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. 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, rocky check dam.

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

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

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

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.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

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.

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.

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.