Predicting estuarine coastal inundation

At the upcoming New Zealand Coastal Society Conference in Dunedin Dr Brett Beamsley will present recent work from a project with Tasman District Council. 

The project was designed to help the council assess coastal hazard risks due to inundation and coastal flooding over the coming 100 years. Normally, coastal flooding risks are calculated using a combination of predicted storm tide and sea level rise. However, this technique produces overly conservative estimates in estuarine environments where the non-linear nature of tidal propagation affects inundation levels. 

The study quantified the likely inundation within the Moturere Estuary, a complex inlet which includes a barrier spit. A numerical model was set up for the site and a combination of ‘worst-case’ water levels were run over several tidal cycles to examine the non-linear propagation of the open coast water levels into the inlet, their effect on inundation levels, and the effect of sequential high-stands on water levels. The findings suggest that because of the tidal wave lag within estuaries the actual inundation extents will likely be significantly different than those predicted if simplistically assuming that land lower than or equal to the storm surge level will become inundated. If the storm surge event spans multiple tidal cycles, or multiple storm surge events are predicted to occur in quick succession, inundation extents are likely to increase significantly. 

Inundation levels for a 3.5 m combined Sea Level Rise + Storm Tide + Mean High Water Springs for the first high-tide stand (left), a high-tide stand 6-days later (centre) and assuming all land sub 3.5 m becomes inundated (right). 

Inundation levels for a 3.5 m combined Sea Level Rise + Storm Tide + Mean High Water Springs for the first high-tide stand (left), a high-tide stand 6-days later (centre) and assuming all land sub 3.5 m becomes inundated (right). 

The conference is held in Dunedin 16-18 November and has as its theme 'He waka eke noa - Linking science, engineering, management and community'.  'He waka eke noa' is a Maori proverb which means 'We are all in this boat together'.

For further information about the conference, see http://www.coastalsociety.org.nz/NZCS_Conference_2016/