A Flood Two Months In - A Waiāri Resilience Story

Rodney Clark (Tauranga City Council), C Olivier (Beca)

Tauranga City Council encountered challenges that are widespread among water utilities while selecting and planning the new Waiāri Water Treatment Plant's location. The site's stream intake works were influenced by factors beyond just the physical characteristics of the area. This paper presents the strategy used in this project for enhancing resistance against multiple natural hazards and explains how the intake system performed in a real-life scenario involving an extreme natural event. 

The Council commissioned its new Waiāri Water Treatment Plant, with water first being delivered to customers in December 2022. The plant treats water from the Waiāri Stream, a pristine waterway with typical turbidity less than 1 NTU and an average summer flow of approximately 4m³/s. The intake site is susceptible to several natural risks such as liquefaction, slope instability, flooding and debris impacts. Significant effort was therefore placed on balancing resilience while maintaining affordability during the intake design. Within the first two months of operation, a significant rainfall event occurred within the catchment, causing the stream flows to peak at approximately 250 m³/s and flood levels to rise 5m above normal stream level. The flood also carried significant amounts of silt and debris and changed the stream bed significantly around the intake site. The event occurred between midnight and 1am. Although the plant was offline at the time, the proof of performance of the intake lies in how well it survived the impact of the event and its ability to be brought back online.

During the design process, designers and plant operators worked collaboratively to consider natural hazard risk and resilience. Not all risks carried the same impact on the plant’s ability to operate post a natural disaster, and a weighted resilience rating was developed to inform the design. This methodology and collaborative approach can be applied to any similar infrastructure design. 

This paper also discusses the effects of the flood on the plant, immediate measures taken to maintain operation and then to restore the stream. It also compares the design versus actual resilience performance during and after the extreme event. The key consequence from this event was silt management. The steps taken to be better prepared to manage this into the future and practical actions to be implemented will be presented.