Source Documents
These are the council documents used to build the TCC calculator. If a link is broken, the document may have moved on the council website.
DS-5 Design Standard for Stormwater (Jan 2026) + SMG-7 Hydrological Design (2012)
Calculation Methodology
Design event: 50-year (2% AEP)
Method: Site-Specific Design (DS-5)
Tauranga requires site-specific design per DS-5. No simplified acceptable solutions. Storage volume calculated as V = 1.5 x Q x D. Primary piped system designed for 10% AEP, overall system for 2% AEP (50-year). Water quality volume based on 90th percentile rainfall (33mm).
Pyes Pa West over-mitigation zone: 50yr peaks must be reduced to 50% of pre-dev, 2yr peaks to 30% of pre-dev.
Design Requirements
- Primary piped system: min 10% AEP (10-year)
- Overall system: 2% AEP (50-year) incl secondary flow paths
- Overall system with CC: 1% AEP (100-year)
- Storage volume: V = 1.5 x Q x D
- Stream erosion control when imperviousness > 3%
- Extended detention: 24hr release of 2yr 1hr storm volume
- Unstable streams: multiply 2yr volume by 1.2
- Water quality: WQV = 0.033 x Awq (90th%ile = 33mm)
- Max velocity: 1.5 m/s at 10% AEP (unless erosion protection)
- Freeboard: 500mm habitable, 200mm garages, 300mm commercial
- Pipe economic life: 100 years
Submission
Required Documents
- Site-specific stormwater design report per DS-5
- Pre/post development flow calculations
- Storage routing: V = 1.5 x Q x D
- WQV and EDV sizing
- Orifice sizing and outlet design
- Water quality treatment design
- Engineering plans and drawings
- Resource consent application (BoPRC if required)
- Manufacturer's tank specifications
Contact
Last verified: April 2026 · NZ Detention Tank Calculator