SHELTERING—SHOALINGDREDGING.

Timaru Herald, Volume XIIC, Issue 13880, 17 April 1909, Page 2 (Supplement)

SHELTERING—SHOALING—DREDGING.

To the Editor of the "Timaru Herald." Sir, —The slopes of rubble breakwaters are the angles of repose' assumed by the loose stones, after having been knocked about and consolidated under the influence of storm waves and groundswclls. From low water up to high water and slightly above it, the best known breakwaters in Britain run 4, 5, (i and 7 feet horizontal to 1 foot vortical on the weather side, but are steeper on the inside —so .the best works, of reference tell ■us. Hubble breakwaters steep and narrow throughout, and built Up to IS feet above high water (as is now suggested here by "Herald" correspondents and others) are not mentioned in works of reference and are beyond my ken. The heights and slopes i of rubble breakwaters at ports similarly situated to Timaru would be interesting, and should be ascertained. Ours is said to be 9 feet above high water under the centre rail-track, the. slope is not given. This is higher than the best known breakwaters in Britain. It is very significant that in the storm of March 30th, rubble was displaced where the upper part had been packed steep and narrow. The slight injury clone to the superstructure, should bo taken as a caution of what might have happened if the seas had been forced up high enough to strike the 'square' beams of the staging. The power of waves on striking a 'flat' surface has been seen iu Timaru. On one occasion six 32-ton blocks of concrete were lifted from a sunken offset on the breakwater, and rolled into the inner harbour by waveblows. Of course, the staging is much higher above water. ! Fifteen years have not yet passed since it was first mooted that overlap-, ping the entrance and fairway to the harbour might soon seriously affect the navigation if great care was not exercised. A Roval Commission report dated October '.3rd, 1894, states chairman objected to any extension at the end of the existing breakwater on the ground that such an extension would induce shoaling in front of the entrance by the sand now carried past the harbour in suspension during rough weather being deposited in the lee of the proposed extension. The reason requires careful consideration, etc.'" "With a view of accounting for the shoaling and indicating possible factors in prospective harbour troubles, the same objector in November 1895, sent the following memorandum to Messrs' Bell and Maxwell—(a) -'That much pure sand and silt is brought down to the sea by the Canterbury rivers when in flood, (b) That the adjacent coasts arc flat and the sea shallow, the bottom composed of soft alluvial and clayey soil, easily worn by seas breaking on and disturbing the bottom, (c) That the fine silt and sand lying on the sea bottom is carried in suspension along the coast wherever the sea bc-r comes rough and yellow in colour, and only sinks again when the sea becomes calm, or where circumstances are favourable for sinking, (d) The area between the elbow of tho north mole and the shore opposite, has not only been converted into comparative still water, thus favouring precipitation, but acts as an effectual trap in retaining all the silt that is ever lodged there." Mr C. Napier Bell, in December, 1895, reported, " It appears to me that the danger to the harbour by permanent shoaling of the entrance comes from the shoaling out of Caroline Bay. The advance of the shoals of Carolino Bay towards the entrance is a feature of the case which is difficult to deal with, for the reason, that not being an isolated shoal, but being founded on. dry land, it will extend its shoaling influence along way out and certainly far enough to involve shoaling all round the entrance." Again, " but the serious shoaling which is most to be feared is that which is slowly creeping out from Caroline Bay." Tho rapidity of sand deposition m some recently constructed harbours has been almost incredible. A few years ago green seas rolled over the twofathom lino in Carolino Bay, whero children now run about on the seaborne sand at high water. Presumably the solo reason offered in advocacy of extending the rubble wall 500 feet nearer to "the north beach, is to give ' more cover' to tho entrance of the inner harbour on the very rare, occasions of a north-east storm. There, will always be an occasional ' range as long as our inner harbour has an entrance. Some Home ports (Dover) are now adopting ' wave-traps ' inside of tho entrance to lessen the rango before reaching the shipping berths. Another ' wave-trap'—(cost about £250) situate say 20 yards inside of the entrance would materially help in intercepting the ' run ' at Timaru on the rare occasions alluded to. Now that still water within the lee of the .outer rubble wall is favouring the sinking of sea-borne silt arriving in suspension, and the scouring action of the prevalent southerly sens has almost ceased there; it h apparent as stagnation'is''.still further increased that efficient mechanical dredging must bocome the sole factor in keeping the port onen to the regular uninterrupted trade of vessels of large tonnage. The maintenance of the " present depth on the route of larpc vesscjs in a wide track from the end oi the rubble wall to tho

entrance of tho inner harbour "niav alone keep the WOO ton dredger, pretuv busy yet, although almost everybody siuu sue was twice too big-for'ail' our prospective requirements. ' ' 'liie toree 01 circumstances in a few years may cause tne guardians of the inner harbour to roll wf tne top stones on the rubble nail, commencing, say, at about VUU lect oift, making it a'jiuiiticlo wall to tile 'end' las was originally sanctioned by all nanus;, and so allow tne triemily southerly seas to do a share ot ''dredging" icee of cost. The policy of "peace i,i our time" may be popular, out 1 think common honesty demands that wo shouiu pause, waten, and iearn the effect of "silt-shoaling under present conditions, m tlio interests of the other fellows coming up, who, wo may bo sure, do not want a "sand trap" instead or a hiirbovr. 1 undcrstiuid that tlio resident engineer is engaged in taking- soundings inside of tliu oU'ccr wall. The result may show that other unseen aspects of comparatively calm water require prompt consideration, say the amount of silt lodging in tne precincts |or the fairway to the entrance of the inner harbour, etc., since the previous soundings were registered. There can bo no doubt whatever that the straight portion of the outer rubble wall commencing at the root has conferred a permanent unalloyed benefit on the port of Timaru, and on the inhabitants of South Canterbury. The portion thence, curving round to the north appears to the eye to be of benefit, but, it is too early yet to guugo whether its effects will bo a benefit or a costly permanent disability. To extend it still further towards the beach would in my opinion be imprudent. Durban is a sand-haunted harbour. Timaru ■ is : a sand-haunted one. Our Board should ascertain and make public without delay, say, the present dredging power at Dnrban, and the yearly upkeep and cost of dredging, etc. Drcdgo 350, since arrival, lias, in addition to other good work, deepened a, portion of tlio entrance channel by lifting about 200,000 tons of the original bottom. I await with curious interest Mr Clarke's report of present depths over this deepened channel to the world's highway. — 1 am, etc., l) x . STUART. Timaru, April 14th, 1909.