A culvert which is liable to run full and which has a steep approach channel (fig. 52) may become suddenly drowned on the upstream side. As soon as the water rises to the crown of the arch, the wet border of the culvert increases and this reduces the velocity and discharge. The water coming down the approach channel then rises abruptly, and the increased section of the stream causes a reduced velocity of approach, and this further reduces the discharge through the culvert. The heading up continues until the difference in the upstream and downstream water-levels is great enough to readjust matters (_Min. Proc. Inst. C.E._, vol. clxxxvi.). The possibility of this heading up occurring should be attended to in the design. In the case of a culvert in a railway embankment where heavy floods have to be passed, the culvert may be made bell-mouthed by a curved embankment constructed on its upstream side. [Illustration: 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. [Illustration: 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.