TIMARU Harbor works,

Temuka Leader, Issue 2186, 9 April 1891, Page 4

TIMARU Harbor works,

(Continued ) Report of Commissioners dated 2nd April, 1891, in compliance with Chairman’s letter dated 21st ” IOT. This 'is signed by Mr O’Connor only, Mr Goodall pending in separate memorandum. “ Timaru, 2nd s April, 1891, Re Timaru Harbor Works. “ Sie,— In compliance with request ■ contained in your letter of 21st ult., I have the honor to enclose herewith a report on the matter therein defined, which. I drafted for;,signature of Mr Goodall ahcl myself, Add which I have now signed as lully expressing my views on the subject. “ Though written in the plural number, however, this report cannot be taken as entirely concurred in by Mr Goodall, who proposes sending you a separate memorandum indicating the points in which he disagrees. It should.(therefore be taken, in its entirety-, as expressing simply my own views on the subject, “ My only reason for retaining the plural number in the report is, that many of my views therein expressed, are the joint production of Mr Goodall and also of Mr Mqychant in conjunction with us both, and J could not therefore properly lay claim to them as original conceptions of my own. —1 have the honor, &c., “0. Y; O’Cohnoe, M. Inst. C.B. “ The Chairman, “ Timaru Harbor Board,” ' " Timaru, 2nd April, 1891. "J2e Timaru Harbor Works, “ The Chairman ; of Timaru Harbor Board. “ Sib, Having carefully studied the questions referred to us in your letter of 21st ult., we have the honor to report as follows: “As to whether or not the danger from .travelling shingle is imminent, “ We are clearly of opinion that the magnitude end velocity of shingle travel is such, that it will inevitably overlap the works in a short time, if steps are not taken to counteract it. “During the last four years, ending January 1891, the accumulation has progressed 600 feet along the breakwater. : .

“ That this result is not spasmodic or exceptional may be judged from the fact that it almost exactly agrees with the calculation of probable result made in August. 1881. “It was then estimated that the high water line would reach to a point Bso feet, froqi root of the breakwatei by January 1893. “As compared with this estimate, the high water line, had got to 560 feet from root of break water in January last ; and, if it progresses for the next, two years at the, same rate. as. during the last four years, it will have got to"'810 ieet from root by January, 1893. Thus the calculation made ten years ago seems likely to be very closely borne out.

“It is true that the estimate made

in August, 1881, is not exactly verified in aj| its; results in all its details. The visible accumulation averages only 50,000 cubic yards per annum, instead of 100,000 cubic yards, as estimated (a good deal of the balance having probably been carried round the bfeakwater in the form of sand), but, as against this, the triangle of gather ing ground, oyer which the shingle has spread itself in the vicinity of the breakwater, is somewhat more acute than was estimated (also probably due .to the grinding of the shingle into sand), and, as it happens, these two discrepancies very nearly balance each other, so that, the position ■ of the shingle along the edge of the breakwater itself is, as already stated, almost exactly what was estimated *• Ihe conditions on which these

calculations of August, 1881, were Based still continue to prevail, and, seeing that these calculations so lar are fully borne out by the results, it would be deliberably shutting one’s ©yes to the inevitable to ignore the probability of corresponding result continuing to accrue in future. V Tfaat the prpgress during short periods of time 'is not regular and uniform proves nothing. It would, i > fact, under ..the, circumstances be unreasonable to expect uniformity, and it is not inconsistent with the conditions that, there should at times be retrogression rather than accumulation “ There have been such retrogressions during the last four years, hut we have nevertheless the fact that the accumulation along the edge of the breakwater during the first two years of these four yeiays vfas ,260 feet, and during the second two years 240 feet.

“ This undonbtably indicates a close

approach to uniformity of progress over reasonable intervals of time, and -jria^ly_J^Bortg_.,the > , contention already advanced, that the progress during the last foiir years thould he regarded as probably normal and reasonable father than spasmodic or exceptional., ‘‘To be reason ably careful, therefore, ; it should be assumed that the rate of progress of shingle accumulation along the edge of. the breakwater will be fully 125 feet per annum, and that it is even quite possible that it may be considerably more than that. “ The toe of the shingle spit is now within 630 feet of the end of straightcut mole. Beyond that point there is sl curve for 340 feet, and a kant for 078 feet, total 1018 feet, trending tpwdrds the nopth.

“Presuming that the conditions up to end of the straight-out mole remain as hitherto, the end of shingle spit, advancing at the rate of 125 feet per annum, may be expected to reach the end of straight-out mole in 4i years. There is, of course, no positive certainty that it will do so, but, on the other hand, there is no positive certainty that it will not, if unimpeded, get there very much sooner. “The volume of retarding influence, due to the back-wash from the breakwater, is continually getting less and leas, and the triangle of gathering ground may quite possibly become more acute, leading to the formation of a narrow spit which would travel along the breakwater with great rapidity, “As to what value should be put upon the 1018 feet of curve and outer kant, in the way of postponing the time at which the shingle would begin to block the entrance to the harbor, it is difficult to say, but we cannot see our way to allowing much for it.

“ Under somewhat similar circumstances at New Plymouth, the accumulation of sand, which bad taken several years to get to the end of straight-out mole (1400 feet) went pn the remaining 550 feet, along curve and north-easterly kant, in about 4 months.

“ Also, where the circumstances favor a rapid travel, as along the beach between Patiti Point and Timaru, it was found, by experiments made by Mr. Balfour, that boulders traversed from say 500 to 5000 feet in a day. “In view of all the circumstances of the ease,therefore, it is clearly evident that steps should at once be taken to avert the danger of the shingle overlapping the worfes, which seems to be at present imminent, and that the measures to be adopted should be such as to admit of this being carried out promptly, effectively, and continuously,, for many years, or possibly for generations, To wait longer before taking action would be very unwise.

i "In view of the uncertainties and contingencies involved in the situation, it is highly desirable to have an ample margin of time to come and go upon.

Best Means op a vesting the Dangee.

“ Only two means of preventing destruction to the harbor from the growing accumulation of shingle appear to be practicable, namely,— (I) the retention of the shingle at the south side of the breakwater, either by eztending the mole in its original direction (or possibly in a direction a little more to the east), or by isolated groins, or else (2) the dredging of the shingle accumulation as it accrues. “ Of these alternatives, we may say "at once the dredging is the preferable one, as being the cheaper, and also the more likely to give satisfactory results, in view of all the circumstances of the case. It is desirable, however, that reasons should be given for this opinion, which we proceed to give accordingly.

Alteenative No I— Extending the Mole.

“To prevent the shingle overlapping the works, by extending the mole, would involve a length being added each year, equivalent to the probable growth of shingle bank, say 125 ft. This, at the low price of £BO per foot, based on last contract, but allowing for rise in price of cement, would coat £IO,OOO,

“ As against this, there would possibly be a credit from value of land reclaimed, but it is very difficult to say what it would amount to. “If triangle of accumulation prej; serves its present shape, the area o accumulation between rock island and breakwater, due to each 125 feet of accumulation along the breakwater would be about three acres. Of this in view of the shape of the land, fully one acre would be required for streets, leaving two acres available for sale or lease. T his, if worth say £2500 per - acre, would be value for £SOOO.

“ The net cost of annual extension would thus be £SOOO, which, as will be shown hereafter, is much more than the probable cost of equivalent dredging.

“In addition to this too, there are reasons why it would be desirable to dredge the shingle and convey it round the harbor rather than retain it all to the south of the work, as, for instance, the desirability of affording protection to the beaches to the north, which might otherwise become denuded. Also the desirability of avoiding an excessive grinding of the shingle into sand (probably becoming greater and greater the further it extends along the breakwater) which would involve the danger of shoaling outside the harbor, by the sand being carried in suspension by the sea. “It may possibly be claimed that the cost of extending the mole should be subject to a rebate, on account of the advantage which it might have towards a future extension of the area of the harbor, but we think very little could reasonably be allowed for this, in view partly of the long time to look forward to before it could be utilised, but more especially in view of the direction which it would probably have to take in order to be effective under existing circumstances,

“ Had the straight-out kant been originally prolonged in its original line it would have been a different matter, as the cost of the works as a whole, including, say, 1000 feet of mole extension, would thus have been some £50,000 less than if such extension were made now ; but, taking the circumstances as they stand, an

extension of the mole for the stoppage of the shingle travel (even omittiiig all consideration of its other disadvantages) could not well be justified (ip view of its additional cost over cost of dredging) on the basis of any use to which it might be put towards extending the hsrbor area hereafter. Alternative No. 1 (Continued) Goins, “ As correlative to the alternative of extending the breakwater another method which suggested itself for stopping the shingle travel was to construct isolated groins on the beach to the south of the breakwater. This, however, on investigation turned out lo have nothing to recommend it While involving all the disadvantages which an extension of the breakwater would involve, it would at the same time be much more costly That is to say, the cost of retaining a given amount of shingle by means of such groins would be considerably more than it would cost to retain the same quantity of shingle by lengthening the breakwater. (To be Continued.)