Water quality management

Water quality management includes both source control (avoid) and runoff treatment (mitigate). It also includes the assessment of contaminant sources and modelling the loads which run off from these areas.

Water quality design criteria

There are two main water quality design criteria which are based on either flow or volume.

Water quality volume

Water quality volume is also referred to as the ‘first flush depth’. Designers need to check with the relevant TA to see what is applicable locally (for instance in Christchurch this is taken to be the runoff from impervious surfaces with a rainfall depth of 25mm). NZTA’s Stormwater Treatment Standard for State Highway Infrastructure has a map of the 90th percentile rainfall depths across New Zealand which can be used to calculate the water quality volume in the absence of local guidance.
Rain gardens are a partial exception to the volume rule, as these are also heavily dependent on taking into account the infiltration component to reduce the footprint. As a result, Christchurch City Council has accepted that the rainfall depth used to size rain gardens can be reduced by 5mm to 20mm.

Water quality flow rate

Some devices, such as proprietary filtration, require a rainfall intensity to generate a flow rather than a volume. Christchurch City Council analysed the historical rainfall record and determined that for the flat areas of Christchurch a rainfall intensity of 5 mm/hr would result in the capture of >80% of all rainfall events. A design rainfall intensity of 5 mm/hr is likely to be suitable for most areas of Canterbury where the 90th percentile rainfall depth is less than 25mm.

Water quality modelling

Contaminant loads and removal efficiencies

WWDG provides contaminant loads and removal efficiencies for urban areas and for a range of devices. These are considered the de facto standard across Canterbury, except where either the contaminant loads or device configuration falls outside of what is provided in WWDG.
Note that the maximum removal efficiencies should only be used where the device is configured in precise accordance with WWDG. A sensitivity test should be undertaken to determine the effect of using the lower bound of the removal efficiencies.
Likewise contaminant loads should assume the worst case, unless it can be demonstrated that using the lower bound is acceptable.

Continuous simulation modelling

For complex situations, such as unusual contaminant characteristics or for devices outside of standard configuration, consideration should be given to continuous simulation modelling. This allows a more site and device specific analysis.

Source control

Source control is the preferred means of managing runoff quantity and quality as it prevents runoff and contaminant generation at the source. This is principally achieved by specifying materials which have low contaminant generation. Building source control into the design is generally the most cost effective means of mitigating the adverse effects of urbanisation on water quality.

Examples of source control are:

Requiring coated metals for all buildings to reduce zinc loads.
Reducing impermeable hardstand areas (e.g. through the use of permeable paving) to reduce peak flows.

Risk Radar – Managing stormwater and trade wastewater

Treatment device detailed design

This is a specialised topic that is covered in design manuals 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.

In addition, there is an additional guide on rain gardens (and other bioretention devices) suitable for use in Canterbury that has not yet been incorporated into WWDG, the Rain Garden Design, Construction and Maintenance Manual.