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Infrastructure Development Code

DS-5.4 Design

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City Plan Performance Standard: Appendix 12B 
  1. Each stormwater management system shall be designed and constructed to ensure:
    1. Consistency with any relevant structure or urban growth plan;
    2. It is compatible with the design and construction of the existing stormwater network; and 
    3. Compliance with any comprehensive stormwater consent where applicable.
  2. Each stormwater management system shall be designed and constructed to ensure:
    1. All components of the stormwater management system are pre-approved by the Council and provide for an asset life that is suitable for its intended purpose;

Note: Components that are pre-approved by the Council are identified in the Council’s Infrastructure Development Code.

  1. Gravity is used as the method for reticulation unless pump stations have been pre-approved by the Council;
  2. The upstream catchment is provided for and the downstream receiving network has the capacity to cater for the design scenario;
  3. They comprise both primary and secondary flow systems which include components to collect, convey, treat and safely discharge the stormwater associated with the development; 
    1. Secondary stormwater systems shall not include piped systems;
    2. Minimum velocities for pipes and lined channels are 0.6 metres per second (m/s)
    3. Maximum velocities are as follows:
  4. 4.0 m/s for pipes;
  5. 1.5 m/s for lined channels;
  6. 1.5 m/s for unlined channels. 
    1. The overall stormwater management system shall be designed to accommodate and contain flows from a 2% AEP (1 in 50 year) rainfall event with the primary conveyance system being designed to accommodate flows from a 10% AEP (1 in 10yr) design rainfall event unless b(i) applies;
    2. The stormwater management system for the Wairakei Urban Growth Area and the Te Tumu Future Urban Zone shall be designed to accommodate flows from a 1% AEP (1 in 100 year) return period rainfall event;
    3. The post development stormwater run-off rates shall not exceed the pre-development run off rates.

Note: pre-development run off rates exclude land uses, buildings and structures that have not been lawfully established.

  1.  In addition to iv) and v) above where a development includes pump stations or road bridges these structures shall be provided with a minimum freeboard of 500mm above the flows from a 1% AEP (1 in 100 year) return period rainfall event;
  2. Secondary stormwater systems are only provided for on:
  3. Local and collector roads;
  4. Accessways and reserves;
  5. Private land where it is protected by an easement in favour of the Council.

The following shall apply:

  1. Unless otherwise approved by Council, the design of the stormwater system shall be in accordance with the IDC and may be supplemented by the documents noted in DS-5 Appendix A.1 General
  2. For catchment areas less than 50ha, without any significant attenuation storage upstream of the point of investigation, peak flow may be determined using the Rational Method in accordance with DS-5 - Appendix E The Rational Method.
  3. For catchment areas greater than 50ha, peak flow shall be determined using a Council approved hydrodynamic model which utilises a nested rainfall profile in accordance with DS-5.4.4 Rainfall Intensity
  4. For catchment areas with significant attenuation storage upstream of the point of investigation, peak flow shall be determined using a Council approved hydrodynamic model that utilises a nested rainstorm profile in accordance DS-5.4.4 Rainfall Intensity
  5. Low Impact Design for stormwater management is the preferred approach where appropriate. Well-designed and well-maintained systems that replicate the natural pre-development hydrological regime cannot only mitigate adverse environmental effects but can also enhance local amenity and ecological values. Low Impact Design methods typically include swales, ponds, rain gardens, rain tanks, a reduction in impermeable surfaces, permeable paving, filter strips, green roofs and constructed wetlands. The use of Low Impact Design methods shall be discussed with Council at an early stage. 

Detailed design is required at the time of Development Works Approval. Where no Development Works Approval is required, Council approval shall be obtained at a time required by Council and before construction of the system. 

DS-5.4.1   Alternative Design

Refer to DS-1.3  Alternative Design.

DS-5.4.2   Stormwater Management Systems

The following shall apply:

  1. The stormwater management system is an overall system that manages the collection, conveyance, treatment and discharge of stormwater. The overall stormwater management system shall provide a minimum standard of flood protection for the community and mitigate any adverse environmental effects.
  2. The overall system shall accommodate flows from a 50 year return period storm event (2% AEP) unless otherwise stated in a comprehensive stormwater discharge consent.  The post development run off rates shall be no greater than pre development run off rates.  Ponding areas shall be provided for only on public roads and public land.
  3. Stormwater designs shall consider the system as a whole, ensure the upstream catchment is provided for, ensure the downstream receiving network has the capacity to cater for the design condition and be appropriately engineered to suit the site-specific requirements. An appropriate mix of solutions comprising Low Impact Design and conventional solutions shall be considered as well as short and long term maintenance responsibilities/requirements and the whole of life cost implications (i.e. cost of construction, maintenance, renewal or replacement). The overall system also needs to provide for a range of storm events.
  4. A subset of the overall system is the primary system. The primary system shall accommodate flows from a 10 year return period storm event (10%AEP). When the primary system overloads by either blockage, malfunction or rainfall events in excess of the design capacity then the part of the overall system that compliments the primary system is known as the secondary system.
  5. Water Quality considerations are required to control potential for damaging environmental effects to our waterways, habour and aquatic life. Water quality shall be included in design where required by:
    1. Bay of Plenty Regional Council Water and Land Plan requirements. 
    2. Bay of Plenty Regional Council Resource Consent conditions.
    3. Council's Comprehensive Catchment Consent Conditions and associated Management Plans as approved by Bay of Plenty Regional Council.

The 90% rainfall storm shall be used as this is the maximised point of runoff volume capture. The 90% rainfall for Tauranga is 33mm for a 2 year / 1 hour storm.

DS-5.4.3   Hydraulic Design

The following shall apply:

  1. The hydraulic design of stormwater pipelines shall be based on tables for the hydraulic design of stormwater drains and pipelines or on graphs or other representation of the same methods based on the Colebrook White or Manning's Formula.
  2. The pipe roughness coefficient Ks used in the design shall be as shown in NZS4404 Table 4.2.  

DS-5.4.4   Rainfall Intensity

The following shall apply for use with:

  1. The Rational Method:
    1. Designers shall use the Design Rainfall Tables located in DS-5 – Appendix B: Design Rainfall Tables.
    2. These tables have been developed from the rainfall records from the rain gauge stations in the Tauranga City area and have been adjusted to allow for climate change to the year 2055.
    3. The Rainfall Intensity Curves are included in DS-5 - Appendix C: Rainfall Intensity Curves.
  2. Hydrodynamic Models:
    1. 10-minute time interval shall be used.
    2. A centrally weighted nested rainfall profile will be applied. 10-minute rainfall depths for the nested rainfall profile are provided in DS-5 - Appendix D: Rainfall Depth Information Sheet
    3.  Rainfall event duration of six (6) hours shall be used except for catchments where a longer duration event is required to account for the effects on peak water levels and flows of catchment storage, tidal downstream boundaries or long times of concentration.
    4. Any departure from the use of the rainfall depths in the Design Rainfall Tables or the six (6) hour event duration shall be agreed with Council before modelling commences. 

DS-5.4.5   Freeboard Above Design Flood Level

This freeboard measurement applies to the nominated building platform. The minimum freeboard height applied to the calculated top water level shall be as outlined in Table 2: Freeboard Heights. 

Where designs require a nominated building platform, the minimum freeboard shall be measured from the top water level to the finished level of the nominated building platform. This provides an easier, cost efficient situation to meet the freeboard requirements of the Building Code when placing a building or structure on top of the nominated building platform. 

Table 2: Freeboard Heights

Type of Structure Freeboard to Applied Design Storm
Non-habitable residential buildings, detached garages and carports  200mm
Commercial and industrial buildings 300mm
Habitable dwellings/structures and attached garages 500mm

For structural designs related to a Building Consent, the requirements of the Building Code shall apply.

DS-5.4.6   High Levels of Groundwater

If there is a need to lower groundwater levels then designers shall provide a method of allowing the groundwater into the stormwater system in a controlled manner. Such a solution may incorporate perforated inlet pipes together with drainage rock and geotextile.

DS-5.4.7 Calculating Water Quality Volumes

Calculating the water quality volume can be done by the following calculation:

2 year / 1hour storm event for Tauranga = 33mm. 

Awq = 0.9(imp. %/100) x total site area + 0.15(pervious %/100) x total site area.

Where total site area = m2

The water quality volume Vwq = 0.033Awq

Where 0.033 = 90% storm depth (m)

Examples of this method can be found via case studies provided within the Stormwater Management Guidelines.


Definitions in this section

AEP

Bay of Plenty Regional Council

Building platform

Council

Design

Development works approval

Freeboard

IDC

Low impact design

Road

Stormwater

Swale

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