DREDGING AT THE PORT

Taranaki Daily News, 16 January 1931, Page 11

DREDGING AT THE PORT

MAINTENANCE OR CONSTRUCTION’? DIFFERENCE OVER THE AUDIT. In connection with the balance-sheet for the year ending September 30, 1929, the New Plymouth Harbour Board yesterday passed a resolution charging £6062 17s dredging to the maintenance Account instead of £2500 as estimated by the board. There had been a difference of opinion between the Audit Department and the board as to the estimate of the proportion of the total dredging costs to be allocated to maintenance. The board estimated £2500, but the engineer, Mr. G. W. B- Lowson, gave a certificate for £6062 17s. A committee comprising the chairman (Mr. C. E. Bellringer), Messrs. E. Maxwell, C. H. Burgess and J. H. H. Holm went very exhaustively into the matter and prepared a voluminous report dealing w'ith the w'hole question of the sand°drift, and this accompanied by plans, documenta and data bearing on the subject, -was *forwarded to the Audit Department. The report states, inter alia:—* It is necessary to make clear that as opposed to a natural harbour, in one portion of which shipping accommodation is provided and into which generally there is natural drainage by streams and rivers and deposit of sewerage and water from city and numerous ships, from all of which sources there is siltation, the Port of New Plymouth consists of a small area of once open coast now very effectively protected by a breakwater. It is not subject to any siltation arising from sewerage, drainage or waste from the town or from any streams or other' natural sources, and therefore the only possible cause of siltation, the removal of which only can be considered “maintenance,” is from drift of sand or other material around the end of the breakwater into the 1 harbour. This and the cost of its removal is the whole question. “SUMPS” IN HARBOUR BED. The obvious and inevitable result of dredging out what amounted .to great sumps, in increasing the depths at the Moturoa -wharf from 13J and 15J and in one place to 22ft. to generally over 30ft. and. from 9it. to 10ft. at the Newton King wharf to 25ft. on the shore side and°3sft. on the western side and the narrow channel of 12ft. to 15ft. depth to a wide fairway of 25ft. to 27ft. depth •was the general movement of all loose material from the sides of the dredged parts and from all shallower parte into it., deeply dredged parte until the whole floor of the harbour would be lowered, so far as the depth, of loose material would permit, in conformity to the natural inclination of such to the bottom of the deepest parte. This general deepening is what would be desired, and it would greatly assist in carrying oiit the improvement work and lessen the cost as the dredge’s, actual operations could ■ be ; confined to comparatively small areas of the total. This is what actually took place. ■ None of this material coming from other parts of the harbour than those actually. dredged- over can; by any stretch of imagination be actually separated -from : the actual improvement dredging any more than can the material necessarily ? removed from the side of a cutting to, .reach a safe battei', however excessive that- batter might be owing to the nature of the material to be treated as a separate cost to that of making the cutting a stipulated width at the bottom.

The cost of maintenance dredging cannot with any fairness be placed at anything above the cost of the removal of the actual drift into the harbour. Mr. Lowson, since this question had arisen as a result of his giving a certificate greatly in .excess of the board’s allocation, has provided an instance most clearly showing the fallacy on which he bases his estimate. It is thoroughly established and now a well-known fact that shingle and sand drifts are not due to ocean currents but to the action of waves on a drifting base provided by a beach or shoal. The progression being somewhat saw tooth in form, the waves as they reach shallow water stirring up the material and carrying it up at an angle inclined to the equator —to the north in the southern hemisphere—and on receding drawing it straight back.

DRIFTING OF SAND. It is also a thoroughly established and well-known fact that sand and other heavy material deposits in deep water will not drift at any considerable depth below low water, varying of course according to the material and the weather. No heavy material will drift at such a depth as 20 feet below low water, except under most extraordinary conditions, though such material will move gradually, but only during storm weather or deep disturbance by th© movement of vessels, from heights into depressions in any direction at such depths. The progressive movement of a drifting base along a shore or shoal is required with depths from nothing at high water to comparatively little depth below low spring tide. The harbour has been dredged out to depths, right from the outside along the fairway to the root of the Newton King wharf with considerable width and around both wharves, at no place less than 25ft. and up to 30ft. and over, thus a complete and impossible barrier is provided to any drift entering the harbour crossum to the eastern side on the Newton Kang wharf. The plan shows that a l>erth of considerable width has been dredged out on the eastern side of Newton King wharf—this was some of the latest work and much of it is quite new. Recently the engineer was instructed to clear this berth of the deposit superj icum’bent on the floor of the recent dredging, with the result that the accompanying letter dated June 19, 1930, was written by him to the board in which he pointed out that the work referred to would raise the cost of maintenance dredging for the year by not less than £2OOO.

This berth is close to the shore. The wide part occupies a position where there was naturally only from a very few feet of water down to 6ft. or 7ft. or 9ft. at most; therefore it is in fact in the nature of a deep sump—2sft. low water spring tides, close to a sandcovered beach and surrounded with a deposit of sand of considerable thickness.

The inevitable result is that in rough [ northerly weather, or even at other times, every wave would wash down as it recedes from the beach or shallow surroundings its quota of sand from the shallows into the sump, where it is trapped until the whole of the material that is movable by wave .action is washed in. This process may not be completed for a long time; in other words, until the whole of the surrounding*'or the cutting have reached a stable inclination which will be nothing short of absolutely baring the rock. It is obvious for the reason previously referred to, that of the depths of 25ft. and over acting as an impassable barrier to drift from the outside, that the soft material washed into this berth c< uld not to the very least extent, let alone the 5 per cent, estimated by Mr. Lowson, consist of any deposit from drift into the harbour —-otherwise what may be termed “maintenance” material -—and the removal of these deposits washed in from higher surrounding into the deep sump is s simply a part of the necessary work and cost of lowering and lessening excessively the material depths and therefore improvement .work and wholly chargeable to loan account. The only question now is the amount of the annual drift into the harbour, the removal of which is necessary to the maintenance of the permanent average depth which has been attained. The average annual drift was estimated by Mr. W- H. Skinner, of the Lands and Survey Department, who had carried I out surveys and soundings of the har- |

hour spread over a period of years, tq be 16,500 tons —cubic yards. The Paritutu was built and purchased for carrying out work undei” loan expenditure and was of a class much more costly by several times than necessary for removal of sand drift—maintenance work. The cost of removing the hard conglomerate and heavy rocks involving such extra costs as diving, blasting and tearing up rocky faces, entailing excessive cost in repairs, is obviously many times over that of sand pumping. Therefore whilst the average cost per ton is Is Ojd, of which the cost of repairs represents 4d, the cost of removal of the annual drift into the harbour would not likely represent m jrc than a. quarter (3d) of the total ton per cost. As no absolute exactness is possible the allocation of £2500 for maintenance made by the board was intended to be a very liberal over-estimate, and obviously was so.