Pretreatment features

From LID SWM Planning and Design Guide
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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.

For BMPs serving up to a two hectare catchment area, a significant forebay may not be required. However, for larger catchments or those with heavy sediment loading a properly sized forebay will help prevent clogging of the filter media and ease maintenance requirements of bioretention facilities.


1. The required volume for a forebay (Vf, m³) serving catchments up to five hectares may be calculated as:


  • Ac = The area of the catchment (m²),
  • R = The capture efficiency (default to 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 used to size small forebays [2].

2. a) The area of a forebay (Af) with 80% capture efficiency of particles ≥ 1 mm may be estimated[2] as:


  • Q = Design flow rate (m³/s),

2. b) To size a forebay for a maximum depth (df, m), where df must ≤0.3 m and between 0.1 - 0.2 m is recommended:

It is recommended that both sizing calculations be made and the forebay be designed to meet both targets. In some cases the additional storage required to meet both targets will reduce the expected frequency of maintenance. See below.

Example calculation[edit]

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 area required to keep the maximum head of water within the forebay to 0.15 m is calculated as:

The area required to settle the 1 mm particles is calculated as:

So to meet the target particle removal, the forebay will be 12 m² in area. This gives the storage volume of 1.8 m³, which can be returned to the initial equation to determine the minimum cleaning frequency as:


  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
  2. 2.0 2.1 Healthy Waterways Ltd. (2014). Bioretention Technical Design Guidelines. Retrieved from