Curb cuts

Overview

Curb cuts are a form of LID/BMP inlet. They include a variety of modified curbs and spillways.
They are well suited to retrofit scenarios and to collect runoff from catchments with relatively gentle longitudinal slope, and/or a greater cross slope. This might be the local topography of a parking lot or a piece of parkland.

As this inlet width is directly proportional to longitudinal slope; the required curb cut width increases rapidly on steeper roads.
Standard width (450 mm), as included in OPSD drawings should be compared to and modified for the flow requirements of the practice.

The OPSD collection of standard drawings for curb cuts include

Flow direction	From asphalt curb and gutter	From concrete curb and gutter	From either asphalt or concrete outlet
30 to 45 degree	605.020	605.010	605.040 Asphalt Spillways
90 degree	604.020	604.010	605.040 Asphalt Spillways

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 total width of 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 (typically between 0.015 and 0.04)

The value obtained for total inlet width (W_T) can then be divided by the chosen number of inlets to determine the required width of each individual curb cut inlet. Curb cut inlets draining roadways should be a minimum of 0.45 m in width and are typically no wider than 1.5 m. The chosen number of inlets may be adjusted up or down to ensure individual inlet widths are within this range.

Where the available space for curb cuts is constrained and a greater flow into the BMP is desired, the effective cross slope (S_x) may be increased by adding a depressed concrete apron.

Where the intention is to capture only a portion of flow from the design storm event, the proportion of flow entering the curb inlet under such conditions 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

Three (3) curb cut inlets of 1 m width each, representing a total curb cut width of 3 m, are proposed 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 cross slope ratio is 3% and the longitudinal slope of the road is 2%. The 1 in 25 year design storm produces a peak flow of 0.08 m³/s from the road drainage area.

The total width of curb cut inlets required 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 peak flow conditions during the design storm event would be: $R_{c}=1-\left(1-{\frac {3}{9.71}}\right)^{1.8}=0.48$

So only 48% of the 0.08 m³/s (i.e. 0.038 m³/s) would enter the bioretention cell through the inlets 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.

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Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

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

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

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 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.

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

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.