On the relationship between natural climate change, tidal anomalies and coastal flooding

Coastal flooding can be caused by both heavy rainfall and/or anomalously high tides. Currently most coastal planners use a 100 year Average Recurrence Interval (ARI) flood risk estimate as an acceptable level of risk to plan for. This risk estimate informs planning controls such as floor heights and is also a base for calculating future sea level rise hazard areas. The ARIs are calculated based on the assumption of a 'stationarity' climate - that is, the assumed chance of an extreme event occurring is the same from one time period (season, year, decade) to another. However, previous research has shown that the ARI of extreme climatic events (e.g. heavy rainfall, floods, droughts etc) may be significantly under or over estimated depending on the climate state. Previous studies have focused on the non-stationarity of coastal flooding as a result of heavy rainfall; however the results presented here demonstrate that similar inter-annual variability in the risk of coastal flooding due to tidal inundation may also exist - and that this compounds the risks and impacts associated with rainfall induced coastal flooding. This research will help improve our understanding of flood and inundation risk in the coastal zone and is a vital first step in preparing for projected increased sea levels as a result of anthropogenic climate change. The better we understand our historical and current sea level fluctuations, and what drives them, the better our coastal planning and management will be into the future