Scientists: Three Kaikoura quake landslide dams pose 'highest level of hazard'

Geonet has posted a blog pinpointing the 11 newly-formed dams on rivers in the Kaikoura area causing scientists most concern, including the Waima River. Photo / Geonet

Nearly a dozen dams formed by landslides in the devastating Kaikoura quake are being closely monitored for risk of sudden collapse and fears of flash flooding.

GeoNet has posted a blog pinpointing the 11 newly formed dams causing the most concern on rivers in the Kaikoura area. Three at the highest level of risk are being monitored closely by landslide experts because they may be hazardous to life and property downstream.

The Hapaku River has a dam formed by a rock, sand and silt avalanche. Photo / Geonet

The dams are in the Conway, Gelt, Hapuku, Leader upstream, Leader downstream, Linton, Medway, Ote Makura, Stanton, Towy and Waima Rivers.

Those threatening the most imminent danger are Linton, Conway and Hapuku Rivers.

Linton Stream has a large leaking dam with a lake behind it, Conway River has a lake with a potential for failure and the Hapaku River has a dam formed by a rock, sand and silt avalanche that was filling and becoming more hazardous until it was full.

Scientists warn the likelihood of dam failure was high during steady or intense rain and for the following day.

There was more likely to be a rapid dam failure if there was a lot of water upstream of a small dam.

Conway River has a lake with a potential for failure. Photo / Geonet

Landslide and paleoseismology teams from GNS were now gathering landslide data from each site to undertake dam-break modelling and gauge what would happen if one collapsed.

"This modelling is used to determine where the flood wave from rapid failure will go assuming normal river flow conditions. From this people and assets at risk can be identified and management plans put in place, " read the blog post.

"Landslide specialists at GNS Science are working hard to confirm the initial assessments, model the landslide dams, and determine what might happen if they fail."

At this stage none of the dams looked like they would spill over their flood banks if they did fail rapidly.

"If rapid failure occurs without being associated with heavy rainfall then the level of the water will probably be a maximum of the level experienced during a one in 10-year flood - but no warning will be given."

Linton River is one of three areas threatening the most imminent danger. Photo / Geonet

Landslide dams are formed when debris blocks a stream or river. There is a complex relationship between dam size, lake volume and landslide type that determines whether or not a dam will fail. Dams can last from a few minutes to thousands of years.

The blog said if a landslide dam rapidly failed the flood wave or water would start high near the dam and rapidly lower as it moved downstream. A dam may also gradually erode and the water level in the lake drop slowly with no flood peak surging downstream.

Experts surveying the quake-devastated region estimate up to 100,000 landslides were triggered during last month's 7.8 tremor across Kaikoura and the Southern Alps.