COASTAL DRIFT.

Taranaki Daily News, 27 August 1921, Page 10

COASTAL DRIFT.

EFFECT OF WAVE ACTION. DIRECTION TOWARDS THE ® EQUATOR. '(By E. Maxwrfl, Opunike.) Littoral drift is a subject of considerable interest and one which is not generally understood. It is somewhat generally supposed that the drift of sand and other material along the coast is caused by sea currents. This is, of course, quite wrong. Currents are not the cause of coastal drifts, although it is likely that very strong currents may have some influence, but such influence is as likely as not td be opposed to the direction of drift. Coastal drifts are caused by wave action. So rquch of the sand, shingle and other material as is deposited on the beach anywhere between & few feet below low water mark and up to high water mark come within the influence of wave action, and material deposited deeper than ’a few feet below low water, except where there are deep water disturbances due to dashing among rocks or forcing through narrow passages, is not disturbed by ordinary wave action. As affecting coastal drift the action of the waves is that of disturbing the material and carrying it with the incoming wave up the beach, and when receding back again and the course of the waves and procession of the drifted material may best be described by likening it to saw teeth. The wave dashing on and running up the beach stirs up the sand and other material and carries it, not straight, bat at an angle up the beach. The waves carrying the material back again receded in a direct line, thus describing the outline of saw teeth.

So the drifting material travels on, on an acutely zig-zag course along the beach. This method of transport or drift would be quite simple to understand if only one knew why the waves do not rush up the beach in a direct line and receded over the same course, instead of the incoming wave taking a deflected course, and, what is more interesting, is that each wave l —on the average—is deflected in the same direction.

DRIFTS TOWARDS THE EQUATOR.

What has been said refers' to where there is a drift, and it has to be remembered that there is not always a drift, that there are coasts or stretches of coast where there is no definite drift.

For some reason not fully known the general direction of drift is from the Poles to the Equator, so that in the Northern hemsphere the drift is southward, and in the Southern hemisphere it is northward. Just why this is so, is not clear. It may be that the spin of the earth round its axis has a tendency to incline movement to the Equator, but, whatever the cause, the direction is as stated, and without question the coastal drift in New Zealand, both on east and west coast, is from south to north. The coast lines offer innumerable examples and proof that this is so. The northern trend of the shingle drift along the east coast down south is very clearly shown by the large area reclaimed on the south and outer side of the Timarti breakwater by drift shingle arriving from the south being stopped by the breakwater. The stripping from all along the west coast of the South Island is continuously being deposited at Cape Farewell, forming there a long horn, because there is no more coast running north along which to drift, and it will not drift southward. AT THE NEW PLYMOUTH BREAKWATER. The stripping of part of our Taranaki coast south of the breakwater drifts, as we well know to our cost, to the root of the breakwater deposits there and along the outer side of the breakwater, and drifts .around the end and into the harbor. The drift goes on, forming in its course bars across the river and harbor mouths, finally reaching the North Cape. Now, all this refers to coasts where there is a defined drift, and such coasts or stretches of coast are those the direction of which is more or Jess northerly. Where the coast line is approximately east to west, or more east to west than north to south, there-is either no drift at all, or, if any, it is not defined. We have .an instance of delayed drift in the case of the stretch of coast from the breakwater to Bell Block, the direction of which is more west to east than south to north, with the result that prior to interference by the breakwater, a heavily sanded beach and sand dimes were maintained, as is the case still at Bell Block.

South of Waingongoro to Patea and southwards we have an instance of a coast line direction being on the average, approximately from south-east to north-west, or just midway between south to north, and > east to west, and there the drift is uncertain and/ indefinite, resulting in the sand in a good many places being deposited directly on the beach, later to be blown inland to form sand dunes. But it is probable that wave action and the movement or deposit of material on coast further on down past Wanganui and on is considerably influenced by the gulf like division, with the narrow (Took Strait at its head, between the two islands.

INFLUENCE OF THE SEA-FRONT CONFIGURATION.

Bearing in mind the great influence of direction of coast line or sea-front with reference to drift of material, it is interesting to consider our breakwater proposal and approved sea walls and. suggested sea walls. Whilst the shore immediately eastward of the breakwater presents a direction tending to incline the drift inwards to the roof of the breakwater—which in fact is what happens—the direction of the shore further eastward suggests what might be termed a neutral area, that where little drift occurs and sand will deposit. This, in fact, is the case. Now the direction of the breakwater is one favorable to rapid drift, that is that the send of the waves under ordinary

average conditions will bo inclined outwards towards the end of the breakwater, which in fact is the case. The direction of the proposed and approved Moturoa-Breakwater wall is favorable to a neutral area, and in all probability heavy deposits of sand will take place seaward of the wall when built, and possibly embed the whole in a ridge of sand. On the other hand, ihe direction of the suggested Mikotahi-Moturoa wall would be such as to afford a rapid drifting base, and the sand drift, coming from the south, instead of depositing along and supporting the wall, would probably pass rapidly alona its outer face round

the outside of .Vloturoa and deposit on the line of and inside the position of the proposed Moturoa-Breakwater wall. Now it is not hard to see why coastal drift has a groat bearing or influence on the rate of coast erosion. Where there is no drift at all material undermined, brought down and crumbled up by the sea, except such as actually dissolves, would remain scattered on the beach or under the tide, and tend greatly to retard the rate of erosion and

sea encroachment. Gn the other hand, where there is a drift, all the material, except the larger stones, is 'quickly moved onward, and by keeping the beach bare of protection facilitates the destructive action of the waves.