“GRAB YOUR UMBRELLA!” OR, INVESTIGATING AN EARLY WARNING SYSTEM FOR URBAN FLOODING

W Bird (WSP), D Ahn (WSP), F Macdonald (Auckland Council)

ABSTRACT

Numerous storm events have hit different parts of Auckland in recent years, causing significant flooding. These short duration, high-intensity and often localised events are hard to predict. Traditional preventative measures (e.g. stopbanks, piping) are also becoming less financially viable. Climate change and further intensification is projected to exacerbate these issues and increase costs. The place of early flood warning systems in the flood risk reduction toolkit is being explored for Auckland as a means of increasing community resilience and reducing damage.

Auckland is characterised by many small, rapidly responding catchments so “flash flooding” was necessarily a central focus of the investigation. However, the study covered flooding that is both pluvial and fluvial in nature. Even in its simplest form, any early warning system will require the cooperation of multiple council and water entity jurisdictions. However, an ideal flood warning system would also be capable of extension to cover other hazards such as debris flow or tidal inundation, or extension to other regions of NZ.

Predicting heavy rainfall or river flooding is only one part of an effective early warning system. Equally important is a mechanism to transmit the warning and a corresponding set of responses actioned by the recipients.

This paper shares the findings of the study, addressing each of the elements that make up a flood warning system, from forecasting the rain before it falls, to educating the public so they know how to respond. It identifies key gaps, both institutional and technological and proposes a path for overcoming these. It draws on experience and learnings locally, nationally and internationally, finding that many flood managers are on a similar journey, although most are focused on much larger catchments. While early flood warning is already feasible for parts of Auckland, more comprehensive coverage requires high precision rain forecasting. The paper outlines the major challenges to be overcome – including modifying public behaviour – if an early warning system for flooding is to be successful.