DS-5.7 Stormwater Discharge

City Plan Performance Standard: Appendix 12B

Discharge structures shall be installed at the discharge point of primary conveyance systems and comprise of either factory made outlet headwalls or a specific design outlet solution.
Discharge structures shall include an energy dissipation device to minimize the effect of erosion on the receiving environment and limit exit velocities to ensure public safety.

This section describes the requirements for structures or methods for discharge of stormwater runoff. These are categorised as:

Discharge structures.
Ground soakage discharge.
Open watercourses.
Public roadside kerb and channel connection.
Connection to Road or Bubble Up Sumps.
Discharge Quality.

The selection of a suitable location for these structures may influence the pipe alignment. Generally a minimum clearance of 1.0m shall be provided clear of the opening around any structure that may allow entry for maintenance and rescue equipment. Council may determine other specific requirements subject to individual site characteristics.

DS-5.7.1 Discharge Structures

The following shall apply:

Discharge structures shall be installed at the outlets of pipelines to the detail shown on Standard Drawings.
There shall be a single point of discharge from any one structure.
Shall be designed to minimise the effect of erosion on the receiving environment and include an energy dissipation device where required. (Submitted designs must clearly illustrate this requirement).
No grill shall be constructed on outlet structures. A hazard warning sign shall be placed on all outlet structures 500mmØ or larger as defined on the Standard Drawings.
Where a pipe is to be installed in a swale or open drain to allow for access over the drain, an outlet structure may be required to be installed to prevent erosion around the pipe exit.

DS-5.7.2 Ground Soakage Discharge

Disposal of stormwater by ground soakage or ground water recharge is only suitable in some areas of Tauranga (i.e. areas not located near relic slips or plateau edges) and most areas of Mount Maunganui and Papamoa. The following applies to ground water soakage methods:

Concept approval is required from Council before undertaking detailed design for any use of Ground Soakage. The detailed design is then subject to Development Works Approval.
Where no Development Works Approval is required as part of a Resource Consent, then Council approval shall be obtained at either application for Resource Consent or another time specified by Council and before construction of the system.
All soakage systems shall be specifically designed to meet the performance criteria required by the New Zealand Building Code Handbook and Approved Documents section E1 - Surface Water and include the design parameters outlined in this section of the IDC and DS-5 - Apx D – Appendix D: Disposal of Stormwater by Ground Soakage.
Site specific testing in accordance with DS-10 Natural Hazards and Earthworks determines that the soil, geotechnical and groundwater characteristics are suitable.
The rate of soakage is determined by an assessment conducted by an appropriately qualified and experienced Chartered Professional Engineer in accordance with the accepted methods in DS-5 - Appendix D.3 Soakage Testing. (In some cases, a Category 1 or 2 Geo-Professional will be required).
A soakage rate reduction factor of 0.5 is applied to ensure that the system is designed for what can be expected in its operating environment.
Confirmation that use of the soakage system will not have an adverse effect on other land, property and structures including land stability, seepage or overland flow perspectives i.e. that adjoining slopes, basements, retained and unretained batters are identified and the possible effects on these features quantified.
Confirmation of the expected overland flowpath/s where the soakage system overflows and how this will be managed.
Confirmation that the effects of overland flow from the proposed development will either improve or will not be worse than, the pre-development situation or existing land use e.g. adjacent properties or mapped flooding / ponding areas.
Confirmation that the specifically designed soakage system can service the "maximum potential impermeable area" of the site to cater for future development.

Explanatory Note:

In Councils experience, secondary subdivision e.g. Infill subdivision or on-site surface changes made by property owners at a later time e.g. paving, garden sheds etc. increase the impermeable area.
When ground soakage methods do not cater for this, they can fail because the system can’t take the increase in run-off or the stormwater solutions for an infill can become very difficult to achieve.
The maximum impermeable area provides for these potential future changes and reduces the chances of property owners creating issues for themselves that they may be genuinely unware of.

It includes areas clear of a flood event up to the 50 year return period storm event (2% AEP).
The system contains the volume of storage required where the rate of inflow exceeds the rate of soakage for the design storm.
The method of storage is approved.
Details of how the soakage system will be constructed, accessed and what the long term maintenance regime for the system is.
All stormwater entering the storage/soakage portion of the system enters does so via a silt entrapment device to ensure the long term serviceability of the system.
Soakholes, soakpits or entry points shall be located to allow for adequate and safe access for maintenance.
Each soakhole, soakpit or entry point pit shall be fitted with a removable lid for ease of access and maintenance.
Location of soakholes, soakpits or entry points shall be submitted for Council records before issue of building Code of Compliance certificates.
Stormwater entry to soakholes, soakpits or other soakage mechanism from external hardstand areas shall be through a yard sump (as shown on the Standard Drawings) or other such similar structure so minimal sediment enters the soakage environment, ensuring the long term serviceability of the system.
The entire soakhole, soakpit or soakage mechanism shall be located 0.5m above the static groundwater level in heavy rain conditions.

Areas of soakage suitability, specific design and soakhole decommissioning areas are displayed in Council’s GIS system.

DS-5.7.3 Open Watercourse

This is generally permitted but only where an existing open public watercourse is available. This method may require a Resource Consent for discharge from Bay of Plenty Regional Council.

DS-5.7.4 Public Roadside Kerb and Channel Connection

Drawing Ref: T535

Kerb connections can be made only to vertical kerb and channel and service 1 independent dwelling unit per connection. This option will be approved by Council only providing the kerb connection can be shown not to cause or increase flooding in the downstream catchment for the street and no other reasonable options are available.

DS-5.7.5 Connection to Road or Bubble Up Sumps

Drawing Ref: T425, T426, T427, T525

This option is not a preferred solution. A connection to a road sump or construction of a bubble up sump shall occur only with approval from Council. Subsoil drainage reticulating groundwater seepage or under channel drains may be connected to road sumps.

DS-5.7.6 Discharge Quality

If the discharge is a permitted activity the quality shall meet the discharge standard as specified in the Bay of Plenty Regional Council Regional Water & Land Plan.

Definitions in this section

Chartered professional engineer

Council

Design

Development works approval