Largest wave not biggest coast threat
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 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 midcentury – 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 waves were causing the biggest problems.
They found it wasn’t necessarily large storm waves having the greatest impact, but “breaking” waves that hit a cliff at just the right time.
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 the impact of a wave that broke against the cliff was significantly bigger than either broken or unbroken waves.
Measurements showed the largest impact from a moderately sized breaking wave was seven times greater than the biggest impact from the largest broken wave.
The largest unbroken wave impact was about 19 times smaller than that of the largest breaking wave, the research found.
“From this field work it’s evident that waves that break against the cliff are having the biggest impact,” Thompson said.
“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.