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Concentrated flow inlets are associated with LID practices such as Bioretention, Stormwater planters, Infiltration trenches and chambers. Sheet flow alternatives include level spreaders, gravel diaphragms and vegetated filter strips. Practices such as permeable pavements and green roofs receive precipitation directly, whilst exfiltration trenches are connected directly to conventional storm sewers.

Inlets for BMPs in the right of way should be located:

  • At all sag points in the gutter grade
  • Immediately upgrade of median breaks, crosswalks, and street intersections.

It is recommended to include multiple inlets, sized to distribute inflow along the length of the practice or between multiple facilities, where feasible, rather than concentrating all inflow into a single location. (Offline overflow).

Trench drains[edit]

Trench drains are long, covered channels that collect and direct water into the BMP. They are an excellent solution for streets where walking across the entire surface is to be encouraged. They can be designed as detectable edges or part of a detectable edge, and may be used to help define curbless or 'complete streets'.

Trenches may either be shallow (where runoff volume is less of an issue) or deep and covered by a metal grate. Deeper trench drains may gather sediment and require frequent maintenance.

Drains may be configured either perpendicular or parallel to the flow direction of the roadway, collecting runoff and directing to a single inlet in the BMP.

Curb cuts[edit]

Curb cuts are breaks along the length of a curb system to allow water to flow into a LID/BMP.

Inlet aprons or depressions increase inflow effectiveness of curb cuts. Steeply angled aprons can be hazardous, especially to people bicycling. Curbside and protected bike lanes along concrete aprons should be at least 1.8 m to give cyclists adequate clear width from the curb and any pavement seams. Aprons can also be marked visually to indicate their perimeter. For aprons into bioretention, the curb may angle into the cell to improve conveyance of gutter flow into the facility. Aprons typically drop 50 mm into the bioretention cell, with another 50 mm drop behind the curb to maintain inflow as debris collects. A depressed concrete apron can be cast in place or retrofitted in by grinding down the existing concrete pavement.

Where the curb alignment along the street is straight, the curb opening may optionally have a bar across the top of the inlet.

Inlet sumps[edit]

An inlet sump is recommended to settle and separate sediments from runoff where a large amount of debris is expected. Water drains into a catch basin, where debris settles in its sump. After pretreatment, water drains via a pipe or opening into the BMP. The sump can be directly connected to a perforated underdrain pipe to distribute the flow to the bioretention, supported soil cells or underground practices such as trenches or chambers.

Sump inlets should not be sited where pedestrians will have to negotiate with them.

Depressed drains[edit]

Runoff in the gutter drops into a grate-covered drain before flowing into the BMP. Drain covers must be compatible with bicycling and walking; grid covers are preferred. Depressed drains are a potential solution for bioretention cells on sloped streets where directing runoff into the cell is a challenge.

This style of inlet can be combined with a curb cut, to maintain capacity in case debris clogs the grate. Depressed drains: Gallery

External links[edit]