PROTECTIVE WORKS.

Press, Volume LX, Issue 18217, 30 October 1924, Page 5

PROTECTIVE WORKS.

THE WAIMAKARIRI SCHEME ADOPTED BY TRUST. The Waimukariri ttivor Trust yesterday decided to adopt the scheme which has been v,m,;, t consideration for some time for effecting protective works in the Waimakariri river at an estimated cost of £21.000. At the previous monthly meeting the chairman. Mr J. Wood., of the J'ublic Works Department, submitted a lengthy report on flic proposal, and it was published, in full in "The Press'- the following 'lay (2-nd inst.). At, that meeting the members of the Trust agreed to submit the scheme to Mr A. Dudley Dobson, fho well-known Christchurch civil engineer, for criticism, and at yesterday's meeting Mr Dobson's approval of the proposed undertaking was received. The cost of.tho work is to be borne out of rates spread over three years, and it will be put in liand as soon as possible.

After c:\pktiuiug the methods by which ho fir rived at his conclusions, Mr Dobson states that the cutting into tho old luoraino fun, tho building up upon it of n second one, and also tho filling up of tho present beds in the lower part of tho river, if left to the course of ZVature, would go on indefinitely, the river filling up tho low ground, changing its course continually, now flowing over low ground and forming alluvial plains of great value, such as at present exist in tho vicinity of Ckrisichureh and Kaiapoi, and then covering them with shingle and silt as tho fan was built up. "This action, however, can be postponed almost indefinitely by constructing works which will force the river to carry its loads on straight into the sea," continues tho report. " Owing to the great fall and short course of tho Waimakariri, this is by no ineana difficult to accomplish. By suitable groynes such as you have designed, there will bo no difficulty in training tho river into the course shown on your general plan; tho chiof difficulty will bo tho financial one. This work, of course, cannot be dono all at once, but tho first step is, as you propose, to stop tho great erosions at present occurring on tho north side, and tho proposed works will at tho same time tend to send the rivor back towards the south more into the centre of its bed,, and, what is of tho most importance, to prevent the rapid silting up of the river channels by stopping the great erosion of the north bank, which is at present supplying the greater part of the shingle which is causing the damage. Unless this is done the river may do immense damage by overflowing its banks and spreading either north or south in the old channels.

"The Empire Bridge was built in 1900, and in eighteen years the waterway at the bridge had filled up 42 per cont. At this point the Tiver is only suo feet in width from bank to bank. It is quite certain that with a very littlo more silting up the water-w r ay could not possibly carry any flood water, which must flow over the adjoining lands. The late Sir Julius von Haast, many years ago, when writing on the geological structure of the Canterbury Plains, pointed out that eventually the river would require to ha carried out by a new 'channel straight to sea, to save the low lands it is endangering. "I am of opinion that the plan you have proposed, as shown on yotir general map, will fully satisfy the necessary requirement?. A great deal of the river's burden will be carried out into the Bay, and the silting up of the low lands postponed indefinitely. Restraining the river within reasonable bounds will also reclaim several thousand acres of land,' over which the riVfer is now wandering. As soon as the banks have been protected by the groynes you propose, the planting can be carried out, and will in time give timber which Avill be a very valuable asset to the Trust.

"The class of works you propose, as shown on your plans, have proved by experience to he well adapted for the purpose, and I am satisfied they will fulfil your expectations. "In conclusion, I am pleased to see that at last the control of the river is to be undertaken in a systematic manner, as a whole, and with due regard to the future; as I am convinced the Christehurch public do not in the least, realise what it may mean to the city if the present channels are allowed to continue filling up at the present rate."

THE DISCUSSION

Mr W. P. Speneer said that he could not see his way clear to approve the report. He did not wish to pit his opinion against that of qualified engineers, but he considered that something could be done in the direction of scooping out the centre of the channel. Engineers said that could not bo done, but it had never been tried. The chairman: I've made an estimate of that. Do you know what it would cost?—Xo.

The chairman: £300,000. It would cost about £50,000 a mile to scoop a channel big enough to carry the floods o£ tlint river.

Mr SpeneCr said that the report suggested that they should buy the land required. He (the speaker) was of the opinion that the neighbouring farmers should give the land required, as thev were to benefit. The'chairman said that, his proposal to purchase was only a tentative one. The adoption of the report did not commit them to buying land. Mr Spencer said that they should not go to great expense to protect lowvalue land. It was the land near Clarkville and such places, where it was worth £7O an acre, that they should protect. The chairman agreed with that view. Mr Spencer said that he would haveto vole against the adoption of the schene. as ).e considered it was unjust to the south side of the river. The chairman said that they had gone into all those aspects of the question previouslv. and even the Uty Council had agreed that the allocation of expenditure was just. v Mr (.'. H. Opie supported Mr Spencer's remarks. He s aid that the south bank ratepayers were being called upon to pav over*9o per cent, of the cost ot the work which was required prnu;pallv for the north bank. In the past the "difficulty had been met by raising the banks of the river, and in time ta:u work would have to be continued nght along the low portions. If °" 1 timer" flood came down the river and changed its course, he considered thathe extensive groynes would left standing high and dry. The old Boar had managed to protect the citjuU rioht for 50 vears. and it the Trust did the same for the next , years Ij its protective groynes it would do well, but lie doubted it. Mr J. 11. Blaekwell supported the scheme. Mr Spencer had suggested an untried and very expensive method, and' Mr Opic had only suggested the bu ding up of the banks, he said Mr i>udlevDobson had given* he chairman's' scheme an unqualified support, and it had received the support also

of a practical man in their foreman Mi T. J,-,;.

THE CHAIRMAN'S REPLY.

~i lr "Wood said that he had not th* slightest doubt that the groyne? would stop the erosion. As to what some "'.embers sa i d regarding obtaining the advice of practical men who knew tiie river, their overseer was an intensely practical man. and he approved the undertaking. Mr Wood was also convinced that the protective work< would not scud the river to the south bank any more than it had a natural tendency to go in that direction at present. If the work would protect only the north hank, he would not recommend it. They would have no right to carry out such an expensive work for that sole object. He thought that the south bank should pay its quota of 00 per cent, of the work. Thoy only had to think what was going to happen in the next iiftv years if they allowed the river to till up at the rate of three inches a year. Mr Spencer: What about a canal.'

Mr Wood: It would be ten years before it mould be completed." and in the meantime you have got shingle piling up which you can stop, and should stop. In* 50 years the bed will be 32 feet higher, and then nothing will stop it. In reply to further questions by Mr Spencer, Mr Wood said that the river had always gone towards the south. If the Tnist thought there was no danger in the shingle which was piling up they had no right to undertake the work. In regard to the' allocation of cost bctween the south and north districts, he said that he considered that the landowners should pay in proportion to the benefits received. There was no fairer method of taxation than that.

Mr A. Manhire supported the adoption of the scheme. He said that when the people of Christchurch understood the danger he was sure they would support the work. The chairman then put the motion for the adoption of the report, reminding the Trust members that they were taking on a very big responsibility. The motion was carried, Messrs C. H. Opie and W. P. Spencer being the only dissentients.

MR J.WOOD'S ADDRESS.

There was a largo meeting in the T.M.C.A. Concert Hall last evening when,, under the auspices of the Canterbury Progress League, Mr J. Wood, chairman of the "Waimakariri Eiver Trust, delivered a public lecture for the purpose of fully explaining the whole position. "The Waimakariri Eiver," said Mr Wood, "has a gradient very similar to other rivers until it approaches within 10 miles of the sea. ' This gradient is about 27 feet in every mile, but in the last 10 miles the fall is only 9 feet per mile, and for the last five miles less than one foot per mile. It is this flattened gradient that constitutes the menace to Christchurch and the surrounding districts from the river. Erosion is a minor trouble on the flatter gradient, the immediate cause for anxiety being the Taising of the riverbed and consequent flooding of surrounding land."

Mr Wood went on to say that he considered that the time had now arrived for complete control of the river. The Waimakariri River was raising or aggrading its bed at a comparatively rapid rate along its delta. The aggradation would, if it continued, cause serious and probably disastrous flooding to the rich and prosperous country within its delta. This menace was one common to many rivers in all countries, and the solutions of the problem could bo divided into three principal classes: —

(1) Mood prevention: This is obtained by the construction of reservoirs or detention basins for storing flood waters and allowing them to escape gradually into the river. This is usually an expensive method, and would be particularly so in a shingle-carrying river such as the Waimakariri. To store and regulate the floods of this river would require very large detention basins, and they would be so costly that we need not further consider this class of work for flood relief.

(2) Protection of adjacent country by stopbanks, or levees, and by channel improvements.

(3) Diversion of flood waters into new channels that give relief from floods by carrying off flood waters more quickly than the existing channels. (4) A combination of Nos. 1 and 2, or of ISTos. 2 and 3. Mr Wood summarised briefly what works were necessary to obtain a full and satisfactory control of the river in the order in which these works should be done, as follows: — (1) Protection of banks along concave margins where the river is at present eroding.

(2) liaising -stop-banks where flood waters can escape from the river-bed. (3) Cutting a direct channel to the sea to improve the gradient and flood velocity of the current.

(4) Erecting works to confine flood waters to a channel of a width and alignment necessary to secure a good depth of water for the increase of flood velocities.

(5) Planting wide belts of trees along river banks aud on reclaimed lands. (Tree planting should be started at once where land is already available.) The accomplishment of these works, he said, would take a good many years. Work on the first two, as well as tree planting where possible, should be started at once, aud surveys for the estimation of the cost and for the preparation of detail plans of the others should now be put in hand. Works once completed must be properly maintained so that they would fulfill the function for which they were designed. If this was not done they might not only fail to do what was required of them, but might even do more harm than would have occurred had they never been constructed. It was imperative, therefore, that a local body controlling a river should have an ample income to maintain all its works adequately. He would go further than this, however, and express the opinion that there should also be a fund held by the local body to meet sudden emergencies in expenditure that occurred so often in river works due to the unforeseen destruction by floods. Such a fund should be interest-bearing, and should not be touched except for emergencies. At the conclusion of the address a questioner asked why they could not clear out the obstructions in the existing river-bed. Mr Wood replied that the intention was to steepen the flood slope and increase the velocity. The Trust intended to clear out the existing bed. Other questions were asked and answered. In moving a hearty vote of thanks to the lecturer, Mr W. Goss said that they would have to find the money to protect valuable property. Seconding the vote, Mr A. D. Dobson said that very few people had any real idea of the danger that existed. The motion was carried by acclamation. A vote of thanks was also passed to the Progress League for bringing the address about.