Source Documents
These are the council documents used to build the WMDC calculator. If a link is broken, the document may have moved on the council website.
Engineering CoP Part 5 - Stormwater and Land Drainage (QP-C814, Issue 5, Feb 2025)
Calculation Methodology
Design event: 100-year (1% AEP)
Method: Multi-ARI Analysis (CoP Part 5)
Post-development peak flows must be less than pre-development for ALL ARI events (5yr, 10yr, 50yr, 100yr) across durations from 10 minutes to 72 hours. Uses HIRDS V4 RCP 8.5 (2081-2100) rainfall. Tanks are unlikely to be approved if an economic alternative exists.
100-year asset life for all stormwater infrastructure. 50-year warranty for pipes and fittings. Methodology per CCC WWDG Parts A and B. Stormwater Bylaw 2024.
Design Requirements
- Post-dev < pre-dev for 5yr, 10yr, 50yr, 100yr ARI
- Check all durations: 10min to 72 hours
- Rainfall: HIRDS V4 RCP 8.5 (2081-2100)
- Tanks unlikely approved if economic alternative exists
- 100-year asset life for all stormwater infrastructure
- 50-year warranty for pipes and fittings
- Soakage encouraged but cannot reduce system capacity
- Road ponding must not exceed 100mm
- Methodology per CCC WWDG Parts A and B
- ECan discharge consent may be required
- Consent notice on title for private soakpits
Submission
Required Documents
- Multi-ARI stormwater design (5/10/50/100yr)
- HIRDS V4 RCP 8.5 rainfall data
- Storage routing for all durations (10min-72hr)
- Geotechnical investigation (if soakage proposed)
- Engineering plans per CoP Part 5
- ECan resource consent (if required)
- Manufacturer's specifications
Contact
Last verified: April 2026 · NZ Detention Tank Calculator