Port Hills work ‘helped erosion’

Press, 27 November 1980, Page 6

Port Hills work ‘helped erosion’

A Ministry of Works and Development report released this week lays some responsibility for Mount Pleasant erosion problems at the feet of two Christchurch lotal bodies — the Drainage Board and the City Council. The report covers an. investigation done for the Minister of Works (Mr W. L.‘ Young) on the Billys Forest and Baxters Bush areas of the hillside suburb. Both areas have been at the centre of a persistent controversy since the early 19705. In both areas, one with a slump and the other with an eroded stream gully, the Ministry’s study found that the local bodies’ work had contributed to erosion problems. Because the watercourse below Mount Pleasant Road was eroding, it . would be better for stormwater from further uphill to continue down Major Hornbrook Road instead of discharging into the Billys Forest watercourse, the report said. An alternative to that recommendation, the piping of water away from the slump — but still into the bush gully — was adopted by the Drainage Board earlier this year. Stormwater from Billys Forest goes through a 675 mm pipe laid under Mount Pleasant Road during reconstruction in 1970. The stormwater catchment area may or may not have been enlarged by local-body roadworks, but those works would certainly have affected run-off, the report said.

1 In the Baxters Bush I stream gplly, the pattern of; stormwater. flow-; had changed, apparently since the road above was -upgraded and the drainage pipeline was laid under it. For one thing, water that might have continued down the-hillside by other routes and joined the main gully further downhill was now collected’ by the road with its kerb and channelling. It was taken directly to the 675 mm pipeline, then down the gully. Water also took less time to reach the pipeline. Changes in rainfall since 1970, where there were several unusually wet years, were unlikely to have had a significant effect on the erosion problem. It was felt that the stream channel’s condition was consistent with a rise in the stream’s dominant discharge. “It would seem that the effects of the pipe to discharge as.it was originally installed were underestimated,” the Ministry’s report said. The Drainage Board has installed an energy dis- , sipator below the pipeline Into the gully. The channel , is being monitored down- j stream to determine whether , more work to control the , water velocity needs to be done. | The report said that a ( changed storm pattern in the Mount Pleasant area could 1 have contributed to channel I erosion, but a changed pattern would then be expected . in other natural channels of the area. ’ “This does, not seem to be|r

the case,” the report said. ; Scouring below Mount Pleasant Road resulted from thepipeline installation. < Tne report concluded: "It is? considered that the eroding condition of the channel is mainly attributed to the upgrading of Mount Pleasant Road, the'upper catchment development, and the stormwater system installed at that time.” Earlier this year, board engineers acknowledged that the Ministry’s report would come to that conclusion. But they said several statements in the report were debateable. They also said the report did not take court decisions about the controversy into account In the early 19705, a court opinion said that propertyowners who deepened the gully channel to ward off flooding problems could also have contributed to the erosion. The chairman of the works committee, Mr Newton Dodge, said he did not think there were any moves to change, past board decisions on the catchment. Alternative ways of relieving stormwater problems might look good on their own, but other factors had to be considered, he said. The board had decided that slowing the water in the channel was better than not having the channel take the water. , Investigations were continuing to determine whether more channel work should have been done. |