How the rain has dealt with the landslide dams

While the recent rainfall meant access to Kaikōura was temporarily cut off, the rain did have some positive effects on the landslide dams caused by November’s earthquake.

More than 200 landslide dams were created as a result of the earthquake, causing headaches for people living downstream and those responsible for public safety.

Worldwide, 70 percent of landslide dams are washed out within 30 days of an earthquake, but in Kaikōura, this has taken a bit longer due to low rainfall over summer.

Staff from Environment Canterbury, Kaikōura District Council, GNS Science and Golders flew over the dams on Friday to see what effect the rain had on them.

What we found was that the rain allowed the lakes behind the Ote Makura (above Goose Bay) and Linton Creek dams to overflow and the water began to pour over the top, cutting the channels down through them.

This is a good thing as it means nature has dealt with these two dam hazards, just as she created them in the first place.

It appears that as the Ote Makura dam breached, a small flood of 1-2 metres deep surged down the river, but the water stayed well within the river channel. This is in line with the “most likely” scenario modelling GNS did some months ago.

A small part of the dam and lake is still in place, but there is nowhere near enough water to cause a serious hazard downstream. The flood hazard is now similar to what it was pre-quake.

The water behind the Linton dam appears to have cut down through the dam in a slower, more sedate way.   The water rose, overtopped, and cut down through the dam but no big surge of water showed up downstream on our river level gauges which are monitoring the river flows. The flood hazard for the Linton catchment is also now similar to the pre-quake risk.

Meanwhile, the dam on the Hapuku is still in place, standing around 150m tall, so the dam-breach hazard is still there. Last week the river downstream of the Hapuku dam rose from around 0.1m to 1m over 24 hours- but this was more in the way of a normal flood as opposed to a sudden surge. The dam now has a large nick in it with water flowing through. The lake was still full which suggests the same amount of water is flowing into the lake as what is flowing out of it. There remains a risk of higher than usual flows from the Hapuku lake as the channel continues to cut down through the dam. The dam will be re-surveyed by GNS over the next fortnight. In the meantime it is still recommended to exercise additional vigilance in this catchment.

In all three situations, there is still landslide material on the banks above the river, and there is always a chance these could slide down the banks and dam it again. But the fact that this didn’t happen during the recent rainfall has experts believing it’s unlikely to happen unless there’s another big earthquake.

Marion Schoenfield is a Hazards Analyst at Environment Canterbury.