Source Documents
These are the council documents used to build the HDC calculator. If a link is broken, the document may have moved on the council website.
On-Site Detention Calculator v06 + Engineering Code of Practice (2011)
HDC Stormwater Information Page
Calculation Methodology
Design event: 50-year ARI (Annual Recurrence Interval)
Retention
None - detention only
Method
Rational Method with 5yr + 50yr storms (HIRDS v3). Detention triggered when post-dev equivalent C exceeds 0.5 (residential 5yr) or 0.6 (50yr). Storage routing across multiple durations.
Design Requirements
- 5-year and 50-year ARI design storms
- HIRDS v3 rainfall with 2.06C climate change allowance
- Max C value: 0.5 (residential 5yr), 0.6 (residential 50yr)
- Storage routing across 10min to 720min durations
- For sites generally less than 0.2ha and single lots
Submission
Last verified: March 2026 - NZ Detention Tank Calculator