ON-LOT STORMWATER…A LOT TO LEARN

A. Phillips & N. Young (Hamilton City Council) & S. Joyce & S. Farrant (Morphum Environmental Ltd)

ABSTRACT

Across New Zealand, as councils and communities respond to the need to protect our precious freshwater resources and reflect the principles of Te Mana o te Wai, there is increasing emphasis on stormwater management within private properties. Whether it is on-lot rainwater tanks, soakage devices or raingardens these devices are designed and constructed to provide long term water quality and quantity improvements within greenfield and infill development. Long term functionality of on-lot devices is reliant on maintenance by homeowners.

Across New Zealand, as councils and communities respond to the need to protect our precious freshwater resources and reflect the principles of Te Mana o te Wai, there is increasing emphasis on stormwater management within private properties. At-source treatment allows devices that are tailored to the landuse and improves peoples understanding and interaction with the water cycle. Whether it is on-lot rainwater tanks, soakage, raingardens or other devices, they are designed and constructed to provide long term water quality and quantity improvements for legacy issues and within greenfield and infill development. However, long term functionality of on-lot devices is reliant on maintenance by property owners.

A Hamilton City Council (Council) District Plan rule requiring on lot stormwater management took effect from 2014. As a result, on lot stormwater devices in Hamilton are now being installed, inspected and recorded alongside the Building Consent process.

An on-lot auditing process was needed to ensure that the intended water quality and quantity outcomes were being realised over the lifecycle of these private assets. This was also needed for assurance to Waikato Regional Council (WRC) that the assets are providing environmental protection in accordance with Comprehensive Stormwater Discharge Consent (CSDC) conditions. Further, future infill development across existing urban areas (intensification) may require widespread use of on lot measures due to limited space in the public realm to manage stormwater from increased impervious surfaces.

Council and Morphum Environmental worked together to develop a city wide on-lot audit program and data structure that provided:

• Devices spatially mapped
• On-lot audit schemas to address the specific requirements of each asset type recorded
• A pilot audit of an initial 60 properties
• Development of a semi-automated system to record results and trigger follow up actions and undertake reporting
• Engagement with property owners through calling cards and subsequent summary letters, informing residents of the issues identified relating to the asset(s) on their property
• Development of educational brochures to inform residents of how to resolve the typical issues identified.

The pilot audit was undertaken in mid 2022 over the course of three days. It provided a great opportunity to test out the initial process and identify areas for improvements. The following key findings came from the pilot study:

• Valuable for assessors to have as much information as possible on-hand while on site (i.e., as-built plans) to assist in identifying and assessing assets efficiently.
• There is a clear need to keep educating residents on what assets they have on their property and what they should be doing to maintain them.
• Raingardens had a much higher number of issues when compared to tanks and soakage devices that were able to be assessed.
• Some newer devices weren’t constructed in accordance with Council practice notes, making maintenance and inspections more difficult.

An additional 100 properties have been identified for assessment in early 2023 to further test the processes and tools developed and continue the overall development of the on-lot audit process.