SIR JOHN COODE’S REPORT ON GISBORNE HARBOR.
5, Westminster Chambers, London, S.W., December, 1880. Str,—l beg now to present my report upon the works which I have to recommend as best adapted for the improvement of the harbor of (risborne, in Poverty Bay. The coasting trade of Gisborne is at present carried on partly by steamers, which call and discharge and take in cargo whilst at anchor in the Bay; and partly by small
vessels drawing, for the most part, from 6 to 8 feet of water, which latter, notwithstanding the smallness of their draught, frequently experience great difficulty in effecting an entrance into the Turanganui, owing to the general shallowness of the bar and the narrow and tortuous character of the channel. The normal depth over the bar is only from 3 to 4 feet at low water of spring tides. As the rise at springs is generally only from 4 feet 9 inches to 5 feet, it is obvious that the river in its present condition is not accessible by any other vessels than those of the smallest class of coasters, and by these only at the time of high-water. It is true that the depth on the bar is subject to occasional improvement by the scouring action of floods, but any benefit so gained is but of comparatively short duration; having regard to the surrounding conditions, it could not well bo otherwise.
An example of such improvement and subsequent loss of depth is given in the “ Notes” by Mr. T. Perham, an assistant engineer in the Public Works Department, who made the survey for the purposes of this report. Mr. Perham states that, at the time of completing his survey—2nd May, 1879—there had been no flood in the river for three years; but after a flood, lasting from May 3rd to Bth, the sand was scoured down to the rock (papa), the bar being forced out to about 10 chains from its former position, with 8 to 9 feet at low-water. He adds: “On 24th June I again took soundings, and found the bar working rapidly back to its former position, with from sto 6 feet at low-water. . . . Calling at
Gisborne on 20th August, I again sounded and got from 3 to 4 feet on the bar, which had then shifted in to about 4 chains off the edge of the western reef.” This is precisely analogous to what occurs elsewhere under similar conditions, and I may here state that there would be no possibility of keeping down the bar at Gisborne without the construction of very long and expensive breakwater piers on each side of the entrance; but in this case there is the further difficulty arising from the presence of a ledge of papa rock at 3 feet below low-water, immediately in the entrance. The precise extent of this papa is not defined by the sections; but, from general indications on the plan and sections, I have not the slightest doubt that it extends at a depth of somewhere about 3 or 4 feet below low-water for a considerable length seaward of the entrance. Having regard to the physical conditions in this case, as above described, I have arrived at the conclusion that it would be altogether inexpedient—at any rate, under the present circumstances of the trade of the district—to incur any expenditure in an attempt to improve the entrance of the Turanganui and the channel within, otherwise than on a very small scale, because I feel convinced that the results would not justify the outlay. If harbor accommodation in the ordinary sense of the word—by which I mean a harbor having such a depth of water as would admit of coasting and inter-colonial steamers and sailing vessels entering or leaving and lying afloat at all times of tide, and properly sheltered in all states of weather, with adequate quay accommodation —is to be provided at Gisborne, the proper course will be to construct such works as I have shown by red colour in general design on the accompanying drawing No. 1, and in detail in drawing No. 2, and more particularly described hereunder. Works of Improvement. Having regard to the general extent of sandy beach within the Bay—extending, in fact, for several miles to the southward of the town and river-entrance—and also to aspect and exposure of the Bay itself, I am unable to recommend any solid structure between the shore and the line of three fathoms at low-water, feeling assured, as I do, that a serious amount of sanding-up on the inner or western side would inevitably follow, and to an extent that, to say the least, would be highly prejudicial to the present river entrance and to the utilization of the new work.
By referring to drawing No. 1, it will be seen that I propose to commence the external works at the point A, about fifty yards southeast of the salient angle of Maori Point, and to run therefrom in a south-west direction for a length of 550 feet as a solid structure, or root, shaped in plan as shown on drawing No. 1. The sides of this root would be formed by substantial walls of rubble masonry, set in cement, founded on the bare papa rock ; this work, being about 120 feet south-east of the “ Boat Harbor,” would not interfere with that useful little creek. From the outer end of the root, and in a similar direction, I would construct an open viaduct of 1,410 feet in length, consisting of wrought-iron piles, supporting a deck formed of three rows of longitudinal wrought-iron girders, with proper cross-girders and joists to carry the timber deck, the level of the latter being at least 15 feet above high-water of ordinary spring tides. This viaduct will allow the heaviest seas to pass through and underneath it, with almost undiminislied force; and, consequently, will ensure such an amount of agitation and circulation as will prevent the deposition of sand in the vicinity of the entrance to the river, and shoreward of the propssed outer harbor. As already explained, were a solid structure substituted for this open viaduct, the shelter provided thereby would inevitably conduce to the filling up with sand of the bight northwest of the work itself.
From the outer end of the viaduct I propose to commence a solid breakwater pier, consisting of concrete blocks of about 22 tons each, placed in position by a portable overhanging setting-machine, travelling outwards as the pier progressed. These blocks would be founded on bags of concrete, in the manner shown in detail on figures 4 to 7 inclusive, drawing No. 2. The inner portion of this pier would extend the viaduct in a south-west direction for a length of 500 feet, bending thereafter towards the north-west by a curve of 300 feet radius, the outer portion bearing W. by S. The quay surface of this pier would be 25 feet in width, and protected throughout on the seaward side by a massive parapet, also of concrete, a “ bull-nose” course being provided to throw off the sea during gales. The level of the roadway on the root would vary from 9 feet 6 inches to 15 feet, the latter being the level of the viaduct deck, as already described, the ascent being made by a gradient of lin 100. The coping of the solid pier would be 8 feet above high-water, or 7 feet below the deck of the viaduct, thereby necessitating an incline, which I have fixed at lin 50, to join these two levels. It is requisite that the viaduct should bo kept at the height proposed, in order to raise the deck above the heaviest seas, whilst, on the other hand, the level shown for the surface of the pier will bo more convenient and economical than if at a higher level. Figures 4 to 7, drawing No. 2, show the pier as a solid structure throughout. It will be seen from figure 1 of this sheet that this work, shortly after leaving the end of the viaduct, would pass over the Pinnacle Rock. At this point, level benchings for the reception of the blocks would be formed around the Pinnacle by bags of concrete, in the manner indicated. Seaward of this rock the footings of the pier would be protected by an apron of concrete bags, arranged as shown on figure 7 —/.(?., for such length as may not be founded on the bare rock.
From the western termination of the pier I have shown a west jetty, of 300 feet in length, running N.W. by N., with a return 120 feet long in a N.E. direction. At the eastern or shoreward termination of the pier I have laid down an east jetty, of 350 feet in length, in a N.N.W. direction, with a kantßO feet in length, pointing S.W. The distance between the terminations of the proposed east and west jetties is shown as 560 feet. The jetties would be formed of cribwork, of the character indicated on drawing No. 2— f.c., of piling and strongly-braced timber framing, filled with a hearting of rubblestone. The heads of the jetties would in each case consist of a mass of Portland cement concrete. The total length of sheltered quayage provided would be 1,600 feet, having a depth varying from 21 to 30 feet at lowwater of spring tides, the rise of these tides being taken at 5 feet. A light would be provided at the head of the concrete pier, and a small port lantern fixed at the termination of the west jetty. The viaduct has been proportioned so as to adapt it for carrying the 22-ton concrete blocks required for the construction of the breakwater pier. Railways on the Colonial gauge, so that they might bo connected with the Government system, would be laid down throughout the works. Sufficient information is not available here to enable me to determine, with such cer-
tainty as is desirable, the line of ftp* proach road which it would be re» quisite to construct from point A, at the shoreward end of the works previously described, to the town, including a bridge across the river. Before this can be done, a special examination of the site would be requisite ; but I do not gather that difficulties of any moment would arise in the execution of a satisfactory line of communication between, say, Gladstone Street and the root of the external works at Maori Point. I estimate the cost of the before-mentioned external works as follows :— £ Masonry root to viaduct, 550 feet in length .. ... .. .. 10,750 Iron viaduct, 1,410 feet in length .. 48,980 Breakwater pic ,9)0 feet in length 135,180 West jetty, 430 ieet in length, measured along the outside .. 80,790 East jetty, 430 feet in length, measured along the outside .. 20,700 £246,400 The above amounts include contingencies, and the provision of the requisite plant | they are based upon such prices as I believe would be found sufficient, having regard to my experience of the prices applicable to other New Zealand harbors; although the local rates upon which the prices have been based are taken as much lower than named in the schedule attached to Mr. Perham’s memorandum, to which I observe that Mr. Blackett has appended a note that he considers the rates given “ to be rather high,” in which I concur. The works described, if carried out in their entirety, would afford very perfect accommodation to the steamer and other trade; but important benefits would arise from the execution of the root, viaduct, and pier, omitting for a time the erection of the East and West jetties, in which case the expenditure on the three works would amount to £194,910. Unless this sum can be appropriated for external improvements at Gisborne, it is not advisable that they should be commenced. Interna I Works. With regard to the question of improvements in the river itself, near the town, I may say that the conditions of the site are such as would not, in my opinion, justify the outlay that would undoubtedly be required to produce satisfactory results. For instance, a half-tide training-bank would be required on the east side of the channel, and for its whole length, in order to concentrate the currents in conjunction with the breastwork and wharf proposed by Mr. Drummond, the Town Engineer. With respect to the design for the breastwork, described in the specification and shown in the drawing sent to me from the colony, I may remark that the structure contemplated in these documents would need to be considerably strengthened if it were at any time determined to carry it into execution. Mr. Perham has called attention to the fact that during floods the waters from the Waimata “ head ” back those from the Taruheru. If considered expedient, this might be remedied by the construction of a V-shaped tongue from the point of land lying immediately at the junction of the two rivers, and, in connection therewith, the removal of the wharf and salient point on the west side, where the two rivers unite and form the Turanganui. It is only right to mention that Mr. Blackett, the Marine Engineer of the Colony, greatly facilitated my inspection at Gisborne, as at the other ports and rivers which I examined when in New Zealand; also that Captain Chrisp, the Harbormaster at this place, gave me much valuable information as to the local conditions in respect of seas, tides, &o. I have, &c., Jno. Coode. The Secretary, Marine Department, New Zealand.
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Poverty Bay Standard, Volume I, Issue 82, 4 March 1884, Page 2
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2,255SIR JOHN COODE’S REPORT ON GISBORNE HARBOR. Poverty Bay Standard, Volume I, Issue 82, 4 March 1884, Page 2
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