Source Documents
These are the council documents used to build the WHDC calculator. If a link is broken, the document may have moved on the council website.
Engineering Standards 2022 - Chapter 4: Stormwater and Drainage
Calculation Methodology
Design event: 10-year (10% AEP)
Method: Modified Rational Method (Chapter 4)
Post-development peak flow must be attenuated to 80% of pre-development (not 100%). Uses CN-derived C values (not E1). Climate change adjustment of +20% to all design rainfall. Rainwater tank attenuation credits available per Table 4-12.
Whangarei uses soil-type-specific C values (Table 4-3). Gravity systems only - pumped systems not acceptable. Orifice sizing per Auckland Council GD01.
Design Requirements
- Post-dev peak flow <= 80% of pre-dev (50% AEP + 20% AEP)
- 1% AEP attenuation where downstream flooding hazard identified
- +20% climate change adjustment to all design rainfall
- Modified Rational Method or TR-55 (catchments up to 8ha)
- CN-derived C values from Table 4-3 (NOT E1 values)
- Minimum pre-development Tc = 60 minutes
- System must operate by gravity (pumped not acceptable)
- Dispersed discharge to land: max 2 L/s/m
- Rainwater tank credits per Table 4-12 (roof portion only)
- Orifice sizing per Auckland Council GD01
- Soakage devices where infiltration > 150 mm/hr
Submission
Required Documents
- Engineering Design Approval (EDA) submission
- Pre/post development flow calculations (Modified Rational or TR-55)
- Detention sizing with storage routing
- Orifice sizing per GD01 methodology
- Climate change adjusted rainfall (+20%)
- Minimum floor levels (1% AEP + 500mm freeboard)
- Overland flow path analysis (1% AEP)
- Operation and maintenance manual
- Manufacturer's tank specifications
Contact
Last verified: April 2026 · NZ Detention Tank Calculator