MOTUEKA HARBOUR ENTRANCE

Nelson Evening Mail, Volume LXIV, 21 January 1931, Page 9

MOTUEKA HARBOUR ENTRANCE

ENGINEERS’ REPORT TO THE BOARD RECOMMENDATIONS OUTLINED Wiien the report on the Motueka harbour entrance was received by the Board at its last meeting it was decided to hand the report to the Press with the request that it be published with a view to placing before all ratepayers the latest information obtained by the Board in connection with the harbour. The Board is to consider the report at its next meeting. In a letter concerning the report the Board’s consulting engineers state:—

VYo forward herewith our report on the. question of improving the harbour entrance. There is no doubt the conditions are rapidly becoming worse and will continue to do so, for as expressed in the report we see no reason why there should he any stoppage in the growth of the bank and therefore of the extension of the outer channel.

The Board is thus placed in the ililficult position of deciding whether, as a. result, shipping is likely to be reduced or freight rates increased and, if. so, what extent of loss is likely to the Board and the district generally. It depends on the answer to the above whether any works or of what extent of works are justifiable. Unfortunately, we can see very little good accruing without spending about £25.000, with the chance of having to spend more in the future. On this last account we consider that an ample sinking fund should be provided. Wo agree with your suggestion that the best method of financing would be by loan and rate lather than by unduly increasing harbour charges, though these arc now very low and could hear some Increase.

The report is as follows:—The Secretary, Motueka Harbour Board, Port Motueka.

Sir.—ln accordance with your letter of 29th .March asking us to visit Port Motueka and make further investigations and with the verbal instructions received at the Board meeting attended by Mr Vickcrman on 2nd May, 1950, we forward the following report dealing with proposals for improving the adverse conditions existing at the Harbour entrance and which arc now seriously inconveniencing shipping and threatening to presently materially affect the business of the Port. Previous Investigation and Report: We first reported on this question on 23rd July, 1928. Since then you have on numerous occasions in your monthly reports drawn the Board’s attention to the persistent and rapid growth of the sand bank and to the difficulties being experienced by shipping. A joint inspection was, as a consequence, made hy Mr Furkert, Engineor-in-Chief, Public Works Department, and the writer on the 23rd May, 1929, followed by a marine survey of the harbour and entrance undertaken by the Public Works Department and hy reports by Mr Furkert and ourselves in March 1930. Since that dale we have at the Board’s request still further investigated the matter. All the reports stressed the rapid growth of the sand bank, more particularly since the time it advanced across the old entrance and so turned the tidal stream running out from the Lagoon that this discharged parallel to the coast and thus ceased to lie operative in stilling up and sweeping away the end of the hank. They suggested as remedies the abandonment ol the entrance used at present and the construction of an inner channel to and the development and use of the south Moutere entrance; or, alternately, the construction of a training wall from the southern side of the present entrance to more or less restore the old direction ol flow of the tidal stream and make it once again capable attacking _ and removing the sand. No final decision was made between the two proposals hut 'this rested on their relalivo costs and the likelihood of the training wall proving successful in view of the large quantity of sand which is constantly being driven from the northwards as well as on developments generally in. connection with the sand bank. The reports also recommended deferring carrying out works till further opportunity had been given for studying the growth of the bank and till it was definitely seen that shipping could not be satisfactorily carried on without them. Result of Investigations: The investigations made indicate that the sand is brought along the coast by the trend of the seas, tides and winds from the Motueka river which drains 780 sq. miles of country and enters the sea about 4 miles to the North, and that there is therefore no chance, apart from the limited trapping which might be effected by fostering the growth ol sand dunes, of any cessation in the supply. As was pointed out in our report of 1928 the extension southwards of the end of the bank was then becoming more rapid than formerly, and the condition lias been maintained, from that date until the survey in October 1929, the growth southwards of the end of the spit at low water was ome 20 chains and this growth is still going on. The increased rate doubtless results from the deflection of the stream from Moutere Lagoon, and which now helps rather than obstructs the formation of the bank. , Besides lhe extension southwards there has also been a gradual displacement of the bank seawards, but there are unfortunately not sufficient surveys available to gauge the extent of this accurately or to enable any really definite opinion to lie given as to the rate at which further movement is likely to occur. It would appear that the position has been affected by changes m Llie position of the starting point ol the hank and it may now possibly bo on. a more or less permanent line. At the same time some advance seawards will have to bo expected in the vicinity of the Harbour entrance as the changed conditions due to the great extension ol the spit southwards appear likely to involve this.

MOVING SAND From the soundings and measurements of the hank which have been taken from time to time if is deducted that an average of some 600 cubic: yards of sand have been brought along in each 24 hours. Study of the line ol the bank snows that whereas from the Motueka river to the old entrance this is approximately south, it is now bending round a nillowin'' a south-south-east direction parallel to the coast. This demonstrates and it is very important from " *arliaur construction point of view, that, whilst the flow from the lagoon is, without the aid of training walls, unable to actually pierce, the sandbank it is capable of controlling the direction of its growth, and of maintaining an open channel behind it. If therefore appears unlikely that Urn lagot'u can he permanently (docked, but naturally navigation must as time goes on he interfered with more and more, and must become increasingly difficult.. The channel now being maintained is some 4ft. to sft. in depth at .D.VV.S. 1. and from 1000 to 1800 sq. ft. in cross Sectional area, also at L.W.S. 1. Owing, however, to its uncontrolled condition the depth and direction of the middle of the channel is constantly changing and thus interfering with shipping.

The accepted tide water levels on the Port datum are as follows: 11. \Y .O.S. 1. 100. GO; LAY.O.K.T. 87.00; II.W. Neap Tide 95.00; LAY. Neap Tide 89.00; HAY. Intcrmcdialc Tide 98.00; LAV. Intermediate Tide 88.00. TIDAL FLOW Measurements of the tidal bow passing the wharf show that the bulk of the water passes during the first three and a half hours after the tide has commenced to ebb. During an observed spring tide (range 99.35 to 85.90) the quantity discharged during this period amounted to 230 million, cubic feet with velocities ranging from 12.3 to 3 loot per second, equivalent to a stream oi 21.000 ettsees and to an average velocity of 9.4 feet per second. During an observed poor Neap tide (range 95.50 to 91.20) for the corresponding period the quantity was some 28 million cubic feetwith velocities of from 1 to 1.2 feet per second, equivalent to a stream of 2700 cusccs and to an average velocity of 1.05 feet per second. From the observations of these tides it is deducted that for an intermediate tide (range 97.50 to 90.50) the outflow would be 140 million cubic foot- equivalent to a stream of 12,800 eusecs and to an average velocity of about 4.5 feet per second.

At the harbour entrance these quantifies would be largely increased, probably 50 per cent., bv the water Irom the old harbour, but under the present uncontrolled conditions the average spring tide velocity corresponding to the above is, near the entrance, only about 4 feet per second and the neap tide 1.16. Seeing that the velocity of only .4 or .5 feet per second is required to move sand, the transporting capacity of the. tidal currents, particularly- those at spring tide, if properly directed, will be readily realised. So far‘as the possibility of successfully constructing a training wall is concerned the natural conditions are quite favourable as it would be ill comparatively shallow water, lying end on to the seas, which, owing to the gradually ,shoaling sea bottom and to tin shelter afforded by the sand hank would lie broken before reaching it. Training Wall: Basing our opinion on the evidence outlined above wo consider, as we reported previously, that, a training wall built out Irom the shore near the southern, side of the entrance channel could he made to function quite successfully, provided if is carried a sufficient distance seawards. The exact length to give a reasonable measure of relief is dillicull In forecast and in this experience must largely guide. Our opinion is that the wall ought certainly to extend in the first instance to near' the outer red buoy involving a. length of 1070 I'cct of wall. If must, however, l»e realised that the probable (•float of this will not lie to make a new onlanee by piercing the. bank, but merely to remove it, say, 10 chains seawards, where it will be maintained and continue to grow southwards as at present. .Moreover, except near the wall there is no certainty of obtaining any greater depfli of water, nor is it certain "hat Uie south end of the bank will form any further seawards than is now the case. Except therefore for casing navigation into the wharf channel and giving more seaway at the turn, the situation will be much as it is at present and but little benefit will be derived. Nothing short of carrying the wall at least as far as low water mark, on the outside of the spit, that is lor a length of some 1800 feet, will prevent the bank developing southwards and that only for so long as it does not develop seawards, which, as mentioned above, is quite likely and in which case the wall would have to be correspondingly extended. As pointed out above the probable ratio of growth seawards cannot bo forecasted, but it docs not appeal' to be cheapest type of permanent wall, as well as the simplest to build, particularly if the work were, to be undertaken intermittently and in AngcL would be one constructed of "pen mcll concrete or stone blocks. The alignment must now be largely decided by past events. Probably that proposed for the channel by Mr Austin in 1911 would have been rather better adapted to enable a wall to deal with the sand than that now reasonably possible and giving a straight channel to the wharf. The latter should, however, prove satisfactory, but if cxnericnce indicates the desirability it could he modified to suit as the work proceeds. The level proposed for the top of the wall is 96.00 feet, that is 1 ft. above HAY. level of Neap tides and 4 feet below HAY. level of Spring tides. Hie side slopes could be li to 1 at the inshore end, but would renuire to be £ to 1 at the outer end. The top width adopted is 4 feet with a monolithic concrete slab carrying a tramline for construction purposes. . Estimated Cost of Training Wall: Ino following estimate is based on a wall ol the dimensions mentioned above and on the use of concrete blocks niacle ot 1 part cement tu 8 parts Jaekelt’s Island shingle and which should be capable ot i manufacture for 30s per cubic yard, t is, however, quite possible that suitable stone could be obtained from the loiiga. Bay granite quarry at a lower cost.

This estimate is considerably less than that previously made because the size and height proposed for the wall and the contingency risk have, in conformity with the results ol later investigations, been correspondingly reduced. 1 1? consideration of the proposal the likelihood of having to extend another 300 or 400 feet at a further cost of at least £7OOO should not he overlooked. South Moutere Entrance: . We ha ve again considered the utilisation of this entrance. The position is still as outlined in our report of duly 1928 except for the rather vital point that the sand hank has since then greatly advanced

and now covers a length of 40 chains out of the 120 chains between the harbour entrance at present in use and that at south Moutere.

It therefore appears quite possible that within five years’ time the growth of the sandbank may call for works to repel it at South Moutere also and if that proves the ease no benefit would result from a change of entrance. Conclusion: The position is thus that the sand bank lias overlapped the entrance and its end has reached a point 40 chains south of this and is continuing to grow steadily at the rate of some 17 chains per annum. Shipping is therefore forced to take the dillicull channel behind the bank and parallel to the coast. This is already a very serious matter with the channel only 40 chains long, and must become increasingly so as it steadily lengthens. Fortunately, so far, except that the. position lias rendered shipping very difficult and imposed an undue amount of work and responsibility on the harbourmaster, the revenue has not been affected, nor have freight charges been increased. This cannot be expected to continue indefinitely as delays to shipping due to working to tides or going aground will, if they become, as they must, increasing frequent, be accompanied by higher freight rates. So far as the utility of the port ts concerned the value to the district as gauged by the nett annual saving effected on account of the lower cost of direct shipment from Port Motueka is variously estimated at from £12,000 to £20,000 per annum and with the increasing production from the land this amount .should steadily grow. The best method of dealing with the trouble is, it is now evident, to build a training wall on the south side of the present entrance. This ought to be some 1800 ft. long in order to reach low water mark outside the harbour and would cost about £23,000 and, in view of the rapid southward growth of the bank with its consequent annual increasing interference with shipping, it should be finished within, say, 4 years. This would with interest and sinking fund involve an annual expense of £I6OO. The work could be undertaken in slower stages but, unless' the sand bank can bo pierced to provide a new entrance, and this will probably not result till the wall is about 15001't. long, the shipping will still have to enter and traverse the outer channel as at present, except that each year the channel will be lengthened and therefore very little, if any, real benefit will be obtained from the works.

We have to again, thank you for much valuable survey and other information. Seeing the importance of knowing how the sandbanks are forming we would suggest that aerial photographs be obtained of tlie coast from South Moutere to the Mouteka River, the photograah to lie taken at the time of low water and preferably during spring tides.

Item. Longtl of wall ft. i Cost . of llem. £ IMiinl, Tramways, Wall Wall etc. — 70 500 600 78 2120 W-ill 500 6175 Wall 400 6960 Wall 300 7067 Totals 1770 £23,000