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

The flow in the tidal reach of a river is the same as if the water was alternately headed up by a movable weir and then allowed to flow freely and be drawn down. If the water carries silt, the tendency for deposit to occur is ([Chap. V., Art. 2]) no greater than if there was no heading up or drawing down. The tendency depends chiefly on whether there is, on the average, any reduction in velocity or increase in depth as compared with the non-tidal upstream reach, and whether the water in that reach is fully charged with silt. If both the answers are in the negative, no deposit due to river silt is likely to occur in the tidal reach.

Fig. 67.

If the sea water is charged with silt, it will of course carry silt into the river as it flows up, but the whole volume of water which enters has to flow out again. On the whole, the tendency for silt to be deposited in the river is due only to the period of “slack tide” near the time when the flow ceases. The tendency is seldom marked.

If the sea water carries silt and the river water is clear, the latter assists of course in removing any deposit—that is, it tends to keep the channel clear.

If the river channel is soft and if the sea water carries no silt, it may, in passing up and down the river, become charged with silt and return to sea still carrying it. It thus has a scouring effect on the channel, and may deepen or widen it. If, owing, for instance, to the flattening of the bed slope in its lower reaches, the river tends to deposit its own silt in its tidal reach, the sea water may prevent this deposit. Thus, as regards silting in the tidal reach of the river, the tidal water of the sea has little prejudicial effect if it is silt-laden, and a beneficial effect if it is not. Silt is likely to deposit in the tidal reach of a river of uniform width, only in a case in which the river water carries much silt, and the slope is flat or cross-section great compared to that of the upper reach.

Sea water is heavier than fresh water by about 2·4 per cent., and this, to some extent, prevents their mixing. At all stages of the flood tide the tendency at the point where the fresh water meets the salt water is for the fresh water to accumulate towards the surface and the sea water towards the bottom. When the tide begins to flow up the river there may be a low-level salt water current moving landward and a high-level fresh water current moving seaward, but this is quite a temporary state of affairs. The surface slope is landward, and the water moving seaward is not moving in obedience to the surface slope. It is only moving as a result of momentum previously acquired. The low-level current may have some extra velocity and extra scouring power, but this cannot be much, because the mean landward velocity of the whole stream must, owing to the internal resistances caused by the two currents, be less than it would be if there were not two currents. Moreover, the state of affairs is temporary. The two kinds of water mix eventually, and their temporary separation has no considerable effect on the general tendency of the river in the tidal reach to scour or to silt.

A body of water included at any moment between any two cross-sections of the tidal portion of a river may not reach the sea during the next ebb tide. In this case it will flow back up the channel with the next flood tide, and so be kept moving up and down, getting nearer, however, to the sea at each tide.

De Franchimont has shown (Min. Proc. Inst. C.E., vol. clx.) how a diagrammatic route-guide can be prepared for any tidal river to show pilots or captains of vessels the best times for starting on voyages up or down the river, and for passing each point on it.

3. Works in Tidal Rivers.—If any works are required in the tidal portion of a river, the principles to be followed in designing them are the same as if the river was non-tidal. All that has been said in [Chap. VIII., Arts. 1] to [3], applies to them. The river may be straightened or trained or dredged. Generally training and dredging are combined. Any dredging in the portion of the river nearest the sea will not, of course, alter the water levels near the mouth, but it will alter them further up. The tide will come up in greater volume and will rise higher and extend further up. The ebb will be facilitated, and the low-water level will be lowered. If any narrowing of the channel near its mouth is effected by training walls for the purpose of lowering the bed, the effect on the volume of tidal water entering the river must be taken into consideration. If the narrowing is confined to a reach near the mouth, and if the resulting deepening is not sufficient to counteract the effect of the narrowing, the volume of tidal water reaching the unnarrowed portions of the channel will be reduced, and this may be injurious. Its scouring action may be insufficient. The proper course may be to continue the narrowing upstream. If this is done, then it is obvious that the width of channel in which deep water is to be maintained at high water, or which is to be kept free from deposit, is reduced in about the same proportion as the volume of tidal water is reduced, and no harm is likely to result.