Quantifying the Contribution of Rainfall and Tide Levels on Flooding in Low-lying Coastal Areas

Three rainfall events in three months flooded houses in the coastal area of Cockle Bay, Auckland between June and August 2018. Anecdotal evidence from residents, indicated that there had been no flooding in the previous 20 years. Auckland Council received several complaints from affected residents following each storm event. The experienced flooding issues led to significant political attention and pressure. Resolving the flooding issues became a priority for Auckland Council.

Short term remedial works have been implemented to reduce the likelihood of flooding from frequent storms, with the intention that larger scale mid and long-term options will be developed to further mitigate the risk of flooding in the area. For mitigation options to be most effective, an in-depth flooding study was initiated to better understand the issues, including determining the root causes and contributing factors that led to the recurrent flooding and to remove the opportunity for conjecture. In low-lying coastal areas of Auckland such as Cockle Bay, the respective contribution to pluvial flooding from tide levels is often poorly quantified and understood and is largely subjective.

The comprehensive study, to get to the root causes of the flooding and later to inform solutions development, included analysis of the catchment, long term rain gauge data, rain radar imagery, tide gauges, 1D/2D hydraulic modelling results and an in-depth analysis of the capacity of the main culvert under varying tide levels. The effect of climate change and impervious development in the catchment was also assessed.

Further work was carried out, for four low-lying coastal catchments in Auckland, to better understand the effect of extreme sea levels on pluvial flooding in coastal areas. This paper outlines how to spatially identify coastal areas that are sensitive to extreme tides and rainfall, how to understand what catchments might be susceptible to rising sea levels in the future and what ground elevations will sea levels now and in the future effect pluvial flooding. It can be used to understand the effect of rising sea levels will have on flooding in low-lying coastal areas, better assess development in coastal areas and to minimise numerous joint probability modelling runs.