Catchment Board Delegates’ See Waimakariri Works

Press, Volume XCIX, Issue 29192, 30 April 1960, Page 6

Catchment Board Delegates’ See Waimakariri Works

A bulwark of stones and shingle for the protection of Christchurch city and farmlands from flood overflows of the Waimakariri river at Halkett was the first point of interest shown to delegates to the catchment boards’ conference on a field trip through part of the North Canterbury Catchmeht Board's area yesterday. The field trip, which was held as the final day of the conference meeting in Christchurch this week, gave delegates an on-the-spot illustration of soil conservation and river protection work. The groyne at Halkett is near the site of some of the earliest river protection to be done in New Zealand, where in 1859 W. Dartnail contracted to construct wooden piling for about 12 chains along the river edge. Timber was cut in the Harewood forest at Oxford and dragged across the river bed to the site on the south bank at a place known as Rowley’s. The board’s deputy-engineer (Mr E. B. Dalmer) said that Dartnail’s piling was washed out the following year. The need for such work to be done at that time was borne out by floods in 1865 and 1868 when the river overflowed and escaped into the catchment of the Avon river. The Halkett groyne was a key feature in south bank protection work, he said. The present groyne, built in 1943 at a cost of £2BOB, is 20 miles upstream from the sea. Walreka Valley From Halkett the party travelled to the Tara property of Mr H. C. Deans near Coalgate, where the board’s senior soil conservator (Mr R. D. Dick) described improvement of the land in the Waireka valley which feeds a tributary to the Selwyn. Mr Dick said that the bringing of the land into production from gorse and swamp had increased runoff. and this meant added flood waters into the main reaches of the river. The scrub and swamp had acted as natuAl water reservoirs which drained slowly. Improved pastures, contour cultivation and channelling to increase absorption of the land to reduce runoff, planting of trees on slipping faces and gullies, and general conservation were aspects which must be considered in catchment control of improved areas. At Springfield the party travelled across the Kowai river to the home of the chairman of the board (Mr R. M. D. Johnson) for lunch and walked through an area of native forest which had been regenerated under the trees over the last 20 years. Mr Johnson described the management of the high country and his method of reducing the grazing of the tops, by improving and concentrating on the

hills and flats and the use of cattle.

After lunch the Waimakariri Gorge bridge was inspected, and delegates saw the automatic reeording gauge station and th« nature of the gorge itself, which were described by the chief engineer (Mr H. M. Reid), before travelling down the South Eyre road past the Eyrewell State Forest plantations to Downs road, where the board has been carrying out extensive operations. Work Since 195,1 Since 1956 protective work On the six miles of the north bank of the river from Downs road to Dixons Bay has been carried out. About 190 acres along the edge of the river was bought and fenced off and about 50,000 trees planted. The trees will not only beautify an otherwise barren stretch of country but will tn about 30 years prove a valuable ssset. Of the 31 groynes built to pre. vent further encroachment of the river in floods, 11 were major groynes and 14 of a spur type. The party inspected one of the major groynes which was completed last month at a cost of about £6OOO. The head of the groyne rests on a sub-mattress of manuka scrub roped together over which a levelling skin of about four inches of shingle is placed. Above this is an articulated mattress of 400 concrete slabs each four feet square and nine inches thick linked together with wrought iron loops in both directions. The links permit the slabs to drop down on any underscouring and halt further erosion. The gabion head sits on top of the concrete mattress and consists of a nine foot high bulk of about 300 cubic yards of crated boulders. The Waimakariri contains relatively few boulders of sufficient size for this type of work in the lower reaches and some of the material used was brought from the Ashley river.

To join the groyne head to the river bank a bank of some 10,100 cubic yards of shingle has been thrown up and consolidated. Willow shoots have been planted in the areas between the groynes to give further protection. The buses travelled during the afternoon on part of the 200 miles of access tracks the board has constructed and maintains. Mr Reid said that these tracks were invaluable in ensuring ready acces sat all timw to the protective works on bwth sides of the rivers it administers for rapid inspections in time of flood and for repairs.