SIR JOHN COODE’S REPORT ON MILFORD HARBOUR.

Temuka Leader, Issue 201, 20 November 1879, Page 2

SIR JOHN COODE’S REPORT ON MILFORD HARBOUR.

“ 5, Westminster Chambers, “ London, S.W., “August, 1879. “ NEW ZEALAND HARBORS —MILPORd “ LAGOON, TEMUKA. « Si R| —Having carefully, considered the documents and, data, received ; from the colony relative to the' proposal to convert Milford Lagoon into an efficient harbour, I have now the honor to submit my report'thereon, together with the accompanying illustrative drawing. “ Milford Lagoon is situated near the south-west end of the ‘ Ninty-mile \ Beach 'on the’ seaboard of the great Canterbury. Plain ; it lies about 50 miles north of Cape Wanbrow, and about 10 miles north-north-east from Timaru. : The length of the Lagoon, in a south-south-west direction, is abput 21 miles ; its average breadth between the Opihi and the outfall of the -Mill Creek is about 140 yards at low water, and 180 yards at high water. Northeast of the Mill Creek the breadth is con - siderably less, ranging from 20 to 60 yards at low water, and from 30 , to 100 yards at high water. ' " . - r; . . “Excepting a, small area - opposite Mill Creek, in and near the line of the outlet channel, which existed when the survev was made in October last, the greatest depth was from 30ft to ,40ft at low water of ordinary spring tides, as will be seen by reference to the ac- | companying drawing. , . I At time of my inspection, in April of last year K the connection between the j { lagoon and the'sea was entirely cut off j, by a considerable , barrier of shingle, j,, which then extended completely across [ the line where the channel of communication had pieviously existed. This | barrier had again been cut through at j the time of the October survey, as ap- jpears by the drawing. The existence f or non-existence of such a channel depends, under present conditions, upon ; the relation between the scouring power j - of the tidal water corabined with thafyof I the fresh-water discharge ,of the nver >■ , Opihi and its tributaries om one hand, and upon the quantity of shingle transported along the coast by the vrows of the sea on the other. ' “The conclusion arrived at during my inspection was;to the effect .that little/ or no shingle passed around .Capej Wanbrow from the southward, and thau ■ the shingle travelling along this south!

ss'.era portion of the Ninety-mL Beach Milford Lagoon is situated) is derived mainly from tlie materials brought down by the river Waitaki, and partially from the shingle cliffs - , avesajing about 35ft high, which fringe the s iore northward of Oamaru. the supply from the two source's named, especially from the Waitaki, is very largo indeed, and practically uny—j limited. Being personally acquainted .vith numerous coasts and river outlets hi several parts of the world, I have • ov.'r previously seeu, nor have I any nowledge of such vast quantities of shingle being elsewhere transported by the waves as those which travel along this portion ol the seaboard of New Zealand. “The shingle is drifted or propelled along .the coast in a northward direction by the heavy southerly seas, the preponderating force being admitted on all hands to be from that quarter. The waves impinge on the beach at a slightly oblique angle towards the northward, thereby generating an almost constant travel of large quantities of shingle from south to north across the entrance to Milford Lagoon. ■ “ The quantity of tidal Water flowing into and put from the lagoon during springs, when communication exists between it and the sea, is calculated from the drawings and data sent from the colony, is under three million cubit yards. “ The fresh-water discharge is of course dependent upon the amount of rainfall over the watershed of the Opihi and its tributary streams. From a tabular statement of the areas of watershed of several of the fivers of New Zealand, kindly furnished to me by Dr. Hector, I find this area may be taken of about 890 square miles. “ According to the published Meteorological Reports, the nearest station at which observations of rainfall have been taken ie at Oamaru. From these it appears, as might be expected, that the quantity of rain in the district varies considerably. In Dr Hector’s tabular statement referred to, the average annual rainfall in the watershed of the Opihi is given as 28in. The four years for which I have the published returns ‘••how 16£in in 1871, 20in in 1872, 28in in 1873, and 21fin in 1874. In the last-named year rather more than s|-in, or one fourth of the whole annual rainfall, occurred in the month of September, whilst the total fall for three successive months in the same year (April, May, and June) amounted to only l-|in or a mean of -|in per month. Taking the average of the four years abovenamed, JJiere wese - four months out of the twelve in which less than I in of rain fell within the whole month. It is obvious, therefore, that, as in so many at other cases, it is mainly upon the tidal # -water that dependence must be placed for the scouring power at Milford, al though the fresh-water discharge would occasionally afford valuable assistance on the occurrence of floods or freshes. cal features, in so far as they effect the question under consideration, I now come to the proposal of Mr T. M. Hardy Johnston, C.E.,whose recommendations are described in his final report dated 19th October, 1877, and arc shown on the several drawings which accompanied it. From these it appears that the south pier, as scaled from the plan, is proposed to be 1200 ft in length (from B to D on the accompanying drawing) and the north pier 1050 ft (from G to E). From the inner ends of each of these piers, wharf-walling would be constructed along the eastern shore of the lagoon, to the extent of 500 ft from H to G, on the north side, and 600 ft from A to B on the south side. “ The mode of constructing these piers and wharves would be similar in principle throughout, and consists generally of timber-framing and blocks of concrete within, so placed as to admit of settlement where necessary ; the interstices between the blocks and piles are intended to be filled with liquid concrete either as mass-work or in bags, the outermost or seaward faces of the piers being protected by large blocks deposited pell-mr 11. Mr Johnston further proposes to cut a channel between the River Opihi and Mill creek, at a point about miles westward of the Lagoon entrance (I to J on plan) ro as to direct a considerable portion of the flood waters of the river through Mill ’ creek, with a view to create a more direct and equal scour through the channel between the lagoon and the sea, and facilitating the formation of a large and deep basui within the^entrance. The barks of the Mill Creek, at its lower end, are proposed to be rectified and protected by training walls of fascine work. “ With regard to the cost of the piers and wharf-walling just described, Mr Johnston estimates that, in their first stage, which he describes as provisional, they will cost L 67,837, including L 3670 for th e d iversion of the Opihi river, and 10 per cent for contingencies on the whole ; also, that the measures requisite for consolidating and completing these works would entail a farther expenditure of L 24,090, thus making a gross outlay of 91,927 for the Harbor Works proper, which, added to the sum of L 7500 for a branch railway to Temuka, would bring the total of Mr Johnston’s estimate to, - 1. in round numbers, LI 00,000. —“ The general principle of the design put forward by Mr Johnston is sound, and would be quite suitable,, assuming the forces at command in this case to be sufficient to overpower the shingle in such a manner as to maintain a good navigable entrance channel at all. times. The character of construction which he proposes is also adapted to the con-

ditions, in so far as it would admit of t e sul>.si euce which would be certain to .•cenr as h • work advanced, and subsequent!,', in times of exceptional floods. T % first and cardinal point, however, to be dctermiivd in this case is ; Whether :>r no! the scouring power due to the influx an ; efflux of the tidal and fresh waters is sufficient, under the most favorable conditions of training,controlling and directing works, for securing a permanent approach with a sufficient depth for navigation purposes between Milford Lagoon and the sea ? Upon this I much regret to be obliged t >* say that I am unable to.expx-ess a -favorable opinion, for, having regard to the exceptionally large quantity of shingle continuously passing along this part of the coast, and t alsotothe limited character of the tidal and fresh water discharge under ordinary conditions, and the comparative infrequency of floods, it appears to me that the physical conditions which prevail at Milford are so adverse as to render the success of any works which may be undertaken there so extremely problematical, and the prospect of any adequate benefit being derived from them so uncertain, as, in my view, to render inexpedient and undesirable that the large outlay should be incurred which' would be necessary for their construction. “ Instances can he adduced where a navigable approach of limited depth has been formed, and maintained across the line of shingle drift, but in all the cases with which I am acquainted, where even a moderate amount of success has been achieved, there the travel of the shingle or sand has been much less formidable in extent and less sistent and continuous in character, than along the sea front at Milford. Moreover, were the piers to be complated to the full length contempleted in the first instance—viz, to 1200 and 1050 ft for the south and north moles respectively—l feel assured that by the time these structures had reached the points contemplated for their termin ation, the shingle along the beach would have advanced so far seaward as to travel around the end of the southernmostwork, and, in the absence of the requisite scouring power to which I have alluded would inevitably block up the entrance channel between the moles for a considerable portion of each year. “ A material augmentation in the velocity of the currents might be produced by bringing the heads of the two piers closer together than 500 feet, the distance proposed by Mr Johnston ; but notwithstanding all that it is practicable to do in this direction, the probabilities of success are so remote that, having regard to the great outlay required, I am reluctantly unable to recommend the construction of any harbour works at Milford. t£ As the result of the works at Kakanui has been referred to in support of the proposed harbour at Milford, I may observe that, in so far as regards the quantity of shingle and the rate of movement along the coast, which form such all important elements at Milford, the conditions are so entirely different in the two oases, that no comp prison can be drawn between them, “I may further remark that the piers propose 1 by Mr Johnston would necessitate the erection of a series of groynes for a considerable distance along the north-east side of the entrance, in order to retain the shingle in sufficient quantity to prevent the north pier from being outflanked by the sea, as it certainly would be in the absence of kuch provision, seeing that the supply of shingle would be cut off from the norih whist the beach was gathering southward of the south pier, “ With regard to the sufficiency of the amounts stated as the probable cost of the proposed works, upon moneying out the quantities computed from the drawings furnished by Mr Johnston, and adopting the prices he has supplied a°- the basis, 1 find that the south pier and wharf-walling, together 1800 ft in length if completed to the full height contemplated by Mr Johnston, would cost L 229,700, and the north pier and wharf-walling, 1550 ft in length, likewise completed to the full height, L 173,000, giving a total outlay required, exclusive of the cost of the groynes, of £403,500. “ The greatest depth within the lagoon generally, and that only for about half its breadth, is from 3 to 4ft at low water, or lo to lift at high water spring tides, and about 8 to 9ft at high water of neaps ; a considerable amount of dredging would therefore be necessary in order to accommodate small coasting vessels and steamers, the cost of which should be added to the above named sum. “I have, &c., & c., ££ (Signed) John Goode,” “To the Secretary, \ “Marine Department, “ Wellington.”