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Sediment loading rates from impervious surfaces studied by STEP were between 0.3 - 0.6 m³/ha/yr <ref>Goncalves, C., & Van Seters, T. (2012). Characterization of Particle Size Distributions of Runoff from High Impervious Urban Catchments in the Greater Toronto Area. Retrieved from https://sustainabletechnologies.ca/app/uploads/2013/03/PSD-2012-final.pdf</ref>. In Brisbane a value of 0.6 m³/ha/yr is the default used to size small forebays <ref name ="HW">Healthy Waterways Ltd. (2014). Bioretention Technical Design Guidelines. Retrieved from https://hlw.org.au/u/lib/mob/20150715140823_de4e60ebc5526e263/wbd_2014_bioretentiontdg_mq_online.pdf</ref>.
Sediment loading rates from impervious surfaces studied by STEP were between 0.3 - 0.6 m³/ha/yr <ref>Goncalves, C., & Van Seters, T. (2012). Characterization of Particle Size Distributions of Runoff from High Impervious Urban Catchments in the Greater Toronto Area. Retrieved from https://sustainabletechnologies.ca/app/uploads/2013/03/PSD-2012-final.pdf</ref>. In Brisbane a value of 0.6 m³/ha/yr is the default used to size small forebays <ref name ="HW">Healthy Waterways Ltd. (2014). Bioretention Technical Design Guidelines. Retrieved from https://hlw.org.au/u/lib/mob/20150715140823_de4e60ebc5526e263/wbd_2014_bioretentiontdg_mq_online.pdf</ref>.
2. a) The area of a forebay (''A<sub>f</sub>'') with 80 % capture efficiency of particles ≥ 1 mm may be estimated<ref name="HW"/> as:
2. a) The area of a forebay (''A<sub>f</sub>'') with 80% capture efficiency of particles ≥ 1 mm may be estimated<ref name="HW"/> as:
:<math>A_{f}=120\times Q</math>
:<math>A_{f}=120\times Q</math>
===Example calculation===
===Example calculation===
A parking lot catchment of 1.7 Ha is being routed through a small forebay into a bioretention cell. The design flow rate is 0.02 m³/s. The system should be designed to require cleaning no more often than once per year.
A parking lot catchment of 1.7 ha is being routed through a small forebay into a bioretention cell. The design flow rate is 0.02 m³/s. The system should be designed to require cleaning no more often than once per year.
The volume is calculated as:
The volume is calculated as:
:<math>V_{f}=1.7\times 0.8\times 0.6\times 1=0.816\ m^{3}</math>
:<math>V_{f}=1.7\times 0.8\times 0.6\times 1=0.816\ m^{3}</math>