ST, CLAIR BEACH

Evening Star, Issue 17086, 3 July 1919, Page 3

ST, CLAIR BEACH

That portion of the. Ocean Beach adjacent to St. Clair baa of late been undergoing one of those periods of denudation winch at times during the past 30 odd years have caused considerable uneasiness to those who mside in, the neighborhood of the bench. On three occasions dining the 80's the- sea actually brqke through the protecting barrier of sandhills and flooded certain portions of the Flat near the beach; on the first occasion opposite Forbury Park, and later, on two occasions, a little further on towards Musselburgh. The portion iof the beach which at that time was most dangerous has since- been rendered comparatively safo by tho planting of the sandhills with sandbinddng vegetation and the establishment of a broad belt of sand dunes, which, after all, is the natural and, therefore most effective, protection against the inroads of the sea. Of late years the effect of the sea has been most "marked on the St. Clair end of the beach, fwm the baths to roughly two or three hundred yards east of Forbury road. That the sea, lias encroached considerably at -ibis end of the beach, during the past 30 yeairs' is shown'by a study of the old maps In 1379 sections were laid off with the seaward side four chains from what is nwv the Metropole Corner, and between the sections and high-water mark was a road a chain ill width. The seaward side of the sections was further cat than the present esplanade wall, and further out still was the road. This bears out to la certain extent the assertion of some of the early settlers that during the 60's it was possible, «t low tides, to walk on a iwmdy beach out by the rocks on which the baths are. built.

A study of tho physiography of this portion of the coast .shows plainly that Nature had for hundreds of years prior to the arrival of white people been, building up a barrior which in course of time shut out the waters of the ocean from what is now known as the Flat, over which, as is shown by the marine deposits, there was at one time a eonritkrable depth of water. The 'ancient beach of that period may be traced in excavations around the foot of the hills at Caversham. The erection of tho barrier of sandhills was no doubt aided by a jjradual rise in the coast line. Such were the conditions which prevailed on the arrival of tho white man, and while the great changes in natural conditions consequent on the invasion of a comparatively numerous white population would affect tho work of Nature to a certain iextejit bv destroying the coastal vegetation through the ravages of stock and traffic, there does not appear to have been a serious orosion of tho beach at S't. Clair prior to carrying out of the first of a- series of ..-».._ ' work? in the

late s-ovt-ntios. 'Jn fact, it is trident from a. study of the physiography of the coaat line and the natural condition*; prevailing thai the greater part, at' not the whole, of tho trouble caused by e-iur-oachnipi't of the son. «t St. CJair has "been fjeiierated by the injudicious interference of man with the processes of Nature and an utter disregard of the weli-k'iown axiom that '' he who obey* Nature will he assisted by her, but he who opposes her will surely meet wit!; disaster.'' The natiroal. tendency of the sea is to fill up the indentations of the coast lino with material eroded from the land by the action of the sea and tho streams which How into it. On this coast lino thore is a general trend of the littoral drift up the icoast to thx- northward caused by a combination of the offshore current which sweeps up the coast from tho Southern Ocean, the tidal flow, and the on-shore winds. The Admiralty chart shows a one-knot an Lowr ctwrenb setting up the coast oft 'White Island, but close in-shore The effect of this current, and also that /of the tidal set, is evidently at times more than counterbalanced by the effect of the shore currents caused by wind action, as may he seen by the maimer in which tho Kt.Clair end of the -beach is eroded and the fit. Kilda, end made tip during periods of on-shore winds, while the rever-w pr '..ess occurs during ia spell of off-shore winds, particularly those from north to east. The accretion of the St. Clair beach during off-shore winds is no doubt acov-ntod for to a. certain extent by the well-known action of the sea, by which, generally speaking, on-shore winds cause erosion and off-shore winds cause accretion of the teaches.

The most potent factor in causing denudation of sandy foreshores is a. lessening or failure of the supply of material which goes to maintain the beach. A sandy beach is continually in >a, state of what may bo kerned unstable equilibrium. Under the influence of the waves the sand la continually kept in motion, and when the waves, under the infhienco of the wind, strike (the shore obliquely, the sand is kept travelling along tlio coast in the ■direction of the sot of the wind-generated current. In times of s-torm a, certain amount of erosion will tal« place at different points; but, provided the supply of sand is sufficient, a return to normal weather conditions will soon repair the ■damage. The supply of usind which goes to maintain the Ocean Beach conies from two sources. The first is the wearing action on the shore caused by tho 'continual pounding of the wave?. 'Hie amount annually eroded from tho hard basaltic rock of tho cliffs will not be a very large proportion of the. supply. The softer sandstone cliffs south cf St. Clair will naturally supply a much .greater quantity, but tlie supply from this source must bo small in comparison to that of the second source of supply—viz., the material brought down to tho sea by tho rivers. The Molyneux dircharg-os into the sea about 10J million gallons per minnle, and carries with it enormous quantities of solid matter in suspension. Tho Mataura., further smith, also brings down great quantities of imatcrial, and it is evident that these two rivers, combined with other smaller streams south of the Otago Peninsula, must bring down an enormous quantity of sand and other material, many times greater than that eroded by wave action from the whole length of roastlino south of Thu>odi.ti. Li tho davs of the early settlement prior to tho outbreak of tho diggings, the Molvneux ran blue and clear, excepting in times of flood ; but w'lh the outbreak of the diggings large quantities of tailings -were discharged into the river, and it became discolored during tho greater part of tho year. For many years the iriver has- .not run clear at Balch'tha, and even as for up-stream as Clyde, although the water retains the blue color characteristic of snow-fed streams, a. eonsidcvablfi amount of turbidity is noticeable nil the year round. The same observations apply also to tfio and other streams affected by mining operations. It is evident from these facts that the supply of sand at St. Clair can-not have decreased since tho settlement of the country. It must rather have increased considerably. As already remarked, the supply of sand deposited by the sea in pr*palcehr- days? had been sufficient to build up a bank which shut out the ocean from the Flat, and with the increase of littoral drift which evidently occurred after the settlement of tho country mid tho commencement of mining inland, this defence was doubtless augmented. Perods of denudation no doubt •occurred during _spells of on-shoro gales, but there is no evidence of their having ever been of such a nature, subsequent to .the arrival or the first settlers, as to cause any danger of the Flat being flooded until the" year 1883, and any erosion which took iplaca was no doubt soon made good with a return to norma] weather condition*. In seeking for a cause of the. encroachments of the past 30 years, it becomes plain that wo cannot lay ihe blame at the door of Father Neptune, _ but must rather look to the landward side for the veal solution. The first, work carried out on tho beaoh appears to have been constructed, at the end of the seventies, and was known as Smith's wall—a structure of dry rubble, built a little beyond high-water _ mark. This wall was doomed to destruction, as like the house of old, it was built on a foundation of r-and. and, being gradually undermined, it was eventually demolished during a heavy gale. A second wall built a. little further out fared no better. About tho time these walls wore in existence adrain was laid to carry the -water from the hills about Allandala and discharge it on the beach opposite the end of Forbury. road. Tho scouring action of tho water discharged from this drain in times of

I heavy raii: must have, been responsible I for the removal of a considerable quantity of sand, but the real mischief was doubtless caused by the stone- walla already mentioned. The recoil of the waves from 'contact with a- vertical wall sets tip a ecour at the foot of the wall which will soon undermine the structure and cause its downfall unless its foundaion be in material which is proof against erosion or it is protected by an apron. While the tea was engaged in undermining the early attempts at sea walls at St. Clair a ■ considerable scouring of the beach in front of the wall , would take place,- and the sand tin-own into suspension by the recoil of the waves would in on-shore weather be carried away by the in-shore current, and doubtless resulted in a serious denudation of the beach and the sea floor below low-water mark., and wad the initiation of the series of encroachments by each on-shore gale, which made necessary the erection, for the protection •of property, of the present esplanade wall. It is a pity that private property was ever allowed to encroach so far teaward as high-water mark. Had the land between what is now Bedford .street and the ocean been retained by the Crown right up to the foot of ,the hill a natural defence cculd have teen established against -my encroachment of the sea, as has been done eastward of Forbury road by the building up of the sandhills, and an attractive reserve created which would have added much to the beauty of the .locality. Th-e----erection of a costly'defence against the *ea would have also been unnecessary. With a. little spare ground behind in reserve an intelligent system of groyning would have been all that was necessary to deal with any erosion which might have occurred. An attempt at groyning the beach was made about 1905 but unfortunately thework appears to have been only half completed, resulting in a possible source of danger rather than a protection. The object in constructing groynes is to entrap the material of the beach by meansof low barriers set at right angles to its direction of travel, and 1 thus prevent its being swept awav bv the lateral currents. The groynes |of which the remains still stand on the i beach were reallv of no use for the purpose for which they were intended. To drive a row of piles across tlw beach and expect it to arrest the sand travel is analogous to expecting the framework of :i i house to keep out the rain. Such plank- | ir.e as was put on was done at; a time ! when the Leach was at a comparatively i high level. The consequence was th:i.fc I the least eTosion allowed the sea to find a I way underneath the planking and the i groynes then became worse than useless. !as the constricted area between the piles caused a greater scour than,. ■ ever. If [groynes are' to he a. success it is,imperiiI live that there bo no under-run beneath | them. A time like the present, when tho ] beach is at its lowest, level, is the time to commence building the groynes; then, as the beach rises they can be built up, care being taken that they aw at no time I more than two or three' feet above tho 1 beach level otherwise irregular accumulation is likely to take place. The classical instance of 'Foreshore defence is that of Ttomney Mar.?h, on the English coast This area contains about 60,000 acres of some of the richest pasturage in England. Portions of if- are 10 or 11 ft beiow hi ah water mark. Nearly a hundred vears ago tho were paved at great expense with stone pitching grouted with cement. Scour caused by thu recoil of the waves from the stonework piayod havoc with this work in 1859 ami 1869, and during the next 20 years some £70.000 was spent in patching. The late i Mr Case eventually carried out a system ot low groynes which was very successful, and piled up a defence of millions of yards of drift at a relatively small cost, This is a, striking example of success achieved bv (economical expedients in place ot costly heroic measures which would eventually have led to disaster. Had the roadway built to the eastwards of For■bury road some four years ago been provided with grovnes running out from the rubble wall which fronted it, it would have probably been standing to-day ; but, unprotected as it was from undermining by the lowering of the beach, it was at .the mercy of the first heavy gale from the sea. , The introduction of loose stone work below extreme high water mark or' a sandy beach is a measure which is open to a certain amount of doubt. The sea, breaking amongst the stones causes more or less eddying, which, while it may not actually cause erosion, will certainly hinder deposition of material. In conclusion, it may be as well to say a word of warning against a scheme which has often been suggested for the improvement of the beach. I refer to the proposal 'to build a breakwater seawards from the headland south of the baths, on the assumption that it would cause an accumulation of sand on the lee- side. The assumption would doubtless prove correct, but before that desirable consummation was reached the effect on the beach further along in the direction of_ Lawyer Head would be likely to prove disastrous, and if such a work was ever commenced it would be time, for the residents of St. Kildh, to lake to the hills. The reason is not far to seek. Sand will only travel freely in a certain depth of water. On this coast the limit of depth at which if will travel in any quantity in ordinary weather is probably between four and six fathoms. Beyond that depth it will drop to the bottom, with the result that an accumulation will take place on the weather side oE the breakwater, and until the wator has shoaled sufficiently around the seaward end to allow the material to travel the beach to leeward will he starved of material, and rapid erosion will ensue. Instances of this inwe occurred in all parts of the world, notably on a largescale at Madras on the coast of India, at Ceara, on the coast of Brazil, and, to como nearer home, at Timaru during the vears following the building of the breakwater, and more recently at Hokitika, 'where serious erosion to leeward followed the seaward extension of the breakwater. Groyning would be of little use in a case of this kind, as there would be nothing for the groynes to catch; the breakwater would have caught it all. Eventually tho sand will work around the end of the breakwater, and a broad sandy beach will accumulate- on the leeward side, as has occurred at Caroline Bay, Timaru. -- [Com m unica ted. ]