Scientists have pinpointed the type of wave that does the most damage to cliffs that make up a quarter of New Zealand's coastline, finding that size isn't everything.
In many parts of the country – and especially in Auckland, Taranaki and the Bay of Plenty – shoreline cliffs have been eaten away to the point where homes are threatened.
This naturally-occurring process is being slowly compounded by rising seas driven by climate change, with New Zealand's sea level projected to be around 30cm higher by mid-century – and potentially a metre higher by 2100.
At Onaero Bay, a Taranaki holiday spot where cliffs are dramatically receding at a rate of several tens of centimetres each year, University of Auckland researchers were able to get a closer look at which particular waves were causing the biggest problems.
One of their most fascinating findings was that it wasn't necessarily large storm waves having the greatest impact, but "breaking" waves that hit a cliff at just the right time.
In her award-winning study, PhD graduate Catriona Thompson buried two seismometers in the cliff top to measure ground movement in response to the impact of individual waves.
She also set up two wave pressure sensors at the toe, or base, of the cliff to measure wave height and water depth, and placed a video camera on the shoreline to film each wave as it hit the cliff.
In particular she recorded whether waves were broken, breaking or unbroken at the moment of impact.
She found that the impact of a wave which breaks against the cliff was significantly bigger than either broken or unbroken waves.
Measurements showed that the largest impact from a moderately-sized breaking wave was seven times greater than the biggest impact from the largest broken wave even during storms.
Unbroken waves have even less impact on cliff ground shaking, the largest unbroken wave impact being about 19 times smaller than that of the largest breaking wave.
"Breaking waves generally caused more ground motion regardless of wave height when compared to broken or unbroken waves," Thompson said.
"From this field work it's evident that waves that break against the cliff are having the biggest impact."
"We think that's going to be significant in terms of even modest sea level rise because water depth near the base of the cliff determines what kind of waves the cliff face is exposed to."
Associate Professor Mark Dickson, who was also involved in the research, says that they were very interested to discover that storm waves, which are usually associated with cliff erosion, didn't result in the greatest ground shaking.
This was because storm waves break in deeper water and dissipate a lot of their energy prior to impact.
"This work is quite significant in the way we look at coastal erosion, particularly in the context of global sea level rise, and the way that wave energy will be delivered to coastal cliffs in the future."
The study is the first to look at coastal erosion by measuring ground motion at the seismic level while also recording the impact of individual waves.
It showed that sea level rise was likely to have two different effects: in areas where there is a significant rise in sea level, sea height could increase the number of breaking waves while in other areas it might mean a higher number of unbroken waves, which could actually decrease erosion rates.
"This is the first study to look at different types of waves that impact coastal cliffs rather than just looking at the tide cycle or storm events and the first time we've recorded different types of wave motion and we think it really does provide better understanding of how sea level rise will affect New Zealand's coastal cliffs," Thompson said.
The study has been recognised by the British Society for Geomorphology which has awarded it the Michael Kirkby Award.