BALANCING ACTS: STORMWATER MANAGEMENT AMID BREAKNECK GROWTH

Danika Ferguson (Beca Ltd.) and Helen Shaw (Beca Ltd.)

ABSTRACT

Queenstown Lakes District Council (QLDC) is home to some of New Zealand’s most iconic waterbodies, including lakes Wānaka, Hāwea and Wakatipu, and the headwaters for the Clutha River / Mata-Au, the largest river in New Zealand (by volume and area). While the volumetric contribution of urban stormwater to these large waterbodies might be small, maintaining pristine water quality in these receiving environments is of national importance.

The key stormwater challenges that this small council faces include:

• QLDC is experiencing the highest population growth rates in the Country (Gui, 2023) resulting in rapid land use intensification, and rise in development resource consent applications. This has led to a wide variety of approaches to stormwater management, and an increase in stormwater infrastructure vested to Council for maintenance.
• QLDC currently manages several discrete stormwater network schemes which:
  • span across a large geographic area,
  • discharge into several distinct receiving environments, and
  • have varying levels of asset information available.
• There is an uncertain legislative environment, with draft, notified and partially notified legislation from both central government and Otago Regional Council, resulting in difficulty allocating investment through the Long Term Plan.

With several diverse, rapidly changing catchments, QLDC needs an approach to stormwater management that balances Te Mana o Te Wai, community development, and economic prosperity. In 2023, Beca was engaged by QLDC to create a wholistic CMP for the wider Wānaka area including Hāwea, Luggate, Wānaka, outer Wānaka, and Cardrona, building on earlier work by others and flood modelling completed by DHI.

This paper specifically reviews the development of a consistent CMP for Wānaka, Hāwea, and Cardrona. These three systems were selected as case studies to highlight the variation across the district in the stormwater systems, the receiving environments, and in the quality and quantity of the existing data.

For example, Wānaka is a town adjacent to Lake Wānaka, with recent flood modelling and ongoing water quality monitoring. It is an example of a more urban catchment discharging to a pristine lake with good asset information. Conversely, Cardrona is a mostly rural catchment, with limited stormwater quantity and quality data, discharging to a seasonally influenced river. Hāwea is in the middle, it is a rural township discharging into a dam controlled lake, with older (2010) quantity modelling and some water quality information available.

In order to develop a CMP that provides district wide consistency in stormwater management and can be used for all types of development, Beca has implemented the following:

• a bottom up prioritization framework that can be tailored to the level of information available; and
• utilizing the Urban Stormwater Management Principles to provide a foundation for nature based solution generation.

The paper will explore the development and implementation of this CMP. It will also explore the interaction of the CMP with a wider stormwater strategy and how the CMP information can be kept live to respond to changes in legislation, Council priorities, and stakeholder input.