Extract

Rainfall totals for the 9 May 2023 event were approximately half that of Cyclone Gabrielle. Rainfall intensities for 9 May 2023 were generally less than the 10-year annual recurrence interval (ARI), but heavy bursts exceeding the 10-year annual recurrence interval (ARI) were recorded in the north, east, and west of the region. Despite the rainfall totals and intensities falling well short of previous major events, significant flooding occurred. River water levels peaked at similar levels to Cyclone Gabrielle and in two cases were higher - the Mahurangi River in the north and Mangemangeroa Creek in the east, which rose to higher levels than recorded during the Auckland Anniversary storm.

The extent of flooding relative to rainfall was likely influenced by saturated ground conditions, as evidenced by 7 of 10 soil moisture sites across the region being above field capacity (maximum water retention capacity of a soil). Overall, the 9 May 2023 storm was a moderately heavy rain event superimposed on an extremely wet environment.

Overview

We present a rapid analysis of rainfall and river water level data following the storm on 9 May 2023 to provide a comparison of the severity of the storm against recent extreme weather events, including the Auckland Anniversary storm (27 January 2023) and Cyclone Gabrielle (14 February 2023).

This analysis focuses on six sites each for rainfall and river water level, chosen to represent the spatial distribution of our hydrometric network. Annual recurrence interval (ARI) analyses were based on ARI values derived from the rain gauge at Albert Park, the longest continually operating rain gauge in the region.

Rainfall

Rainfall totals and intensities were calculated for six sites representing north, east, west, central, and southern areas in the Auckland region for four rain events in 2023 (Table 1). The data represent rainfall across the major catchment areas of the region. Rainfall statistics were derived for short to medium duration rainfall intensities (1-hour to 12-hour) due to the relatively short overall duration of the 9 May 2023 storm to allow comparison across the four events.

Event rainfall totals from 9 May 2023 were generally half that of Cyclone Gabrielle and even less than the Auckland Anniversary storm (with some exceptions). The 9 May 2023 storm had heavier 1-hr duration bursts than Cyclone Gabrielle, but 12-hr duration bursts were less heavy. Most rainfall intensities on 9 May were considerably less than the Auckland Anniversary storm with notable exceptions of 1-hr duration bursts in the north and southeast. Rainfall annual recurrence intervals (ARI) were generally less than the 10-yr ARI, but exceeded the 10-yr ARI in the north, east, and west for at least one measure (e.g. 1-hr duration intensity). ...

Auckland Council May 2023.