How much of the coast was uplifted by how much?

Although Covid19 set back some of our planned fieldwork, we put the lockdown period to good use to characterise some of the core earthquake impacts on the coast. One key questions is ‘how much of the coast was uplifted and by how much?’. Knowing this helps us to extrapolate the results from small-scale field surveys to the wider coast, which in turn is the best method for gauging the extent of impacts and how the recovery is looking overall. Being able to report the results as the ‘length of coastline’ affected is an intuitive option for comparing one place to another, but have you ever heard of the ‘Infinite coastline paradox’? Google it up!

In our case we have some very high resolution data for the whole coast, which means we can take into account all of the nooks and crannies. This is important because if you’re a small guy, like a paua, all of those intricacies are relevant to your available space. Another aspect is which part of the shoreline we’re talking about, and in this case we did the assessment for the position of Mean High Water Springs (MHWS). We also looked at two periods of time—the immediate earthquake effects and whether there have been any changes since. This is important since our RECOVER project began two years after the quake and we have been following changes since. The results can be shown lots ways, as in Figure 1. This shows the degree of uplift within four major substrate types that are associated with characteristic habitats and together make up the whole coast.

In the coming months we will be interpreting results from our field survey sites against these ‘big picture’ trends to evaluate how the recovery is going. We also confirmed that most of the changes have occurred during the initial earthquake, at least south of Waipapa Bay. However, in other work we are following the erosion of uplifted reefs—which is significant in some areas. See the next issue of Recover for an update!

A plot of the amount of uplift (‘density’) that was experienced by each of four substrate types across the coast from Oaro to Waipapa Bay according to metres of coastline at the position of pre-quake MHWS (high tide). Plot by Shane Orchard.

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