Part 4 – Stormwater Drainage
4.8 Design Requirements
4.8.1 Stormwater Design Criteria
4.8.1.1 Design Storms
All new stormwater systems shall be designed to comply with at least the AEP as set out in the following table for storm durations of 10 minutes and 24 hours.
| Function | AEP (%) | Equivalent ARI (years) |
| Primary Systems - | ||
| Rural | 20 | 5 |
| Residential and rural residential areas | 10 | 10 |
| Industrial areas | 10 | 10 |
| Business areas | 10 | 10 |
| All areas where no secondary flow path is available | 1 | 100 |
| Secondary Systems | 1 | 100 |
4.8.1.2 Time of Concentration
The time of concentration shall be determined as the “time of entry” plus the “time of flow” from the furthest part of the whole catchment to the point of discharge.
The minimum time of concentration to be used is 10 minutes.
Time of entry to the system shall be calculated from the Overland Flow Graph in Figure 4.2 or an equivalent published graph and the formula from which it was derived.
Time of flow can be calculated from the flow velocity in pipes and channels (note since this is not known initially, an iterative type solution is necessary with time of concentration recalculated from the catchment flow calculation).
4.8.1.3 Rainfall
Rainfall data is to be derived from the HIRDS data for the relevant area of the District and then adjusted as below to allow for climate change factors.
Note: Design rainfall events are shown in the MPDC Soakage Guidelines for the four (4) townships in the MPDC area.
Factors for Deriving Extreme Rainfall Information in Matamata-Piako District
| ARI (years) | |||||||
| Duration | 2 | 5 | 10 | 20 | 30 | 50 | 100 |
| < 10 mins | 16.8 | 16.8 | 16.8 | 16.8 | 16.8 | 16.8 | 16.8 |
| 10 minds | 16.8 | 16.8 | 16.8 | 16.8 | 16.8 | 16.8 | 16.8 |
| 30 mins | 15.5 | 15.5 | 16.0 | 16.4 | 16.8 | 16.8 | 16.8 |
| 1 hour | 14.1 | 14.9 | 15.5 | 16.2 | 16.8 | 16.8 | 16.8 |
| 2 hours | 13.0 | 14.0 | 15.1 | 16.0 | 16.8 | 16.8 | 16.8 |
| 3 hours | 12.4 | 13.7 | 14.7 | 15.8 | 16.8 | 16.8 | 16.8 |
| 6 hours | 11.1 | 12.8 | 14.3 | 15.5 | 16.8 | 16.8 | 16.8 |
| 12 hours | 10.1 | 12.2 | 13.7 | 15.3 | 16.8 | 16.8 | 16.8 |
| 24 hours | 9.0 | 11.3 | 13.2 | 15.1 | 16.8 | 16.8 | 16.8 |
| 48 hours | 8.0 | 10.5 | 12.8 | 14.9 | 16.4 | 16.8 | 16.8 |
| 72 hours | 7.4 | 10.1 | 12.4 | 14.7 | 16.2 | 16.8 | 16.8 |
4.8.1.4 Soakage Systems
All soakage systems shall be designed by a suitably qualified person.
Figure 4.2 (click to enlarge)
All new soakage systems shall be designed to comply with at least the 10% AEP (10 year ARI) 24 hour nested storm where a flow path already exists to accommodate the balance of a 1% AEP (100 year ARI) 24 hour nested storm. The impermeable area draining to a soakage device will be the maximum potential impermeable area for that device.
The system shall be designed to achieve a satisfactory solution recognising that the performance of soakage systems deteriorates over time. A performance deterioration factor of 50% must be allowed for and this is applied to the on-site soakage test results to achieve the “design soakage rate”.
The proposed soakage dimensions and associated area shall be based on the appropriate design AEP and the “design soakage rate”.
Note: The MPDC Soakage Guidelines provide guidelines to assess the design soakage rate and soakage dimensions which meets the requirements of the New Zealand Building Code section E1 and the Waikato Regional Council for individual lots. Council can also provide a “Residential Soakage Calculator”.
Figure 4.4 (click to enlarge)
Figure 4.5 (click to enlarge)
