THE RANGE.

Timaru Herald, Volume CXXIII, 2 June 1926, Page 6

THE RANGE.

DEDUCTIONS FROM THEORY. (By J. Hardcastle.) Pursuing the subject of the range, and its origin from the breaking down of the ends of waves passing the eastern mole, it may be said: “Knowledge of the nature and the origin of the range may be interesting, but is it of any practical value?” Let us see.

I have no theory of range waves except as to their origin, and their being of the class of “long waves,” of horizontal motion. Some of the characteristics of these waves appear to have applicability to certain phenomena of the range at Timaru. One is their comparatively high refrangibility. They are more com-' pletely reflected than sea waves, more completely deflected or turned aside by obstacles in their path, because they reduce their force less by impact. They do not “break” so readily. In a letter of August 6th, 1924, I drew attention to a remarkable convergence of the 3-fathom lines of soundings of 1909 and 1923, at the bend of the north mole. The line of 190 9 appears to mark ail advance of the shoaling of the - bay roughly parallel to that of the 2-fathom line. In. the next fourteen years the 3fatliom line advanced very little indeed at the bend, whereas beyond the bend it advanced sharply sea-" ward. I suggested that this striking halt in the advance of the shoaling at the bend of the mole was due to waves piling against the arm of the mole, and then escaping by deflection to right and left, part of the accumulating wave turning off into the bay, another parf into the entrance. The deflection towards the bay, I concluded, checked the advance of the shoaling just at the bend, but did not affect the silting farther out. The part of the wave deflected into the entrance would cross over it, and reach the arm of the breakwater just about where the harbourmaster said the range does strike it—at about one-third of the distance from the end. There is probably also some reflection of the range waves from the arm of the north njole, but if so, nothing is known about it. The deflection into the entrance can sometimes be seen (I have seen it myself Quite clearly) the deflection into the bay is v r ell evidenced by the sounding lines. There is also a. convergence of the 2-fathom lines' of the 'same years near the ballast wharf. This convergence is probably duo to deflection of range waves reaching the nio’c between the 2-fathom and 3-i'athom lines.

Range waves making straight for the entrance of course run in, and the direct and deflected waves do not interfere with each other in ' their mutual crossing. (I have observed this also.) According to the theory of. the origin of range waves set forth in the preceding article, the angle at.which such waves reach the north mole-arm varies more or less with the direction of the seas outside the eastern mole, and the proportion of the deflection to right .and to left will vary also. Range 1 waves from easterly seas will reach the mole arm at an angle more favourable to deflection into the entrance, than those from southerly seas.

One deduction of practical value can, I think, be drawn from the effects of the north mole arm on the range waves which'it opposes. This is that in designing harbour works under the shelter of an outer work which cuts the seas, as does the eastern mole at Timaru, care should be taken to avoid if possible giving opportunity to any part of the structure to deflect waves into the entrance. This was done, when the original harbour and the north mole were both exposed to sea waves. The arrangement of the two works happens to. bo quite wrong now that not sea waves but range waves have to be provided against. From this point of view the design of the Dashing Rocks mole was defective, in that the inner spur on the eastern mole would have made a large triangle, in which range waves would be impounded, and from which much of them would escape across the channel and into the entrance, in the latter cause augmenting the waves entering directly (as is the case with a north mole deflection now)! Whether the waves deflected across the direct waves would make an injurious combination in the fairway, or not, I do not know. If the spur did deflect much of the accumulating wave into the entrance, its effect would be far worse for the north mole extension scheme, as it would be confined on the west side, whilst under the Dashing Rocks scheme the waves would have a large area over which to expand and lose themselves. (There is a great deni yet to be learned about range waves, one thing being the rate at which they expand—stretch out endwise —and thus weaken themselves.) The outer spur—from the end of the eastern mole —of the Dashing Rocks scheme —would cut seas from due east, and make a large area for the piling up of range waves. Some would be partly destroyed along the spur, but there would be left, to bo reflected and deflected, much more than if the spur were not added. There would be less and less accumulation as the seas were more and more southerly, but still a considerable amount with the most southerly seas. The remedy would be altering the lines, or providing an adequate “spending beach” outside the inner spur. This must be a costly addition. Holmes and Sons’ design, it seems to me, may accentuate the defect of the Dashing Rocks scheme. It facilitates deflection into the outer entrance in the case of some seas, but not all, and leaves a wide opening for the access of range waves to their inner entrance, where mischievous deflections are not provided against. Furthermore their outermost spur from the eastern mole might possibly ensure a dangerous jobble just outside, by causing sea waves reflected from the spur to meet other waves running in trom the open seas.

THE RANGE THEORY,

ANH AIR CLARKE’S SCHEME,

As an introduction to some remarks upon Air V/. If. Clarke’s proposed scheme lor improvement of the hsirbeur —a treble improvement, increasing its area for greater convenience in handling shipping, and for inturo extension of wharfage, and at the same time reducing the range at the berths, winch, he says, is “the principal dillicuity at the present time'’ —a few sentences may bo quoted from the report of the Royal Commission on Mr Maxwell’s eastern mole .scheme (which was adopted and carried out). The Commissioners, .Messrs Hay and Kawsoii, •said: “The scheme will, we think, prove disappointing as regards the diminution of scene! or range in the preedit harbour unless a new north mole is built. . ' . . to exclude the waves expanding from those passing the end of (lie mole.” This refers lo a. north ’wall marked on Air Aloxwcll s plan, but not at that time asked lor: “With a new north mole, in addition to the proposed south mole, it would

he possible to reduce tho rango still further if found necessary, by making tho existing entrance narrower than it now is, which could easily bo done.” The Commissioners • correctly forecast tho clfect of the south molo on the range. It has proved disappointing. Two proposals to reduce tho range by outer works have been turned down by a Royal Commission, ono became' of inherent objection, the other because it was too expensive. Both oi these contemplated the retention of the present jiarbour, unaltered. A third proposal embraced enlargement ol the harbour (with curtailment of the north mole), and reduction of range. This scheme was received with disfavour by the Board. Mr Clarko’s new scheme is a modification of his 1923 north mole extension scheme. The alterations are important as he now proposes an entirely new north mole, and removal of the outer half or more of the present mole. I am not concerned at present with any other aspect of the scheme than its probable influence on tho range at the wharves. I may, however, say that the arguments in favour of enlarging the area for the handling of big steamers arriving or departing, so as to render the work more safe and more simple, are worthy of very favourable consideration. As to the effect of tho proposed new works and alteration of the north mole on the range at tho wharves, any forecast must bo based upon some theory of the origin of the range waves. I endeavoured to explain this origin in a previous article. Range waves do not spring up at random. They have to bo created, and I showed how tho.v are created, by the breaking down of the ends of sea. waves after these have passed tho eastern mole. Tho range waves are strongest at their origin, and this for all seas producing them is near the end of the eastern mole. Further off they are weaker at their source as the sea wave rolls on, with its , end more and more depleted of energy ns the wave breaks down. They vvvi become weaker By expanding into dead water to tho left- of their course, that is, into the lee of tho eastern mole, but not to the right, as the wave is coiv timious on that side. Mr Clarkes report docs not recognise this true source of range waves. He mentions only reflected wave motion from Dashing Rocks, and the swirl in the Bay.” The former, I believe, is a. very doubtful source, the latter an impossible one. One of Mr Clarke's points in favour of tho scheme, is -that the new entrance will ho 800 foot further from the shipping berths. But if my explanation of range waves is correct, that is equivalent to saying that the entrance will be 800 feet nearer to the source of the strongest rango waves. • 'While, however! the range wave must bo stronger at the now entrance tliroiigh missing expansion on one sido whilst it runs 800 feet, it is given opportunity to expand to both right and left after passing the entrance, and therefore must bo weaker than now vdicn it reaches tho end of the breakwater. _ Both these expansions will occur within the harbour. What would become of tho expansion to the left may not signify, as it would be outside the breakwater. What would become of the expansion to the right, into the 100 of the arm of tho north mole, it is difficult to say. There would' bo reflections, in directions dependent upon the direction of.the range wave, and tliev would cause some unrest over the harbour.

A' more important question is the directions of such portion of the range waves as run through-the entrance. These directions depend upon those of the 'generating sea' waves. The latter are practically limited to the angle between -cast and south-east. I have plotted the courses of range waves from seas within those limits, assuming that the waves are 'generated at right angles to the direction of the sea waves, and that apart from sidewise . expansion, and apart from any influence of the deeper water in the channel, they fun straight, and I find that the behaviour of the range among the wharves will be very different from what it is now. When the arm of the north molo is removed, the rango from E.S.E. seas will run down to No. 2 and 3 wharves; from S.E. seas to No. 3 and 4 (if one is mado at north mole), while the range from E. seas will mainly he trapped by the breakwater. For E. seas the outer berth at No. 1 wharf, and for most seas tho whole of this wharf, will be in the “shadow” of the breakwater, and therefore should bo tbe quietest wharf, though reflections of expansion from the arm of the new north molo, and expansions from/ direct waves, might cause confused movements in the water along that wharf. Expansions from entering range waves will make tho space between tho breakwater and the space from the eastern molo a very uneasy boat liarboor.

“The new entrance, will bo 500 feet wide, and the harbourmaster considers this ample, ns the entrance is well sheltered by tho eastern extension.” It is so sheltered from sea waves, but not from rango waves, and these are the cause of tho trouble and expense at the wharves. The present entrance is between 300 and 4 00 feet wide, but owing to overlap of the breakwater, the effective width for the entry of range waves from near the end of tho eastern molo is only about 200 feet. The new harbour would have about two and n-half times the area of the present harbour, and as tho whole of tho 500 feet of the new entrance is open to tho entry of range waves, the proportion of entrance to the harbour is about tho same as at present. In spite therefore, of a double, instead of a single expansion of range waves over 800 feet run tbe estimate that the present range difficulties will bo minimised "at least 50 to 75 per cent,” is sm.dv unwarranted. Many years ago Mr C. Napier 801 l told the Barbour Board that “tho only way to avoid range is to make a largo harbour with a small entrance.” It will not mend matters to make a larger harbour with a proportionately larger entrance. The proposed entrance of 500 feet could, of course, bo mado narrower and still bo ample, as the constricted present entrance lias proved to he. I am of opinion that tho plan before tho Board can be beneficially modified as regards range, by extending the arm of the new north molo so that it shall cut all, or very nearly all, direct range waves from near tho end of the eastern mole, when the spur from the latter might he unnecessary. _ Tills opinion is formed from Ihe plotting of range wave directions, which cannot he described, but must be shown on a chart.