What to consider when assessing a coastal inundation natural hazard

Posted: 12 June 2020

Determination 2019/067 considered what approach was appropriate when calculating coastal inundation.

Background

The owners applied for a building consent to build a house on their property, which is located next to a tidal estuary. They had previously received a building consent to build a seawall along the property boundary with the estuary. The owners had also carried out improvements to the site to reduce ponding and installed a surface water removal system.

However, the building consent authority (BCA) believed the property was still subject to inundation. The BCA considered surface water during storm events could lead to inundation. The BCA also considered inundation from coastal flooding, which included seawater during extreme high tides, storms and sea level rise. The building consent for the house was issued subject to a section 73 notice. This meant the natural hazard would be registered on the title of the property. The compliance of the house itself was not in question and it had mostly been elevated above the site.

Discussion

The determination concluded the inundation from rainfall events was not a natural hazard.

The determination considered whether the seawall would protect the land and the building work from coastal flooding. In doing this, the determination had to assess the different methods used to calculate the coastal flooding levels.

The Court of Appeal in Logan v Auckland City Council stated "a sensible assessment involving considerations of fact and degree" is required when assessing whether land will be subject to a natural hazard. The determination stated when choosing a method to calculate the inundation level a "sensible assessment" approach should be used. The calculation should estimate the level accurately and realistically given the available data, and not rely on a very conservative approach.

The determination noted the difficulty of calculating coastal flooding levels. This is due to having to factor in existing data about extreme tide levels and storm effects, as well as the likely impact of rising sea levels due to climate change. The determination’s expert opinion used a complex analysis to calculate the coastal flooding level, while the BCA used a simpler, more conservative approach to calculate the flooding level. The complex analysis resulted in a lower flooding level than the BCA’s approach.

The determination stated conservatively calculating the flood level to minimise or eradicate risk is not always the best approach. A conservative approach would lead to calculating the worst-case scenario, which is highly unlikely to actually eventuate. This approach would not follow the "sensible assessment" approach set out in Logan v Auckland City Council.

The determination emphasised the natural hazards sections in the Building Act do not seek to remove or address all risk completely. The impacts of climate change and rising sea levels cannot be predicted with total accuracy, so it will never be possible to remove all risk. However, the determination considered the BCA's very conservative approach to the higher predicated levels of sea rise was unreasonable.

Therefore, the determination considered the more complex analysis would provide a realistic estimate of the coastal flooding level compared to the BCA's approach.

The decision

The BCA incorrectly granted the building consent subject to a section 73 notice.

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