ASR - A Global Partner for the Sustainable Management of Coastal and Marine Resources

Waikato River

Location: Waikato River, North Island, New Zealand

Type of Project: Hydrodynamic Modelling

Client: Solid Energy, New Zealand

ASR Projects- Waikato River

From left to right: A current velocity vector plot of the Waikato River North of Huntly; locations where sediment samples were taken and changes in river currents in m/s after deepening.

Project Overview:

Solid Energy extracts coal from its underground mine at Huntly in the Waikato Region of New Zealand. This extraction could lead to a subsidence of the bed of the Waikato River. ASR Ltd were asked to assess the impact of this subsidence on the river flows and sediment transport.

Project Outcomes:

The first phase of the project, examined the hydrodynamic changes resulting from a 12 cm subsidence in the river bed associated with mining activities. The second phase of the project looked at the effects on sediment transport in the River resulting from the change in the hydrodynamics.

The study predicted changes in the current velocity, including expected impacts on the sediment dynamics. The modelling showed that there would be a decrease in river flow velocities as a result of the subsidence of the river bed. Sediment transport numerical model studies were then undertaken to assess the time scale for infilling of the deepened area.

The results of the sediment transport study suggested that the sediment fluxes and infilling rates after subsidence were of the same order as the subsidence induced depression volumes. Thus it was concluded that the River would remain essentially in equilibrium, especially since the subsidence would occur gradually over a span of years. For an assumed worst case of an instantaneous subsidence of 12 cm, the infilling would take on the order of 1-2 years and would not have a significant impact on river dynamics.

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