# Curb cuts

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Curb cuts are a form of LIDLow 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./BMPBest 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. inlet.
They are well suited to retrofit scenarios and to collect runoffThat potion of the water precipitated onto a catchment area, which 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. 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 asphaltA mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces. catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale. From concrete catchmentThe land draining to a single reference point (usually a structural BMP); similar to a subwatershed, but on a smaller scale.
30 - 45 deg 605.020 [1] 605.010 [1]
90 deg 604.020 [1] 604.010 [1]

## Sizing

To completely capture linear flow travelling along a gutter perpendicular to a curb inlet, the inlet must be of width[2]:$W_T=0.817Q^{0.42}S_{0}^{0.3}\left (\frac{1}{nS_{x}}\right)^{0.6}$

Where:

• WT 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
• Sx is the cross slope ratio (typically between 0.015 and 0.04)

Where 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:

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

Where the curb cut width is constrained and a greater flow into the BMPBest 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. is desired, the effective cross slope may be increased by adding a depressed apron.

## Example

A curb cut of 3 m is proposed as an inlet for an offlineRefers 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. bioretention cell receiving runoffThat potion of the water precipitated onto a catchment area, which 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. 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 1 in 25 year design storm produces a peak flow of 0.08 m3/s.

The width of inlet to capture 100% of this flow is:$W_T=0.817\times(0.08)^{0.42}\times(0.02)^{0.3}\left (\frac{1}{0.013\times0.03}\right)^{0.6}=9.71\ m$

The proportion of water entering the bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. 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 bioretentionA shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation. cell through the inlet as designed.