Part 9 – Landscaping Engineered Stormwater Devices

9.0    Introduction

This section applies to engineered stormwater devices that have a planted component (planted stormwater devices or “PSDs”) or works in any way related to the use of landscaping as either an amenity or water mechanism in the development of new engineered stormwater devices and the remediation of existing engineered stormwater devices.

This Design Guide Part 9 is supplementary to Part 7 – Street Landscaping of this Development Manual and is to be read in conjunction with Part 4 – Stormwater Drainage.

Note: Where there is a conflict between the requirements of the PSD Design Guide (Part 9) and the requirements of Parts 4 and 7, this Part 9 takes precedence.

These design guidelines apply to all PSDs that are located in any part of a road reserve, drainage reserve, subdivision or vested reserve, or where required as a condition of resource consent. These include, (but may also include other stormwater devices as required by resource consent conditions):

• Permanent stormwater ponds.
• Rain gardens.
• Swales.
• Vegetated filters.

Proposed landscape designs or works shall be approved or rejected at the sole discretion of Council or its delegated officer.

Landscape Plans will be submitted for approval with the Engineering Plans.

Applicable Engineered Stormwater Devices
Examples of devices that require a landscaping component are as follows:

Note: The Auckland Regional Council publication TP10 Stormwater Management Devices: Design Guidelines Manual provides in-depth definitions of engineered stormwater devices.

Permanent Stormwater Ponds
These are either wet ponds (where there is a permanent pool of water) or detention ponds (which have temporary pooling dependent on rain events).

Note: Council opinion is that “wet pond” type detention dams should be avoided if possible.  Both are used to capture and store stormwater in rain events and then release it at a slow rate. This rate is intended to be consistent with pre-urbanisation stormwater flow from the catchment. Landscape planting is used to: 

• Prevent erosion and stabilise the pond slopes and flood zone.
• Minimise long-term maintenance costs.
• Increase run-off water quality.
• Increase local amenity values.
• Increase the quality of ecological greenbelts throughout the city for regional flora and fauna.

Raingardens
These are temporary stormwater detention devices that retain water and release it into the stormwater catchment over a designed period of time, such as 24 hours, when peak stormwater flows have diminished. Water is absorbed by the plants and released through vegetative evapotranspiration. Plants also use trapped sediments for nutritional requirements. Depending on the design, stormwater flows and the trapped sediment loading, these last for 15-30 years, after which they will need to be excavated and replanted.

Swales
These are mechanisms to control stormwater flow velocities from or through a site.  Engineered ditches, they reduce the impermeable area of sites and assist with groundwater recharge. To be effective, they need to have low water velocities and are normally planted with a grass cover that is maintained at a calculated height so that velocities are slowed but not impeded. On slowing, suspended solids drop and aid in the soil nutritional value. Other plants may be used depending on the swale design.

Vegetated Filters
Vegetated filters act as stormwater water quality filters that rely on a distributed waterflow to produce a thin layer of water passing through the vegetation to be effective. They are often used in conjunction with other stormwater treatment practices.