Each stream in a state of nature fluctuates in accordance with the rapid changes of weather and alternates between high and low water, having usually a spring flood due to increased temperature, the melting of snow and frequent rains. The factors which com bine to produce floods vary in intensity from year to year; occasionally the combination of extraordinary rains on frozen ground or with rapidly melting snow produces flood of ex ceptional violence. During their geological his tory the streams during high water periods have built up flood plains by deposits from the muddy waters. Such lands are of exceptional fertility and their level character has invited settlement. The tendency has been not merely to cultivate these lands hut to build manufac turing establishments, railroads and towns upon the level surface. During periods of low water or even of ordinary flood there is no difficulty, but at times of high flood, the bridges, factories and other buildings along the bank interfere with the free flow. The river of necessity at such times spreads out and endeavors to take possession of its ancient flood plain, with con sequent destruction to property or even life. The immediate answer to questions, which are presented to the hydraulic engineer by these flood conditions, is to remove from the river channel and the flood plain the obstructions placed there by man and to erect permanent buildings only on higher ground, saving the low land for such agricultural purposes as will not be seriously injured by the occasional floods. This, however, has often become impracticable and it is necessary to consider other solutions for the many flood problems. In attacking these there are two lines of effort : First, flood prevention; second, flood protection.
In flood prevention the remedy is to be sought by careful surveys and examinations on the drainage basin to discover possible reservoir sites and by storing the flood water in suitable basins, the natural ponds or lakes or making artificial reservoirs where the floods may be restrained for a period of days or weeks, the excess being let out slowly in accordance with the capacity of the channel to receive it. There are not many localities where adequate reservoir capacities have been provided by nature or where dams can be erected creating a reservoir at a cost commensurate with the im mediate benefits. Investigations have been made, however, and it is evident that in the future many reservoirs will be constructed to reduce the flood crest.
In flood protection the object sought is to build near the points of danger large dykes or walls, shutting off the river from its ancient flood plain, and confining it in a relatively nar row channel. This is the most immediate and direct method of solving the difficulties for any particular locality, but of course does not assist other threatened points as in the case of reservoirs or similar works built for flood pre vention. In fact the protection of one area may jeopardize another by increasing the flood heights. The combination of flood protection by reservoirs and of flood prevention by dykes offers many interesting problems and is the sub ject of continued study by engineers in many parts of the world.
River All these hydraulic en gineering problems of (a) domestic and munic ipal supply, (b) irrigation and drainage, (c) disposal of sewage and waste, (d) manufactur ing, (e) water power, (f) navigation, (g) flood protection and prevention, may be con-. sidered as part of the general hydraulic en gineering problem of river regulation. As a rule each of these items has come up as a separate matter to be handled by the hydraulic engineer within certain territorial limits or re-. strictions of funds available. The artificial re striction of property lines, or of State or other boundaries, have usually prevented any broad consideration of the subject, but it is obvious that for the full development of any country each river system must be considered as a whole and examined, beginning at its head waters, with the intent of utilizing all possible storage reservoirs for holding water for municipal sup ply, irrigation, water power or flood prevention considering in this connection also the forest cover and cultural conditions of the drain age area. There should be studied also the
requirements of disposing of sewage and manu facturing waste, the employment of water for manufacturing and water power and finally in its lower courses its use for navigation. Regula tion in the interest of the commonwealth de mands broad consideration of all such problems by qualified hydraulic engineers and requires a broad control, usually by national authorities, because of the fact that the larger river systems are independent of artificially drawn State lines.
In river regulation in general and in the individual problems a wide field is thus offered to the engineer. The capacity and arrangement of storage reservoirs must be proportioned to the fluctuations of the natural flow of the stream. The channels for the conveyance of the water must be proportioned for the various proposed uses and with reference to the mate rials of which they are composed. When used for irrigation a low velocity of flow is desir able to avoid abrasions of the banks, and the sizes of the channel are proportioned to convey definite quantities of water steadily. In other cases the channels must be maintained in such Tanner as to constantly varying quan tities of water at different speeds of current, and the banks must be fortified against injury from sudden fluctuations of level in the water surface and from abrasion by floating material or accretion by deposit.
The construction of levees or embankments along the banks of a river and parallel to its current is the earliest and simplest of the methods of river improvement; but it is only within the last half century that this has been reduced to a science, and the most effective and economical methods of design and construc tion formulated. If the stream improved is of sufficient size to be navigable, the conditions of the problem are again changed. The course of the channel in_ places may have to be altered to avoid rapids of too steep descent for pas sage of boats, and a sufficient depth of water must be maintained at all times and in all places for boats of definite dimensions and draft Wherc such artificial channels are necessary it is important that they should be so propor tioned that the passage of the largest boat at the maximum permissible speed should not create a wave which would injure the banks or retard the progress of the boat.
The problem of the resistance of the water to a vessel passing through it enters into the consideration of navigation, especially of canals and inland waters. The more rapidly a vessel passes through a small channel the greater is the work to be done in the displacement of the bulk of water occupied by the hull. The dis placed water is prevented from flowing off by the adjacent banks and shallow bottom, and the pressure required to propel the boat is so increased that it is found that a greater ve locity than three miles per hour is not eco nomical. All such factors must be listed for consideration in the general plans for improve ment of inland waters.
From the above review of hydraulic engi neering it is to be noted that it is a profession which has numerous•ramifications and under lies the health, comfort and prosperity of the human race to a greater and greater degree as population and industry increase. Each year is showing notable advances in methods and results of work. The literature on the subject is already voluminous and each month shows some notable contribution either in the proceed ings of engineering and scientific societies or in textbooks or treatises on one or another of its many details. See HYDRAULICS ; HYDRO STATICS; HYDRODYNAMICS; HYDRO-ELECTRIC DE VELOPNIENT.