West Coast Harbour Improvements PATEA AND WANGANUI.

Progress, Volume I, Issue 9, 2 July 1906, Page 240

West Coast Harbour Improvements PATEA AND WANGANUI.

Describing the course of the Patea river, the late Sir John Coode, in his report of 1879 upon to improve the harbour, states . " The Patea " river which rises under the slopes of Mount Egmont " is about 60 miles in length ; its course is extremely " smuous and its bed encumbered by many obstruc- " tions. The influence of its tidal flo-u is said to " extend to 20 miles above the town of Carlyle " Sir John Coode estimated the capacity of the tidal compartment, with a 6ft. tide-rise, at 4.5,500,000 cubic ft., to which should be added during a portion of the ebb the fresh-water discharge which amounts to 150,000 cubic ft. per minute under ordinary conditions. Upon this data the mean discharge on outgoing tides of tidal and river water combined may be taken at about 280,000 cubic ft. per minute. At the time of Sir John Coode's visit, which was a year before he formulated his report, no works had been undertaken for the improvement of the entrance. The outfall channel up to this time was directly subjected to the prevailing northwesterly winds which caused the waves to impinge upon the beach with an easterly bias. This bias during heavy weather piled up a sand spit which often forced the outlet to the eastward for a distance of some 1200 to 1500 feet. With a following set of weather from the eastward the river would cut out through the spit towards the western headland ; the position of the entrance therefore ranging between the boulder bank and the western bluff, according to the weather conditions and directions. With the entrance thus constantly changing, combined with the heavy sand drift which prevails along the coast, the ebb waters upon leaving the river fanned out, and, consequently, much of their useful effect in maintaining a channel was lost. At the time of Sir John Coode's visit the Harbour

Board had entered into a contract for the construction of an arm or guide pier from the boulder bank, which was the eastward limit of the river's range. Sir John Coode, however, at once, and before his report was compiled, recommended that the position of the work then about to be commenced should be modified by placing it on the line indicated by the eastern pier shown in the plan herewith. The construction of this work, v Inch was very thoroughly earned out, fell to the lot of Mr. John Thompson, M.I.C.E , then engineer for the Patea Harbour Board, and at present Harbour and Public Works engineer at Greymouth. As an immediate result of this work the oscillation of the entrance to the east was checked, and the ebb currents held to a more direct course seaward of the work. The works remained in this position until some six or seven years ago, when the guide pier, also shown upon the plan, was built ; but this work, which should be considered as practically an internal work had not much effect upon the entrance. Sir John Coode, in his scheme submitted with his report, m 1879, intended that the eastern pier should extend seaward for a distance of 1000 ft. beyond the point the work was constructed to by Mr. Thompson, and, also, that a western piei commencing at high-water mark about 1000 ft. to the westward should extend in a south-ersterly direction to converge upon the east pier, leaving an entrance 220 ft. wide, which would give according to that eminent engineer, a depth at the entrance of about 16 tt. at high water. These works complete, including some short lengths of training walls in the river, were estimated to cost £197,000, which sum was far beyond the reach of the district ; and until the beginning of 1903 no further works than the

guide pier and eastern training wall, as shown upon plan, had been undertaken. Early in the year 1903, a commencement was made with a western breakwater which had been advised and designed by Mr. Leslie H Reynolds when his services were called in by the Harbour Board a year

or two previously. The work, which consists entirely of concrete, built to high-water level, and of the design shown in our illustrations, is now practically completed at a cost of some The effect has been to shut off the run of the westerly seas from the entrance and the ebb currents m consequence take a very much more direct course seaward than heretofore, and the conditions of the entrance have been considerably improved. The Harbour Board have now invited Mr. Reynolds to report upon further improvements of the entrance, including an extension of the eastern pier ; and it is probable that they will now devote their energies to this work, together with the removal of a shoal composed principally of papa reef and boulders, situated about iooo ft. within, the entrance, and which interferes with tidal influx and efflux. Some very severe weather has been experienced during the construction of the western breakwater, but, with the exception of a block or two being overturned at the extreme end, no damage has been sustained. The Wanganui river is not only one of the most beautiful rivers m the colony, but it is one of the finest, and offers facilities for improvement which few, if any others, possess. The tidal influence extends up the river for a distance of 23 miles from its mouth ; and during the influx of an ordinary spring tide some 428,000 000 cubic ft. of water accumulates m the estuary. The normal freshwater discharge of the river has been estimated

by Mr. Leslie H. Reynolds and Mr. Lloyd Hassell at some 350,000 cubic ft. per minute. The average discharge at the entrance, due to fresh water and tidal water combined is, according to Mr. Reynolds' investigations, 1,228,000 cubic ft. per minute, or twice the estimated outflow of the Buller river. The river is occasionally subjected to heavy 'floods, which arise principally owing to warm rams melting the snow on Mt Ruapehu. The discharge during heavy flood is estimated by Mr. Reynolds at from 9 000 000 to 10,000,000 cubic ft. per minute. Long stretches of the tidal reach above the town are of considerable depth. For instance, an unbroken stretch, with a depth of 25 ft. at high water, extends as far as Upokongaro, a distance of some seven miles. The surface inclination or fall of the river for a distance of twelve miles above the town bridge amounts to only 7 inches, or not quite f inch to the mile ; and as in all cases where the fall of a

river is small, its waters are deep. Below the town, however, very different conditions appertain ; the total fall from the town bridge to the heads, a dis tance of four miles, is 3 ft., two -thirds of which fall occurs within a distance of if miles of the town bridge. Over this stretch the depth in the fairway at low water only amounts to some 5 ft. The width of the river above the town is generally from 500 to 600 ft., but upon reference to plan (p. 242) it will be seen that it widens immediately below the town to a width of some 2,000 ft., that width being practically maintained to the entrance. For half the distance to the heads the depth in the fairway is at low-water spring tide about 5 ft., and over the remaining portion from 8 to 10 ft. The stretch with the lesser depth is known as the flats, and the illustration depicts several vessels aground on this reach, the bottom consisting of shingle, snags and clay on which the scouring action of the river has little or no effect. With a view to improving the sailing track from the heads to the town, Messrs. Barr & Oliver, about the year 1878, made investigations and advised the Board to confine the river by training walls, winch were approved by Sir John Coode ; they also advised the construction of a sea mole to extend from the seaward extremity of Castlechff towards the bar upon which the depth of water is generally from 5 to 6 ft. at low spring tides. Of this mole, some 900 ft. were executed some years later by Mr. Lloyd Hassell. Immediately following the advice

of Messrs. Barr & Oliver the training walls extending from the town to Landguard bluff k were constructed to half-tide level, with the result that the depth between the walls that was previously from 2\ to 3 ft., was, through the walls, aided by a little dredging, increased to 5 ft. at low-water spring tides. The dredge during heavy flood broke from her moorings, and after being carried over the bar was stranded on the beach some miles to the northward. After Messrs. Barr & Oliver the charge of the harbour was entrusted to Mr. Lloyd Hassell, who was appointed resident engineer — a position he resigned in 1885. In the beginning of 1894 Mr - Leslie H. Reynolds was called in to advise upon the deepening of the channel up to town ; and in the end of the same year he was appointed to formulate a scheme for the improvement of the entrance and the harbour generally. In 1899 Mr. Napier Bell was engaged to report, but until last year nothing further was done, when the Board adopted Mr. Reynolds' recommendations and instructed him to prepare the detail drawings necessary to execute the work. Mr. Reynolds' scheme comprised north-east and south-west moles at the entrance, as shown upon plan, together with a training wall within the estuary for the protection of the south spit against the tendency of the river to break through during floods, and also for the purpose of training the outgoing waters with the object of obtaining a better sweep through the narrows at Castlecliff. In conjunction with this he advised systematic dredging of the channel through the flats up to town, and in addition outlined a future floating basin for the accommodation of deep-draught vessels. Mr. Reynolds estimated that were the moles extended seaward a little beyond the three-fathom contour line the depth maintained by scour would be equal to 28 ft.

at high-water spring tides. As the funds of the Board would not admit of the whole of the works referred to being undertaken, it was decided to proceed as far as possible out of revenue during the present year, after which it is proposed to borrow £30,000 and construct portions of both moles at the entrance. The works, now being undertaken under Mr. Reynolds' supervision, comprise the construction of the training and protective wall to the south spit, together with the dredging of the channel through the flats to give a depth of 9 ft. at low-water spring tides, which will be equal to about 16 ft. at high water. For this

purpose the Board purchased a gold dredge, to which was added a centrifugal pump, to discharge the finer material over the southern training wall, the heavier material being delivered into hopper barges and utilised for the construction of the training wall at the south spit. The addition of the centrifugal pump was left to Mr. Edward Roberts, of Dunedin. The hopper barges, plant and material necessary for the work have just been completed, and a commencement made to the

training and piotective wall to the south spit, This consists of fascme crib work to within a foot or two of low-water level, and from that to 3 ft. above high water of ironbalk piling and planking. The fascine crib work will be filled in with heavy shingle and other spoil delivered from the dredge, the whole being protected by a layer of rubble obtained from the Shell Rock quarries some 16 miles up the river. In concluding our article it should be stated that the Patea scheme, although a small one, has given complete satisfaction to the owners of vessels trading to the port, and that the work at Wanganui is certainly destined to provide that prosperous centre with a harbour capable of allowing vessels of considerable draught to enter or leave without risk.