The severe storms which hit the Waikato and Coromandel in June and July added more than half a million dollars in repair costs for the Region’s flood schemes.
This week’s Environment Waikato Operations Committee heard that the storm in June hit the Region on several fronts from the Coromandel to Putaruru, bringing very strong winds and intense rainfall which resulted in widespread and exceptional damage.
Works Manager Owen Passau said the event in late June produced rainfall intensities of about 100mm in an hour, which is estimated at a one in 100 year event, and creating river floods ranging up to 100 year events in many of the smaller rivers and streams. The July event produced further very heavy rainfalls on sodden catchments, creating additional damage and making existing problems worse.
Thames urban streams handled up to 50 year flood flows with limited damage to property, urban areas in Paeroa and Te Aroha did not suffer major damage from scheme streams, the schemes handled the widespread surface flooding and ponding effectively and some past problem areas operated very effectively, he said.
The Ohinemuri river levels rose dramatically, making it necessary to erect Criterion stoplogs across State Highway 26 at Paeroa.
The storms involved major Environment Waikato assets and those for which other agencies and private residents were responsible. The Waihou Valley Flood Control Scheme operated well in such a large event, minimising damage to properties in most areas protected by the scheme from Thames to Putaruru. However communities in the Thames coastal reach outside the Waihou scheme area and north of Tararu suffered from major stream flooding, erosion, channel infilling and debris deposits.
Urgent works were needed on the main channels of Tapu, Te Mata and Waiomu. Scheme assets suffered widespread and severe damage, with more than 100 sites yet to be examined. Power cuts caused problems for pump stations not serviced by diesel.
The Te Aroha urban area main scheme streams coped very well, with some erosion and channel infilling of the Tunakohoia Stream and debris traps filled on the Tutumangao Stream.
Damage was moderate in the Piako River Scheme area, confined mainly to flooding and drainage failures with the Waitoa River, and overtopping and flooding the Springdale area. The June event caused major damage in the Thames Valley Drainage systems from Matamata to Springdale, with more than 50 sites to still be addressed.
Asset Management Group Manager Scott Fowlds said the sheer number of incidents had stretched resources, so that works have had to be prioritised. While all requests had been acknowledged, it could be several months before all remedial works could be completed. Environment Waikato was working closely with landowners on issues, including addressing issues on a priority basis, and consideration of areas outside of the present schemes.
Such very high intensity storms were out of the ordinary and had affected catchment stability of some streams, requiring a change of focus to include upper catchment erosion control, he said.
While repair costs were more than $525,000, about $126,000 of this would be covered by normal maintenance expenditure, and also part covered by the Ministry of Civil Defence and Emergency Management. However, the damage and repair costs will have some implications for scheme rating in the future, he said.