Halswell River drainage district earthquake damage

The Halswell River and drainage system, south-west of Christchurch, has undergone significant changes as a result of the September 4 earthquake and aftershocks, say Environment Canterbury river engineers.

“Water levels are flowing much higher as a result of the wet winter and the earthquake elevating groundwater levels for several days following the quake,” said Environment Canterbury regional engineer Ross Vesey. “Along much of its length, ground movement has reduced the river’s draining ability. As a result water levels have remained high.

“Environment Canterbury has already begun emergency works to help remedy the problem and minimise flooding. A significant part of the work involves removing material from the bed and banks of the Halswell River to restore its capacity. This will involve heavy machinery working beside the river.

“We have written to people living near the river, advising them that it may be necessary for machinery to get to the river across their land. Spoil may also need to be stockpiled on land temporarily. Some stretches of river may need access to be improved and this may include removing trees and fences. Individual property owners will also be contacted as works progress.”

Ross Vesey said that the work was necessary to restore drainage for properties in the Halswell and Tai Tapu area. “The cost of recovery could be high for these communities,” he said. “Although it is anticipated that Government funding will be made available for part of the cost, rate increases for this rating district are probably inevitable if people want to continue to farm this area and live here without the risk of high groundwater levels.”

The main reason for the reduction in the width of drainage channels was due to bank collapse on either side (lateral spread). Liquefaction has been widespread throughout the catchment resulting in ground level changes, with silt filling up channels and changing the water levels creating ponding in several places.

The restoration of drainage capacity is being tackled in three stages by Environment Canterbury: Firstly, liquefied material from the affected side drains is being removed and selected locations of the Halswell River cleared with drag lines. A drag line has begun working between Tobecks Drain and the Leatham Swamp outlet. The second stage involves medium term stability and restoration of capacity. The final stage is to achieve long term stability of the total system.

ENDS

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