Part 9 – Landscaping Engineered Stormwater Devices

9.1    Minimum Requirements

Minimum PSD design requirements are as follows:

• Stormwater Devices may be permanently grassed if the slope ratio is more than 1 (vertical) to 4 (horizontal).  Should Council deem the slope too steep for safe maintenance, the slope shall be mulched and planted with permanent landscaping.
• Any embankment that the engineer determines is either too inaccessible or unsafe for regular grass mowing shall be permanently planted.
• All inlet pipes must have a 1 m wide concrete apron or band around the external portion.  Between this apron and 2 m from the inlet pipe, Carex virgata and/or Carex germinata shall be planted on the embankment with mulch matting, unless otherwise specified by the engineer.  No other plant species, including grassing, may be installed within 2 m of the inlet pipe.
• PSDs need to enhance and strengthen the existing character and intended future character of neighbourhood areas
• Any landscaping shall provide maximum long-term benefit with minimum ongoing maintenance. It shall not compromise the safety of adjacent property owners nor the local community
• PSDs are to be landscaped so that they:
  • Comply with engineering requirements;
  • Improve stormwater water quality discharge where possible;
  • Become a community asset and positive visual amenity; and
  • Provide, where possible, forage and habitats for native flora and fauna.
• Safety of the site is paramount by incorporating CPTED (Crime Prevention through Environmental Design) principles (refer to Part 7 – Street Landscaping).
• Plant species allocations are to be specific to soil type and conditions, site topography and exposure, post-development groundwater table levels and alignment with local indigenous native plant species.
• Plant species are to be indigenous to the Waikato Region, although native New Zealand grasses are permitted.  Likewise, plants are to be eco-sourced where possible from the Waikato Region.
• Planting plans are to be detailed, indicating different mixes and/or individual planting as applicable to the different Planting Zones, topographical, ecological and amenity zones within the PSD and surrounding environment.
• Pond planting may be staged to minimise slope erosion. The initial stage shall be grassing the site with the Council approved PSD grass mix, followed by landscape planting once grass has established.

Minimum planting provision requirements are:

• Quick establishment of plant cover is required for PSDs throughout the site, as engineering requirements permit.
• PSDs are to be landscaped with groundcovers, shrubs and trees where site conditions and engineering requirements permit, otherwise the PSDs are to be grassed or turfed as site design permits.
• Of the vegetation mix in Stormwater Ponds, at least 10% and no more than 25% must be staked 1.5 m high grade trees.
• Avoid planting woody vegetation near the slope toe of Stormwater Ponds to prevent future bank stability issues when the plant reaches the end of its lifecycle and its root systems decompose.
• Swale surface treatments shall be preferably established with low maintenance treatments such as rolled turf (for short lengths) or planted up in approved Carex species or laid with loose river rocks.  Swales sown with grass seed, though low cost to construct are often difficult to establish and maintain.  Acceptance of grassed swales or other swale surface treatments shall be at the Engineer’s discretion.
• Where mulch is used, it is to be contained within the plant area that it is providing cover for. Other mulch applications are to be utilised on slopes greater than 1:3.

Appropriate maintenance is required post-landscaping.

Note:  The MPDC Infrastructure Code of Practice includes a PSD Defects Liability minimum standards and Maintenance Schedule.