Are 1800 Kapiti homes really threatened by sea level rise?Guest author | September 3, 2012
Seemingly sloppy science seems to have sullied our coastal planning process. Dr de Lange describes, in the polite, scholarly way of his, a scientific blunder in a Kapiti Coast erosion report that anyone less courteous than him would call a dereliction or worse. Why? Because the wrong formula was used to calculate the amount of foreshore vulnerable to damage from sea level rise, and many hundreds of properties are now apparently at risk. The report explains correctly why a certain formula should not be used, but then, in a stupefying about-turn, goes ahead and uses it anyway. Prices for those properties will plunge, yet the new risks just aren’t justified.
The author (or principal author) of the Kapiti Coast Erosion Hazard Assessment 2012 update is Dr Roger Shand, of Coastal Systems Ltd. He said the report was peer-reviewed by “Coastal Scientist Dr Mike Shepherd” – who effectively works for Dr Shand. Why didn’t they admit that they’re colleagues? This isn’t a peer review, it’s a pal review, and if values plummet, land owners will descend on the High Court demanding compensation. Does the District Council realise its exposure? - Richard Treadgold
Recent news stories have highlighted the redefinition of coastal hazard zones along the Kapiti Coast. The populated region is concentrated on a coastal landform known as a cuspate foreland, which has formed due to enhanced accretion of sediment in the lee of Kapiti Island over the last 7500 years. Examination of the coastal landforms in this region indicates that there has been long-term accretion over the Holocene disrupted by storm-induced erosion associated with large waves from either the southwest or northwest.
So has that pattern changed recently? The Kapiti Coast District Council commissioned a report that was published in August. This report is a reassessment of an earlier report in 2008 and suggests that:
“Around 1,800 properties in Kāpiti, including most beachfront properties, are shown to be at risk of potential erosion or inundation from coastal hazards within 100 years. Up to 1,000 of these may be at risk within 50 years.“
Although you could argue that a tectonically active coast such as this is more likely to be threatened by earthquake-induced subsidence, liquefaction and tsunami inundation, the focus of the study was coastal erosion associated with sea level rise.
The study includes an analysis of historical shoreline trends, and found that the Kapiti Coast has undergone long-term accretion with episodic erosion associated with storm events; a finding consistent with several earlier studies and the evidence provided by the coastal landforms. It is clear from the available evidence that this long-term accretion occurred while sea level was rising.
If the response to sea level rise to date has been long-term accretion, how could the study find that sea level rise threatens
so many homes? The answer lies in the methodology used. In line with the earlier 2008 report, the 2012 report states that a commonly used approach for assessing the shoreline response to sea level rise, known as the Bruun Rule, is inappropriate for the Kapiti Coast because many of the assumptions of the rule are violated. The mathematical concept behind the Bruun Rule is very simple for anyone who can remember basic trigonometry, being based on similar right-angled triangles (see sketch).
Essentially, the Bruun Rule states that the shoreline retreat is equal to the ratio of the sea level rise to the slope of the shoreline. There are different ways of defining the slope ranging from the slope of the continental slope, to the slope of the beach face or offshore bar. The ratio indicates that a gentle slope will result in more retreat than a steep slope, so by selecting the “right” slope it is possible to get a desired retreat, which has led to a few Environment Court cases.
It is also evident from the ratio that a higher sea level estimate will produce more shoreline retreat. Again, there is the opportunity to choose the “right” sea level rise to get a desired answer. The key difference between the 2008 and 2012 assessments for the Kapiti Coast is the use of a much higher estimated sea level rise with no consideration of the probability of that occurring.
Finally, it should be obvious that this Rule cannot predict shoreline accretion, as R will always be positive for a positive sea level rise. The problem is that for the New Zealand beaches where the rule has been tested, in the long-term R should be negative because the beaches are accreting. From the evidence presented in the Kapiti Coast assessment, R should be negative for the Kapiti Coast, and the authors are correct to state that the Bruun Rule is inappropriate.
Instead they have used a method proposed by Komar et al. (1999) and applied to the Oregon coast of the USA. Dr Jeremy Gibb has also applied this method to coastal hazards in New Zealand. The method assumes that the key factor driving dune erosion is saturation of the sand at the base of the dune, and it was developed to predict shoreline erosion due to storm events (see sketch).
The method does not include a sea level rise term at all. It is based on the extreme water level relative to the elevation of the dune toe, and the beach lowering during the storm. It is not intended to predict the effects of long-term sea level rise. Consequently the authors of the Kapiti Coast study modified the Komar et al equation by replacing the numerator term with the sea level rise (Equation 3 in the report). Therefore, instead of using the equation as defined, they used the ratio of the sea level rise to the slope of the beach. I have already discussed this ratio – it is known as the *Bruun Rule!
Therefore, despite saying the method is inappropriate, the revised coastal hazard zones are based on the Bruun Rule after selecting the “right” sea level rise.
So, are 1800 homes threatened by sea level rise? Based on the evidence presented in the reports to the Kapiti District Council, the answer is almost certainly not, although some homes are at risk from storm-induced erosion. If the Kapiti Coast District Council will quantify the probabilities of their “right” sea level rise, then I can quantify my “almost certainly”.
* Equation 3 (p18): R = S/tan β