THE SHINGLE QUESTION.

South Canterbury Times, Issue 4984, 17 April 1889, Page 3

THE SHINGLE QUESTION.

At the March meeting of the Timaru Harbor Board Captain Sutter moved and it was carried 11 That] the engineer lay on the table at next meeting a report as to the quantity of shingle drifted from the south, the annual drift of which has been obstructed by the breakwater, the quantity to be given in cubic yards, and to include the quantity used in the construction of the breakwater ; the calculation to be made from the commencement of the work.” Captain Sutter said there were bo many opinions about the drift of the shingle, some saying it was enormous, others that it was nothing at all, that it was advisable to have a reliable estimate of the actual rate of accumulation. In pursuance of the above resolution, Mr P. W* Marcbant presented to the board to-day the following : REPORT. To the chairman, Timaru Harbour Board. Sir, —According to the instructions contained in the board’s resolution of March 20lh, 1859,1 have the honour to present to tou my report on the accumulation of shingle beside the Timaru breakwater, and also to lay before you my observations and opinion concerning the shingle question general!;. j; description. After the commencement of the breakwater in 1878 a considerable quantity of shingle collected on its southern side. As the work advanced so Hid tho shingle, until the breakwater had projected a considerable distance seaward, when by opposing a vertical face to the prevailing direction of the seas at a favourable angle, waves of reflection were created which drove the shingle to the south and caused a seaward projection of the beach at a point about 16 chains south of the breakwater at the termination of the reflected \V2IV6S« The" seas also in meeting the [breakwater heaped themselves up against it, and sweeping inshore along it plunged on the beach beside if with great force, thus drifting the shingle inshore and southward After the establishment of these waves of reflection and concentration, the whole of the beach was occasionally scoured away from beside the breakwater as far as tho blockyard. Further south, by the rock island at the termination of the reflected waves, the beach steadily and continually advanced seaward until October 1885. Southward from the rook island to the bight near Patiti Point the beach continually advanced seaward until in April 1883 it stood 175 feet opposite Heaton street and 130 teet opposite Grey street in advance of the original beach line. In the bight no extra accumulation had taken place. • , _ . From April 1886, the date of my first survey of the beach, and for 12 months thereafter, during which time no existing conditions had been modified in tho slightest degree, the beach steadily advanced 90 feet beside the breakwater and receded about tho same distance by the rock island. . ... Towards the end of 1886, during a violent S. F. storm, part of the breakwater on tho first kantjwas underscoured. This defect has been experienced in all the vortical sided breakwaters exposed to heavy seas where not founded upon rock or protected by a wave breaker. I therefore advised the board to construct a random block work wave breaker alon» tho wholejof the first kant similar to thatwhich had been laid further out,. After the completion of the upper part of the wave breaker the beach advanced rapidly beside the breakwater and receded by the rock island, thus From January 1887 to January 1888 it advanced 200 feet beside the breakwater and receded 90 feet at the rock island. From January 1888 to January 1889 it advanced 40 feet beside the breakwater and receded about 30 feet by the rock island. South of the rook island it has made about 30 feet since April 1836. Taking Sir John : Coode’a excellent survey of the beach as my basis, I estimate the total accumulation of - shingle between the breakwater and Heaton Street —a distance of about a. mile at 750.000 cubic yards, including in this amount the quantity of shingle used in tho construction of tho concrete breakwater. Of this total, 600.000 cubic yards were deposited in the first seven years after the commencement of the breakwater and 150,000 cubic yards have accumulated in the last 3 years.' Of this last named amount, I find that 60,000 cubic yards accumulated in 1836, 50,000 cubic yards in 1887 and 40,000 cubic yards in 1888. I have not included in the above amounts, the current|removals of shingle for railway ballasting and other purposes, nor have I estimated the cubic measurement of the accumulation between Heaton Street and the bight to the south, as it does not affect the question under consideration and is moreover of but small quantity, the water being shallow there. CAUSES OF PRESENT ACCUMULATION BESIDE THE BSEAKWATEE. During 1887 and up to tho present date the following causes have combined to effect the accumulation of shingle beside the breakwater:— 1. That since the completion or the concrete work some 18 months ago, no shingle has been removed from the beach beside the breakwater. . 2. That during the ’ construction' of the breakwater a long row of concrete blocks nearly always stood on the parapet offset, thus opposing a broadside six feet high to the action of waves during storms and so greatly increasing tho force of the waves of concentration. Wince the remoyal of these blocks for the completion of the works, the •naves sweep lover tho breakwater in great volumes during storms, thus reducing their scouring effect on the beach. 3. That tho upper blocks of tho wave breaker standing above low water mark, have reduced the inshore run of the waves along the breakwater. This is .the only reductive effect of tho wave breaker, as the strength of the reflected waves south of the breakwater is as great as ever. 4. during the past two years, and more especially so during the last'nine months, wo have had unfavourable weather in so far as shingle accumulation is concerned. Hardly any heavy S B, swells have occurred during that period, and it is these which scour awoy tho shingle from beside tho breakwater and distribute it over tho beach to the southward. The steps which’ have lately been taken to reduce the accumulation of shingle beside the breakwater are the following :—^ We have arranged with the Eailway Department to remove in future all tho ballast it requires from the beach beside the breakwater instead of taking it from tho ballast pit by tho rock island. A cart road has also been opened across the blockyard so that shingle for town purposes can be removed from this point instead of from tho former place of removal further south. We are lifting the uppermost blocks of the wave breaker and stacking them on the parapet offset in order to increase the waves of concentration. About one half of this work has been completed but its value has not yet been tested by a heavy S.E, sea. While I do not lay any stress on tho effect of the removal of the beach for ballasting etc. still it will help in the long run._ The removal of tho random blocks and their formation into a parapet, will, I believe have a most benficial result in scouring away the accumulation of shingle beside tho breakwater and enabling it to be distributed over the beach by the reflected waves. Part 2, DESCRIPTION OF COAST LINE, ETC, Southward from Timaru to the Pareorc river, a distance of about 7 miles, the coasl is of a much more irregular outline than ii the case south of the latter point, owing t( the fact that an underlying stratum of basal tic rocks projects as a sunken reef at severa points far distant into the boo, thus forminj headlands with intervening bays. South wan bgain of the Pareora river tho coast line i one long stretch of shingle beach. Owing to the configuration of the coas between the Pareora and Timaru the shingl gaqnotbave any steady progressive north

ward drift. It is trapped at the headlands during S.W. seas, driven southward by -N.E. ones, and drifted northward by S. B. waves ; from this quarter come the only really heavy seas to whicn the breakwater is subjected. During tho three years I have been Engineer to your Board I have continually made experiments and observations in connection with tho subject under consideration, and have carefully watched the effects of heavy seas on tho beach between the Pareora and Timaru, in order to arrive as far as possible at a right understanding of tho natural laws governing the existence and movements of tho beach in question. Without going into details, I may-say that 1 have satisfied myself of the truth of the following statements :

1, That the movement of the beach is intermittent and varying, sometimes stationary, sometimes from north to south and at other times from south to north : the greatest movement is from south to north, but these movements are not on the immense scale that has heretofore been supposed by some. 2. That when the beach is projected into deeper water by any obstacle and is thus exposed to a heavier wave stroke its reduction into sand is much accelerated.

3. That a great quantity of beach accumulation is destroyed by being ground into the finest sand, is deposited on the sea bottom and drifted northwards in suspension by a strong current which is created by every B.E. storm or heavy sea. Tho undestroyed balance ultimately accumulates near the breakwater. FUTUBE LINE OF ACTION RECOMMENDED. At present I advise the board not to take anj furthersteps than these described as being in course of adoption, but to wait and closely watch their effect under ordinarily favourable circumstances. The beach has yet 1000 feet to advance before it is possible that any ill effect can arise from it, and as it has assumed an outline near the breakwater almost at right angles to the prevailing run of the sea, I am confident its progress must be very slow in future, if not stationary or retrogressive. Should it continue to advance it will encounter waves of much greater strength, even than those which beat upon it now, with a correspondingly quicker reduction into sand. Eventually should the board desire to check its possible further increase, I recommend the construction of two timber groynes,one at Patiti Point and one at the rock island, the first about 150 feet long and the latter about 200 feet; the cost of these would not exceed £I6OO,

These groynes would act in the following manner. They would carry the adjacent beach into deeper water and so expose it to a heavier wave stroke, consequently causing its speedier reduction into sand, also by causing the beach immediately southward of them to assume an outline more nearly at right angles to the prevailing direction of the waves ; the northward travel of the shingle would be much checked at these points and tho accumulation would ho liable to be drifted southward again by tho next N.E. sea. To this proposition it may bo objected, that the breakwater already acts ns an enormous groyne and has as yet not effected the balance of the accumulative and destructive forces. This is so, for the reason that tho breakwater as a groyne is unfavourably situated ; all the undestroyed drifting beach is trapped at the one point only, and by reason of tho configuration of the coast, once there, it remains practically in tho one place. Were this accumulation spread along the beach in the manner groynes would effect, the destructive action of the sea on it would be much increased.

CONSIDERATION OF MECHANICAL REMOVAL OF SHINGLE. In my opinion, mechanical removal of the beach should not be thought of, until every effort has been made to effect the balance of the accumulative and destructive forces. If, in spite of all such efforts, mechanical means of removal must be resorted to at last, I recommend the following scheme : That the Priestman crane should dredge from the breakwater and discharge the shingle into a raised hopper running on the breakwater railway; the capacity of this hopper should bo equal to that of the dredge Taniwha. From the breakwater hopper a shoot could extend under the wharf, and from this the Taniwha could he loaded with shingle in a few minutes. By this means no capital expenditure worth speaking off would be entailed and the working cost would not exceed 2Jd per cubic yard, allowing for discharging tho shingle off Dashing Rocks or elsewhere in the vicinity. To arrive at the annual expenditure which this means of removal would entail the following explanation is necessary. While it is shown that 750,000 cubic yards of shingle have accumulated in 10 years, it must be borne in mind that the first deposits were made in much shallower water than at present, and were consequently less exposed to the destructive action of heavy seas than now. I have shown that as the beach advances into deeper and rougher water, so the undestroyed balance is less year by year. Allowing that 50,000 cubic yards per annum is the average accumulation at present, and that it is considered desirable to prevent the further seaward advance of the beach, I am of opinion that this could be effected by the means described for an annual expenditure of £SOO for working expeases ; repairs and depreciation of plant would increase it to about £9OO per annum at the most. The

rate of removal would be between 400 and 500 cubic yards a day. CONCLUSION. This question of shingle accumulation and its future effect on the well-being of the harbour is of such paramount importance to this district, that I have felt it desirable to leave nothing unsaid that I could say in order to relate the facts of the case and the deductions I have drawn from my experience and observation. In spite of the predictions of many engineers, I desire to record my full confidence in the ultimate success of the work.