SCOPE: SURGES IN CANALS
This chapter is confined to the problem of surges which occur in navigation canals as a result of lock operations, since a full discussion of other unsteady flow phenomena which may occur in open channels would be feasible only in a large treatise.
a. Surge Problems. In canal design, the study of surges is necessary because of the following three problems which are given in their probable order of importance: (1) to an extent governed largely by traffic density; (2) surges which depress the water level reduce the effective channel depth, those which raise the water level encroach upon the freeboard; and (3) surges may impose sudden large loads upon miter gate operating machines. It should be emphasized that in most canals, there is no serious difficulty with surges. Unless traffic density is great, an existing surge problem can quite easily be alleviated. Nevertheless, some consideration of the surge problem is warranted in design, particularly for long, excavated canals controlled by high head locks.
b. Limitations of Theoretical Analysis. The theoretical laws of unsteady flow in open channels are well established but, except in a few fields such as flood wave transmission and tidal hydraulics, the techniques of applying the laws to practical problems are not yet well developed. Certain simplifying assumptions which are permissible in ,the study of canal surges are entirely inappropriate for the analysis of unsteady flow phenomena which may occur in channels used for other purposes. For example, surges of large amplitude cannot be tolerated in practical navigation canals nor can the occurrence of high velocities. Therefore, methods applicable exclusively to surges of small amplitude may be used. In navigation canals, the velocities of flow are so low that they may, for some purposes, be neglected. Limitations exist as to the accuracy which can be realized, regardless of the computation procedures adopted. This is true for several reasons; minor changes in canal cross section produce partial reflections which cannot be evaluated; traffic in the canal has a similar effect; interference between newly generated and reflected surges occurs. The latter problem would not be a serious obstacle if the strength of reflected surges were known, because the effects of two or more waves can readily be combined. However, basic data as to the rate of energy dissipation in unsteady flow are unavailable. For these reasons, and others discussed in detail in several of the references listed at the end of this text, the results of any analysis of canal surges should be treated as an index, comparable with similarly determined indices for canals in successful operation. To avoid presentation of ideas which cannot be supported by observed data, this chapter is confined to the case of level canals which are reasonably regular in form. Surges in canalized rivers are less important than those in canals because the river widths are generally much greater. This is fortunate because the analysis of the effects of major channel irregularities is very speculative. Traffic-induced surges in navigation canals are significant mainly from the standpoint of vessel power requirements and are not considered in this chapter. Surges which occur in lock chambers are likewise excluded as a special problem in lock design and are discussed in chapter 4 of this part.
