Stormwater quantity effects can range from degraded habitat in streams due to increased velocities to nuisance flooding of property to life-threatening flooding. The hydrological management of stormwater needs to consider all these effects and the prevention or mitigation of these.
Key considerations for hydrological management are:
- Meeting regulatory requirements
- Understanding hydrological design objectives (e.g. stream channel erosion, building protection, high hazard zones, climate change adaptation)
- Hydrological modelling (different for small or large sites)
- Flow, volume and velocity mitigation (typically set by the relevant territorial authority)
- Detailed design of hydrological management devices
Links and examples
Meeting regulatory requirements
Key documents outlining regulatory requirements are:
- NZ Building Code, Clause E1 – Surface Water
- E1.3.1 Except as otherwise required under the Resource Management Act 1991 for the protection of other property, surface water, resulting from an event having a 10% probability of occurring annually and which is collected or concentrated by buildings or sitework, shall be disposed of in a way that avoids the likelihood of damage or nuisance to other property.
- E1.3.2 Surface water, resulting from an event having a 2% probability of occurring annually, shall not enter buildings.
- Regional Policy Statement – Chapter 11, Policy 11.3.2 – This sets out the requirement to avoid development in areas subject to inundation in a 0.5% AEP flood event.
- Each territorial authority has it’s own requirements within the District Plan which sets out requirements development must meet in each area:
Understanding hydrological design objectives
Hydrological design objectives vary according to the receiving environment and territorial authority requirements. Some points for consideration are:
- Stream channel erosion
- Building protection (refer to Meeting regulatory requirements)
- High hazard zones (refer to Meeting regulatory requirements)
- Climate change adaptation
Stream channel erosion
Stream channel erosion generally increases when impervious surfaces increase due to the increase in runoff volume and velocity. Generally devices which impound and slowly release water mitigate the effects of this better than devices with low volume and/or retention time.
Stormwater Management Devices in the Auckland Region (GD01) contains comments on which devices mitigate stream channel erosion.
NZTA’s Stormwater Treatment Standard for State Highway Infrastructure contains recommendations for stream erosion control (Section 6.2.4).
Hydrological modelling requirements are different depending on the size of the contributing area and the sensitivity of the receiving environment. Chapter 21 of the Waterways, Wetlands and Drainage Guide provides guidance on this modelling, suggesting that:
- For sites on non-hill catchments up to 15 hectares and hill catchments up to 5 hectares the Rational Method may be used (as outlined in the NZ Building Code Clause E1)
- For larger Christchurch sites, the method outlined in section 21.4 provides input into modelling packages
- Banks Peninsula sites can use the regional flood method outlined in section 21.5
Larger sites outside of Christchurch and Banks Peninsula, or where there is a particularly sensitive receiving environment will likely require the use of a modelling package to calculate the stormwater runoff.
Note that wherever possible do not use methodologies developed for localities outside of where the method is being applied. If it must be used then all hydrological input parameters must be adapted for local conditions.
Even small sites can have a negative effect on stormwater, and when this is combined with hundreds of other small sites the effect can be significant. It is therefore important to mitigate these effects to reduce erosion and flooding. Consideration should be given to:
- Source control (e.g. minimising impermeable surfaces such as using permeable paving)
- On-site management (providing mitigation at site such as through roof tanks with orifice outlets)
- Large devices (e.g. detention basins)
Christchurch City Council has a Small Site Stormwater Attenuation guide available online.
Detailed design for hydrological management
This is a specialised topic that is covered in design manual such as Christchurch City Council’s Waterways, Wetlands and Drainage Guide, NZTA’s Stormwater Treatment Standard for State Highway Infrastructure and Auckland Council’s Stormwater Management Devices in the Auckland Region.
Note that where standards are used from outside the region, then care must be taken in applying a particular design methodology as this will be tailored to meet the receiving environment objectives of a particular area. Therefore any hydrological management design methodology used must be checked (and adjusted) to ensure that it is applicable.