Simulated flood in Christchurch stream study

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NIWA scientists have been simulating flooding in Milne Stream near Halswell to study the effect of plants along the banks of Christchurch streams. The study is a joint project between NIWA and the Christchurch City Council, and field work wraps up tomorrow (Friday, June 4).

The amount and type of weeds in the city’s streams and of plants along their banks can affect flooding in adjacent streets and properties. When a stream is in flood, trees and shrubs on the bank can slow down the water and increase its depth. NIWA scientists have tested the effect of two types of plants: small sedges that bend with the flow; and upright branching kanuka, which does not bend readily.

Rather than wait for a natural flood, NIWA simulated one in Milne Stream near Halswell last week, by pumping water into the stream from a fire hydrant, in conjunction with the city council.

“In order to test the effect of varying amounts of both types of plants, we had to put thousands of them along the banks of the stream then take them away again in just a few hours,” said NIWA river ecosystems scientist, Dr Scott Larned.

Large or dense aquatic weeds can also slow down the streams. To study this, NIWA is taking detailed measurements of water flow, plant sizes and the shape of the stream channels on five tributaries of the Avon and Heathcote Rivers. The final measurements will be done tomorrow (Friday, June 4).

The city council’s project manager, Nina Nikora, said “The knowledge and methods from this study will help us manage weeds, enhance the streams, and improve our ability to predict flood levels.”

NIWA is due to present the results to the Christchurch City Council in October.

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Chief Scientist - Freshwater and Estuaries

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