Methods of Lifting Water by Permanent Works

METHODS OF LIFTING WATER BY PERMANENT WORKS early saw that he could assist nature by taking water out of a river at a point higher upstream than immediately opposite his own particular field, and thus take advantage of the fall or slope between these two points. In the case of the Nile the fall is only about 6 in. in i mile. As some slope must be given to the water in a canal to make it flow, and if this is, say, 3 in. to i m., only 3 in. can be gained in each mile of length and thus land can be irrigated that is somewhat higher in level than could be commanded by the river directly opposite any particular field. Canals, to be really effective, must take off a river usually many miles upstream of the areas they serve. In the case of the Nile they are only dug deep enough to catch a supply from the river when in flood or artificially heightened by weirs.

Weirs or

the weir, or, as it is often called, the barrage, as an aid to lifting water is old, there is no doubt, yet only in modern times has the idea been used to any great extent. A weir or barrage is a structure built across a river, not for the purpose of storing water, but simply to produce a higher level of water on the upstream side of the building, to enable it to flow easily into canals taking off from either bank and command the land with free flow of water. Thus the weir or barrage is a

method of providing for doing, practically for ever, without further labour, what hitherto had to be done continuously by the aid of manual or mechanical work. The artificial upstream level is produced by shutting as much or as many as may be necessary of the doors or gates with which each structure is pro vided. As these buildings are most often necessary in those parts of a river's course where there is no other foundation than sand or silt, very naturally no great heading up can be provided for. A head of 20 ft. is seldom reached. More usually the upstream levels are only increased by i o ft. to 14 feet. Whatever heading up of the waters is required, it is never so great as to interfere with the silt-carrying property of the river. Fortunately in such situations great heights of lift are seldom necessary.