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. [Illustration: FIG. 49.] In any case in which the water rises above the crown of the arch, the bridge becomes a syphon, and a floor is probably necessary unless the foundations are very deep, or unless the rise of water above the crown is temporary. [Illustration: FIG. 50.]