WSUD in streetscapes – designing for flood

Many WSUD assets can be adapted to maximise stormwater retention and provide flood storage in the small to medium flood events.

This includes:

Sizing the WSUD asset to be 5% of the total catchment area
Typically WSUD assets are sized to be 1-2% of the total catchment area. Increasing the surface area of the WSUD asset to 5% is required to achieve optimal flood management benefits.

Maximising the extended detention to 300 mm where possible
The space between the filter media surface and the drainage overflow is referred to as the ‘extended detention’ area. Stormwater pools in this area as it infiltrates to the drainage below.

In areas with groundwater level concerns smaller extended detentions will be required to keep the system’s invert above the groundwater level

Combining streetscape systems to maximise flood volumes
The combination of WSUD tree pits and permeable pavement on structural soil retention trench is a great example of how multiple WSUD assets can be configured to maximise storage volumes.

No lining is applied between the interface between the tree pit and the unlined portion of the passive irrigation tree trench. This allows for tree roots to access retained water within the trench. Other sides and the base of the tree pit can be lined with liner or root barriers to protect nearby infrastructure.

Estimating flood storage capacity

Flood storage volumes of WSUD assets can be estimated using the following calculations.

Note – As submerged zones are not free draining, their volumes cannot be considered as providing any flood mitigation volume.

Side of road passive irrigation tree trenches
The amount of flood storage volume provided by a trench is:

(trench depth – submerged zone depth) × trench width
× trench length × void ratio of trench fill material

For example, with a configuration of a 600 mm deep trench with a 300 mm submerged zone that has a void ratio of 0.35, such an asset offers 0.105 m3 ((0.6-0.3)×1×0.35) of storage per square metre of trench.

WSUD tree pits
The amount of flood storage volume provided by the tree pit is:

((tree pit depth – submerged zone depth) × tree pit area × void ratio of filter media)
+ (extended detention depth × tree pit area)

For example, with a conservative configuration of a 600 mm deep tree pit with a 300 mm submerged zone, 50 mm of extended detention depth and a filter media void ratio of 0.35, such an asset offers 0.155 m3 (((0.6 – 0.3) × 0.35) + 0.05) of storage per square metre of trench.

Raingardens
The amount of flood storage volume provided by the raingardens is:

(filter media depth × raingarden filter area × void ratio of filter media)
+ (extended detention depth × raingarden filter area)

For example, with a filter media depth of 500 mm, 200 mm of extended detention depth and a filter media void ratio of 0.35, such an asset offers 0.375 m3 ((0.5 × 0.35) + 0.2) of storage per square metre of the raingardens filter area.

Access to downstream local drainage

Where access to downstream local drainage is not readily available, balance pipes, set at the desired submerged zone level, can connect raingardens to one another creating a system of raingardens until the downstream end of the system is able to connect to existing local drainage.