Pretreatment features

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Without a forebay (top) the flow is concentrated around the inlet, potentially causing erosion and not optimally spread for infiltration; A three sided forebay (centre) with a level spreader on all sides will distribute the water and reduce the energy, sediment will collect in the pad of the forebay (shown darker); In a narrow cell, the forebay may extend across the whole facility (bottom).

Forebays are a form of pretreatment for open inlets such as curb cuts. Energy of the incoming flow is dissipated, causing suspended particles to drop out of the water. These accumulated particles/sediment can then be easily swept or vacuumed during routine maintenance and doesn't end up clogging downstream filter media or material. A well designed forebay will distribute the flow, reducing erosion around the inlet. One effective way of achieving this is by surrounding the pad with some form of level spreader on all sides. The level spreading could be a sharp crested weir in metal or concrete, or be soft edged with irregular landscaping stone.

Design[edit]

1. The required volume for a small forebay, serving up to 2 Ha (Vf, m³), may be calculated as:

Where:

  • Ac = The area of the catchment (m²),
  • R = The capture efficiency (suggest 0.8),
  • Lo = The sediment loading rate (m³/Ha/yr), and
  • Fc = The cleanout frequency (yrs)

Sediment loading rates from impervious surfaces studied by STEP were between 0.3 - 0.6 m³/ha/yr [1]. In Brisbane a value of 0.6 m³/ha/yr is the default value used to size small forebays <refname ="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. The area of the forebay (Af) designed for 80 % capture efficiency (i.e. R = 0.8) may be estimated<refname="HW"/> as:

Where:

  • Q = Design flow rate (m³/s), and

3. Check that the forebay depth (df, m) ≤ 0.3 m:

Example calculation[edit]

Gallery[edit]

  1. 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