River and Canal Engineering, the characteristics of open flowing streams, and the principles and methods to be followed in dealing with them.

by E. S. Bellasis

Fig. 52.

3. Training Works.—The object of the upstream and downstream protections already described ([Chap. X.]) is to prevent damage to the structure owing to the disturbance caused by the structure itself. When a river is given to shifting its course ([Chap. IV., Art. 9]) and cutting away its banks, protection of another kind is required. The stream, if left to itself, may cut away one bank upstream of the structure for a long distance, and eventually damage, or destroy by undermining, the upstream pitching and the abutment itself. This is known as outflanking. If in the neighbourhood of the line A B ([fig. 53]) there is nothing for the river to damage,—if, for instance, the structure is a weir with a canal, if any, only on the opposite bank of the river,—and if the land is of no particular value, the case could conceivably be met by protecting the abutment on all sides, but even then there might be a chance of the erosion of the bank continuing until the stream had formed a connection at C with the downstream reach. This, of course, in the case of a weir, would render the work useless and might even destroy it.

Fig. 53.

In the case of a bridge carrying a road or railway, or of a syphon or aqueduct carrying a canal or other stream, it is wholly inadmissible to allow the stream to cut away even as far as the point A for fear of its severing the line of communication. Thus in every case it is practically necessary to prevent any serious erosion of the bank upstream of the structure. In ordinary cases it is sufficient to protect the bank C D by any of the methods given in [Chap. VI., Art. 3], the protection being turned inwards, as shown at D, to prevent the end of it being damaged.

In the case of railway bridges across the great shifting rivers of India, protection used at one time to be afforded by various systems of spurs. This has now been abandoned in favour of Bell’s guide banks ([fig. 54]), which are found to be far more satisfactory. These guide banks are discussed in the paper by Spring quoted above (Art. 1). The spaces behind the guide banks become filled with water, at least during floods, and are meant to be silted up. An opening in the railway embankment should be provided at A, and another on the opposite side of the river, to ensure a constant flow of water ([Chap. V., Art. 3]), but they should not be large enough to cause high velocity. The chief danger to which a guide bank is liable is outflanking when the stream assumes the position shown. To guard against this danger it is necessary to have very strong and massive heads to the guide banks. When the bank of the eroding stream, downstream of the guide bank head, becomes a semicircle or thereabouts, the stream takes a short-cut across the sandbank, and to encourage this an artificial cut can be dug, at the season of low water, on any suitable line.

Fig. 54.

If the guide banks were made with an increased width of opening at the upper end, this would reduce the chance of outflanking but would increase the danger from a direct attack such as indicated, in the figure, on the left bank. It has been suggested that the width at the upstream end should be less than at the bridge, but this seems undesirable. Probably the form shown in [fig. 54] is the proper one. The length of the guide bank upstream of the bridge is made about equal to the span of the bridge between the two guide banks. If made less than this, the river might cut into the line of railway. The length of guide bank downstream of the bridge is generally 300 to 500 feet, being greater as the velocity of the river is greater and the sand of its bed finer.