supervision, particularly during floods, and availability of enough labour and material on the spot are necessary to detect and plug breaches in the levees, if they occur.

The method of flood control by levees is fairly simple and economical as it uses locally available material and labour for its construction. Besides, levees can also be extended gradually to cover more and more area.