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Barrage Flushing Implications (Monitoring Sediment Releases from the Rangipo Hydro Barrage)

Author(s): G. M. Smart

Linked Author(s): Graeme M. Smart

Keywords: Suspended sediment; Flushing management

Abstract: Rangipo Hydro Barrage is fed by volcanic catchments in the centre of the North Island of New Zealand. Its sluice gates were used to flush out an average of around60 kilotonnes of sediment per year prior to 1995. Volcanic activity in 1995-96 more than doubled the annual volume of sediment trapped by the barrage so that flushing had to be carried out more frequently and at lower flows than originally envisaged. Sediment movement in the Tongariro river below the barrage has associated environmental effects which are controversial as the river is famous for trout fishing and rare Blue Ducks. Suspended sediment monitoring has been carried out under varying flow conditions. With the barrage sluices closed, sediment concentration is typically higher on the rising flood than on the falling flood and concentrations fluctuate from one sample to the next. During a flood with the barrage sluices open, there is less fluctuation and concentrations are similar at the same river stage on the rising and falling flood. Following the volcanic eruptions, measurements of sediment concentrations during floods with the sluices closed, resembled pre-eruption floods which had the sluices open. Eruptions and barrage flushes do not greatly increase sediment concentration levels but result in more uniform levels similar to the peak levels found under natural conditions.


Year: 1999

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