REPORT OF MR GOODALL'S HARBOR SCHEME.

Daily Telegraph (Napier), Issue 4006, 24 May 1884, Page 4

REPORT OF MR GOODALL'S HARBOR SCHEME.

Acting on the instructions which we had the honor of receiving from you by telegram, requesting us to place ourselves in communication with Mr Goodall, we proceeded to Timaru and there inspected, the breakwater and the adjacent coast. We then came on to Napier, and, receiving your instructions by letter dated the 14th, instant, wo proceeded to examine the site of the proposed works, and afterwards the coast to tho westward as far as Waikari Point, and further by steamer for several miles from tho northward. We subsequently examined the coast to within three miles of the black reef at the Kidnappers. We were assisted in the investigation which we made by evidence kindly given us by Mr C. Weber, Mr J. Goodall, Mr Kraeft, and Mr Saunders, and from the information thus collected we now submit the following report : Mr Goodall's design on which we are to report is the last of many proposals for improving the harbor at Napier, all previous ones being considered more or less unsatisfactory, chiefly from not affording sufficient guarantee of security against the assumed danger to the works from travelling shingle, or from the deficient depth of water secured by the proposed works. The works proposed by Mr Goodall consist of a breakwater starting from the north-cast corner of the Bluff, and running in a northeasterly direction for 1410 feet. The effect of this position is to receive the waves at such an angle as to cause them, after striking the pier, to rebound in a direction nearly parallel to the shore line. It is assumed that this action will drive the accumulations of shingle away from the works. This length of 1410 feet Mr Goodall assumes to be sufficient to secure the required amount of reflection from impinging waves. From the extremity of this length the work is carried in a northerly direction a further length of 1060 feet, which provides shelter for shipping lying inside the breakwater. The works are capable of future extension in a north-easterly direction if required for any distance, as far as the Auckland Rock, but this is not provided for in the estimates. The design provides a railway wharf 1900 feet long, on the inner side, and the positions are indicated of three jetties not included in tho estimates. There is a line of rails laid to Spit town, with a road six feet wide alongside. Provision is also made for the protection of the shingle beach from the railway in the eastern mole of the present works, with sheet-piling. The proposed harbor shelters a water space, in which the depth varies from 15 to 32 feet at low water, with a rocky and uneven bottom. The large rock known as the Auckland Rock rises to within Bft of low water at a point 1400 feet north-west of tho end of the proposed breakwater. The berthage which this work will_ provide is, approximately, for six ordinary ships or large coasting steamers, and one largo steamer of, say, 400 feet in length, besides affording anchorage for five or six ships under the shelter of the works. The breakwater is proposed to be built of concrete blocks, weighing about 27 tons each, and capped above low water with larger blocks of concrete, in mass measuring 36 feet by 20 feet by 9 feet 6 inches in greatest height. The width is 36 feet over the blocks up to the height of 3 feet above high-water mark, but the outer portion rises 3 feet higher, or. 6 feet above high- ' water mark; being 20 feet wide, which leaves a shelf 3 feet wide on each side. After reaching a depth of 18 feet to low water, the concrete blocks will rest upon a mound, of rubble or loose stone, and on the seaward side large blocks of stone are to be deposited to protect the foundations of the concrete blocks The wharf is of timber, the piles being of totara, which is found to resist the seaworm better than most other timbers. The position chosen for the work is fully exposed, and receives no shelter from the Bluff, the object being to secure the work from the encroaching shingle, it being assumed that this should be encountered on the beach trending to the southward. This gives room for the back-wash of the waves reflected off tho pier to drive the shingle as far as its effects extends, as is seen in tho case of Timaru. We consider the site to be well chosen. It is easily accessible, and would occupy a very similar position as regards the present port as the breakwater does to the town of Oamaru. Tho structure also appears to us sufficiently solid to resist any storms that are likely to occur iv this locality. The experiment of placing the concrcto.on a foundation of rubble stone where the water is over 18ft. deep, has been practised successfully in Europe and elsewhere. We consider }hat the height of tho breakwater, namely, six feet above highwater, is not sufficient for the safety of tho ships inside, or the people or merchandise which may happen to bo on it in rough weather, and that it will bo found necessary sooner or later to raise a parapet to keep heavy seas from sweeping over the works. But this again requires that the rubble foundations on the weather side should be protected with heavy stone, and it may be found that the slopes shown on the plans are too steep to render them secure. The wharf which runs along the inner line of tho broakwator, for a length of 1900 ft., is raised 3ft. 6in. higher than the breakwater, or 9ft. Oin. above high-water, with tho object of allowin" the waves which overtop the concrete to pass under the wharf. This is attended with some risk, both to shipping and to people, Avhich would be mitigated by the construction of a parapet of sufficient height. Mr Goodall, recognising the necessity for a parapet, recommends that it be dispensed with for the present. With this we are not. disposed to disagree, it being understood that sooner or later either the breakwater must be raised or a parapet erected, and much greater stability would be attained by raising both. The line shown on Mr Goodall's plan as . to limit of protection from heavy seas, must not be taken as indicating the boundary of sheltered water in rough weather. On the contrary, and especially in north-east gales, the waves will curve round into the area enclosed by the works, so as to mako very rough water along the wharves and in the anchorage under shelter of the works. This may not endanger the shipping, but it will give i-iso to much trouble and inconvenience, and loss of time. In the work estimated there is no shelter from north-west winds, and we have the evidence of Mr Kraeft, tho pilot, to the effect that north-westerly gales are frequent and of long continuance in summer. The stretch of water in this direction is from four to five miles, and produces a short chopping sea which would be dangerous to vessels lying at the wharves. From these effects it is evident that tho works under consideration do not form what may be considered a sheltered harbor, although we do not think this is condemnatory of the design. It shows that before a secure harbor is obtained, new works must be undertaken in the shape of extendingthe breakwater as shown by tho dotted lines on the plan, and constructing a detached mole enclosing the open space towards the northwest. It is necessary to take into consideration tho effect which the construction of the breakwater will have upon the coast between the base of the breakwater and the present harbor works. The action of south-cast seas will be limited to tho extremity of the shore nearest to the present harbor, but the east and north-east seas will continue to wash this part of the beach in a diagonal direction, carrying the shingle along with them towards the west. When the supply of shingle has been stopped by the breakwater, the waves will have only the naked beach to act upon, which will be rapidly stripped unless some means are taken to prevent it. This action is taking place at Timaru to an extent that has beon considered alarming. It is therefore sufficiently evident that it will occur here. Mr Goodall has provided a concrete retaining wall, with a facing of loose rubble blocks, as far as the railway, and that with a wall of sheet-piling, extending to the present harbor works. We are of opinion that tho beach in front of the sheet-piling will be washed away to the extent of making deep water in front of it, which would lead to its destruction unless protected by a bank of rubble stone. But this rubble, as well as that in front of the railway, will be washed and ground up by the sea, as is found to be the case with much of the harbor stone at Timaru. Consequently the maintenance of

tho whole length of these protective works will be a constant source of expense. This may be remedied by substituting crib work filled with rubble and shingle for the sheetpiling, but tbe concrete wall in front of the railway must depend for protection on heavy blocks of stone, because the foundation is proposed to be on soft limestone rock. The plans show a railway from the Spit to the breakwater with a foot-road opposite it 6ft in width. We do not think this is sufficient for the trrffic of what will become the principal port, and a road 20ft wide will be necessaiy sooner or later. It may happen that the accummulation of shingle will ultimately give a good wide access to the breakwater round the southeast side of the Bluff, by which both the railway and the road to town will be considerably shortened. The shingle to tho north of the present works will become exposed to the same eroding action of the waves as that anticipated above, and we have little doubt that in the course of time it will bo quite removed, and the Ahuriri lake become connected with the sea. To attempt to protect this would bo out of the question, but all the anticipated effects consequent upon the strippage of the shingle by the construction of the breakwater, may be remedied by excavating the shingle v/hich accumulates on the windward side of the breakwater, and transferring it to the exposed part of the coast on the lee-side, as inhereafter noticed. The danger from travelling shingle is that which has most engaged the attention of the many persons who have undertaken to make designs for a harbor at Napier. There can be no doubt that it is the most serious obstacle to bo considered. There is not mnch to bo gained from experience on this subject in New Zealand, and therefore the question of what will happen to works obstructing _ tho course of travelling shingle is still a matter of theory and conjecture. Predictions as to the fate of works to obstruct the shingle have been emphatical from engineers of tho highest standing, and tho breakwater at Timaru was designed and undertaken in defiance of such warnings. The design for Napier is justified by the experience gained during the construction of the Timaru works. We therefore examined those works carefully, along with the plans showing the alterations in the shingle beach (prepared by Mr J. Goodall and Mr J. H. Lowe, of tho Railway Department). The evidence to be gained from examinations of this work were considered as far from complete at present, and we cannot help feeling that there is considerable risk of error from theorising on it. Mr Goodall assumes that the breakwater will cause the shingle to keep of the work which is the case as seen at Timaru. Therefore, if the supply is continuous, the beach will swell out into a triangulaf form. This will cause the new line of beach to become more normal in the direction of the southerly waves and more oblique to easterly waves, with the effect of impending the travel of the shingle. By this combined action, the point of the triangular mass of accumulations may reach a long way down the coast Southward, and the further it reaches the greater will be the quantity accumulated, and the danger to the works will be SO much the longer delayed. It must be understood that the backwash, on which Mr Goodall lays so much stress, is not a phenomenon that will in any way prevent the travel of tho shingle, but will only keep it temporarily away from the works, leaving it still a matter of conjecture how long it will take before the accumulation of shingle covers them up. As to this question, in Napier and Timaru, we have precisely opposite evidence. The 2>ier at Napier, 1100 ft long, was buried in shingle nearly as fast as it was construated, while at Timaru the accumulations form only a swelling of the beach some distance away from the works, and extending for 2700 ft down the coast. If this difference of result is not due to greater quantities of travelling shingle at Napier, it may bo owing to the direction at which the waves strike the shore of that part of the coast lying between the Bluff and the eastern pier of the harbor mouth, or to the less depth of water at this place. We think that if the easiern pier had been extended into mnch deeper water, the features attending the accumulations of shingle at Napier would have resembled those at Timaru are such as would lead one to think that a great many years must elapse before any harm is done by the shingle to the works there, but we think it might be advisable, before undertaking any works at Napier, to wait a fow years and see the evidence of the dangers of the accumulations of shingle at Timaru become more decided or not. The success of artificial harbors constructed in somewhat similar situations in Europe has not been encouraging, but there are some instances, such as Port Said and Kurrachee, and others, which so far have been successful. Some are only kept in use by continually dredging the sand or shingle which is washed into the harbor. In the uncertainty that prevails on this subject it would be vciy rash to offer decided opinions on the subject of the durability of the harbor at Napier from the fact of the travelling shingle, and all that we can venture to say that it will probably take much longer to accumulate to the extent of endangering the works than is generally supposed. If the work could be made to last for 50 years, it would no doubt be satisfactory oven financially considered, as the progress of population and production is much more rapid in New Zealand than in most old countries. If it came to the worst, and the shingle was found to be increasing, it could be removed from the weather side to the lee side of the works, which would only be equivalent to the dredging operations carried on in many harbors of this character.

Mr C. Weber estimates that in a period of 32 months shinglo had accumulated to the amount of 240,000 cubic yards on the east side, and, say, 60,000 cubic yards on tho west side of the present harbor, which would give au annual amount of 113,000 cubic yards. This quantity is not more than has to bo removed from many harbors to maintain the necessary depth, but in addition to. tho shingle there is likely to be a considerable quantity of sand yearly washed into the harbor which must be removed by dredging. If the estimates above given of the quantity of shingle yearly accumulating are to be relied on, and wo see no reason to doubt their being approximately correct, then the cost of maintaining the breakwater can readily bo ascertained. We think it could be removed for about" lOd per cubic yard. This would amount to an annual expense of £4708, to which must bo added a sum not yet ascertained, for dredging sand inside the harbor, and it is for the Board to decide •whether they will undertake the construction of the work under the liability of haviug to incur this annual expenditure. We have taken out the quantities, and estimatad the cost of tho work as shown on Mr Goodall's plan. The total cost conies to £235,470, to which must be added additional rubble under foundation which we consider will be necessary for security, making the correct estimate £260,126. We have also estimated the cost of extending the breakwater, as shown by dotted lines on the plans, and to complete a detached mole to shelter the area enclosed ; and also to add a parapet, and to raise the breakwater 9ft Oin above high-water mark, thus making a perfectly sheltered harbor. The total cost of this we estimate at £486,823. The above estimate wo think is fairly correct, although many small items may be omitted by reason of the limited time at our disposal. We have allowed a considerable sum under the item of contingencies, which wo consider necessary in estimating works of this character, on account of the unusual risks to which they are exposed."