Stormwater management of urban catchments rely on effectively controlling the volume and flow of runoff. Greenfield and brownfield sites in areas where planners and developers are providing intensification often create more impervious area, meaning more runoff at a faster flow rates compared to pre-development scenarios. Stream habitats and existing infrastructure are sensitive to changes in both flow rates and total volumes: so flood management and catchment often call for detention and retention.

Understanding the nuances between detention and retention are key to successful stormwater design and meeting consent requirements throughout New Zealand.

Detention

Detention seeks to reduce the peak discharge flow rate through the temporary storage and controlled release of stormwater. Most councils require post-development flows to not exceed pre-development flows, with some consents requiring a reduction in runoff. Mitigating frequent storm events helps manage limited pipe capacity in downstream networks and provides stream bank erosion protection, while detention of larger storm events is useful for flood mitigation.

Detention structures will typically have an inlet, outlet with flow control (often an orifice), and an overflow. During a rainfall event, water builds up in the tank while the flow control releases the water slowly, and the overflow prevents flooding during a large storm and regulates the total storage volume. Requirements for detention volumes can be found in local stormwater management guidelines, APD can provide assistance if you need help for your local project.

Retention

Retention of stormwater reduces the total volume of runoff from a site. This can be achieved by storage for reuse, soakage or evapotranspiration: unlike detention, water entering a retention device will not leave via an outflow pipe. Retention helps manage demand on town water resources, and reduces the total volume of water discharged to sensitive aquatic environments.

Retention tanks will typically either allow water to infiltrate into the ground, or contain a pump to allow for non-potable reuse like toilets, garden irrigation or fire suppression. Retention can also be achieved using green assets like permeable paving, living (green) roofs and rain gardens.

Combining Detention and Retention

Some councils in New Zealand require a combination of retention and detention. Auckland’s Stormwater Management Area – Flow 1 and Flow 2 (SMAF) areas require a combination of retention and detention. For impervious areas where hydrological mitigation is required this typically this involves:

• Retention of at least 5mm runoff depth, and
• Detention and drain down period of 24 hours for the difference between the pre-development and post-development runoff volumes (minus retention volumes)

Stormwater storage tanks are a space and cost efficient way to meet detention and retention requirements on intensive urban developments. APD has a market-leading suite of stormwater management solutions to suit any site requirements.

• StormLite tanks can be installed under driveways, parking spaces and lawns
• StormSlab systems can be placed within the building footprint, under a deck or garden
• StormSlim above ground tanks can be used where there is limited depth to stormwater connections.

APD’s in-house design tools and technical support allow us to provide detention and retention within the same tank. We give accurate tank dimensions and pipe level information to specifiers, contractors and councils, adjusted for each tank. Ask APD how we can help meet the detention and retention needs for your next project.