DS-5.6 Stormwater Treatment / Detention

Stormwater ponds are an accepted method of improving stormwater quality, reducing downstream flood potential and peak downstream flow rates. Council is not in favour of encouraging a proliferation of small stormwater ponds but prefers a total catchment/treatment train approach.

DS-5.6.1   Permanent Pond Type

Council accepts the use of dry and wet ponds as a part of an engineered solution and/or treatment train. However, the selection and design criteria (short term as well as long term functionality, maintenance, ownership) must be approved by Council before construction. Access from public carriageways shall be provided for maintenance.

DS-5.6.1.1   Regional Council Resource Consent

The following shall apply:

1. Permanent ponds may require Resource Consent from Bay of Plenty Regional Council as part of an earthworks and/or stormwater discharge Resource Consent. Refer to DS-5.3 Regional Council Requirements and contact Bay of Plenty Regional Council for more information.
2. Temporary ponds may require Resource Consent from Bay of Plenty Regional Council as part of the earthworks Consent and/or discharge Resource Consent. Refer to DS-5.3 Regional Council Requirements and contact Bay of Plenty Regional Council for more information.
3. Pollutants and contamination issues shall be identified and managed to the satisfaction of Council and any Resource Consent conditions related to the pond.

DS-5.6.1.2   Permanent Pond Ownership and Location

The following shall apply:

1. If acceptance of ownership of a pond is agreed by Council all stormwater ponds and the access to them shall be located on land either:
   1. Owned by Council.
   2. Proposed to be vested in Council at completion of the development. 
2. Ponds that are not proposed to be vested in Council shall be discussed with Council prior to application for Resource Consent or, if no Resource Consent is required, prior to construction of the pond to ensure the pond construction has minimal or no impact on Council’s existing stormwater system.

DS-5.6.1.3   Detention Pond Design

The following shall apply:

1. Detention Ponds shall be designed to the following minimum requirements:
   1. Compliance with DS-1 - Apx A.1 General.
   2. Stormwater ponds shall be designed to ensure that retention and treatment to achieves removal of 75% of total suspended sediment smaller than 120µm diameter on a long term average basis. 
   3. Any pond shall be designed to ensure any discharge has a suspended solids concentration no higher than 150 g/m³. 
2. Specific matters to consider in pond design:
   1. Side slopes with safety considerations - maximum 1:5 slope above and below water level.
   2. Ease of access for and of maintenance including mowing and silt removal and disposal and access to public roads.
   3. Shape and contour for amenity value.
   4. Effectiveness of inlet and outlet structure/s. 
   5. Overflow design and scour protection.
   6. Pest control (rats, mosquitoes etc.) and wind blown debris.
   7. Sustainability of aquatic life.
   8. Depth of water.
   9. Use in treatment of stormwater.
   10. Minimise dead zones and short-circuiting to improve the treatment performance of the pond.

Dependent on catchment water quality, local ground conditions and soil properties, Council may require, (in exceptional cases), that ponds shall be lined. 

The designer shall refer to Councils Stormwater Management Guidelines for more information regarding designs of this type. 

DS-5.6.2  Constructed Wetlands

Constructed wetlands are large shallow planted ponds which are designed to provide stormwater quality improvement, reduce downstream flood potential and peak downstream flow.  
They differ from Detention Ponds in that they can be shallower, provide greater contaminant removal (including nutrients) and support a wider variety of aquatic plants and wildlife. They can also be more attractive and designed to provide greater amenity, ecological and safety benefits.  

It shall be demonstrated that a constructed wetland complies with the following:

1. A catchment area greater than 1 hectare.
2. Size is calculated to achieve water quality volume.
3. Includes a littoral and main wetland area at a depth not exceeding 1.0m.
4. Shall be appropriately planted.
5. Includes inlet pipework with appropriate erosion control.
6. Includes an emergency overflow.
7. Designed for a controlled discharge rate that allows for continuous flow through a wetland to prevent stagnation. 
8. Provides access for maintenance.

Council approval shall be obtained prior to design and construction of any wetland. All Resource Consent, location, functionality, maintenance and ownership issues shall be confirmed prior to any detailed design commencing.

The designer shall refer to Councils Stormwater Management Guidelines for more information regarding designs of this type. 

DS-5.6.3  Permeable Pavements

Permeable Pavements are hard surface paving systems that reduce stormwater runoff flows and improve runoff water quality. Their porous surface allows stormwater to soak through them before slowly draining away. 

The 3 main types that exist are:

1. Open cell grid of concrete or plastic with sand/gravel or grass cover.
2. Solid interlocking blocks with drainage gaps. 
3. Porous interlocking blocks.

Permeable Pavements shall only be used in low traffic areas such as carparks, driveways and footpaths.
Council approval shall be obtained prior to design and construction of Permeable Pavement.

The designer shall refer to Council's Stormwater Management Guidelines for more information regarding designs of this type. 

DS-5.6.4  Bio-Retention Tree Pits

Bio-Retention Tree Pits collect stormwater from small car park areas or roads. They can reduce stormwater runoff flow velocity, improve runoff water quality and contribute to the overall urban design and aesthetics of an area. 

It must be demonstrated that a Tree Pit complies with the following:

1. Appropriate functionality and adequate capacity.
2. Has a ponding area at a level which sits below surrounding land and a maximum ponded water depth of 300mm deep.
3. Filtration layers shall comprise of the following:
   1. Filtration soil media layer: 400-600mm deep.
   2. Final drainage layer of 2-5mm washed gravel: 300mm deep.
4. Filtration soil media shall be sandy loam or loamy sand, free of rubbish, plants and weeds with a minimum permeability of 0.3m/day.
5. Ponding area should drain completely within 24 hours.
6. An underdrain shall be provided in the final drainage layer with a minimum cover of 50mm. 
7. An overflow bypass system shall be provided which sits below the surrounding ground surface. 
8. Achieves all other relevant performance standards for the primary system. 
9.  Provides access for maintenance.

Council approval shall be obtained prior to construction of Bio-Retention Tree Pits.

The designer shall refer to Councils Stormwater Management Guidelines for more information regarding designs of this type. 

Definitions in this section

Council

Resource consent