Flood Mitigation and WSUD Solutions

It’s important to consider designs for flood mitigation on new and exiting properties and the application of Water Sensitive Urban Design (WSUD) solutions. In recent times many new houses and developments will have onsite detention systems constructed, such as OSDs.

These are designed as large concrete basins usually underground, that capture stormwater runoff from a residential site and holds it for a little longer to reduce the impact of downstream flooding. The stored water runoff is released at a slower rate through a small opening (orifice plate) near the base of the tank which is discharged to the council curb and the larger, public waterways. learn more about Onsite Stormwater Detention and its associated requirements in our other article here.

Onsite stormwater detention tank (OSD) water sensitive urban design WSUD flood mitigation system

Simply put, OSD facilities capture stormwater runoff to release at a slower rate to avoid flooding public waterways. They have a trash screen to collect large debris and require periodic maintenance.

Consider Water Sensitive Urban Design (WSUD)

WSUD is viable on all sites, although, the degree of application may vary depending on the site’s constraints. Every site should be able to maximise permeable surfaces such as vegetated areas like garden beds, lawns, and porous surfaces and paving (such as gravel). If you’re not at all familiar with WSUD check out our article, what is Water Sensitive Urban Design?

Before constructing subsurface systems on your sites such as soak wells, agricultural pipes, and infiltration trenches, considers the following matters:

Type of Soil

  • Identify the type of soil on your site, as this effects the efficiency of certain WSUD devices. Sandy soil is great for infiltration, whereas clay soil can get waterlogged. WSUD in heavy clay soils may need to be supplemented with traditional conveyancing methods.

Soil Depth

  • Make sure the soil has a sufficient depth. Shallow areas underlain by impervious material such as granite, shale or limestone may block infiltration and require stormwater pipes to remove water for discharge off-site.


  • Determine the depth of groundwater. A high groundwater table may cause infiltration methods to be less effective during rain events.


  • Slope is always something to consider when ensuring your stormwater design is accounting for the terrain. Large slopes increase the speed and velocity of water runoff. Councils will commonly require that runoff is discharged to council water ways via gravity and without the use of mechanical pump systems. As such, the stormwater catchments must be sloping from a higher point and flow downstream off the site.


  • Always check with your local council before employing WSUD solutions. Certain components may conflict with drainage regulations.

If you would like a comprehensive inspection and report of your new or existing site, advising on your council’s regulations and what WSUD solutions suit your property, contact Stormwater Sydney via this form.

Stormwater Sydney provides Inspection, Cleaning, Maintenance, Repairs, Construction, and Advisory on stormwater systems across Sydney, ensuring compliance with Sydney standards and Council regulations.

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