Design criteria Print

Design Outcome

​​​​When the design process requires hydrological or hydraulic modelling, a design report will be necessary. 

The design report shall include all underlying assumptions such as runoff coefficients, time of concentration, catchment areas, roughness coefficients and losses. These assumptions shall be clearly presented so that an appropriate check of all calculations is possible. A copy of the model or calculations will be required by Auckland Council for review and/or record keeping. Models developed in proprietary software need to be submitted with temporary licenses.

The designer shall undertake the required design and prepare design drawings compatible with Auckland Council's design and performance parameters. Designers shall ensure the following aspects have been considered and included in the design where appropriate:

a) The size of pipes, ponds, swales, wetlands and other devices in the proposed stormwater management system

b) How the roading stormwater design is integrated into the overall stormwater system (please refer to Auckland Transport Code of Practice (ATCOP)). For works within the road reserve, liaison with Auckland Transport is required to confirm design parameters. Where these are different to those of Auckland Council, the most stringent parameters shall apply.

c) The type and class of materials proposed to be used

d) System layouts and alignments including:​
  • Route selection
  • Topographical and environmental aspects (see Watercare’s Water and Wastewater CoP)
  • Easements
  • Clearances from underground services and structures (see Watercare’s Water and Wastewater CoP)
  • Provision for future extensions
  • Location of overland flow paths, and checks against depth and flow criteria (see Section 4.3.5.6)
e) Hydraulic adequacy (see Section 4.3.5.3)

f) Property service connection locations and sizes (see Sections 4.3.9.5 and 4.3.11)

g) Accessway, including vehicle access for future operation and maintenance activities

h) Cost, including whole of life cost

i) Other specific requirements such as specific geotechnical conditions, fish passage, debris loading/blockage as necessary.

Where necessary, the designer shall liaise with Auckland Council prior to commencement of design, to ensure that sufficient prerequisite information is available to undertake a robust design.

4.3.5.1. Hydrological Design of Stormwater Systems

For most catchments, estimation of surface water runoff shall be derived using Auckland Regional Council technical publication TP108 (Guidelines for Stormwater Runoff Modelling in the Auckland Region), adjusted for climate change as stated in Section 4.2.10. For larger catchments, or where significant storage elements (such as ponds) are incorporated, surface water runoff shall be determined using an appropriate hydrological and/or hydraulic model to the approval of Auckland Council.

Runoff factors are to be based on the underlying geology, as defined on the geological map for the Auckland Region and confirmed by site inspections. Runoff factors will also be influenced by land use.

4.3.5.2. Design Standard

All new public stormwater systems shall be designed to cater for design storms of at least the Annual Exceedance Probability (AEP) set out in Table 4.3 (adjusted for climate change – see Section 4.2.10) unless specific approval has been obtained from Auckland Council.​

Table 4.3: Design Standard ​

Function

Annual Exceedance Probability (AEP)

Primary systems

10% AEP

Secondary systems

1% AEP 


Development is generally not permitted in areas with no secondary flow path.

Secondary systems shall be designed to accommodate the 1% AEP design storm event assuming the conditions listed in Section 4.3.5.6.​

4.3.5.3. Hydraulic Design of Stormwater Systems

The primary piped system shall be designed to cater for the peak design flow, without surcharge, determined by the water surface profile throughout the piped system. Secondary stormwater systems shall be designed as open channel flow.

The hydraulic design of stormwater pipelines shall be based on either the Colebrook-White formula or the Manning formula. System capacity shall be determined from the Colebrook-White or Manning coefficient. The Colebrook-White and Manning formulae can be found in Metrication: Hydraulic data and formulae (Lamont, n.d.). The roughness coefficient used when determining the system capacity shall consider the aged condition of the new and the existing stormwater network. Manufacturers’ specifications shall also be referred to.

Examples of appropriate ranges for Manning’s roughness values for materials are provided in Table 4.4.​

Table 4.4: Manning’s Roughness Values for Closed Conduits and Overland Flow Path (Chow, 1959)​
 

Classification

Manning’s Roughness Values

Metal (coated, uncoated, galvanised)

0.010 – 0.017

Plastic

0.010 – 0.012

Corrugated metal

0.017 – 0.030

Concrete (finished)

0.011 – 0.014

Concrete (unfinished)

0.012 – 0.020

Overland flow paths along roadways

0.02 – 0.03

Overland flow paths through properties/parcels

0.10 – 0.20 


Any obstruction to flow in a pipeline, such as fish passage, will require specific design to the council’s approval.

Hydraulic design of precast concrete stormwater culverts shall be in accordance with CPAA Design Manual Hydraulics of Precast Concrete Conduits (Pipes and Box Culverts). For other materials use Guide to Road Design: Part 5B – Open Channels, Culverts and Floodways (Austroads, 2013). Also see Section 4.3.9.8.​

4.3.5.4. Energy Loss through Structures

Energy head loss (also known as head loss) in a pipeline or an access chamber typically consists of entrance, exit and bend losses.

Energy loss is expressed as velocity head:

Energy loss He = KV2/2g (m)

where:

K is the empirical entrance loss coefficient

V is velocity (m/s)

g is gravitational acceleration (m2/s).

The entrance loss coefficient table (Table 4) and energy loss coefficient graph (Figure 12) in New Zealand Building Code (NZBC) clause E1/VM1 (Compliance Document for New Zealand Building Code: Clause E1 Surface Water (Department of Building and Housing, 2011)) provide appropriate K values for flow through inlets and access chambers respectively. Figure 4.1 below shows coefficients for energy loss due to bends through manholes.

The exit loss coefficient has a range of values from 0 to 1. A free discharge exit has a K value of 0 while an exit submerged in a pond has a K value of 1.

When modelling catchpits, either of the following scenarios may be used:
  • Where catchpit grating losses are allowed for, water levels at design flow shall not exceed kerb level at catchpit positions.
  • Where catchpit grating losses are neglected, design water level shall not allow standing water above the catchpit grating.​

​​
4.3.5.5. Determination of Water Surface Profiles

Stormwater systems with a subcritical flow shall be designed by calculating backwater profiles along the pipe starting from an appropriate outfall water level. Computer modelling shall be utilised where the system is complex. On steep gradients with supercritical flows, both inlet control and hydraulic grade line analyses shall be used and the more severe relevant condition adopted for design purposes. At manholes and inlets, the water levels computed from the design flow shall be low enough to prevent overflow and to allow existing and future connections to function satisfactorily.​

Request shall be made to the council to provide outfall levels from the relevant hydraulic model when the discharge is to an existing stormwater network. If the discharge is to a river or other body of water, the council shall be consulted for relevant hydraulic model information; however, in the event that this information is unavailable, the outfall water level shall be determined using the tailwater depth calculation in E1/VM1 (Compliance Document for New Zealand Building Code: Clause E1 Surface Water (Department of Building and Housing, 2011)). For areas with tidal outfalls (including tidal rivers) designs will need careful consideration in terms of the nominated receiving body water level which will be used for the backwater curve calculations. Each situation will need to be individually assessed by the council.​

In principle, each step in the determination of a water surface profile involves calculating a water level upstream i.e. reduced level of water surface (h2) for a given value of discharge and a given start water surface level downstream (h1).​

This can be represented as:

h2 + V22/2 g = h1 + V12/2 g + Hf + He

where:

V is velocity (m/s)

g is gravitational acceleration (m2/s)

h1 is downstream water level

h2 is upstream water level

Hf is head loss (in metres) due to boundary resistance within the reach (for pipes, unit head loss is read from Manning’s flow charts, for example)

He is head loss (in metres) within the reach due to changes in cross section and alignment (see Figure 4.1 for loss coefficients for bends through manholes).​​

4.3.5.6. Secondary Flow Paths

Secondary overland flow paths shall be designed with sufficient capacity to accommodate the 1% AEP storm event assuming the following conditions:
  • For pipelines up to and including 600mm diameter, assume that the pipeline is 100% blocked.
  • For pipelines between 600mm and 1050mm diameter, assume that the pipeline’s capacity has been reduced by 50%.​
  • For pipelines in excess of 1050mm diameter, assume that the pipeline’s capacity has been reduced by 10%.
​The following matters need to be taken into account when considering the design of all secondary flow paths:

a) ​​Secondary flow paths shall not be obstructed in any way. Buildings or structures, including fences and retaining walls, shall not be built within a secondary flow path or form an obstruction to any part of a secondary flow path. This includes works which do not require a building consent.

b) Any property owner is legally required to accept stormwater runoff that would naturally flow onto their property. The plotted secondary flow path entry point on the upstream boundary and the exit point on the downstream boundary shall not be altered by site development. However, it may be possible to relocate the flow path entry and/or exit point by mutual agreement with neighbouring properties.

c) Where modification to a secondary flow path is required by a development, applicants shall submit a detailed design of the overland flow path when the catchment exceeds 4000m2 and for smaller catchments at the council’s discretion.

d) Where the road reserve is to be used as the secondary flow path, Auckland Transport shall be consulted at an early point in the design on a number of issues such as road and reserve width, road profile, lifelines access, and health and safety.

e) Where flow paths traverse pedestrian or vehicular accessways or public carriageways, the guidelines are that the expected flow has both:​
  • A maximum of 200mm depth
  • A maximum velocity for pedestrian safety of:
         o 0.6m/s where there is no obvious danger
         o 0.4m/s where there is obvious danger

f) Secondary systems shall be designed to avoid land instability and to reduce the significance of erosion during significant events. The design shall incorporate erosion protection measures as appropriate.

g) Careful management of stormwater on the site (for example landscaping, placement of driveways) can sometimes be used to convey secondary flows. All applications potentially affecting secondary flow shall include design details showing the layout of the secondary flow path across the site including flow depth and velocity at critical locations.

h) All modifications to secondary flow paths shall be documented on as-built plans.​

4.3.5.7. Freeboard

Floor level requirements in relation to floodplains are set through rules in the Proposed Auckland Unitary Plan (PAUP) and operative district plans. Refer directly to these plans to be certain of requirements. Examples of relevant permitted activity thresholds for floor levels are in Chapter H of the PAUP under Section 4.11 (natural hazards) and Section 4.12 (flooding). Where more than one requirement applies, the most stringent will be used.

The minimum freeboard for overland flow paths shall be as per Section 4.3.1 of E1/VM1 (Compliance Document for New Zealand Building Code: Clause E1 Surface Water (Department of Building and Housing, 2011)), using the flow generated by the 1% AEP flood event, except where the flow is in excess of 2m3/s. Table 4.5 shows freeboard requirements for different situations.

Table 4.5: Freeboard Requirements for the 1% AEP Event​

Freeboard

Minimum height

Vulnerable Activities*

500mm

Less Vulnerable Activities*

300mm

Overland flow paths where flow is less than 2m3/s

 500mm where surface water has a depth of 100mm or more and extends from the building directly to a road or car park, other than a car park for a single dwelling

150mm for all other cases

 

Overland flow paths, where flow is equal to or in excess of 2m3/s

 500mm for Vulnerable Activities*

300mm for Less Vulnerable Activities* 



* As defined in the PAUP​

​Alternative specific designs for freeboard within overland flow paths may be considered for approval at the council's discretion.

Freeboard shall be measured from the top water level to the finished floor level.

4.3.5.8. Coastal Areas

In coastal areas, design criteria shall be discussed with Auckland Council at an early stage. Coastal processes including storm surge, tsunami hazards, climate change, sea level rise and coastal vulnerability need to be taken into account in accordance with the following guidance documents:
  • Coastal Inundation by Storm Tides and Waves in the Auckland Region (Stephens et al., 2013)
  • Coastal Storm Tide Levels in the Auckland Region (Stephens et al, 2011)
  • Coastal Hazards and Climate Change: A Guidance Manual for Local Government in New Zealand (Ministry for the Environment, 2008)
  • Assessment of Potential Sea Levels Due to Storms and Climate Change Along Rodney’s East Coast (Rodney District Council, 2005)
  • North Shore City Sea Inundation Study (North Shore City Council, 2004)
  • Kaipara Harbour Hydrodynamic Modelling (DHI Water & Environment, 2006).
​The worst case from any of the above documents shall be considered, where relevant.​

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