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The Pamphlet Collection of Sir Robert Stout: Volume 80a

The Westport Harbour Board. Mr Bell's report on the effects of the works on the river and on further improvements required

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The Westport Harbour Board.

Mr Bell's Report
on the
Effects of the works on the River
And on
Further Improvements Required.

To the Chairman, Westport Harbour Board.

Sir,—I have made a careful inspection of the works now completed, comprising the breakwaters, the training walls in the river, the dredging, and the effect of the walls on the navigable channel.

Since the river walls were completed several floods have occurred, that in March last being one of the heaviest ever known. This flood was above the Buller bridge about one foot lower than the great flood of 1877, and below the bridge 1 foot 8 inches lower; at the staitbs it was the same height, and continued at the same height towards the sea. The velocity of the flood was by ships' logs 10 knots or 11¼ miles an hour. At the Inangahua Junction, 25 miles up the river, the flood of last March is said to have been seven feet lower than the flood of 1877.

The effect of this great flood inside the river was less than might be expected, the fact probably being that if it scoured extensively during its height it filled up correspondingly as it subsided; a vast quantity of it poured over the training walls and flowed back into the river through the west lagoon, and it was noticed that all drift timber passed over the walls and did not come among the shipping. It was remarked that the shipping at the wharves had no difficulty in holding, and apparently were in no danger during the height of the flood. No scouring took place along the staiths, or if it did, it filled up again.

This flood scoured out the channel between the ends of the breakwaters to a depth of over 40 feet, and at the present time there is over 30 feet at low water. It also scoured off the top of the great shingle bank at the end of the east training wall to the extent of 6ft.

The vast quantity of sand and shingle washed out of the river by this flood has accumulated in a bank at sea lying to the east of the direct line of the opening between the breakwaters and extending out to sea for over 2000 feet beyond the end of the east breakwater. The sailing course in the line of the beacons passes over this bank of deposits, and after the flood the depth was much reduced; but the bank is being slowly washed ashore and the water deepening on the sailing course. At present there is about 13 feet at low water spring tides, in line of beacons at 1300 feet outside the tip-head; but westward of this line there is deeper water, In a few months the same change will take place as occurred after previous floods; the depth between the ends of break waters will shoal up to about 13 feet and this outlying bank will be washed ashore.

Inside the river, from the entrance of the west lagoon to 1200 feet below the present end of the west lower training wall, there has been extensive scouring and deepening, so much so that the 9 feet line of soundings has scoured back on the shingle bank an average of 180 feet, and where the 12 feet line was in 1893 at the back of the walls it is now 150 to 200 feet in front of it. From this it is seen that the great shingle bank at this place is scouring away as the training wall advances.

Opposite the opening of the west lagoon is the shoalest water in the page 24 fairway, although the depth has im proved lately, and as the whole of the tidal and flood water of the lagoon flows down the opening at the land end of the lower west training wall, this probably accounts for the want of efficient scour in the fairway opposite the lagoon.

Opposite Cobden and Bright streets the width of good water remains unchanged, and in fact from the lower end of the staiths to the base of the lower east training wall is the best water in the river.

Along the training wall opposite Henley street the shoal which occurred during the construction of the wall has been scoured away. The spit opposite Wakefield street has advanced further down the river, and shoaling has taken place at Riley's wharf to the top end of the merchandise wharf.

Above Wakefield street the deep water pool which existed on the present site of the training waill has advanced from opposite the top of Riley's wharf to opposite the bottom of the merchandise wharf, a distance of 900 feet, and it appears to be slowly advancing down stream alongside the training wall.

Along the curve of the training wall below the bridge a shingle bank has accumulated from 6 chains to 20 chains below the bridge; this is as high as the wall and about 100 feet wide; also opposite this accumulation the great shingle bank on the east side of the river has scoured away to about the same extent.

The general effect of the construction of the walls appears to be: That opposite the wharves, from the merchandise wharf to lower end of staiths, the depth where ships lie is increased, but the width of deep water has con tracted; thus from Pakington street to Brougham street the 6 feet line of soundings has changed very little; the 9 feet line has advanced towards the wharves 30 feet at Pakington street, and 60 feet at Brougham street, the 12 feet line 10 feet at Pakington street and 60 feet at Brougham street. Below Pakington street the depths and widths have improved.

It is impossible to say that the pre sent condition of the port as regards depth and width of waterway is permanent; the shingle banks move very slowly; it may still take some years before the waterway assumes a fixed position, as a result of the construction of the training walls, and I am of opinion that improvements in depth and width should be assisted by dredging for some time to come.

Where dredging is desirable or where unnecessary is best decided by observation of the Engineer on the spot, as beneficial or injurious changes which may arise from the construction of the walls are best seen by the Engineer residing in the locality.

In my report of 1892 June,, I recommended that the end of the shingle bank opposite Wakefield and Rintoul streets should be dredged off. It was found that there was considerable difficulty in doing so, from the strength of the current, the shallow depth of water, and the narrowness of the channel to admit dredge and barges.

I have mentioned above that shingle is accumulating in the curve of the wall and that the shingle bank opposite this deposit is showing signs of scouring away. If this deposit of shingle in the curve of the wall increases greatly the opposite shingle bank will be correspondingly scoured; but failing this the shingle bank can be scorned away by making several groins to project out into the river from the curve of the wall below the bridge. This means of straightening the channel, with the object of giving a better run of the river along the upper part of the wharves, would cost about £1400, whereas to dredge right through the shingle bank as proposed by Sir John Coode (even if practicable, which I doubt) would cost over £12,000.

This shingle bank did not exist 30 years ago; at that time, as I am informed by Captain Leech, it commenced to grow in consequence of the wearing away of the banks on M'Padden's side and the great overflow above Martin's Island. Now that the river is confined by the wall, the channel can be pushed back again nearly to its course of 30 years ago, by means of groins built out gradually as the opposite shingle wears away, and I am inclined to believe that if the track of the old channel, which passes through the slipway and extends nearly to the bridge, was cleared out, all the snags taken out of it, and a ditch cut with a plough and horses-shoved through the top end of the shingle to the water under the third pier of the bridge so as to let the tide flow throughthis old channel, that in a short time a large part of the river would be diverted and flow in a straight course from the east end of the bridge through its old course. There is, however, some risk in thus diverting part of the river water, because for probably many years the river would occupy two channels, one flowing along the training wall, and one down the channel just mentioned; in each channel the current would be more feeble than as at present in one channel, and the result might be the extension of the spit between the two channels to the extent of injuring the depth of water at the upper wharves, I mention this because it has been proposed to clear out the old channel close to the East bank and let the river flow down it, but I think it would be safer to push the channel gradually over by groins, and this method is very commonly employed in Europe for such purposes.

In the same report I indicated a straight channel to be dredged, midway between the east and west lower training walls. Some attempts have been made to cut this channel, but it was found to silt up as fast as it was dredgod, and great quantities of snags were found embedded in the shingle. There was also a difficulty in dredging, owing to the waves from outside, which roll in here with a force sufficient to frequently interrupt dredging. I have mentioned above how extensively this shingle bank is scouring away as the training wall advances; it, therefore, appears that the best way of straightening the channel would be to extend the west lower training wall for 700 feet; the scour, assisted by dredging, would keep a deep channel open for the traffic, and the fairway would then lead by the shortest distance to the existing deep water along the east breakwater. The resulting channel would be nearly straight, and would be quite suitable and convenient for navigation. This extension of the wall would cost no more than would dredging the originally proposed channel, which your Engineer considers to be almost impracticable, or, if practicable, tedious and costly.

In the plans I submitted with my report of June, 1892, I showed this west training wall as not connected to the shore at its base, but I showed a channel which was to be dredged to page 3 divert the current from the lagoon into the main river. This has not been dredged; consequently the current flows with full force through the opening between the shore and the base of the wall. The flood of March, 1896, piled a great mass of drift timber against the trestle spanning this opening with the result that it was carried away, and the current scoured the opening to a depth of 13 feet below L.W. I find now that all the tidal water of the lagoon, and even some of the water of the main river, flows through this opening, thus doing no good to the navigable channel, as the water all passes behind the wall. I would, therefore, recommend that this opening be closed by the half tide wall, instead of re-erecting the trestle.

During the flood of March the water of the east lagoon overtopped the sea beach, cutting a wide channel into the sea, and the river water rushed through with great force. The tides now regularly flow through the opening thus made, and the sea does not seem to have the power to close it. This passage takes so much water from the bar, and to that extent it is injurious. There is also no telling what damage may be done by this overflow during heavy floods, and, for these reasons, it should be closed by throwing across it a bank of sand high enough to prevent the floods overtopping it.

The east breakwater has suffered some damage: first, during a continuance of northerly seas, the sand was scoured away from the east or seaward side of its outer part, causing the stone to sink down, but this has been made good with more stone. During the last flood the west, or river side, suffered extensive scouring, causing the stone to sink down, and it will take about 5000 tons to make good this damage. With these exceptions, all the works are in good condition.

The breakwaters have now been completed about six years, and, as yet, there is no sign of any change for the worse in the position of the beaches or the entrance. The beach on each side has not altered since the break-waters were completed, and there is yet no appearance of increased shoaling in the sea bottom outside the breakwaters. The bar varies in depth between dry weather and floods, but the average depth remains the same. After each heavy flood vast quantities of sand are piled in a shoal to the east of the works. Occasionally this deposit encroaches on the depth in the entrance, but, after a month or two, the waves wash the sand ashore and the normal depth is maintained. After a long absence of floods the depth on the bar gradually decreases, but it is again restored after the first floods occur. In 1892 I reported on a progressive shoaling of the bar, which occasioned some uneasiness. I came to the conclusion then that it was due to want of floods, and this has proved to be case, as the bar has, on the average, been deeper since then.

I remarked at that time that the breakwaters do not produce their best effect, from the west one being 400 feet longer than the other; the effect of this in dry weather is that shoals grow between the splayed opening, which would be prevented, and deeper water secured, if the two breakwaters were of the same length, and I again repeat my recommendation to extend the east wall 400 fet, which will certainly give two feet more water on the bar.

I examined the Orowaiti overflow, which has always been a threatening danger to Westport. Since I saw it last a great quantity of stone has been placed, in the form of pitching, on the banks of this low place, and it has every appearance of being perfectly safe. The flood of March last flowed through here, a raging torrent 12 feet deep, carrying vast quantities of sand over the high stone banks, and lodging drift timber among the trees and bushes. The stone pitching, however, stands without damage, and a dense growth of willows and gorse protects and holds together the loose soil behinds the banks. There are places where more willows might be planted with advantage, and, before the lease expires of the quarry, it would be advisable to bring down more stone to repair weak places and possible damage.

I was not able to see the relief channel, but I would recommend that some men be sent up to cut away snags which may possibly have lodged in the entrance, and may be doing some damage.

I would call your attention to the Government having declared the west lagoon a sludge channel, and it is already being rapidly silted up by tailings. Any silting up of the tidal area of the harbour is injurious and should be strennously resisted.

Now that the works are nearly completed and the good effects made evident in increased and permanent depth of water both over the bar and in the river, the subject of increased accommodation for the shipping has become urgent. The coal trade has been steadily growing for years past, but lately the Westport Coal Company has been making every effort to secure other markets outside the colony, and having made a very promising commencement the Company has hopes of securing a largely increased sale.

As yet very little Westport coal has has been sold outside New Zealand, but the demand for the coal has risen steadily, as this table will show:—
Year. Export. Tons.
1885 78,094
1886 119,779
1887 115,942
1888 130,219 *
1889 169,915
1890 160,214*
1891 201,736
1892 203,081
1893 223,794
1894 229,969
1895 222,928

This shows an increase of 72 per cent. since the bar was deepened.

The Westport Coal Company is opening up the fine seam of coal at Granity Creek and expects in a short time to be able to put out about 1500 tons a flay from this mine. The production of the Denniston mine will be maintained as now at 800 tons per day. The Cardiff Company expects to be soon in a position to produce 350 tons per day, and the Mokihinui mine 250 tons per day. The total of these would be 2900 tons a day.

At present the total export is about 750 tons a day, almost the whole of which is consumed in the colony. In 1895 the Westport Coal Company produced 650 tons, the Cardiff 115, and the Mokihinni 22 tons per day. It is therefore seen that there is a great gap to be filled from 790 tons a-day, the product of 1895, and 2900 tons a day which is

* Bar deepened by breakwaters

* Great strike

page 6

Estimated Revenue Account, 1896.

RECEIPTS. £ s. d. EXPENDITURE,

Taking the coal output as doubled in the next few years, the yearly estimate will be as under:—

RECEIPTS. EXPENDITURE. Railway and wharf 42,500 0 0 Interest and charges on

Westport:

Printed by Tyrrell and Co., at the "Times and Star" office, Palmerstou-street.