THE ECONMIC WEEKLY October 26, 1954

Preventing Floods by Channel Improvement Kumud Bhushan Ray

In rivers, a narrow and deep channel develops a greater velocity and has a greater discharging capa­city than a ivide and shallow channel of the same sectional area. A narrow and deep river is thus able to discharge adequately its primary junction of drainage from the whole of the catchment area and also its secondary function of transporting all its sediment to its outfall in the sea,

For flood control, it is, therefore, necessary to improve the channls of rivers by narrowing and deep­ening them. To ensure a cheap as well as a permanent method of channel improvement, the author has developed a form of bamboo bandalling, known as " permeable: screens " These are described in this article.

FROM the report of the Indian engineers who visited China

in 1954 appears that in the Yellow, Huai and Yangtze rivers, adjoining lands are subject to inun­dation by Hoods of even ordinary magnitude, that the old marginal embankments have been strengthen­ed, new ones constructed wherever necessary, and that these are being properly maintained.

A new departure has been made in the method of construction, in that instead of using modern earth moving machinery, " a total of .4,600,000 workers and peasants are said to have thrown themselves into the struggle,'' These peasants " worked in the dry season for bare sustenance-, and there was none of the indignity of coolie work about their effort ".

From ancient tunes in China, the Yellow river has been causing flood damage, and flood embankments had been constructed to confine its waters and prevent the submergence of river-side lands. These embank­ments were, however, found to create more problems than they solved. In the sixteenth century, the fomous engineer, C II Plan (1512 .1595)

proposed confining the flow of the lower course of the Yellow river within a narrow channel by two parallel ' louti ' or near dykes for downscouring the river bed."

Such narrowing and deepening would have increased the velocity and consequently the discharging capacity of the river. W i t h quicker drainage, flood heights would have been lower, and there would have been no overflow and no flood dam­age. In 1946, the Yellow River Commission prepared a comprehen­sive plan to regulate the lower course of the river, with a system of permeable and solid spur dykes and training walls to guide its channel to a width of 1,640 feet, " w i t h the ultimate aim of scouring the channel bed down from about 19 to 35 feet."

As strengthening of the old mar­ginal embankments and const ruction of new ones are being attempted at present, it is evident that the older Chinese ideas of flood prevention by channel improvement, by confining the Yellow river to a narrower chan­nel and thereby downscouring the river bed, have been abandoned.

OLDER IDEAS ABANDONED

After the floods in the Kosi in North Bihar and the Brahmaputra in Assam in 1954, it has been de­cided to construct flood embank­ments on both banks of these rivers on the lines of the embank incuts that are being strengthened and the new ones that are being built along both banks of China's rivers. Attempts arc also being made in India to follow the Chinese method of construction by trying to " muster 230,000 able-bodied persons tor a period of four months each veai' ", in order that the kosi em­bankments may be completed " w i t h i n a period of two years, in stead of the scheduled period of six "

From 1777 to 1927, flood embank­ments have been maintained along the Mississippi river to confine flood waters and thus prevent the sub mergence of riverside lands and flood damage. Owing to a gradual increase in flood heights, these em bankments had to be raised by as much as 17 feet. But in spite of this, there have been several breaches in the embankments. In the 1927 floods, there were more than 20c breaches, and about 20,000 square miles of land were submerged with enormous flood damage.

Hence from 1928, instead of plac­ing reliance on flood embankment alone, it was proposed to construct four overbank embanked floodwayj to carry the surplus water that could not be drained through the embank ed Mississippi. Out of these four only three were constructed. But up to 1950, one of these floodway? came into use only once in the

1937 floods, the second only twice m the 1937 and 1945 floods, while the third has not come into use at all. Channel improvement works were also started. The Mississippi River Commission has made thirteen cut-offs between the mouths of the Arkansas and Red rivers which short­ened the Mississippi river by 1 15.8 miles. Al l projections, above and below a cut-off, wore removed so as to develop an alignment having straights joined by flat curves.

A single, narrow and deep chan­nel is maintained by dredging, the dredged sand being, utilised to fill up side channels. These works have shortened the Mississippi by another 20 miles. Revetments have been constructed along the eroding banks to prevent erosion. The single, narrow and deep channel, wi th a shoitcned river course, develops a greater velocity and consequently has a higher discharging capacity. W i t h quicker drainage of water, flood heights have become lower, so that the maximum flood discharges are now confined within the improved Mississippi channel.

EMBANKMENTS NOT ENOUGH

It will thus be seen that the flood embankments along the Mississippi continued to be breached in spite of their heights having been raised, and it became necessary to carry out channel improvement works at a considerable cost annually. From the experience on the Mississippi, it is evident that without channel improvement works, prevention of flood damage cannot be ensured by flood embankments alone. Thus even ordinary flood, not to speak of major floods, are likely to cause breaches in flood ernbankments in China or India in some year or other, as had happened in the Missis sippi embankments before 1927.

Moreover, the fact that embank­ments along the rivers in China have now been strengthened and that they are being well maintained wil l

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October 26, 1954 T H E E C O N O M I C W E E K L Y

give inhabitants of river-side lands a .sense of security, so that they wi l l cease to be on the alert. Hence when flood embankments arc breach­ed and lands are suddenly submerg­ed, there is the danger of greater flood damage along the embanked reaches of the yellow, H u a i , and Yangtze rivers in China and the Kosi and Brahmaputra in India.

All water on land comes from piccipitation. Water, evaporated train the seas and other water sur­faces on lard , passes into air as invisible water vapour. As the air rises and cools, water vapour con­denses to form clouds. Blown by wind over land masses, clouds, under favourable conditions, are pre­cipitated as rain. In regions with a colder climate and in mountains and table lands with a high altitude, pre­cipitation is in the form of snow which on melting forms water.

Part of the water evaporates and a part is absorbed by the land to form underground ''water, while the bal­ance runs together on the surface of the land. The flow of water is due to the action of gravity down the inclined plane of the land from a higher to a lower level. Running water has the power of erosion and of transporting the sediment load, the power of erosion varying as the square of the velocity and the power of transportation varying as high as the sixth power of velocity. The water running down the surface of the land erodes the soil, forming tiny channels through which water flows as tiny streams. Many streams combine to form a river. By erosion of its bed and banks, a river deep­ens and widens its channel. The fundamental law of hydraulics pro­posed by Domenico Guglielmini is:

An alluvial river automatically adjusts its channel to correspond to its flow requirements.

A river is thus able to adjust its channel to drain away all water from its catchment together with the underground water that seeps into its channel back to the seas, thus re plenishing the water that was eva­porated from the seas and maintain ing Nature's " movement of water in a circle " beginning with evapo­ration of water from the seas and ending with drainage back into the seas. The flow of water down a river channel into the sea is thus not a waste of a country's water wealth; it is of vital importance in maintaining Nature's balance.

RIVER BANK EROSION The question naturally arises:

how is it that when an alluvial river automatically adjusts its channel to correspond to its flow requirements, it is not able to drain away its maximum flood discharge? The answer is that uncontrolled erosion of river banks causes the progressive widening and shallowing of a chan­nel and development of a tortuous alignment, in which there is gradual decrease in velocity and consequently in the discharging capacity, so that a river is not able to drain away quickly its maximum discharge. There is a rise in flood heights, over­flow from the river and submerg­ence of river-side lands. In the shallow depth of water, flowing over the river bank, there is a sudden de­crease in velocity, so that practically the whole of the sediment load is deposited on a small width of the river bank, raising its level, while a few hundred yards away there is l i t t le or no deposit, so that the level of

river-side land is not raised appre­ciably.

Moreover, the river channel below the point of overflow, carrying a lesser volume of water, emaciates to correspond to its lesser flow require­ment, and the level of its bed gradu ally rises. The river banks and bed thus gradually rise, while river-side

'lands remain practically at their ori­ginal level. Thus owing to uncon­trolled erosion of river banks, in the course of some years, the river flows high on a ridge of the alluvial plain; its emaciated channel cannot drain away quickly even ordinary flood dis­charges; and there is overflow and submergence of river-side lands and flood damage. Flood embankments make such conditions worse, as there is a gradual rise in flood heights in the embanked river channel which makes the depth of the submerg­ence of river-side lands greater.

A narrow and deep channel deve­lops a greater velocity and has a greater discharging capacity than a wide and shallow channel of the same sectional area. A river, with a straight channel b e t w e e n two points, has a steeper slope of water surface than one with a tortuous channel, so that greater velocity is developed through the straighter and steeper channel. In a narrow and deep channel, with uniform width and depth and a uniform slope of water surface, the motive force of gravity is not dissipated in the erosion of river banks and in disturbed flow, so that the whole of the force is utilised for the drainage of water and for the transportation of the sediment load. As none of the sediment load is deposited per­manently on any part of the river, its channel is maintained in good order.

There is thus quicker drainage of water, so that flood heights remain low; there is no overflow, no sub­mergence of land and no flood damage. The lower flood heights and lower dry season water level in a deep channel ensure natural drainage of low river side swamps, which are thus brought under cultivation. The drainage of swamps prevents the breeding of mosquitoes and there is eradication of epidemics of malaria, while the relief in water-logging improves agriculture. A narrow and deep river is thus able adequately to perform its primary function of drainage from the whole of its catch­ment, including the underground water to its outfall into the sea; it is also able adequately to perform its secondary function of transportation of the whole of its sediment to its

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