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

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

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 flow would enter the bioretenteion cell through the inlet as designed.

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.