What Constitutes an On-Site Stormwater Detention (OSD)?

Urban planning and development necessitate effective stormwater management to mitigate the adverse impacts of flooding, erosion, and water contamination. On-site stormwater detention (OSD) represents a pivotal technique in the management of stormwater runoff. This involves the temporary storage of rainwater and runoff on the premises where it originates, as opposed to immediate discharge into local drainage systems or neighbouring properties.


The Advantages of OSDs

OSDs offer a range of advantages, prominently including the reduction of peak flow rates for stormwater runoff. This reduction aids in the control of water movement, thereby minimising downstream flood risks, particularly in the context of severe storms. OSDs can adopt diverse configurations, either positioned above or below the ground. They can be seamlessly integrated into various structural elements like car parks, driveways, landscaped areas, subterranean tanks, and pits, contingent on the specific design and requirements of the site.

It is vital to distinguish rainwater tanks from OSDs, as rainwater tanks are primarily intended for the collection and storage of non-potable water for applications such as irrigation and toilet flushing.

Categories of Developments Mandating OSD Implementation

OSDs are typically obligatory for a variety of development categories, encompassing:

  1. Subdivisions
  2. Single dwellings, along with their expansions, extensions, and enhancements
  3. Townhouses, villas, duplexes, and home units
  4. Dual occupancies
  5. All commercial, industrial, and specialised use developments and structures
  6. Tennis courts
  7. Roads, parking areas, and other paved surfaces
  8. Public edifices

Situations Exempt from OSD Requirements

Instances exempt from OSD demands consist of:

  1. Developments leading to an increase in impervious site area of fewer than 100 square metres, with a single exception allowed per site.
  2. Development within the confines of the 20% Annual Exceedance Probability (AEP) flood extents.
  3. Subdivisions of existing dual occupancies that don’t escalate impervious area.
  4. Allotment boundary adjustments and consolidations that do not result in additional lots.
  5. Alterations of use that don’t amplify impervious area.
  6. Building additions or internal modifications within the existing dwelling’s present footprint.
  7. Novel developments within subdivisions where OSD provisions encompass the entire subdivision.
  8. Structures within non-urban or rural locales.
  9. Developments within OSD Concession Zones, where the runoff can drain to watercourses without affecting other properties.


In scenarios where an OSD isn’t integrated into a development application, the applicant must provide a written rationale. This justification should elucidate the reasons for not implementing an OSD, highlighting the infeasibility or lack of necessity for the particular project.

It’s crucial to recognise that these stipulations and guidelines can diverge based on local ordinances and development control strategies. Therefore, developers and applicants should actively engage with pertinent authorities and planning departments to ensure adherence to stormwater management protocols.

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