Source Documents
These are the council documents used to build the HCC calculator. If a link is broken, the document may have moved on the council website.
Three Waters Practice Notes HCC05 and HCC06 (Rev 3, Sep 2016)
HCC Stormwater Information Page
Calculation Methodology
Design event: 10-year ARI (Annual Recurrence Interval)
Retention
(Ar + Ai) x 10 litres
Method
Zone-based discharge rate. Discharge at 80% of pre-dev 2yr ARI rate. Check 10, 20, 30, 60 min and 2hr durations, adopt worst case. Above or below ground tanks accepted.
Design Requirements
- Discharge at or below 80% of pre-development 2yr ARI rate
- Above or below ground tanks accepted
- Min orifice diameter: 10mm
- Pre-dev runoff coefficient C = 0.30
- HCC05: Only roof water to reuse tank
- HCC05: Lilac pipes for non-potable supply
Submission
Last verified: March 2026 - NZ Detention Tank Calculator