Book 2 | Planning and Management - WSUD

Water Sensitive Urban Design
Figure 3b - Example of bioretention raingarden
3.1.4 Sizing bioretention systems
Figure 5 presents sizing curves for
bioretention systems in Eastern and
Western Sydney, showing typical
performance for different catchment
impervious fractions including 40, 60, 80
and 100%. Different curves are provided
for Eastern and Western Sydney to
accommodate the influence of different
rainfall patterns and volume on treatment
performance. The sizing curves are
presented in terms of the bioretention
filter area as a percentage of the total
catchment area.
The curves assume that the bioretention
system included 0.2m extended
detention, 0.5m filter depth, and a
sandy loam filter material, with
a median particle size of 0.5mm
and hydraulic conductivity of
100mm/hr. It can be seen in Figure 5
that in Western Sydney a bioretention
system to treat a 40% impervious
catchment can be sized at approximately
1% of the catchment, whereas this
increases to 1.5% in Eastern Sydney.
For catchments with higher impervious
fractions, a larger treatment area is
required; at 100% impervious fraction,
1.5% of the catchment is required in
western Sydney and 2.1% in eastern
Sydney.
Other than the area, key aspects of
bioretention system design that have a
strong influence on pollutant removal
performance include the extended
detention depth, filter media depth
and type of filter material. Bioretention
performance improves with increasing
depths of extended detention and the
filter media, however deeper systems may
not be feasible on all sites.
16 Book 2 | planning and management