Curb cuts

From LID SWM Planning and Design Guide
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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[edit]

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To completely capture linear flow travelling along a gutter perpendicular to a curb inlet, the inlet must be of width::

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::

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.

The proportion of water entering the bioretention cell under these flow conditions would be::

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[edit]