SOLVING THE “SCEND” PROBLEM

Timaru Herald, Volume XCVIII, Issue 18084, 22 September 1923, Page 12

SOLVING THE “SCEND” PROBLEM

To tiie Editor of the “ Timaru Herald.’’ Sir, —Tiie origin and nature of the “sceud”‘ or range wincu has for a great, number or years proved such a troublesome factor against the safe working of the shipping in our port apparently is one ol those subjects winch our Harbour authorities do not understand so 1 think that it would not be out of place on my part were i at this stage of proceedings to place before them particulars which may lead to their enlightenment as to its evidently peculiar propensities. The origin of that “scend ’ is to be found in tiie very strong southerly currents which set into the roadstead from the North and its nature during storm conditions is to drive its attendant sea-horses (in the form of deflected wave forces, into the Harbour and so keep the Harbour Master and his staff on the tenterhooks of expectation. Tiie powerfully cog tinuous force of that southerly current acting in conjunction with tiie intermittent pulsations of the wave forces deflected by the Eastern Extension and of the surge from the wave power on tiie South side of the Extension—which not oiliy expends its troubled waters through and over that structure but acts like a hydraulic ram in forcing through the Extension the silt which it dredges up from the sea bottom in the surf hole, and the small shingle—forms the driving , or motive power which governs the greater o. less degree of disturbance manifested on the occasion of a sea storm. The converging point of these forces s over the dredged-out ship’s channel directly opposite the entrance to the Harbour and as in rough weather that southerly current is usually of abnormal - power. Its action behind, and in unison with the directions of the pulsations of the converged forces, accelerates those pulsations and drives those converged forces, in the form of a “scend,” into the, in comparison, slack waters of the iner harbour to expend its force down the face of No 1 wharf. If it were not for the direct driving action of that southerly current the “scend” (if there happened to be one then, which is very doubtful) would be felt in an equal degree over the whole of the inner harbour, but outside of that direct course its effect is apparently scarcely appreciable except under exceptiona. circumstances. The driving force of this current, which is a recurred off-set from the northerly current running some twenty miles or more East of Timaru from Otago Heads to past Banks Peninsula, varies considerably. It is invariably more strong during a heavy sea storm _from the South than at other times, unis variableness beiti K accounted for by the fact that the sea storm accelerates the main northerly set of the current and necessarily puts more energy into the circular currentwhich is diverted back down the otherwise slack waters—or back water—of the lower part of the Canterbury Bight, and past our port. To instance the great power of this southerly current I would point out that oti the memorable 14th of May 1881 before the “City of Perth” broke adrift and was driven ashore, she was lying at her anchorage with her head to the north and her stern to the sea a state of affairs which meant that the southerly current was at that time far stronger than the tremendous force of the waves that eventually snapped her cables. After she stranded close to Splashing Point the southerly current, drove her along the sea bottom —as if she had been travelling on a moving platform—for about half a mile nearly due south until she finally brought up at the wreck of the “Benvenue.” The movement of the “City of Perth” was watched by scores of spectators witn intense interest and wonder that she should drift at practically a right angle to her original drifting course. Her movement was so steady and consistent in its evenness that she sustained no injury to her hull or rigging from that’cause, but if she had been rising and falling only a foot, the seas would have bumped her masts out of her before she bad drifted half that distance. Another lack worth mentioning is that on the day in question the Norwegian barque “F. C. Punch"’ lay at the outer anchorage during that day “with her stern to the sea. straining at her cables with her head to the north.” Her stern got such a battling from the heavy waves that the sea actually washed the letters of her name off of her stem board. If the current that dav was sufficiently strong to perform those wonderful miracles so much at variancy with the then accepted laws of nature there must be something more strange about the sea action on our port than the wiseacres are qualified or competent to appreciate. When considering these lacts it must clearly be borne in mind that as less than 1200 feet of the breakwater had them been constructed it had not been advanced sufficiently far out to sea to even give anyone an excuse to say that the work had deflected the current or even affected the natural run of the wave power. And when it is considered that the “F. C. Funch” was anchored about a mile and a-half south-east from the, end of tiie breakwater the only conclusion one can come to is that the southerly set of the current was then and is liow sufficiently powerful to perform more wonders than we ever dreamed of in the philosophies oi those who profess io know—but do not. As the break water,and later the 'Eastern Extension, advanced in a north-easterly direction the nature m the construction of (hose works caused year bv voar a deflected wave force or power 'of increasing strength and radius to develop and flow into Caroline B'iv until the Extension was finished. The effect of Hint deflected wave force or power could be plainly observable in practically all weathers.

This wave force is caused by tlie Extension confining and 'thus compressing a certain body of water or wave action on its weather side which cannot, until it lias readied the end of the structure, free itself fretail the pressure of the waters behind it that is acting like a compressor, but which as soon as it reaches the end of the. Extension is released and eventually finds its normal pressure in the surrounding waters. Before properly doing so, however, and by reason of its previous compressio.. such deflected wave force—particularly so in tlie case of a heavy sea—lorms a lan-shaped wave force which spreads itself over the area of waters intervening between the end ol iho Extension and the western shore. Jim middle or main portion of that fanshaped wave force now strikes tho northern end of the AYaimataitai Spit and is then deflected or distracted into, and sweeps round Caroline Bay until it and the balance of the inner half of that fan-shaped wave force meet the surge ot waters forced through and over the Extension —that is, as 1 have before said immediately’ in front of the entrance to the harbour to become the co-adjutors and assistants of that southerly current in iormuig the “scend.’’' If the very conspicuous southerly set of t-lio current on the 14th May 1882 was sufficiently powerful to hold that Norwegian barque with her stem to the son mul it* dii\o uio “City of Perth” half a mile in a southerly’ direction practically against the run of the wave .power when there was not sufficient ol Hie breakwater and no Eastern Extension to eauso a deflection of the wave force, if becomes very’ much more than an “inlereuco to assume that, under present day conditions, the deflected wave lorce. when j it reaches the entrance of the harbour must be augmented and aceeleraled by a directing and controlling force, smficientlv great to overcome ils waning powers and to force il lo expend its evident rehabilitated powers ol sell m assisting lo form that “scend which is an everlasting nuisance to shipping. And that accelerating force cannot be other than that southerly’ current lor iho very simple reason that- there is no other force in the viciiufy of our port from which an argument con'd he construed to account for any of Ihe phenomena that T have mentioned There is onlv one arnumoidafn e poinwhich could he considered in this mailer and that is—-T a southerly cnrrenl had never existed to crea e a drmm force into Hie harhoim H'o;’" oF'the defleded

wave force liave not in , themselves sufficient body when they reach thu , entrance to tho harbour to create more j than a slight surface rulllo or disturbance and would thus spread over the harbour in a fairly even and systematic manner instead of as at present maintaining a strictly defined course. The whole question, therefore devolves itself into a matter of cause and effect, tho keystone on which _ everything nautical and otherwise is based or built. Without a comprehensive knowledge of the laws and causes no mn.. can arrive at an approximate estimate of what the effect will be, even in tin, smallest matters, but by a careful study of the effect of any given power a very substantial groundwork of theory can be constructed on which to build up a practical knowledge of the workings and forces of that otherwise incomprehensible course.—T am, etc,- — A. J. MOBTUS.