River and Canal Engineering, the characteristics of open flowing streams, and the principles and methods to be followed in dealing with them.

RIVER AND CANAL ENGINEERING

RIVER AND CANAL ENGINEERING THE CHARACTERISTICS OF OPEN FLOWING STREAMS, AND THE PRINCIPLES AND METHODS TO BE FOLLOWED IN DEALING WITH THEM BY E. S. BELLASIS, M.Inst.C.E. RECENTLY SUPERINTENDING ENGINEER IN THE IRRIGATION BRANCH OF THE PUBLIC WORKS DEPARTMENT OF INDIA 72 ILLUSTRATIONS London E. & F. N. SPON, Ltd., 57 HAYMARKET, S.W. New York SPON & CHAMBERLAIN, 123 LIBERTY STREET 1913

PREFACE

RIVER AND CANAL ENGINEERING - CHAPTER I INTRODUCTION - 1. Preliminary Remarks.—River and Canal Engineering is that branch of engineering science which deals with the characteristics of streams flowing in open channels, and with the principles and methods which should be followed in dealing with, altering, and controlling them. It is not necessary to make a general distinction between natural and artificial streams; some irrigation canals or other artificial channels are as large as rivers and have many of the same characteristics. Any special remarks applicable to either class will be given as occasion requires. - 2. Résumé of the Subject.—Chap. II. of this book deals with the collection of information concerning streams, a procedure which is necessary before any considerable work in connection with a stream can be undertaken, and often before it can even be decided whether or not it is to be undertaken. Chap. III. deals with rainfall, and describes how rainfall figures and statistics can be utilised by the engineer in dealing with streams.

CHAPTER II RAINFALL - 1. Rainfall Statistics.—The mean annual rainfall varies very greatly according to the locality. In England it varies from about 20 inches at Hunstanton in Cambridgeshire, to about 200 inches at Seathwaite in Cumberland; in India, from 2 or 3 inches in parts of Scinde, to 450 inches or more at Cherrapunji in the Eastern Himalayas.

CHAPTER III COLLECTION OF INFORMATION CONCERNING STREAMS - 1. Preliminary Remarks.—The information which is required concerning streams depends on the character of the stream and on the nature of the work which is to be done. For the present let it be supposed that the stream is large and perennial. Other kinds of streams will be dealt with in Arts. 6 and 7. In dealing with a large perennial stream it is nearly always necessary to know the approximate highest and lowest water-levels, and these can generally be ascertained by local inquiry, combined with observations of water marks; but a higher level than the highest known and a lower level than the lowest known are always liable to occur, and must to some extent be allowed for. If navigation exists or is to be arranged for, the highest and lowest levels consistent with navigation must be ascertained. The highest such level depends chiefly on the heights of bridges. A plan to a fairly large scale is also necessary in most cases.

CHAPTER V METHODS OF INCREASING OR REDUCING SILTING OR SCOUR - 1. Preliminary Remarks.—Most important works which affect the régime of a stream have some effect on its silting or scouring action, but this is not generally their chief object. Such works will be dealt with in due course, and the effects which they are likely to produce on silting or scouring will be mentioned. In the present chapter only those works and measures will be considered whose chief object is to cause a stream to alter its silting or scouring action. It does not matter, so far as this discussion is concerned, whether the object is direct, i.e. concerned only with the particular place where the effect is to be produced, or indirect, as, for instance, where a stream is made to scour in order that it may deposit material further down the stream. The protection of banks from scour is considered in Chap. VI. Dredging is dealt with in Chap. VIII. - 2. Production of Scour or Reduction of Silting.—Sometimes the silt on the bed of a stream is artificially stirred up by simple measures, as, for instance, by scrapers or harrows attached to boats which are allowed to drift with the stream, or by means of a cylinder which has claw-like teeth projecting from its circumference and is rolled along the bed, or by fitting up boats with shutters which are let down close to the bed and so cause a rush of water under them, or by anchoring a steamer and working its screw propeller. It is thus possible to cause a great deal of local scour, but the silt tends to deposit again quickly, and it is not easy to keep any considerable length of channel permanently scoured. The system is suitable in a case in which a local shallow or sandbank is to be got rid of and deposit of silt a little further down is not objectionable. It may be suitable in a case in which the bed is to be scoured while a deposit of silt at the sides of the channel is required, especially if some arrangement to encourage silt deposit at the sides is used (Art. 3, par. 4; also Chap. VI., Art. 3).

CHAPTER VI WORKS FOR THE PROTECTION OF BANKS - 1. Preliminary Remarks.—The protection of a length of bank from scour may be effected by spurs, which are works projecting into the stream at intervals, or by a continuous lining of the bank. A spur forms an obstruction to the stream (Chap. IV., Art. 1), and when constructed, or even partly constructed, the scour near its end may be very severe, even though there may be little contraction of the stream as a whole. If the bed is soft a hole is scoured out. Into this hole the spur keeps subsiding, and its construction, or even its maintenance, may be a matter of the greatest difficulty. A high flood may destroy it. If it does not do so, it may be because the stream has, for some reason, ceased to attack the bank at that place. A continuous lining of the bank is not open to any objection, and is generally the best method of protection. Spurs made of large numbers of rather small trees, weighted with nets filled with stones, have been used on the great shifting rivers of the Punjab which swallowed up enormous quantities of materials. The use of spurs on such rivers has now, in most cases, been given up. If L is the length of a spur measured at right angles to the bank, the length of bank which it protects is about 7 L—3 L upstream and 4 L downstream,—but the spur has to be strongly built, and its cost is, in many cases, not much less than the expense of protecting the whole bank with a continuous lining.

CHAPTER VII DIVERSIONS AND CLOSURES OF STREAMS - 1. Diversions.—When a stream is permanently diverted the new course is generally shorter than the old one, and the diversion is then often called a cut-off. The first result of a cut-off is a lowering of the water-level upstream and a tendency to scour there, and to silt downstream of the cut-off. Fig. 23 shows the longitudinal section of a stream after a cut-off A B has been made. The bed tends to assume the position shown by the dotted line. If both the diversion and the old channel are to remain open, the water-level at the bifurcation will be lowered still more, and the tendency to scour in the diversion will be reduced.

CHAPTER VIII THE TRAINING AND CANALISATION OF RIVERS - 1. Preliminary Remarks.—When a stream is trained or regularised it is generally made narrower, but sometimes narrow places have to be widened. Deepening has also very frequently to be effected. The object of training is generally the improvement of navigation, but it may be the prevention of silt deposit. Some natural arms of rivers which form the head reaches of canals in the Punjab are wide and tortuous, and they are sometimes trained. Training often includes straightening or the cutting-off of bends, as to which reference may be made to Chap. VII. - 2. Dredging and Excavating.—When a flowing stream is to be deepened, the work is usually done by dredgers. Dredgers can remove mud, sand, clay, boulders, or broken pieces of rock. The “bucket ladder” dredger is the commonest type. The “dipper” dredger is another. Both these can work in depths of water ranging up to 35 feet. The “grab bucket” dredger can work up to any depth and in a confined space. The “suction dredger” drawls up mud or sand mixed with water. A dredger may be fitted with a hopper or movable bottom, by means of which it can discharge the dredged material—this, however, involves cessation of work while the dredger makes a journey to the place where the material is to be deposited—or it can discharge into hopper barges or directly on to the shore by means of long shoots. For small works in comparatively shallow water the “bag and spoon” dredger, worked by two men, can be used.