Source Documents
These are the council documents used to build the SDC calculator. If a link is broken, the document may have moved on the council website.
Engineering Code of Practice Section 8 - Stormwater and Land Drainage
Calculation Methodology
Design event: 50-year (2% AEP)
Method: Total Detention (Storage + Overflow Soakage)
Total detention (storage plus overflow soakage) at 2% AEP (50-year) critical duration for ground disposal. Post-development flows must not exceed pre-development for 2yr, 10yr, and 50yr events. First flush depth 15-25mm.
Selwyn provides township-specific rainfall tables. Soakage may not be available at depths less than 5m. Stormwater and Drainage Bylaw 2025 applies.
Design Requirements
- 2% AEP (50-year) critical duration for ground disposal
- Post-dev <= pre-dev for 50%, 10%, 2% AEP events
- Total detention: storage + overflow soakage
- Min design rainfall intensity: 10mm/hr (flow-based)
- First flush: 15-25mm rainfall depth (Canterbury)
- Soakage may not be available at depths < 5m
- Use Selwyn township-specific rainfall tables
- Stormwater and Drainage Bylaw 2025
Submission
Required Documents
- Site-specific stormwater design per CoP Section 8
- Soakage/percolation test results
- Total detention sizing for 2% AEP critical duration
- First flush treatment design
- Township rainfall data from Selwyn tables
- Engineering plans and drawings
- ECan resource consent (if required)
Contact
Last verified: April 2026 · NZ Detention Tank Calculator