Section 6: Council Activities

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Stormwater

Responsibility

Brent Manning (Manager Water and Wastes)

Description of service

The public stormwater network has progressively expanded with development of the urban roading network and growth of the towns. Flood protection schemes were later introduced to protect the Central Business District of New Plymouth and urban properties. New Plymouth District Council is hence responsible for the provision of effective systems for the drainage of stormwater in urban areas of the district including:

The maintenance and monitoring of existing flood protection schemes.
The planning of future flood protection and drainage measures for urban areas.
The reduction of stormwater discharge impacts from Council systems as and where possible.
The effect of drainage and treatment of stormwater is essential to the protection of the local economy and quality of life in the district.

Scale of service

This describes the size and scope of the activity.

6,600 hectares served by systems.
Three earth flood protection dams (Huatoki, Waimea, Mangaotuku).
Eight earth detention bunds (Huatoki Street, Brois Street, Waimea Street, Rugby Park, Sutherland Park, Vogeltown Park, Ainslee Street, Magnolia Drive).
Three diversion tunnels (Bonithon Avenue, Queen Street, Gilbert Street).
One wetlands area (Mangati) and one planned (Peringa Park).
One major pump station (Waitara).
265km stormwater pipelines.
$118 million network replacement value.

Rationale for activity

New Plymouth District is subject to high intensity rainfall events. Control of stormwater flows is essential for the protection of people and property and contributes greatly to the community well-beings, in particular economic and social well-being. The most efficient means of achieving this is through a single coordinated system for each catchment area, utilising natural characteristics such as ponds, watercourses, wetlands and vegetation, urban infrastructure such as roads, drains and culverts, and piped systems. As many of these systems are in public roads and drainage reserves, it is logical that they be vested in and maintained by the Council. The Council also defines and ensures a consistent standard of design and level of protection.

Contribution to Community Outcomes

Stormwater management primarily contributes to the Connected and Secure and Healthy Community Outcomes by providing reliable infrastructure which helps ensure the safety of property and people. Stormwater also makes a significant contribution to the Sustainable Community Outcome.Return to top

Connected

A district that delivers accessible and integrated infrastructure, transport and communication systems which meet the needs of residents, businesses and visitors.

C1 Effective, efficient, safe and reliable infrastructure is provided and maintained.

Secure and Healthy

A district that provides a safe, healthy and friendly place to live, work or visit.

S&H6 A sustainable management approach is taken to hazards and risks across the areas of reduction, readiness, response and recovery.

Negative impacts on community well-being

The installation of piped systems, if not designed to the appropriate level and/ or if undertaken on a piecemeal basis, can lead to concentration of the flow of stormwater run-off at a point further downstream, thereby aggravating the effects on downstream properties and the environment.

Stormwater systems inevitably pick up pollutants from roads, e.g. particles from car brake linings, and run-off from other surfaces. In New Zealand this run-off is not often treated before it is discharged downstream into the environment. Usually the impact of this run-off is within acceptable levels. Where the impact of these pollutants becomes unacceptable, treatment of stormwater or other options will need to be considered.

Increase in subdivision for residential development, if not managed properly and designed to appropriate standards, can cause negative impacts.

Facing the challenges – assumptions and risks

Assumptions

It is assumed that stated design levels of protection for stormwater services will not be changed during the 10-year life of the plan.
For new residential property and other developments, the range of site coverage of buildings and hard standing (impervious areas) will be maintained at current levels.
Rainfall intensities for longer duration events (> 4 hours) are estimated to increase steadily over the next 50-years at an average rate of 0.21mm per annum due to climate change effects.
Climate change will increase overall rainfall and high intensity storms, which reduces the ability of stormwater systems to cope, and thereby reduces levels of protection.
No significant change is expected in contractor target response times or reactive maintenance levels of service.

Risks

A major flood exceeds the design criteria of the stormwater and flood protection systems.
Customer expectations exceed the current level of service.

Managing risks

The risks can be managed by ensuring that the Council’s development contributions can fund any increased capacity brought about by urban growth or increased site coverage effects. The Council is to ensure that its planning and codes of practice incorporate assessed climate change impacts.
Customer expectations can be managed by ensuring that levels of service are appropriate to meet customer needs whilst being cost effective. Return to top

Levels of service and performance measures

The following levels of service and performance measures outline the objectives we will set and how we will measure our progress.

Levels of service and performance measures

Asset Information

Assets required

The diagram represents those infrastructural assets both currently employed and planned to be incorporated in service delivery for the stormwater and flood protection activity area.

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Managing improved and/or expanded services

To properly mitigate adverse effects from peri-urban growth and up-catchment development, emphasis is placed upon stormwater treatment and control using an integrated system over a catchment area from source to discharge. Use of in-catchment attenuation of increased flows as a result of development, and natural treatment solutions to avoid degradation of discharge water quality, is encouraged.

Funding

Stormwater operating expenditures are funded through general rates.

This means that all district properties pay for stormwater according to their land value and the rate in the dollar levied on that valuation.

Capital improvements (new or improved infrastructure) are either funded from loans, to spread the costs over the asset’s life, or from rates. New vested assets are created and funded by individual developments and vested in the Council as part of the public stormwater drainage system on acceptance of the subdivision or development.

Renewals and replacements are funded from depreciation reserves which are funded from annual contributions from operating revenue (rates) each year for this purpose. The annual contribution to reserves is calculated by dividing the asset’s replacement value by its expected useful life, i.e. the contribution equals annual depreciation.

Managing change

New subdivision areas are required to provide stormwater systems that meet Council standards as a consent condition. These assets are then vested in the Council and maintained, replaced and renewed by the Council thereafter. Infill subdivision is expected to generate stormwater flows that do not exceed current system capacity. This requirement may drive more on-site containment and treatment measures in future.

Maintenance, renewal and replacement of assets

Existing stormwater assets are maintained, renewed and replaced by the Council and external contractors according to the specific service lives of the component parts of the system and as described in the Stormwater Asset Management Plan (SWAMP). The Council’s asset management plans can be viewed at the Civic Centre, Liardet St, New Plymouth at any of the libraries and service centres at Bell Block, Inglewood or Waitara or district libraries. Maintenance expenses are funded from the general rates and used to fund all other operating costs. Asset renewal and replacement expenses are funded from depreciation reserves. Depreciation reserves are calculated annually as a percentage of the total asset value, and are funded from operating revenue (rates).