LITMIR.BIZ

      It is the tidal current which is doing this, and these currents vary in direction with the state of the tide. At any given locality they can be investigated, but on an open beach it may happen that with a rising tide the movement is, say, eastward, whereas on a falling tide it is in the opposite direction. If you continue your observations while bathing, and if you peer down a little under the water, you will note that the sand is disturbed by the waves about your feet, and that individual grains so stirred up may be carried a short distance one way or the other by the tidal current. The disturbed particles soon fall to the bottom, but the next wave may stir them up again, and another lateral jump takes place. In rough weather this action takes place on a much greater scale. The particles “jump” sideways, or if you like, move laterally in a saltatory manner. If the tidal current is moving fast, it may carry material with it just as a river does. In both of these ways—beach-drifting and tidal currents—vast amounts of sand and silt are carried. It will be appreciated that current action, which can go on at all states of the tide, is quite distinct from the beach-drifting that takes place on the beach itself. What is more, since tidal currents along a stretch of coast may flow in opposite directions at different times, the balance of movement may be quite small. Often, however, the flood current, i.e. the current associated with a rising tide, is the more powerful.

      To current action of this sort, impossible effects are occasionally ascribed. A current which can move an individual pebble in a smooth trough is by no means necessarily capable of moving the same pebble over a sand surface, and still less of moving it from a group of similar stones. The mass of pebbles in a sense behaves as one large one. Hence, a five or six knot current is not necessarily moving shingle over which it may be flowing. If, however, the shingle were put into motion in some other way, it might continue to move for some time. The method by which a current causes the movement of material on the sea floor can take place in one of two ways. Either, the current causes a direct thrust against any particle projecting above the general level, or, because of swirls and eddies a downward motion of the water is caused. It is for this reason that large stones lying on the bottom are often direct aids to erosion. It is therefore important to know the bottom speed of a current; it may be quite different from the surface speed, and the surface and bottom currents, even in shallow water, may run in opposite directions. Assuming that the direction of movement is the same at all depths in moderately deep water, the speed close to the bottom is about 85 per cent of the surface speed. On the other hand, anomalous factors may arise.

Owens found that where sand exists in quantity all currents up to 2·5 feet per second, or 1·7 miles per hour, are ineffectual in moving shingle, whereas at about 2·5 feet per second the current suddenly acquires the power of moving stones up to nearly 3 inches in diameter over a sandy bottom. Complications naturally occur if the bottom is not smooth, and in nature it seldom is. Owens’ conclusion, with which I fully concur as a result of my own observations, is that “… since the sea bottom is nearly always irregular, and stones are seldom perfect spheres, the effect of currents alone, unless of exceptional velocity, is chiefly limited to the transport of fine matter such as sand and mud.”¹

Before turning to various types of beach, two other factors in wave action need attention. It is perhaps too great a simplification to assert that, even for our present purpose, waves are of two sorts, but it is helpful to note the suggestion of Lewis,² who differentiated between constructive and destructive waves.