As shown in Figure 8, even though ponds control the discharge rate, there is still an increased total volume of water entering the receiving environment over a longer time period. The total stormwater volume discharged from the pond is represented in a hydrograph as the area under the graph.
A larger total volume of stormwater effectively results in increased hydraulic 'work' on the receiving environment, especially when a large storm event leads to a prolonged peak flow period. In addition, where there are multiple ponds in a single catchment, the extended period of peak flows can overlap, leading to a greater overall peak flow in the receiving environment. This coincidence of prolonged peak flows can lead to flooding in the lower catchment and potentially extend the time that a stream is exposed to erosive flow. The hydrograph in Figure 9 illustrates this situation, known as superposition of peak flows.
Auckland Council has developed design requirements in the PAUP that require stormwater management devices to provide a specific detention and retention volume in some catchments. Detention reduces peak flows by capturing and attenuating stormwater volumes and controlling the discharge rate. Retention reduces flow volumes discharged directly to streams by capturing flows and allowing them to infiltrate to ground or to be re-used for non-potable supply. These detention and retention recommendations are based on 'continuous simulation' and ensure that devices are targeted at mitigating a range of storm intensities and volumes. For further information on the PAUP rules, please refer to Auckland Council Technical Report TR2013/035 Auckland Unitary Plan Stormwater Management Provisions: Technical Basis of Contaminant and Volume Management Requirements.
Climate change resilience is also considered. Details on climate change requirements for design are in the Auckland Council Stormwater Code of Practice (AC Stormwater CoP, 2013).
A WSD approach reduces the risks associated with storm variations and weather anomalies by limiting the generation of stormwater runoff at its source, and providing opportunities for stormwater retention in multiple smaller catchments. Ways to reduce stormwater runoff using WSD approaches include:
- Retaining mature (or planting new) tree canopy to intercept and transpire rainfall
- Protecting and remediating porous soils to improve groundwater infiltration
- Reducing impervious surfaces through site and infrastructure layout
- Retaining or reinstating natural drainage systems to slow and detain stormwater flows
- Increasing pervious surfaces through pervious paving, living roofs, etc.
- Disconnecting stormwater from piped networks and redirecting this runoff to landscaped areas and WSD devices (swales, raingardens, etc).
All of these approaches are discussed in this guideline document in Sections C to E, including the means to optimise their effectiveness through combined application.