THE ST. OLAIK SEA WALL.

Otago Daily Times, Issue 9182, 31 July 1891, Page 4

THE ST. OLAIK SEA WALL.

MR G. M. BARR'S REPORT.

The following is Mr G. M. Barr's report on the condition of the sandhills, with recommendations regarding protection of the Bame :— To Messrß Gourley, Bridgman, and Wardrop, mayors of St. Hilda, Cayeraham, and South Dunedin. Gentlemen,—ln accordance with your request I have examined the sandhills between Musselburgh and St. Olair, with the view of reporting upon possible risk of the waters of the ocean breaking through at any part to the damage of the properties on the Flat to the westward, and alto as to what preventive measures require to be adopted. Further, as to the manner and cost of the restoration and protection of the esplanade of St. Clair, The first part of the subject may soon t>3 disposed of, seeing that both the hißtory of the beaoh for the last 10 years and present conditions confirm the opinion that there is a very appreciable danger unless certain points of the sandhills are raised in level and secured at that increased height. In March 1880, on the motion of the then Mayor of St. Kilda, 1 reported upon the same subject, more especially In connection with certain proposals to remove large quantities of sand for reclamation in Dunedin, and then pointed out the possibilities of an inroad by the ocean upon the low-lying grounds, I need scarcely remind you that large areas of these lands, in their natural condition, are hardly above the high water level, while other parts just touch it, and others again are really under the height of our highest tides. It is evident, therefore, that a break in the natural barrier of the sandhills may be the cause of a serious disaster over a wide ares. Up to the date of that report there had tan notorious encroaohment by the sea, but just three years thereafter in March 1883, considerable quantities of water forced themselves through the sandhills, about opposite thecentreof the Forbury racecourse, and cauied considerable injury to railway, road, and private proparty. Again we find that in July of that year heavy seas caused a very marked denudation of the tsach, and began the undermining of the esplanade wall at St. Clair, which in due course was completely destroyed by the severe gales of August 1880. In April 1888, and again about the Bth of last month, the sea waters found their way through a low portion of the Sandhills close to Musselburgh, and on the latter. occasion especially have flooded a large area, though, fortunately, not to any great depth. It is thus on record that in eight years the beach has b3en Beriouely lowered over a large extent by the aclldn of the oc;an, and it may be added that at present there are no indications of natural forces tending towards the restitution of the old levels. It may be that we have been passing through a period of denudation, to be succeeded by one of restoration, as has bean experienced in other parts of the world at different times; but our position, viewed from a practical point, is that in eight years there have been three inroads of the sea waters upon the Flat, and without giving way to needless alarm, it may be considered that provision againßt a recurrence of these or worse experiences would only be a proper preventive of possible disaster. Fortunately, I am in a position to assure you that all which urgently requires attention is comprised within two or three places at most between Musselburgh and St. Clair, and that the i danger at these places can be sufficiently provided against by an outlay not exceeding £ICO, to be expended in the erection of light fences in wood or ecrub, against which the sand wouldbepiledup by the aotionof thewind. With regard to the restoration of other parts of the beach, I do not suppose you will undertake such at present; but I will point out that the proper principle to adopt in this will be to endeavour to induce the Sandhills to extend from the beach outwards, and not to attempt the erection of works within the influence of the waves, under the Impression that they will do the building np of a new and higher slope. Much better to depend upon the action \j{ the wind, and stretch the Blope seawards, when the waves will at the proper time indicate the limit b9yond which the work thould not be extended. The works required for the restoration of the esplanade at St. Clair will involve fr much larger expenditure. It isnot necessary to recite the history of the alterations on the beach at this part, but simply to point out that during the last eight years the reducing forces have predominated over the restorative, and that in designing and estimating the necessary works, it is requisite to bear in mind that the wave force haß already besn instrumental in causing the destruction of -two walls, and in removing the principal part of the embankment over 11 chains of road. The width of this road was generally one chain; but, as I understand that a width of 40ft would furnish ample space for the preient requirements ol traffic the following estimates have been made foi both widths. This has been done with the feeling that if the funds can be found for a structure of undoubted stability, with thegreaterwidth.it would be. a pity to restrict the roadway to the narrowe> one; for, even if it b3 ample for the needs of to-day, the time will come when it would be felt to ba inadequate to the wants of the larger population which will bj-and-bye make use of Its area for purposes of traffic and recreation. At the same time it must be birne in mind that, besides involving greater first cost, the extra width also means the extension of the foundations of the work further seawards, and therefore they will ba exposed to a greater wave shock. In the else of two of the designs hereafter described—viz., that of a rubble slope, and that for a dwarf wall with a long Blope aboveit—the outer extremity is designed to coincide with the line of Mr Smith's wall, and quito as far towards the ocean as it is prudent to go. The drawings accompanying thiß report will show you the details of the various designs, and the following few notes, with the estimate of cost, will enable you to decide upon the scheme most likely to suit present circumstances. ESTIMATES. Scheme A.—Main roadway 40ft, and footpath at lower level 20ft, maximum width; masonry founded upon conorete in bags, and faced with sheet piles on ocean front; £2500. . Scheme B.—Roadway 40ft wide, and work similar to above; £2120. Scheme o.—Dwarf wall, with long slope partly pitched, 40ft road; £1760. Scheme D.—Bubble, with Blope towards the ocean; roadway 10ft in width; £1180. Soheme B.—Roadway 64ft maximum, with facing of piles and boarding; £1810. Scheme F.—Roadway 40ft ; timber facing ; ' Scheme GK—Main roadway 40ft, and footpath 20ft maximum width at lower level; timber facing; The three first deiigns are in permanent materials, and so far as to ensure against a scour under the foundations sheet piling is provided for on the seaward face. If a selection be made from this group, then design C would be the best value for the money, and it is also capable of being supplemented by additional height of wall, so as to get a roadway of 60ft when tbat is desirable. _ The next one, scheme D, is also in durable material, being wholly stone and clay, but as it is Intended to be constructed with large stoneß, roughly placed in position, its appearance would not be bo favourable aB in the drawing unless considerable care were bestowed in facing off the slopes. Further, as great care will have to-be exercised m selecting proper sized Btones, and in placing these in due proportion of large to small to secure the greatest stability, it is essential that this should be carried out by somo syßtem of day labour, bb the conditions are suoh as could not be satisfactorily met by contraot. Also, until the structure is up to some height above high water there ia risk uf damage to the work by wave aotion —a risk which of course tenderers would estimate at a lavish rate, and the cost be correspondingly increased, The schemes X, F, Q, in timber, though not so durable as the foregoing, would still be good for a quarter of a century at any rate. It is proposed to have the piles in manuka.anativetimber which possesses valuable qualities for thiß class of work, and the other timbers to be in Australian gum. I f Australian piles be used the extra cost will be from £70 to £100. It is necessary to Btate that owing to the high Bess about the beginning of June, and the consequent reductions of the road embankment, these estimates provide for more work than other calculations placed before you. Aleo, it may be pointed out that further inroads upon the embankment mean outlay still in excess of what is in these estimates. In all the schemes I have calculated upon the removal of the portion of roadway and wall now projecting at the south-west end of the esplanade, and the formation of an outline of easy curves towards the Bea. In addition to the abDve work, it will be advisable to construct a low rubble wall upon the lines Bhown upon drawing at the north-east end of the esplanade, to as to break the force of the wave and divert it from the sandhills at Mr Mitchell's fence, where recent storraß have made a dangerous inroad upon the slopes of the Sandhills. Further, the accumulation of sand in this corner should be encouraged by light fences, bearing in mind the eeneral principle which I have laid down in the first part of the report. The coßt of the rubble wall should not exceed £50. Acceßs to the beaob by a footpath at the southern end is a detail which could be easily carried out when the works are in hand, and a carriage road may be constructed at almost nominal cost af, the northern end after the slopes of the Sandhills have been extended by the procesß above recommended. A fence along the seaward boundary of the road for the 11 chains could be erected for £50 or £60. Q. M. BABR, M.1.0.8. In connection with the foregoing the Mayors of Caversham, St. Kilda, and South Dunectin met on Wednesday evening, and after due consideration of the report, agreed to recommend to their councils the proposal contained in Bch»me F, the cost being £1115, to which has to be added £100 for gaps, £50 for fonce, and £50 for rubble wall. The total cost of the work is put down roughly at £1400.