Climate change challenge
Dr. Willem de Lange, a Coastal Oceanographer from the University of Waikato, spoke to a packed meeting of the Ōmokoroa Residents and Ratepayers Association on Wednesday 17th March, on the effects of projected climate change, sea level rise, land instability and coastal inundation on Ōmokoroa and how best to manage these risks.
Ōmokoroa has had several major deep-seated regressive landslides in its history. These can occur when rainfall results in the groundwater depth exceeding 7m in the unconfined aquifer above the sensitive Pahoia ash layer about 23m below ground level.
Extreme rainfall events, delivering greater than 25 hours rainfall at greater than 4 mm per hour, are required to initiate a deep-seated landslide such as occurred in Bramley Drive (1979) and McDonnell Street (2017).
These extreme rainfall events have occurred randomly in the Tauranga historical rainfall record and there is no evidence that their frequency and/or magnitude has changed in response to the observed warming of 1 degree Celsius since the record began.
Based on this observation, it is unlikely that a similar increase in temperature will result in an increase in the incidence of extreme rainfall events necessary to initiate landslide.
A reduction in the likelihood of landslide occurring may be achieved by managing the water table depth (below 7m) and/or by stabilising the sensitive ash layer using a potassium salt shown to be effective in the laboratory.
Dr de Lange stated that the projected sea level rise of 12.5mm/year, based on the exceptionally unlikely (borderline impossible) RCP 8.5 scenario, is inconsistent with the measured sea level rise of 1.4 to 1.8 mm/year over the last 100 years.
An extreme projected sea level rise has been combined with extreme high tide forecasts and an extreme storm surge estimate to define an estimate of coastal inundation of RL 2.8m (in 100 years’ time) at Ōmokoroa for present day planning purposes by WBOPDC. This approach has been rejected by the Environment Court recently.
Dr. de Lange concluded that predicting the future is very difficult, particularly for a complex chaotic non-linear system such as climate. Responding now using scenario based models that are not fit for purpose, is likely to be costly and ineffective, particularly when using exceptionally unlikely worst-case scenarios (RCP 8.5) as is the case for New Zealand.
Identifying thresholds at which potential impacts are unacceptable for affected communities (Trigger Points) should be the first step, followed by consideration of what options can be employed when thresholds are reached, then identifying when the community’s preferred option should be implemented (Transfer Point).
Assessment, consultation and design process should be periodically repeated to review new evidence. In other words – community focussed Dynamic Adaptive Policy Pathways, not central government laws and regulations.