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LID SWM Planning and Design Guide β

Curb cuts

Revision as of 19:29, 31 October 2017 by Jenny Hill (talk | contribs)

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.


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


  • 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:\[R_c=1-\left ( 1-\frac{W}{W_T} \right )^{1.8}\]


  • 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. <math>


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.

Curb cuts gallery