Curb cuts

Sizing

To completely capture linear flow travelling along a gutter perpendicular to a curb inlet, the inlet must be of width:: $W_{T}=0.817Q^{0.42}S_{0}^{0.3}\left({\frac {1}{nS_{x}}}\right)^{0.6}$

Where:

• W_T is the width of the inlet for complete capture (m),
• Q is the design flow perpendicular to the inlet (m³/s)
• S₀ is the longitudinal slope ratio
• n is Manning's 'n' (between 0.012 and 0.016 for concrete, depending on surface treatment), and
• S_x is the cross slope ratio

Where the inlet width is constrained or the intention is to capture only a proportion of the flow, the ratio of flow entering the curb inlet may be calculated:: $R_{c}=1-\left(1-{\frac {W}{W_{T}}}\right)^{1.8}$

Where:

• R_c is the proportion of flow entering the curb cut, and
• W is the available curb cut width (m)

Example

A curb cut of 3 m is proposed as an inlet for an offline bioretention cell receiving runoff from an adjacent roadway. The gutter and the curb are made from smooth concrete with Manning's 'n' = 0.013. The x-slope is 3% and the longitudinal slope of the road is 2%. The design storm produces flow of 0.08 m³/s.

The width of inlet to capture 100% of this flow is:: $W_{T}=0.817\times (0.08)^{0.42}\times (0.02)^{0.3}\left({\frac {1}{0.013\times 0.03}}\right)^{0.6}=9.71\ m$

The proportion of water entering the bioretention cell under these flow conditions would be:: $R_{c}=1-\left(1-{\frac {3}{9.71}}\right)^{1.8}=0.48$

48% of the 0.08 m³/s (i.e. 0.038 m³/s) would enter the bioretenteion cell through the inlet as designed.

Curb cuts gallery

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  OPSD 605.040 Asphalt Spillway inlet to biofilter swale and road catch basin overflow outlet. County Court Blvd., Brampton, ON.

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  This curb cut has been sawn into existing concrete as part of a retrofit. Note the temporary (erosion log) and permanent stone erosion control measures in place. Mississauga Road, ON.

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  Curb cut used as a controlled overflow route from permeable pavements to a bioretention facility with monitoring well, Lake Simcoe Region Conservation Authority, Newmarket, ON.

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  Curb cut into a bioretention facility in Hinsdale, IL. Stone in the center of the facility reduces erosion and dissipates power inflow around the inlet area. A monitoring/maintenance well can be seen in the foreground. Photo credit: CNT

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  Curb cut into a bioretention facility in Brown Deer, WI. Stone is used to reduce erosion around the inlet area. Photo credit: Aaron Volkening

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  Curb cut into a bioretention facility in Ajax, ON.

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  Stone lined inlet at IMAX site in Mississauga

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  The grading around this inlet prevents flow in the correct direction. i.e. from the pavement onto the grass. Not too critical in this example, as the surface is permeable pavements.

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  Curb cut into a rain garden on a green street in Newmarket, ON.

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  Curb cut leading into a small bioretention cell in Brampton, ON.

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  Curb cut leading to a bioswale in Brampton, ON.

Refers to a system that when full, stormwater will bypass the practice. Offline systems use flow splitters or bypass channels that only allow the water quality volume to enter the facility. This may be achieved with a pipe, weir, or curb opening sized for the target flow, but in conjunction, create a bypass channel so that higher flows do not pass over the surface of the filter bed.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

A bioretention BMP featuring an impermeable liner and underdrain that prevents infiltration of runoff into the underlying native soil; provides sedimentation and filtration of urban runoff as it passes through the mulch layer, engineered filter media and vegetation root zone.

A bioretention cell that features an impermeable liner that collects and treats stormwater through sedimentation and filtration only (i.e., no infiltration).

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

Includes the protection of soil from dislocation by water, wind or other agents.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.