Development can generate increased runoff rates and contaminants, with corresponding negative effects on receiving environments.
Stormwater devices can be used to reduce the effects of changes in stormwater quality and quantity.
Refer to relevant planning provisions for stormwater quality and quantity requirements that need to be met for new developments. Auckland Regional Council technical publication TP10 (Design Guideline Manual for Stormwater Treatment Devices) provides guidance for stormwater device design using a Best Practicable Option approach.
126.96.36.199. Stormwater Devices
There is a wide range of stormwater 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 Auckland Regional Council technical publication TP10 (Stormwater Management Devices: Design Guidelines Manual) for guidance on the appropriate selection and design of stormwater devices. Final selection of any devices to be vested is to be approved by the council.
In addition to the devices outlined in TP10, proprietary devices may also be considered in re-development or ‘brownfield’ development 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 council ownership. A cost-benefit analysis of all devices is required before vesting.
188.8.131.52. General Design Approach for Stormwater Devices
The success and cost effectiveness of stormwater devices is enhanced by considering them in the very early stages of planning and design of a development. The developer shall enter into discussions with the council regarding the selection of these devices as early as practicable in the development’s design prior to sign-off by the 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 devices. The devices chosen need to match the water quality and quantity objectives.
b) Secondary objective. Stormwater 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 devices with other infrastructure such as parks, reserves, wastewater, water supply and buildings as part of the Water Sensitive Design of the development. For example, a green roof may function as a quality device, roof and open space.
- Maintenance of stormwater devices shall be considered early in the design process. This will assist in the identification of features that will facilitate the ease and efficiency of on-going operation and maintenance of devices. 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 train”. The respective position of the various components in the treatment train is an important consideration in ensuring effectiveness of the system throughout its lifecycle. Treatment trains are needed where a single device does not meet all of the water quality and quantity objectives. Refer to Auckland Regional Council technical publication TP10 (Stormwater Management Devices: Design Guidelines Manual) for further guidance on the treatment train approach. The whole of life cost of devices, including maintenance costs, shall be considered.
e) Device location. Any stormwater device shall be located in a readily accessible location, preferably on public land or land to be vested in the council. In cases where this is not possible and the device is located in private land, easements are to be provided for maintenance and access purposes. Generally, the location of stormwater devices in trafficked locations is not acceptable. Deviations from this approach may be considered by the council, for example 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 the council’s approval. Refer to Section 184.108.40.206 for access requirements.
Devices located in high amenity open space areas require additional consideration to achieve a sympathetic and unobtrusive design to the 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 number of devices proposed in relation to the 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 the council to purchase such items from a single supplier.
h) Design for safety. Ensuring that the device is safe both for the public and for operational and maintenance staff is of critical importance. Devices involving open water storage such as wet ponds and wetlands require particular 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 the council’s approval that risks to public safety, in particular falls from height and drowning, have been appropriately addressed.
i) Other issues. Additional items that may need considering include aesthetics, biodiversity, site topography, underground devices and future decommissioning of the device.
220.127.116.11. Maintenance of Stormwater Devices
The design and construction of any stormwater device shall take into consideration the future ownership, access and maintenance requirements, and whole of life costs, 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 the following:
a) Safety in design to enable safe operation and maintenance
b) Access arrangements for operation and maintenance purposes shall be in accordance with Section 18.104.22.168.
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 the council
e) Where necessary, an appropriately sized and located drying/storage area shall be provided for 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 the Auckland Council Plant and Pest Management Strategy. Plant maintenance for vegetation shall be included in the maintenance plan.
An operation and maintenance manual for all stormwater devices, public or private, shall be submitted to the council and approved, prior to issue of certificates such as the Code Compliance Certificate (CCC) or Resource Management Act (RMA) s224(c) Certificate for subdivision consent. This 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.
22.214.171.124. Access Requirements for Maintenance of Devices
The following minimum criteria shall be met in order to allow anticipated future maintenance works for treatment devices:
a) The minimum accessway width shall be 3.0m.
b) The maximum gradient of the accessway should be 1:10.
c) The minimum track specification including the design, construction and choice of surfacing for access tracks shall be discussed with the council. Requirements will vary according to device type and the use of the area in which the device is located.
d) A turning circle or equivalent shall be provided, of sufficient size to accommodate the intended vehicles for all maintenance activities.
e) Where necessary, provide a suitable platform for an excavator to undertake any maintenance work as required.
f) The approved access shall remain available at all times in perpetuity or until the council confirms in writing that the access is no longer required.
g) If the device is located 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.
126.96.36.199. Stormwater Device Ownership
New stormwater devices may be vested in the council’s ownership if it can be demonstrated that a significant flow from the public stormwater network discharges to that treatment system. The council may at its discretion approve public treatment systems where there is considerable public benefit, that is, treatment is available for stormwater runoff from public land or from properties outside of the immediate development site. Prior to vesting such assets, a comprehensive Net Present Value (NPV) analysis shall be submitted to the council. Stormwater devices shall otherwise remain in private ownership.
Stormwater devices either created by the council or by a developer then vested in the council’s ownership shall satisfy a number of criteria, including the following:
- Public stormwater devices shall be located on Auckland Council-owned land.
- The stormwater discharge consent and any operation and maintenance consent, if issued to the developer, shall be transferred to the council at the same time as the land and assets are vested in council ownership.
- The developer shall enter into an agreement on vested assets with the council prior to resource or building consents being issued. Granting consent is conditional on signing the vested assets agreement.
The ownership and maintenance responsibility of stormwater devices (e.g. rain gardens) installed as part of a road project lies with Auckland Transport, and should comply with Auckland Transport Code of Practice (ATCOP) requirements.
Catchpits and associated pipe systems located on private land will remain private assets and will not be maintained by the council.