Co-operation between Environment Waikato and power companies during July’s flood substantially reduced flooding downstream in the lower Waikato River, this week’s Environment Waikato Operations Committee heard.
Asset Management Group Manager Scott Fowlds said Mighty River Power and Genesis Power had worked very closely with Environment Waikato in holding back water under Environment Waikato’s Flood Rules during the week-long event. While this had cost Genesis Power hundreds of thousands of dollars in earnings from power generation, the company regarded the controls as part of responsible operation, he said.
Natural Hazards officer Adam Munro told the Committee that the cumulative effect of multiple rain bands passing over the Region from the northwest in the six weeks before the event had saturated soils and raised river levels above normal.
Most of the rain fell in the western and southern parts of the Region, and there were more than 10 weather watches or warnings issued by the MetService. The Waikato Waipa Flood Control Scheme ran well throughout the event, with lower Waikato areas benefiting from reduced flood levels.
Very high river levels were sustained in both the Waipa and Waikato over a long time, with the Waipa contributing well over half the flows recorded downstream of Ngaruawahia. In spite of huge increases in inflows into the hydro lakes, Karapiro outflows remained steady, with the hydro system absorbing a significant amount of floodwater.
Environment Waikato issued 45 flood alarms to district councils, land owners and key agencies and the event resulted in flooding of farmland next to the Waipa River fringes, and flooding of 700 hectares of unprotected farmland in the Lower Waikato. The total cost of damage to scheme works exceeded $80,000, excluding $25,000 spent during the event for emergency works, loss of productivity on unprotected farmland, and loss of power generation.
The Lower Waikato Waipa Flood Control Scheme performed exceptionally well, providing effective protection to most areas. The flood itself was generally lower than the design of the scheme – about a 20 to 50 year event, with peak levels in the lower Waikato on average 0.70m below the 1998 flood.
Assets requiring maintenance and upgrade were identified and Project Watershed funding provisions are adequate to carry out the work within present budgets.