WEST COAST SINKINGS.

Taranaki Daily News, 20 August 1921, Page 9

WEST COAST SINKINGS.

SEA COAST EROSION.

WHAT IS GOIXG OX. (By E. Maxwell, Opunake). The damage caused by the recent storm has brought into -prominence the seriousness of the erosion of the sea front, but it is doubtful whether even now the authorities or the 'average citizen, with the evidence before them of what destruction can occur within the space of a few hours—as is to be seen, for instance. on the sea front from the eastern end of the railway breastwork along to, and past, Henui—realizes what a serious menace sea encroachment along our coast is. It is of the utmost importance that the community should not remain lulled in a sense of immunity because of the almost imperceptible progress in the past, especially in some parts, such as near the Sugar Loaves, but should realize what a serious menace threatens, so that, though some of the causes are beyond their influence, every possible effort be made to retard, if not to etop, the destruction where sea encroachment is most rapid and the loss resulting most serious.

To realize the menace one has only to reflect on the many examples in other countries of loss and devastation caused by sea encroachment, marked instances of which are. afforded in the Old Land, where certain parts of the coast, once the sites of villages and fertile fields, now lie under the waves. Whilst encroachment there has been slow, we see, on turning to our own coast here, and viewing such places as the clifl's eastward of the railway embankment, 'where, within the last few years, considerable areas have been undermined, 4 have fallen and disappeared. And again at the Henui, where in quite a short time areas amounting to acres have vanished, and now last week's storm surely should be convincing proof of the ever-increasing rapidity of encroachment.

SLOW BUT SURE. Encroachment has, without doubt, for long period in the past been on the average very slow indeed; how slow one cannot say. Along parts of the coast, possibly only a few inches in the century, whilst in other places the loss has amounted to many feet, or even chains, in the same period. But due to causes (which later will bo indicated) that no human effort can alter, the loss by erosion or the rate of sea encroachment on the land has -now alarmingly increased so far as considerable portions of our coast line are concerned.

A necessary preliminary to guarding against or to a proper treatment of the evil is a full appreciation of its magnitude and a knowledge of the various causes.

Erosion by the sea affords a most interesting subject for observation and study, and our coasts, especially the part, say, from Patea to the Cape, forming the South Taranaki Bight, and the parr, from the Cape northward, forming the North Taranaki Bight, offer exceptional evidences of cause and effect. And the configuration of the parts of the coast is in pointed contrast to coast lines where opposite conditions prevail. Coast erosion in the main is caused by the waves dashing on the shore, and at the bases of banks and cliffs where they exert hydraulic forces, both explosive and by suction, in the cavities, crevasses and fissures, and also by the grinding action of rock, boulders, ehingle and sand, moved by the advancing and receding waves. Generally speaking, erosive action is less pronounced on a gently rising sand-covered shore than where the sea reaches bank or cliff bases. The apparent immunity of the smooth sandy beach is due, among other causes, to the fact that the waves roll over the smooth surface, and have nothing to undermine, and also that a sandy beach generally indicates a position of deposit of drift material which is always tending to make good any loss.

A NATURAL PARTIAL PROTECTION.

Where a hard or rocky barrier is presented to the seas from low to highwater mark, erosion will be very slov*. Where the material is a mixture of hard and soft it will be more rapid, but delayed proportionately* to the amount of hard material that is deposited in the form'of rocks, boulders, etc., from the masses as they crumble down, and where, the material is nearly all such as will quickly dissolve and wash away, the erosion will be very rapid. Of course, the rate varies considerably, independent of the material, with the varying configuration of the coast and its inclination to the currents and the. trend fit wave action. Usually the various strata near sea level are composed of hard compacted material, superimposed by softer material, such as clay, mud and soil.

Now, if the reader will think of the coast lines o'f many countries, such as Great Britain, where there are many capes and headlands, bays and large est'uaries, and compare with these the line of sven curves of the South and North Taranaki Bights, which are clear cut, without a single estuary, inlet, or bay, no promontories or headlands. Next examine the cliff faces at many places, such as off Hawera and at Opunake particularly, and it will be found that right at sea level a stratum of soft material, swamp debris, mud, clay and soil, imbedded in which is a decaying fort st, tree trunks protruding into* the water at high tide. Above this are. strata of sand, sandstone, conglomerate, containing large boulders, and above this again are clay and soil. At New Plymouth along the sea front eastward can be seen mud with shingle in it, right at tide level, overlaid W’ith clay. Along the coast south and north of Opunake, and near the White Cliff, can be seen small portions of Maori pahs, just a few feet only remaining of what living Maoris knew as large, intact, important pahs.

WHERE ONCE FORESTS GREW. What do all these things-—the clean cut coa«st line without indentations of any kind, the buried forests at sea level, and the few feet of Maori pahs— mean? Just what scientists have told us, that this west coast of ours is steadily but surety sinking, slowly very slowly, but surely inch by inch, dipping down under the sea. The one time headlands and promontories have vanished, being now only represented by reefs of hard material (all else has been washed away), such a.s we see near the breakwater, or of boulders running far out into the ocean, the present coast line being where once forests grew well inland. . " This sinking of the coast htu> caused an immense acceleration of the erosive action of the sea, for much of the hard stratum that offered a barrier to encroachment is now below high tide, and the softer material, being lowered to tide level, is an easy prey to the sea's devouring action, Further, as New

mouth, conditions have been made much worse, and sea encroachment greatly speeded by the fact that for about eighty years now, much, above all, the needs of the community in sand, gravel, boulders and stone for walls, kerbing, channelling, road metal, and for building, have been taken from the foreshore, or, in, other words, much of what would have otherwise still remained to protect the foreshore has been removed. The penalty, in addition to land already lost, will be that for every shilling’s worth of material that has been removed from the foreshore, many pounds will Kave to be spent in placing rock or other barriers. If much of the present site of the eastern portion of New Plymouth and Bell Block is not to go under the waves, each generation will have to provide its quota of foreshore protection. •Coastal drift has a great bearing on the question of erosion by the sea, but that is matter for a special article.