INUNDATION.

West Coast Times, Issue 107, 20 January 1866, Page 3

INUNDATION.

(From Hie T.i/tliHi>n Times'.) What is to bo done with tho rivers ? This is the question which Christmas Day has left us to answer as best wo can. Tho country is liable to floods ; some districts are severely injured by them, while others have received no more than a warning that thoy aro exposed to dangor. Kaiapoi is in tho former category — Christohurch in the latter ; and tho warning to Chrislchurch will not bo in vain if it serves to draw more particular attention to tho* case of Kaiapoi than has hitherto been given to it. Tho damage from tho rivers occurs in two ways. In some places the course of the river -.bends, or from some local cause tho current is directed towards one of the banks, which it undermines and crumbles away piecemeal, making the bend deeper and deeper, ■ and sweeping away acres of the surfaco. lv other places, simply from the bank being low, the water rises above it, and pours over the top out on to the plain. Tho former process is ono which goes on constantly ; the, latter is confined to periods of "fresh," of which Christinas Day offered a > striking example. The two descriptions of injury have to be carefully distinguished, because not only their causes but their effects also are completely different from one another ; and therefore* the remedies which may bo devised arc likely to bo no less dissimilar for the two cases.'

The fir.it case seems to us by far the most difficult to grapple with. A large river like tho Raknin or the Waimnkuriri takes a set in a certain direction after a " fresh." Whether this change of direction occurs in consequence ot a local and temporary obstacle, or is in obedience to certain fixed and ascortainttble natural laws, we need not at present inquire, but we believe that too much stress has been laid upon the latter, and too little on Ihc former by the scientific inquirers who have reported on tho subject. At any rate tho problem lo be solved in finding a remedy is the stuno in either case. Either the direction of the stream must be changed again, or the threatened bank must bo protected artificially. Of the two plans (he latter has been not unfrcquently tried. But the result has been failure in a majority of cases. The groat pressure on tho bank of the whole stream in time of fresh, extended as it often is over a considerable distance, onmiot be resisted by any ordinary means ; and engineers have not scrupled lo assert in some cases that any attempt to construct an artificial bank must be hopeless.

We let the discussion on this point alone for a while, because the case immediately beforo us does not involve tho same considerations. If we ore rightly informed on tho facts, the iloods which so constantly injure Kaiapoi Island, and have even threatened to do damage to (Jhristchuvch, belong to the second kind abovo discribed. That is we are dealing with an overflow at a comparatively low but not necessarily a weak point in the banks. We do not think the same difficulty arises in this case as in the other. In the one, the whole body ot a rapid river is tumbling against' an upright bank ; in the other, a few inches of water in depth escape over a wide, and horizontal surface. The conditions are entirely different. We see no reason at all why tho works necessary for stopping back an overflow should be of great strength 'and si/.c, or in any way expensive. At the same time, they ore very necessary. Not only is the surface injured, especially in all the depressions, by tho water flowing upon it, but if it is permitted to flow long, a channel will gradually form, and valuable land will be converted into a river bed. Besides, as tho water must have ttp outlet at last down the ordinary watercourses, the swelling of these will bo a sourco of increased danger to the country through which they puss. We see no reason why a bank not exceeding in the average a few feet in height, constructed at the proper place, should not .stop every disaster likely to arise from this cause.

Kaiapoi Island, wo have always believed, is being sacrificed to a somewhat superstitions worship of scientific theory. That watov, nnd Uievefovo rivers, will naturally ilow at right angles to tho Hues of level course is perfectly hue as a theory. But if an indentation in tho surface' occurs at any point of a river's course, the water will ilow down it, no matter at what angle it may lie to tho general slope of the country. At tho lowest point in tho banks tho river will first begin to overflow, no matter what has caused that depression. It is difficult to make good the whole height of a river bank against an impinging current ; but it is very easy to stop back the few inches at tho top, or even to embank to a considerable height were the result of the embanking is to stop tho current altogether from flowing in that direction, nud so to produce still water. We venture- to suggest that the first object to aim at in confining tho Waiinakaviri to its proper channel is to stop the overflow above, the banks wherever it occurs, which may be done by vory simple and cheap embankments being extended wherever tiovo is a depression in the natural surface along tho r.ivur b'4<H>