LAND DRAINAGE.

Hauraki Plains Gazette, Volume XXXIII, Issue 4426, 12 June 1922, Page 4

LAND DRAINAGE.

NEW ZEALAND AND AMERICA. MR J. B. THOMPSON’S REPORT. I ■ 1 EXCAVATION COSTS, 1919-20. | No. 5. ; Analysis of Attached US.A. Costssheet.—dome fifty isix contracts are listed, representing typical work by various classes of floating dredges, walking-dippers, drag-line excavators, etc. The highest cost is 1 dollar (4s 2d) per cubic yard fpr an improvement work of 12,173 cubic yards extending over a distance of 2.1 miles. The lowest cost is 11 cents (sKid) per cubic yard for excavating 1 059,640 cubic yards extending over a distance of 13.25 miles. The average cost ,;t the whole of the fifty-six contracts works out at 22.78 cents per cubic yard ' For comparative ' purposes it is necessary to exclude from the cost-sheets all small and extremely large works. The average cost of ordinary-sized jobs, therefore, is worked out. as being at the rate of 15.8 cents (8d) per cubic yard. New Zealand Dredging Costs.—Our average costs covering .the same pej riod as those above is 6.99d'per cubic yard, and fpr. the period 1920-21 amounts to 8.12 d per cubic yard. It will thus be seen that the comparative costs are identical. ANNUAL MAINTENANCE. It is an astonishing fact that neither in Canada nor in the United States has the question of maintenance been solved. This subject has exercised the minds of all engineers ami controlling authorities, and they express surprise that this item - received so much practical attention in

New Zealand. Annual maintenance of drainage ditches is considered an absolute neP , sS ej ty in America -by all engineers who design such projects, and they feel that the benefit at first derived is' very soon discounted by allowing di’tehes to deteriorate. Some of the districts carry out maintenance, but others neglect it. , It would appear that the comparatively short currency of some of the bonds is in cases largely responsible for the neglect to strike a maintenance rate in addition to the rate and sinking funds necessary to extinguisn the original bonds. The result' of non-jnaintenance has been largely instrumental in inducing engineers to make ditches over size, and thus meet silifag, erosion, and vegetation in this manner. This does not. appear thoroughly satisfactory, as many of these ditches became choked with all classes of vegetation, and are theoretically much beyond the cross-sec-tion necessary, and have not sufficient water to keep them scoured out under normal conditions. It therefore frequently happens tnat the rej tirement of the original bonds heralds i a new issue to re-excavate- the old ditches.

The annual growth of vegetation in Canadian and American ditches does not appear to be so great as in Ne v Zealand. Practically all the wellknown vegetation, suc'hi as cat.-tails (raupo), convolvulus, saw (cutty) grass, duckweed, etc., were observed, and so far, no light machine has been designed that will successfully and economically deal with it. The various engineers m r t did not consider it at all advisable to place he-vy expensive machines in competition witlii hand cleaning of ditches at the price the work could be manhandled in New Zealand. This is correct in so far as it goes, but the trouble is to have sufficient labour at the right period 'in New Zealand.