Studying Rangitata’s silt and sediment

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Scientists are expected to begin work soon on looking at how the Rangitata River may be affected by the accumulation of fine sediments, such as sand and silt.

University of Waikato researchers are still waiting for a settled period of weather to conduct the survey, with low flows of less than 50 cumecs in the river needed before undertaking the monitoring work.

The best time for the work is often late summer, when there are naturally low flows and high water abstraction, or mid-winter, when the upper catchment freezes, causing low flows.

Low flows are needed to maximise the area of dry riverbed able to be surveyed.

“The study is part of a wider water science and river engineering project being undertaken by Environment Canterbury (ECan) and fits into our increasing emphasis of work on braided river management.

“This includes work on understanding river flow models (Rakaia River), sedimentation (Rangitata) and vegetation growth in our river braid plains. These all contribute to a better understanding of braided rivers to protect their special values,” said ECan principal surface water quality scientist Adrian Meredith.

“We are also interested in the overall topography of the braided rivers, where old channels and vegetation islands are – this is important for assessment of riverbed and instream habitat and flood-carrying capacity.

“We’re collaborating with University of Waikato researchers to assess whether helicopter mounted ultra-high intensity LiDar surveys are a more effective way of mapping these features of rivers, rather than a traditional physical survey of random points in a riverbed,” he said.

A small trial flight had successfully been conducted last year and funded by NZ Salmon Anglers.

The ECan-funded project was now scaling this up to a survey covering the full riverbed from the gorge to the sea. ECan has set aside $68,000 for the Rangitata monitoring.