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.

CHAP. IV. explains the laws of silting and scouring action, a subject of great importance and one to which the attention ordinarily given is insufficient. The general characteristics of streams, being due entirely to silting or scouring tendencies, are included in this chapter. CHAP. V. describes how silting or scouring may be, under some circumstances, artificially induced or retarded.

CHAP. VI. deals with various methods of protecting banks against erosion or damage. CHAP. VII. treats of diversions or the opening out of new channels, and with the opposite of this, viz. the closing of channels, a feat which, when the stream is flowing, is sometimes very difficult to achieve. This chapter also deals with dredging and excavation.

CHAP. VIII. discusses the subject of the training of streams, a class of work which is generally undertaken in order to make them navigable or to improve their existing capacities for navigation, but may be undertaken for other reasons. The main features of this kind of work are the narrowing and deepening of the stream, often the reduction of the velocity and slope, and generally the raising of the water-level. In this kind of work a channel may be completely remodelled and even new reaches constructed. CHAP. IX. deals with artificial channels of earth or masonry, and includes navigation canals.[6]

In CHAPS. X. and XI. the chief masonry works or isolated structures--as distinguished from general works which extend over considerable lengths of channel--are dealt with, and those principles of design discussed which affect the works in their hydraulic capacities. General principles of design applicable to all kinds of works, such as the thicknesses of arches or retaining walls, are not considered; they can be found in books on general engineering design.

CHAP. XII. treats of storm waters and river floods, and shows how works can be designed for getting rid of flood water and how floods can be mitigated or prevented, one of the chief measures, the widening of the channel and the lowering of the water-level, being the opposite of that adopted for training works. Embankments for stopping flooding are also dealt with. CHAP. XIII. deals with reservoirs, including the design of earthen and masonry dams.

CHAPS. XIV. and XV. deal with tidal waters, river mouths and estuaries, and works in connection with them, viz. the training of estuaries and the methods of dealing with bars, the object being in all cases the improvement of the navigable capacities of the channels.

3. =Design and Execution of Works.=--After obtaining full information concerning the stream to be dealt with, careful calculations are, in the case of any large and important work, made as to the effects which will be produced by it. These effects cannot always be exactly foreseen. Sometimes matters can be arranged so that the work can be stopped short at some stage without destroying the utility of the portion done, or so that the completed work can be altered to some extent.

In works for controlling streams there is, as will appear in due course, a considerable choice of types of work and methods of construction. In practice it will generally be found that there are, in any particular locality, reasons for giving preference to one particular type or kind of work or, at all events, that the choice is limited to a few of them, either because certain kinds of materials and appliances can be obtained more cheaply and readily than others, or because works of a particular type have already been successfully adopted there, or because the people of the district are accustomed to certain classes of work or methods of construction. In out-of-the-way places it is often undesirable to avoid any type of work which cannot be quickly repaired or readily kept in order by such means as exist near the spot.

It is sometimes said that perishable materials, such as trees, stakes, and brushwood, cannot produce permanent results. They can produce results which will last for a long time and which may even be permanent. By the time the materials have decayed, the changes wrought may have been very great, deposits of shingle or silt may have occurred and become covered with vegetation, and there may be little tendency for matters to revert to their former condition. If the expense of using more lasting materials had had to be incurred, the works might never have been carried out at all. On the Mississippi enormous quantities of work have been done with fascines.

4. =The Hydraulics of Open Streams.=--When any reach of a stream is altered, say by widening, narrowing, or deepening, so that the water-level is changed, there will also be a change in the water-level, a gradually diminishing change, for some distance upstream of the reach. Also in the lowest portion of the reach the change will gradually diminish and it will vanish at the extreme downstream end of the reach. In the next lowest reach there is no change. Thus if it is desired that the change in the water-level shall take full effect throughout the whole of a reach, the change in the channel must be carried further down. If a weir is built there is no change in the water-level downstream of it except such as may be due to loss of water in the reach upstream of it. The above points are mentioned here because, although they are really questions of hydraulics, they are of much importance and of very general application.

Matters connected with the hydraulics of open streams seem to lend themselves in a peculiar way to loosely expressed remarks and fallacious opinions. The set of a stream towards a bank is sometimes supposed to profoundly affect the discharge of a diversion or branch. Its effect is simply that of “velocity of approach,” which, as is well known, is quite small with ordinary velocities, and is merely equivalent to a very small increase of head. Narrow bridges or other works are sometimes said to seriously “obstruct” a stream without any observations being made of the fall in the water surface through the bridge. This fall is the only measure of the real obstruction.[7]