WHAT THE SWOC? – POST CYCLONE STORMWATER STRATEGY DEVELOPMENT

K. Hess (Beca Ltd) & B. Severinsen (Central Hawke’s Bay District Council)

ABSTRACT

Central Hawke’s Bay District, a largely rural district in New Zealand, covers three main townships: Waipukurau, Waipawa, and Otāne, with the Tukituki River and its tributary, the Waipawa River, being the primary recipients of stormwater from the District. Central Hawke’s Bay District Council (CHBDC) is a small council (with a population of ~16,000) with limited funds for stormwater management and maintains a network comprising largely open waterways draining into these two rivers, with some pipe networks and swales within the townships.

In the aftermath of Cyclone Gabrielle in February 2023 and subsequent heavy rain, widespread flooding occurred, much due to river overtopping stopbanks. With this, the towns were flooded, with stormwater flowing overland through them, highlighting the need for improved flood protection beyond the stopbanks. The Council saw this not just as a challenge but an opportunity to address immediate concerns of localized flooding and plan long-term management through a phased approach to a Stormwater Strategy.

This paper will explore the approach taken to developing the Stormwater Strategy, including initial stages (already complete) and next stages (currently underway at time of writing).

The first stages of the Strategy development were:

• Setting up a Severe Weather Outlook Checklist (SWOC) to clearly set out Council’s response to forecasted rain events.
• Creating a maintenance plan.
• Identifying short term “Quick Wins” projects.
• Setting up a framework for understanding and prioritising works.

The SWOC was one the first documents to be developed as it gave Council a tool to implement immediately for all future extreme rain events. The checklist notes critical assets for Council to inspect for and perform maintenance needs any time a heavy rain event is forecasted.

The SWOC became part of a regular Maintenance Plan for Council to implement to help improve the function of the stormwater network. Details of how the Maintenance Plan was devised and the specific inspection and maintenance activities are also included in this paper.

In addition to the SWOC and Maintenance Plan, a list of projects was developed to improve the stormwater network that could be implemented over the following year (“Quick Wins”). The short-term projects included waterway maintenance works, a variety of network upgrades, flapgate installations, bank stabilization, culvert upgrades/ replacements, and transformation of storage areas into treatment wetlands. As a method to help Council select the order of completion for these works a prioritization framework was included.

By establishing these first parts of the Strategy and implementing them the council was able to start meaningful work quickly, mitigate the risk in future events, and build its relationship with the community. An example of this is how the SWOC was put into action in Nov 2023 in response to a Met Service heavy rainfall warning, although flooding did not eventuate.

The Stormwater Strategy is now progressing to the second phase, setting future-focused objectives for stormwater management in the district.

The paper will set out how, using a staged approach, a council can use a Stormwater Strategy to achieve both short term needs and longer term aspirations for stormwater management.