THE WAIMAKARIRI.

Press, Volume LXII, Issue 18631, 4 March 1926, Page 11

THE WAIMAKARIRI.

TO THE EDITOR OE* TUB FREoS

Sir, —Ab on© interested in the problem, of river control, I attended 1 ast; week to hear an explanation of the proposed regulation scheme for the Waimakariri which, we are told, is to cost approximately £200,000, and w.IL do away with tho flood menace tor some considerable time. At the close of the lecture a questioner asked what the throttle was for, and tho reply was that it w catch tho shingle. The question was then asked, ''Why does not the gorge above, a natural tlnottle, stop (he shingle?" The reply was that there was no shingle coming thiough tlu.

" This is the crux of the whole pro_bU'in The proposed' scheme is. as iviM show, based on three fallacies, that shingle does not come through the gorge, that- the only shingle travelling in the. river, is from erosion, ol banks, and that stones travelling miles or so are ground to grit at the sen coast. . . Dunn 0- the- "Waima-kann power scheme " investigation, measurements and records were definitely obtained of the amount of gravel continuousb travelling, and since the Trust, are apparently ignorant of these records, it might * interest them to know that in, sav, a flood of 8700 cubic feet per second there is a continuous underflow of gravel of 2-j cubic per second. While in a flood ot l<,o!)0 cubic feet per second the flow of gravel approximate to 7 •"> cubic feet pet: second. It' tliis latter flood extends for ji clay, and it often extends for several days, in one day approximately SoO.OOO cubic, yards arc moved bodily, that is a bank -~>o yards by 1000 yards and one yard deep. In tiie recent disastrous flood considerably more than the above amount- will be moved.

If tlie narrow rock-bound gorge above does not stop the travelling shingle. and there is conclusive evidence to support this statement, although the Tr>ist completely disregards it. how then will experimental and artificial groynes made of li;j;ht soil and small pravel do so? I will now predict with certainty what will happen. Owing to the artificial constriction caused by the throttle, the oncoming shingle will deposit above the groynes, will raise the bed which will lead to the river in a short time breaking info the banks above and rebounding in'o an entirely different direction. Liter will follow the complete disintegration of th.e groynf The Trust s reply to this is that they will construct another throttle higher lip- and when ji 'is ineffective, still another one. ff ibis is the plan, wliv not go vitiht n[> to Otavama and build a proper throttle between roc': <1 iiYs 1-y' feet liic.li y The Trust maintain that by stoppin p: the erosion of '■anks they wii! stop the travel of shinitle. the only shingle travelling ni th-: river is the result of scour and erosion of hanks. This is 'definitely disproved of'harks hv the above record-, and also by the fact that "the riverled is above the snrro'tndine country. This is a feature of all alluvial tivers. namely, that the adjacent lands sjone airav front, and not towards the man) stream. Hence this aggregation must be accounted for in some other way. In reznrd to the grinding of stones in the oou;*p of travel downstream, tafias been definitely established by actual experiment, and measurement that average stones lose only half their v.v.).-rbt over a distance of 'O miles, ivhiHt it iskes distance of .300 utiles to reduce them to.grit. Cones ally the*

law is that the wear is- proportional to the weight of the pebble and the* distance travelled. A complete account of this phenomenon is to be found in Bulletin No. '2, Vol. 3ti, Geological Society, U.S.A., where the researches of the late Joseph IJarvdll, an eminent geologist ami other. Continental experts, are detailed:, 11, in accordance with the theory of the Trust, gravel moving in a riverlied is turned to grit after a journey of .10 miles, how is the presence ol gravel opposite Kairaki and along the !K)-mile heath the .result' of the out-' pourings of the l'akaia, Rangitata, and \Yait«ki Rivers to be accounted for?

The proposed scheme. adopts the straight lino instead of t lie curve in, defiance of the laws'of Nature and in defiance of tiio proved continuous travel, of shingle yhrough the gorge. One never sees the straight line in .Nature, not even in a rock-bound gorge; all the more, therefore, is it impossible to maintain the Waimakariri in a straight course, owing to the shingle burden'causing the formation of banks and bars and consequent lneandering. "Oscillation of flow" as termed by French engineers is one of the moat important of principles in connexion' with river How. By it definite tendencies are imparted to the current without which the regulation of complicated channels would be little else ' than guesswork. It is supposed to be dependent on the principle.of economy of natural forces, manifested in the tendency of Nature to accomplish her results with the least labour. The chief function of a river being to discharge the flow with 1 the accompanying burden of sediment, it should strive, in accordance with this law, to create that cross-section of channel '-which will permit; it to fulfil its labours - with the least waste of effort. The chief requisites of such a channel would be stability and depth, since a shifting course leads to constant and useless toil in the erosion and building of sand-bars, ami a shallow one gives rise to a friction from which a deeper channel would be largely free. These two requisites appear to be secured l»y a river more easily with motion curve than along a straight line.

A river in an alluvial basin such as the Waimakariri is in the grasp of two contending influences, the one that of gravity tending to drive it down the shortest route, and the other that derived l'rom the necessity of its keeping as far as possible an equilibrium beliweeu t.lie velocity of flow and the nature of the. bed. The more this eeuilibrium can be preserved the less will he the labour of a river in carrying out its work, because the erosion will be less. The stream 'therefore lengthens its course in order to reduce its Blope, and therefore its speed, and then there appears to come into play the farreaching principle which guides it in moulding the type of channel which will best facilitate its labours, and allow it to transport, both How and sediment. wit li the least waste of energy. The only sure way to hold up the gravel is to build a dam in the Gorge. This dam is not now justified from the power point of view, since Coleridge 33 duplicated, and will supply tho needs of Canterbury for some years. However, a golden opportunity was lost before the duplication of killing two birds with one stone, supplying the- future power fur Canterbury, and doing n real service to prevent aggregation of gravel in the lower reaches.

hi flic absence of the dain, the uliiugle wil! .-(inliniie to collect in the lower readies, and the safest way is to distribute it. over the riverbed as evenly and as quietly as possible by controlling the river in its present course in a series tit' natural bends of a radius from one and :i half to two miles, rebounding from side to side. Meanwhile, stopbanks must be raised along the banks where necessary to keep pace with the certain raising ■of the bed, 3 process which is not going to stop for tsome thousands of vears.

The erosion at Stewart's Gully on the. i south hank has been caused by neglect; to block a channel which in time of! flood leads water behind the island above on the north bank and directs ' it at an angle of 45 degrees on to the south bank. Until this channel is" blocked it is certain that successive floods wit! completely isolate the pre- \ sent railway bridge. Groyne after groyne has been put in, and each only serves the better to assist the impinging current- to erode the hanks,. The source

of the trouble, that is, ttyj deflection of the current behind the island, still remains.

However, before this happens, the first serious damage to be anticipated is the cutting out of the south bank just above White's bridge. Here a large island lias been allowed to grow and forced the river over on tho south bank. Many other things will happen when the straight cut to the sea is made, chief of which will be the conversion of a few square miles good land into riverbed. Sooner or later Nature's laws will reassert themselves, and the Waimakariri, in spite of the Trust's order to remain straight, will curve out for itself the course it-likes and whether this will be at the expense of Christchureh or not, I, for one, would not like, to say.

The proposed scheme is fouaded on insufficient data, and the wrong data, and defies Nature in many respects, It completely ignores snagging and dredging, the -chief agents of regulation, and on the above grounds I hope that the scheme will not be sanctioned. —Your 3, etc..

U. M. CHRYSTALL.