Pretreatment features

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 also spread the flow somewhat, 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 more like a soft edged check dam.

Design

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

$V_{f}=A_{c}\times R\times L_{o}\times F_{c}$

Where:

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

2. The area of the forebay (A<subf) designed for 80 % capture efficiency (i.e. R = 0.8) may be calculated as:

$A_{f}={\frac {12\times Q}{V_{f}}}$

Where:

• Q = Design flow rate (m³/s), and
• V_f = Volume of forebay (m³).

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

$d_{f}={\frac {V_{f}}{A_{f}}}$

Gallery

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  Solid splash pad preventing erosion from the flow from the inlet. Image credit Dylan Passmore

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  Forebay connected to drainage area in the roadway with a curb cut, overflow visible in the centre of the feature, level spreading is encouraged with the rock check dams, Milwaukee, WI, Photo credit: Aaron Volkening

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  Rounded 'river rock' and a series of check dams slow water from this inlet.

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  This forebay has a rock splash pad to slow water down before it reaches a bioswale.

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  Incoming water from these curb cuts is slowed by river rock splash pads.

A pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

The land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

A pretreatment basin at the inlet of a practice that allow settling out of sediment and associated contaminants suspended in urban runoff.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.