Increases in stormwater runoff volumes associated with land development can put significant stressors on receiving environments. In order to avoid potential adverse effects, concept designs for a site should seek to direct stormwater flows through a series of mitigation practices, all of which contribute to the retention, attenuation or infiltration of a component of stormwater volume. This will prevent the rapid accumulation or superposition of peak flows at the bottom of the catchment. Refer to Section B2.0 for a discussion of peak flows.
A WSD approach increases the 'initial loss' of rainfall through a preference for pervious surfaces, restoration of forested conditions, and rehabilitation of soils in the catchment. Concept designs can also directly cater for stormwater volumes through directing runoff to landscape areas, raingardens or wetlands for retention or detention. For more information, please refer to Auckland Council Technical Report TR2013/035 Auckland Unitary Plan Stormwater Management Provisions: Technical Basis of Contaminant and Volume Management Requirements.
WSD approaches also influence the time of concentration of stormwater flows within a catchment. The primary factors affecting the time of concentration include the saturation of soils, and the length, slope and roughness of flow paths. Conventional developments often utilise kerb, gutter and reticulated systems, which rapidly concentrate flows to the point of discharge. WSD approaches promote dispersed flows across landscape areas or vegetated swales that are rough rather than smooth. This decreases velocities of stormwater flows in the catchment, slowing the time of concentration, and subsequently reducing the peak flow duration.