# Curb cuts

Curb cuts are just one form of BMP inlet. 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 increases rapidly on steeper roads.

## 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:

• 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 BMP is desired, the effective cross slope may be increased by adding a depressed apron. [itex]

## 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:$W_T=0.817\times(0.08)^{0.42}\times(0.02)^{0.3}\left (\frac{1}{0.013\times0.03}\right)^{0.6}=9.71\ m$

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.