A CASCADE OF TENSIONS: FLOOD RISK MANAGEMENT UNDER CLIMATE CHANGE

F. Macdonald (Auckland Council)

ABSTRACT

There are two key ways through which flood risk is managed in Auckland:

Landuse planning. This includes the Auckland Unitary Plan (AUP), which directs where and how development may occur, with avoidance and mitigation of risk in floodplains and overland flow paths. The AUP states that climate change must be taken into account.

Infrastructure provision. This includes the Stormwater Code of Practice (SWCOP). The SWCOP provides design guidance for primary and secondary infrastructure, to a specified capacity[1]. The current SWCOP directs designers to apply a 2.1deg temperature rise to rainfall, resulting in higher peak flows and volumes.

Auckland Council’s Regionwide Stormwater Network Discharge Consent provides best practice to manage all public stormwater discharges across the Region.

These controls have been based on designing for a maximum future demand determined by a fixed future rainfall and a maximum impervious area. Given recent information on the range of possible climate futures, selection of a scenario has become a three dimensional problem – selection of climate scenario, time horizon, and rainfall return period. Central government has not yet provided clear direction as to appropriate scenario/s to be used.

When considering flood risk management under climate change, various tensions need to be balanced, including the following:

• Having declared a Climate Emergency, there is an expectation that Council will act on climate mitigation and adaptation. However the framework and tools for flood risk management under climate change do not exist, nor does central government direction. Existing tools need to be retrofitted to provide short-term process.
• Climate mitigation vs adaptation – providing larger stormwater systems to convey future increased flows can increase embodied carbon.
• Community expectations – New Zealand does not have a defined level of service for flood management. The expectations of today’s communities are unclear. The expectations of future communities living in a much warmer world are unknown.
• Historically the focus of stormwater design has been on the primary system. Under climate change, secondary systems become more important. The owners of these assets are rarely the entities responsible for flood risk management.

Many councils will be wrestling with these same tensions. There is no ‘right’ answer, as many of these factors are interdependent. We as an industry need to move forward despite uncertainty, and determine an approach to creating resilient solutions that don’t lock us into a high carbon future.

This paper will discuss Auckland’s journey to date. The paper will outline the impact of climate change on extreme rainfall. It will consider available information on climate projections, including recent advice commissioned from NIWA, and outline how this information has been used in determining appropriate climate scenarios for flood risk management. Different scenarios are proposed to be used for floodplains and overland flowpaths, for which a precautionary approach is reasonable, versus the primary system for which precautionary reasoning does not necessarily apply. The implications of the proposed change from 2.1deg to 3.8deg on floodplains and secondary systems will be presented. Analysis done on appropriate primary system design criteria will be presented, including impact on level of service.

[1] Conveyance of 10% AEP and 1%AEP flows respectively