Stormwater quality and control Print

Development can generate increased runoff rates and contaminants, with corresponding negative effects on receiving environments. Stormwater management devices can be used to reduce the effects of changes in stormwater quality and quantity.

For stormwater quality and quantity requirements that need to be met for new developments, refer to the relevant planning provisions. GD01 provides guidance for stormwater device design using a best-practicable option approach.

4.3.6.1   Stormwater management devices

There is a wide range of stormwater management devices available for managing the quality and quantity of stormwater. Each of these devices may be appropriate for varying sites with different land use types and other constraints. Readers should refer to GD01 for guidance on the appropriate selection and design of stormwater management devices. The final selection of any devices to be vested is subject to the approval of Auckland Council.

In addition to the devices outlined in GD01, proprietary devices may also be considered in areas where spatial constraints and practicality require their use. In these instances, only devices that have been approved by Auckland Council may be used. In general, the use of proprietary devices in 'greenfield' development areas shall not be permitted for vesting in Auckland Council ownership.

4.3.6.2   Stormwater device ownership

New stormwater management devices may be vested in Auckland Council's ownership if it can be demonstrated that a significant flow from the public stormwater network discharges to that treatment system. Auckland Council may, at its discretion, approve public treatment systems where there is considerable public benefit, i.e. treatment is available for stormwater runoff from public land or from properties outside of the immediate development site. Stormwater management devices shall otherwise remain in private ownership.

The developer shall discuss and agree with Auckland Council any requirements for vesting stormwater management devices and/or infrastructure funding agreements early, as part of the pre-application process. As part of these discussions, the developer maybe asked to prepare and submit to Auckland Council a comprehensive whole-of life cost analysis for the proposed device(s). The developer shall be responsible for providing suitable access for all future maintenance operations. Any financial burden, including costs associated with easements and/or land being vested in Auckland Council, shall fall on the developer (unless an alternative arrangement has been agreed as part of an infrastructure funding agreement).

Stormwater management devices either created by Auckland Council or by a developer, then vested in Auckland Council's ownership, shall satisfy a number of criteria, including the following:

    • Public stormwater management devices shall be located on Auckland Council-owned land
    • Any private stormwater diversion and discharge consent, or other relevant RMA consent
      (e.g. to dam water or for physical works associated with ongoing operation and maintenance) issued to the developer, shall be transferred to Auckland Council (in accordance with
      RMA s136) at the same time as the land and assets are vested in Auckland Council ownership
    • In general, treatment devices should be constructed after the catchment is fully developed to ensure that no contaminants from construction works can enter the device
    • Where the above is not possible, any devices constructed prior to a catchment being fully developed shall be reinstated to its as-new condition upon the completion of the maintenance period, once the catchment has been fully developed, prior to vesting. This is to ensure that the quality of the device vested to Auckland Council is not compromised by construction works during a latter phase of a development
    • All devices shall be operated and maintained with the fully developed catchment for at least 12 months by the consent holder prior to vesting
    • The device is vested to Auckland Council with no service contract
    • The minimum maintenance periods for proprietary devices are 12 months
    • For screen and gross pollutant traps, the following must be provided:
      • Safe access during storm events
      • Bypass for flows larger than the target flow
    • Whole-of-life costs including capital, maintenance and rehabilitation costs using a lifecycle of 100 years are to be submitted to Auckland Council. Replacement costs associated with components with less than a 100-year life shall also be accounted for in the whole-of-life cost.
    • The Operation and Maintenance Plan and maintenance records must be submitted to Auckland Council prior to vesting.

For stormwater management devices (e.g. rain gardens) installed as part of a roading project, liaison with Auckland Transport will be required to confirm locations, connection requirements and design parameters. All stormwater management devices to be vested in Auckland Transport shall be designed and constructed in accordance with Auckland Transport Code of Practice requirements and the Auckland Transport Design Manual.

Catchpits and associated pipe systems located on private land shall remain private assets and will not be maintained by Auckland Council.

4.3.6.3   General design approach for stormwater management devices

The success and cost-effectiveness of stormwater management devices is improved by considering them in the very early stages of planning and design of a development. The developer shall enter into discussions with Auckland Council regarding the selection of these devices as early as practicable in the development's design, prior to sign-off by Auckland Council. The following key items shall be considered:

a)    Primary objective: Having a clear understanding of the statutory requirements regarding water quality and quantity is crucial in identifying appropriate stormwater management devices. The devices chosen must match the water quality and quantity objectives.

b)    Secondary objective: Stormwater management devices offer many opportunities to deliver multiple benefits in addition to their stormwater functionality.

c)     Integrated approach: An integrated approach considers several aspects of stormwater design including the following:

    • Consideration of stormwater management requirements in the early stages of a project
    • Integration of stormwater management devices with other infrastructure such as parks, reserves, wastewater, water supply and buildings as part of the development's integrated stormwater management, e.g. a green roof may function as a quality device, roof and open space
    • Maintenance of stormwater management devices shall be considered early in the design process to facilitate the ease and efficiency of their on-going operation and maintenance. All maintenance requirements shall consider current resource consent requirements.

d)    Device selection: The proper design and position of a device within the stormwater catchment is critical. Several devices are often used in series, in what is called a "treatment suite". The respective position of the various components in the treatment suite is an important consideration in ensuring effectiveness of the system throughout its lifecycle. Treatment suites are needed where a single device does not meet all of the water quality and quantity objectives (refer to GD01 for further guidance on the treatment-suite approach). The whole-of-life cost of devices, including maintenance costs, shall be considered.

e)    Device location: Stormwater management devices shall be located in a readily accessible location, preferably on public land or land to be vested with Auckland Council. Where this is not possible, and the device is located in private land, easements are to be provided for maintenance and access. Generally, the location of stormwater management devices in trafficked locations is not acceptable. Deviations from this approach may be considered by Auckland Council, e.g. where the device is in a very low traffic volume location, access is on the berm rather than the carriageway, and there is sufficient area. Device location, type, size and maintenance requirements are subject to Auckland Council's approval.

    • Refer to Section 4.3.6.5 for access requirements
    • Devices located in high amenity open space areas require additional consideration to achieve a sympathetic and unobtrusive design for Auckland Council's approval. Reference should be made to the Auckland Design Manual (www.aucklanddesignmanual.co.nz) for guidance with respect to design in high-amenity open space.

f)      Device quantity: Applicants shall optimise the proposed number of devices in relation to treatment effects and whole-of-life costs. There are situations where fewer and larger facilities are preferable to many smaller ones; there are also situations where the opposite applies.

g)    Device replacement parts: Applicants shall demonstrate that any spare parts anticipated being required for routine maintenance activities are commonly available on the open market and are not subject to any licences or other restrictions that would bind Auckland Council to purchase such items from a single supplier.

h)    Device design: The device must be designed for a 100-year design life, and no contaminant leaching material can be used (e.g. uncoated treated timber). Generally, retaining walls shall be kept out of the permanent water pool.

i)      Design for safety: Ensuring that the device is safe both for the public and operational and maintenance staff is critical. Devices involving open water storage such as wet ponds and wetlands require attention to water safety, including inlet and outlet location and levels. Pond fencing requirements will be subject to careful risk assessment which shall demonstrate to Auckland Council's approval that risks to public safety, especially falls from height and drowning, have been appropriately addressed. If there is a potential risk of vehicles rolling or driving into stormwater infrastructure which has open water surfaces, sufficient barriers in accordance with Auckland Transport's Code of Practice must be provided or an alternative design that is agreed and accepted by Auckland Council.

j)      Emergency spillways: The requirement for, and the design (and consideration) of spillways to protect upstream and downstream receiving environments during extreme weather events (outside of the design storm) to ensure a safe and resilient community.

k)     Other issues: Additional items that may need considering include aesthetics, biodiversity, site topography, underground devices and future decommissioning of the device.

4.3.6.4   Maintenance of stormwater management devices

The design and construction of any stormwater device shall take future ownership, access, maintenance requirements and whole-of-life costs into consideration and shall ensure that maintenance can be carried out with little or no disturbance to the surroundings or neighbouring properties.

Elements to consider in the design for the maintenance and operation of the device include:

a)   Safety in design to enable safe operation and maintenance

b)   Access arrangements for operation and maintenance purposes shall be in accordance with Section 4.3.6.5

c)    Procedures for the removal and disposal of sediment, including the required frequency. This shall include any consenting issues that are considered likely to occur in the future associated with the removal and disposal of silt

d)    Obtaining consent(s) for any maintenance activities for assets to be vested in Auckland Council

e)    Where necessary, an appropriately sized and located drying/storage area shall be provided for any litter/silt/media etc. that is removed from the device

f)     Wherever practical, it should be possible to drain the device and forebay by gravity flow

g)    Maintenance requirements of mechanical parts

h)    Vegetation maintenance requirements. Weeds shall be controlled and removed in accordance with Auckland Council's Plant and Pest Management Strategy (2007). Plant maintenance for vegetation shall be included in the Maintenance Plan.

An Operation and Maintenance Manual for all stormwater management devices, public or private, shall be submitted to Auckland Council for approval as part of the Engineering Approval process, and prior to the issue of certificates such as the Code Compliance Certificate or RMA s224(c) Certificate for Subdivision Consent. The manual shall include a detailed technical data sheet and shall state the methodology for the ongoing and long-term maintenance of the device, including:

    • Inspections required and frequency
    • Maintenance needs and frequency
    • Recommended ongoing control methodology to eradicate established pests and invasive weeds from both terrestrial and aquatic areas.

Additional operation and maintenance information that is needed for detention ponds and wetlands (and their surrounding drainage reserve) are:

    • Details for permanently wet areas
    • Details for the surrounding planted area
    • De-watering methodology for the main pond and the forebay
    • De-silting methodology for the main pond and the forebay
    • Consent(s) for operation and maintenance.

4.3.6.5   Access requirements for maintenance of devices

The following minimum criteria shall be met to allow anticipated future maintenance works for treatment devices:

a)     To allow for access for maintenance, a Vehicle Manoeuvre Plan shall be provided with the Maintenance Plan. This must show that maintenance vehicles as required as per the Operation and Maintenance Plan, can access and exit the site safely, and that sufficient space is provided to manoeuvre the vehicle on site. General requirements are:

    • Minimum accessway width shall be 3.5 m
    • The maximum gradient of the accessway should be 1:8.

b)     The minimum track specification including the design, construction and choice of surfacing for access tracks shall be discussed with Auckland Council. Requirements will vary according to device type and the use of the area in which the device is located.

c)      Where necessary, provide a suitable platform for an excavator to undertake any maintenance work as required.

d)     The approved access shall remain available at all times in perpetuity or until Auckland Council confirms in writing that the access is no longer required.

e)     If the device is in a trafficked location, sufficient area shall be allowed to establish all necessary traffic management controls, or any other requirements of the Traffic Management Plan.

The suitability of the access and all other requirements listed above shall be demonstrated in the device's design and in the Operation and Maintenance Plan.

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