In many conventional urban developments, stormwater collects on impervious surfaces before 'running off' to kerbs, catchpits and pipes.
Based on international literature, a catchment containing 10-20% impervious surface will generally experience a two-fold increase in stormwater runoff volumes during a storm event; a 35-50% increase in impervious area will experience a three-fold increase in stormwater runoff; and a 75%+ area, a fivefold increase (Paul & Meyer, 2001).
Figure 7 is a hydrograph illustrating the discharge of stormwater quantity over time. The developed situation assumes stormwater runoff rapidly coalesces on impervious surfaces, realising a higher peak flow.
This increased runoff rapidly concentrates at the bottom of the catchment via piped systems, leading to significantly larger peak flows. Apart from immediate concerns from surface flooding, there can be serious consequences when these flows reach streams, wetlands or estuaries (receiving environments), where they can cause increased erosion, bank slumping and the subsequent deposition of transported sediments in low-energy downstream environments.
Rainfall infiltration is important to sustain vegetation and groundwater flows to stream environments during dry periods. Impervious surfaces and piped networks reduce the ability of rainfall to infiltrate to groundwater.
Stormwater ponds have traditionally been used to manage these increased flows. They capture and hold stormwater runoff at the bottom of a catchment and provide for a controlled discharge rate to the receiving environment. The hydrograph in Figure 8 illustrates a scenario where a pond is used at the bottom of the catchment, showing the same quantity of stormwater released over a longer time period.
Ponds generally require large areas of flat land that would otherwise be available for development or open space reserve. WSD looks at alternatives to ponds by directing runoff from impervious surfaces to many and dispersed stormwater devices or purpose-designed landscape areas such as vegetated swales, raingardens and pervious paving.