Heavy rainfall fills Karāpiro dam, forcing river levels up

Karāpiro dam on the Waikato River near Cambridge is releasing water following recent heavy rains across the country.

Tom Lee/Stuff

Karāpiro dam on the Waikato River near Cambridge is releasing water following recent heavy rains across the country.

The Karāpiro dam will be discharging at maximum consented flow to relieve the backlog of water in the Waikato river system as more rain is expected in coming days.

Heavy rainfall this week has impacted the region and saturated catchments with river and stream levels high and in some cases still increasing.

The Waikato Regional Council is warning that some Hamilton river paths will be inaccessible due to the rising levels of the Waikato River.

The council, and power company Mercury, are working together to manage flows in the Waikato Hydro System in response to the recent rain, as well as forecast wet weather later this week.

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To create storage through the system the Karāpiro dam will be discharging at maximum consented flow of 500 cubic metres per second into Thursday, the council’s resilience team leader Rick Liefting said.

“That means water levels will rise downstream of Karāpiro. In Hamilton, some river paths will be underwater and the Grantham Street car park may also be inaccessible due to flooding.

“The Waipā River is also continuing to rise naturally as a result of the recent rain. Combined with the higher levels on the Waikato River, it means flows downstream of Ngāruawāhia will also be high.”

Liefting said they’re well within the capacity of their flood protection schemes despite the high flows, and all their modeling at this time indicates that homes will not be impacted.

Ultimately, the Waikato River flows to the Tasman Sea at Port Waikato.

Tom Lee/Stuff

Ultimately, the Waikato River flows to the Tasman Sea at Port Waikato.

The majority of rivers within the Waikato region are natural systems with no control of flows. However, there are some rivers that are modified with flood protection, such as stopbanks, to keep the river water within the channel and not flood communities.

“The lower Waikato River has a flood control scheme managed by Waikato Regional Council, which also includes structures that control flow from the Waikato River into and out of Lake Waikare, as well as flows from the Whangamarino River into the Waikato River.”

Liefting said Mercury controls flows from Lake Taupō and past the dams in the Waikato Hydro System, with the last control at Karāpiro Dam.

The Waikato Hydro System is a “run of river” system with very little storage in the hydro dams. During flood events, Mercury has limited ability to hold back flows in the river catchment.

“Waikato River water levels downstream of Ngāruawāhia are also significantly influenced by uncontrolled flows on the Waipā River, as well as other water from other uncontrolled tributaries.”

The Waikato hydro dams have very little storage .

Tom Lee/Stuff

The Waikato hydro dams have very little storage .

Ultimately, the Waikato River flows to the Tasman Sea at Port Waikato, but there are storage areas that can be used as part of the flood protection scheme of the lower Waikato River, with Lake Waikare and the Whangamarino wetland being the greatest storage area.

MetService meteorologist Angus Hines said the region is expected to get even soggier with more rain forecast for Thursday afternoon and Friday morning.

“There is a chance for heavier rain overnight into Friday morning, but it is looking to clear up from about mid-morning onwards. The day could be a bit brighter with a brief shower in the afternoon,” Hines said.

The start of the current wet stretch began on July 5, and since then 142mm of rain has been recorded at the Hamilton Airport weather station.

“It is a lot. It’s been a very wet stretch for that part of the country after what had been a couple of dry months to start off the year,” Hines said.

There could be a small reprieve from the wet over the weekend, but people shouldn’t put the umbrellas away anytime soon.

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