In designing a set of sluice openings or regulators, it is sometimes the custom to make the total area of waterway the same as that of the stream in its unobstructed condition. There is no particular reason why it should be the same. In a description of the Assiut Barrage (Min. Proc. Inst. C.E., vol. clviii., p. 30), it is mentioned that one of the reasons for placing the floor lower than the river bed was that the width of the waterway of the barrage was less than that of the river. The bed has to be heavily protected in any case, and the proper principle is to fix a velocity which is considered to be safe and, the maximum discharge being known, to determine the area of the waterway accordingly. In the case of a very wide river like the Nile, with a well-defined channel, it is inconvenient to make the distance between the abutments of a work much less than the width of the channel, but so far as velocity is concerned, the floor need not usually be lower than the bed. The protection given to the channel on the upstream side of the barrage ([fig. 41]) seems to be rather greater than necessary. The thickness of the floor (9 feet 10 inches) seems excessive. The thickness originally proposed was much less.
Of the many kinds of apparatus described in this chapter each possesses some advantages and disadvantages. Gates require a bridge with powerful lifting apparatus, and are suitable for large bodies of water and great depths. Comparing needles with planks, the former can be worked by one man and admit of rapid removal, and require far fewer piers. Planks require two men, and are sometimes liable to jam, but obstruct floating rubbish less than needles, and in shallow water give rise to less leakage. Whether needles or planks are used, masonry piers are most suitable where sand or gravel are liable to accumulate on the floor, or where there is much floating rubbish. The hinged frames are suitable in other cases. Falling shutters of the Chanoine type admit of very rapid lowering, and can be used without a foot-bridge. The drum weir is perfect in action, but its cost is high.
At any system of sluices the regulation should be so arranged as to minimise the chances of damage to the bed and banks where this is at all likely to occur. If the gates are opened only near one side of the structure, there will be a rush of water on that side, and serious damage may occur. The opening should be done symmetrically and, as far as possible, distributed along the whole length.
Until experience has shown it to be unnecessary, soundings should be taken at regular periods of time downstream of every important work where scour can occur. When scour is found to have occurred at any particular part of the work, the rush of water at such places should, as far as possible, be prevented, and a chance given for silting to occur.
Unless experience shows that damage is not likely to occur, a stock of concrete blocks, sandbags, or other suitable materials should be kept on the spot ready for use. Life-buoys should be provided on any work where large volumes of water are dealt with, especially if it is unfenced in any part, or if any of the men employed are casual workers.
Regarding works for preventing a river from shifting its course so as to damage or destroy a weir or similar work, see [Chap. XI., Art. 3.]
CHAPTER XI
BRIDGES AND SYPHONS
1. Bridges.—Bridges are of many kinds. In this book only those parts of them are considered which are exposed to the stream. If a bridge has piers, there must be some disturbance of the water. The disturbance will be least when the area of the waterway of the bridge is at least as great as that of the stream, and when its shape is as nearly as possible the same. For small streams, a single span clearing the whole stream may be adopted, especially when the channel is of soft material, but for a large stream the cost of intermediate piers, even with a certain amount of protection for them or with deep foundations, will be more than counterbalanced by the smaller thickness of arch or depth of girder.
Generally a bridge has vertical abutments which limit the waterway, but it may have land-spans, and in this case the stream as it rises can spread out. Piers and abutments should be so designed that abrupt changes in the section of the stream are, as far as possible, avoided, the piers being rounded or boat-shaped at both ends and the abutments suitably curved ([fig. 49]). Boat-shaped piers, besides presenting the neatest appearance, cause the least amount of disturbance.
A bridge can be made safe against scour either by giving deep foundations to the piers and abutments or by adding a floor and, if necessary, pitching. The former course is usually adopted and is the best. But in a case in which the discharge of a stream is to be increased or has been underestimated, it is often far easier to add a floor to an existing bridge than to increase the span of the bridge. In order to increase the waterway the floor can be “dished,” i.e. made at a level lower than the bed of the stream[17] and gradually sloped up—the slopes being pitched—both upstream and downstream of the bridge, to meet the bed.