|Neap tide.||Average tide.||High spring tide.|
|Tidal Range||5 ft. 8 in.||7 ft. 8 in.||10 ft. 10 in.|
|River water entering tidal basin during flood tide in cubic feet||130,000,000 = 6,000 cusecs (low)||134,000,000 = 6,200 cusecs (average)||233,000,000 = 10,800 cusecs (high river)|
|Outflow of upper layer of fresh water during flood tide in cubic feet||107,000,000||98,000.000||133,000,000|
|Balance of river water that would be stored during tide in cubic feet||23,000,000||35,000,000||100,000,000|
|Inflow of sea water during flood tide in cubic feet||163,000,000||150,000,000||237,000,000|
|Totals of fresh water that is stored and inflow of sea water during flood tide||186,000,000||185,000,000 = 14,300 cusecs||337,000,000 = 26,500 cusecs|
|Volume of tidal compartment as calculated from lagoon areas and normal tide levels in lagoon, as a check on line immediately above||150,000,000||196,000,000||269,000,000|
During a spring tide on the flood there is about 269,000,000 cubic feet in the tidal compartment, which more than doubles the amount of water discharged on the ebb. Similarly at neap tides and with a low river, the tidal compartment increases the outflow on the ebb from 130,000,000 cubic feet on the six-hour flow of the river to 280,000,000; and yet there is great difficulty in convincing the public that increasing the tidal compartment will cause improvement.
Sir John Coode recommended increase in the tidal compartment by dredging in 1884, and successive engineers have reiterated the advice right up till the present day, and yet no action has been taken by those responsible.
Now the local conditions are not such that at any reasonable expense a tidal basin could be obtained which would solve all the bar problems, but benefit would surely result from this work either regarded singly or in combination with any other works.
3. The moles could be extended. Further storage space for drift would be made by extending the moles, but the condition of the sea floor around and opposite the entrance is such that any extension much less than 1,000 ft. would hardly be worth considering. This extension would do no more than enable the present depth to be maintained, as at 1,400 ft. out, in good times, and even at 1,800 ft., after adverse weather conditions, the sea bed is as high as the bottom of the river inside the moles with all the intervening distance about half-river depth. The west mole is now approximately 4,500 ft. long, and for the 28 years since its completion it has only sufficed partially to hold the drift, so that less than a quarter of that length could have only a moderate life, its full effect being lost before its cost
could be recovered by ordinary financial measures. Within two years of the last extension of 360 ft. the increased depth observable after the extension began to diminish and within 10 years all effect was considered to have passed away; by some observers, six years is regarded as the period. There is another difficulty about extending the moles to any considerable distance, not connected with their high cost or limited life, and that is the risk that the lessened shelter afforded by Cape Foulwind and its outlying islands and reefs which would be available in the more seaward position of the entrance and the bar may result in the dredger having to work in a location subject to a swell which would restrict its actual dredging time to a greater extent than is now the case. If a decision to extend the moles were made it would be advisable that a fresh start be made from the beach clear of the present “tips” so as—
To provide another wave basin.
To avoid some of the strong scour existing around the present end which experience has shown to deepen the water ahead of the work and thus to greatly increase its cost.
To avoid the considerable extent of tipped rock beyond the end of the moles into which it would be most difficult, if not impossible, to drive staging piles.
To take advantage of the opportunity to turn the entrance more into the line of the heaviest seas as Sir John Coode intended.
Another matter which merits very serious consideration is that of reducing the width of the channel, as a moderate reduction in width would improve the scouring effect of the ebb tide and of floods. Sir John Coode proposed 600 ft., and though he allowed for a certain amount of latitude plus or minus, as experience might indicate as work proceeded, he undoubtedly admitted the possibility that this width might be narrowed. Unfortunately, it was widened, though Durban, with a width of 600 ft., has dealt with a trade of 6,000,000 tons per annum carried in the largest vessels which trade south of the Equator, and had the Westport entrance faced the heaviest seas a width of 600 ft. would certainly have been satisfactory.
At East London the distance between moles is 700 ft., but the navigable channel is only 500 ft. The Sulina mouth of the Danube is dredged to a bottom width of 300 ft. with depth of 23 ft. Vizagapatam channel is 300 ft., widening where it meets the open sea to 500 ft.
Now that mariners for half a century have enjoyed the 700 ft. Westport entrance they will be averse to accepting anything less. When the details of any new extension, if adopted, are being settled, the very widest enquiry should be made as to the experience elsewhere with channels of 650 or 600 ft. in width.
4. An island breakwater could be built under the shelter of which the dredger could operate in most weather, making in addition to the actual channel a considerable hole into which the drift material might drift and be held for removal from time to time. This system has been employed at Vizagapatam, where a million yards per year are dredged, but it has elements of uncertainty about it.
If the island is placed too near the shore it may produce so much shelter that the drift will settle and build up a connection between the island and the shore; if too far off, the drift may not be carried into the sheltered area behind the island and seas may swing round too freely. The use of a floating pipe-line would be impracticable at Westport, so that self-propelled hopper barges would be required to remove the dredgings. As at Vizagapatam 30 in. pipes with a pump having a 6 ft. 6 in. impellor driven by a 900 h.p. engine plus 300 h.p. on the cutter have been used, it will be realised that Westport would require decidedly larger equipment with the much heavier drift obtaining there. This seems a scheme which should be tried out with a large model before it is seriously considered.
5. A groyne might be run out from a headland a few miles south of Cape Foulwind which would trap the littoral drift for a period long enough to justify the cost of the groyne. In other words, if the interest and sinking fund can, without placing a crippling burden on the port, pay off the cost of the groyne before the storage space thereby provided for drift has been filled up, then it would be a good investment. This problem was discussed very fully by Sir Frances Spring in his paper on coastal drift at Madras in 1913 (Min. Pros. I.C.E.), but in the case of Madras there was a difference of opinion as to the probable storage. At Tauranga Point there is a sudden projection in the form of a granite knob some acres in extent on which the Westport Harbour Board had a quarry during the execution of the main works. This knob was not connected to New Zealand until comparatively recent times, and when it was an island the coastline had established an alignment almost due north and south for some miles. When, owing to the shelter caused by the islet, the sand drift settling behind it eventually connected it to the shore there was a natural groyne about half a mile long which trapped the littoral drift and gradually built out the coast until the present shoreline was established parallel to what it was before the connection between the island and the mainland took place. Tauranga Point now projects about a quarter of a mile from the coast, and if by the construction of a mole the natural groyne could in effect be extended another half-mile, then the whole of the littoral drift would be held up until the projection was reduced to what it is to-day and perhaps for a longer period. That the present shore alignment is not a changing one, but is such as the balance of all the natural forces tends to establish, is shown, not only by the geologically recent old shoreline practically parallel to the present one south of Tauranga Point (which runs due south for three miles and then 4° west of south for five miles), but by a still older coastline parallel to the present and to the above-mentioned one and roughly a quarter of a mile behind (see Fig. 11), and also by another stretch of coastline north of Westport where a rocky bluff stands out square to the beach nearly a quarter of a mile and south of which the alignment runs due south for three miles and then 3° west of south for five miles. These similarities seem to be too close to be mere coincidences. (No beach for 250 miles south or 150 miles north is oriented west of north, even for a short distance.) If in nature there were any beaches on this stretch of
coast where the sandy shore between headlands tended to assume an alignment west of north, the author wonld not be so confident about the storage capacity of the area south of Tauranga Point. The very ancient embayments which extended where now the two coastlines in question exist would have established equilibrium on an alignment more or less parallel to these ancient rocky shores, if such had been possible in the face of forces affecting littoral drift, and the fact that this equilibrium was established definitely four times, thrice south of the Buller and at least once north of the Buller, and where the weather conditions are as nearly identical as they ever are, gives grounds for the belief that any projection of sufficient length on the coast in this locality would tend to develop a beach alignment on the southern side of the projection on a north-south bearing. Similarly, the cross-section of the beach as it advances seaward is likely to remain similar to that now existing. Some opinions have been given that the beach advanced by such obstructions as are now under discussion tends to be steeper the further it is advanced, but with similar weather, currents, tides, alignment and drift material there seems no reason, taking a long view, to suppose that nature would adopt a different cross-section. The observations off the port of Westport indicate that the cross-section, while steep inshore in the early days, tends now to become very closely parallel to what it was 50 years ago. The results of model tests by Major (now Colonel) Ralph Alger Bagnold are here quoted.
“Lateral Stability of the Beach Contours: In all the experiments where shingle was used, the model beaches which were thrown up by the unimpeded action of the waves set themselves at right angles to the line of the wave advance. When more material was added on one side of the tank, this soon became distributed evenly' over the width of the mobile portion of the beach. Prom this it appears that if waves strike a stretch of shingle beach obliquely the beach-line will, provided that coastwise losses of material away from the farther boundary of the stretch are prevented, become re-oriented so that its contours run at right angles to the line of advance of the waves.
“Continued Supply of Material and Constancy of Profile: A further experiment was made by adding a small but continuous supply of fresh material to the shelf after a mature beach had been thrown up. This was intended to imitate the accumulation of material by coastwise drift. The effect was that the whole beach advanced seawards, but its profile remained entirely unchanged. The topmost surface, previously a narrow ridge, became a flat horizontal table as the beach top moved forward.”
If it is assumed that the coast will advance parallel to the present line and that the under-water profile out to seven fathoms will remain substantially as at present, then the storage will be approximately 300,000,000 cubic yards, or adequate for 60 or 70 years, and it may even last 100 years if consideration is given to the fact that some of the drift material is always being ground fine enough to float away into deep water beyond the influences which cause coastal drift, and this tendency would be encouraged by the halt in northward progress and the grinding caused by the run of the waves along the mole and
the headland from the sea towards the shore. It may be contended that a mole half a mile long, which with the present promontory will make a groyne projecting 4,000 ft. beyond the average run of the Nine Mile Beach, will not stop all or even a very substantial proportion of the coastal drift which earlier in the paper has been mentioned as moving over a very wide belt, perhaps 8,000 ft. wide. Notwithstanding the statement by Mr. Zahrtman that on the coast of Jutland the sand moves on a belt 8,000 ft. wide extending into 60 ft. of water and that it moves equally throughout this belt, the author is of the opinion that there is much more transport of material where the water is violently agitated by the breakers than there is in deeper water beyond wave break. The Vizagapatam experience supports this view.
The fact that variations of depth are disclosed by successive soundings in deep water shows that material does travel in these depths, but the movement there is slow, and until the steady advance of the coastline and consequent rise of the sea bottom brings the present deep bottom up to such a level that it is affected by wave action no great disturbance and forward movement of the sand can be expected. The depth into which the suggested groyne would project is between 30 and 35 ft., and if the slope of the ocean three-quarters of a mile off shore along the beach south of the groyne remained substantially as it now is, the shoreline would have to advance 2,700 ft. before the depth in line with the end of the groyne was reduced to such a figure as would permit of the passage of a major portion of the Coastal drift around its end. (See Pigs. 3B, 7, 8 and 9.)
Such portion of the total drift as passes up the coast in water deeper than 30 ft. would continue to do so, but this should have little if any effect on the Westport bar, the 30 ft. contour being about half a mile seaAvard of the end of the breakwater, while the forces of nature should cause material in that and greater depths to roll seaward rather than up the slope of the bottom towards the beach. Even if after passing the groyne this deep-water material did work in to the shore, its volume would be small compared with the mass which now travels in the breakers (ef. Hokitika, where an accumulation between two groynes, six chains apart, of 10,000 cubic yards was observed in one tide in 1917).
The amount of storage on the Nine Mile Beach is such that even if there were an error of 100 per cent, in the quantity of the littoral drift, or if the required projection of the groyne were twice as great as has been assumed—in other words, if half a mile of groyne only stopped half as much as has been set out above—there would still be 30 years in which to pay off the whole of the capital cost; which would be in accordance with sound finance. As the site of the work would be unprotected by any physical features from all storms, the groyne would need to be very substantial, but, on the other hand, it not being connected with any navigation channel, the fact that in a record-breaking storm it was washed down or breached would not be disastrous, as would a similar occurrence on the moles enclosing the harbour entrance. Experience is available as to what is necessary in the works at both Westport and Greymouth, so that there is no
Fig. 8.—Old Sandy Beach oriented by Coastal Drift and Point North of Tauranga Bay. Tauranga Island is now a Point. (For more ancient shore, see Fig. 11.) Scale: 2 miles = 1 inch.
need for trepidation. Damage by storm would only affect the portion assailable by the waves and this could be later restored, and the base would still be effective as a drift retainer in the meantime.
Benefit from such a work would be obtained long before it was completed. This was proved at the commencement of the Westport Harbour works in 1887 and again when the 360 ft. extension was carried out in 1915–16 (see Fig. 3A).
The foregoing sets out the past history and the difficulties in connection with the port of Westport and indicates various ways in which improvements may be effected.
Taking these possibilities seriatum and indicating their costs and advantages, it should be possbile to arrive at a conclusion as to which, if any, should be adopted.