OVERFLOW OF THE WAIMAKARIRI.

Press, Volume XIII, Issue 1728, 19 September 1868, Page 3

OVERFLOW OF THE WAIMAKARIRI.

The following memorandum by Mr Felix Wakefield, civil engineer, Wellington, laid before the Kaiapoi Borough Council at their last meeting, and purporting to have been submitted to his Honor the Superintendent of this province, difi'era somewhat from tho opinions ndvaaced by Dr Hector in his report upon the dangers of Christchuroh and its neighbourhood from tha overflow of the Cjtirtenay river. He requested uto bo read with a former memorandum of lolh July, IS6B, upon the Suniner estuary: —

"Everybody who has examined the country in the neighbourhood of Christehuroh with a view to proposing ti remedy for tho overflow of tlio Kiver Courtenay must agree with the report of Dr Hector in tho certainty of that city being always exposed to inundations, and in all probability through some sudden thaw or other casualty in the mountains to destruction, involving enormous loss of property, unless somn immediate steps ure taken to control the danger. "Mr Dojne made a report on the subject in 1854 , , and if I recollect right it proposed a cutting at Kaiapoi to ward oil" vrhat he called ' the impending calamity. . Other works have been gomgonsince.with expensive surveys.only ending in tho waste of much money ; and any works, however solid, with a view to opposing a mountain toneni liko the overflow of the .Courtenay, must have the same result when the changed circumstances of the area over which the Hood has gradually spread itself for ages are considered. "Dr Hector remarks 'Previous to tho settlement of the country, the escape water would seem to have been distributed over the extensive swamp's which then occupied tho district, end was already absorbed, whereas since the clearing of the land and the formation of surface drains, it flows oil* with too great rapidity for the capacity of the present watercourses.' In this opinion, that Dr Hector is right no one can doubt, but (I say it with great respect) I do not think he goes far enough. Those who have seen the process of clearing raapo swamp, tutu, aud other rank produce of Iho land between Christchurch and the Courtenay, will all agree that the changing a swamp into almost solid cultivated fields (the occupiers subsequently making every effort to get rid of surface water, in order to consolidate the rich soil) 7tas permanently lowered the level of the country. I do not speak scientifically, referring to some internal convulsions creating a change in the earth's surface, I speak as a farmer would speak of his fields when compact ar.d fit for the plough reclaimed from a swamp, and mean from five to fifteen feet. Perhaps fifteen feet may be an extreme case, but there are bolus in the Styx more than twenty feet deep ; and if the fine land through which the Styx percolates were properly drained, the surface level of the land would bo permanently lowered fifteen feet below its original level in a slate of nature. To this cause is due the recurrence of floods which, influenced by the same operation going on over a larger area, must increase in their desolating violence. If this be the case, no defensive worts such as those proposed could resist such an overflow as resulted from the rain deluge that fell early in February last, when the choked formation of the river at Kaiapoi and the porous nature of the south bank of the Courtenay are taken into consideration. Even supposing for a moment that the foundation of such works as proposed stood, the gap leading to the Styx would receive the whole force of the flood, or at any rate as much as was stopped by those works. Then, one might urge that two gaps should be stopped simultaneously ; and this might perhaps be accomplished if the continuous labour of hundreds of men (as in the repair to the breach in the G-omershain dam in the English fens some years ago) could be obtained. If this be done the flood would continue its course and Kaiapoi would be destroyed; but in all probability the flood thus compressed would find vent somewhere at somo fresh gap or lower place, leaving the defensive works high and dry, to take its natural course to a lower level.

" The remedial measures proposed by Dr Hector when the flood reaches Christchurch are, and I say it again with great respect, not sufficiently outspoken. To remove a nuisance in an increasing city, as the mill at Christchurch is obvious, and particularly when the earne convenience could be obtained by a race without creating an obstruction. The storm channel propesed by Dr Hector does not require remark as it would be comprised in the more comprehensive remedy I would take the liberty of proposing.

" The Courfcenay is not singular in its overflowings and the destruction it causes ; in that it resembles many rivers, but in none so much as those in the north of Italy. These after depositing the sediment brought down in their torrential course from the Alps in the large lakes to the north of the Po, they have been made, by skilful guidance, of the greatest help to man, where, prior to human interference, they desolated the country until they found their vent in the Po, which received them all. They ioere controlled by turning them into canals ; some are navigable, some are not, the latter only conveying water for irrigation. The principle of controlling their violence might be applied to the Courtenay, for by extending the line of the canal reserve (laid down in the survey of 1849) to the most eligible spot for taking the water from the Courtenay a navigtia f/rande would be formed on the Canterbury plains j with the following great advantages over every similar undertaking in Italy. • "1. The fall being so slight between the ordinary surface level of the Courtenay and the Sumner estuary, where the canal reserve commences, there would be no danger of the water injuring the banks of the canal. ! "2. For the same reason, the line being almost straight, no high land intervening, the cutting would be moderate, and at the same time, not liable to lose water to a lower level, , such not existing. ! "3. That probably most of the soil would be of the finest qualify, and used upon the land on both sides of the canal in filling up hollows and top dressing, and "4. That the canal would form a drain for all the rich land in the neighbourhood of j Christchurch and of that city.

"If the clear water were taken from the Courtenay in the skilful manner in which the Italian lakes are relieved of their surplus water, no shingle could enter the canal; and the quantity of water flowing into the canal could be controlled according to the height of the rivr.

" Without going into details, perhaps there is probably not more than a fall of fifteen feet between the ordinary surface of the Courtenay and the Sumner estuary. (Would his Honor do mc the favor in return for the trouble I have taken to have this accurately levelled both ways as a check, it would not take a competent surveyor a week making the eectiou at all ?) So that with proper appliances a constant stream could be allowed of two feet in a mile fall alocg the whole length of seven miles. The quantity of water taken from the river being under control, any increase of speed would be due only to the greater quantity allowed to pass in time of flood. " This memorandum strictly confines itself to the remedy that a canal would afford to the overflow of the Courtonsy and the saving of

Christchurch, and perhaps tho winter might be allowed to deter the commencement of defensive works on the banks of the Coiirtenay until his Honor th,> Superintendent shall find it convenient to refcir tin's memorandum to the Provincial Council, in order that the subject may be discussed on its merits. Nor need the writer assure his Honor that unless he had an intimate knon ledge of the process he has described as obtaining in the north of Italy, both by personal investigation on the spot and by ex tensive technical reading, he shall consider the submitting of this memorandum a gros3 impendence; but so satisfied is ho of the certain destruction;«f Christchurch in spite of all defensive measures oa t'je bauk of the Coart tenay unless (he means of affording some relief to that rirer at its ordinary JeveJ, and the power of increasing that relief on the water commencing to rise bo attained, that he could not allow Dr Hector's report to pass without respectfully suggesting what, his experience and observation le;;ds him tto beiieve would bo a remedy for an a'anniug danger. "Herewith I beg leave io send for his Honour's inspection Captain Baird Smith's well-known report on the water system of Northern Italy, with the utlas, where everything is detailed, that I have taken the liberty to advance in a few lines. "F. Wakefjeld, " Civil Engineer." Wellington, Augiut 24, 1883.