Curb Cuts to Bioretention

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    Chad Shaw

    Searching for feedback/examples/details/etc….Currently designing a bioswale between parking/drive lane w/ curb cuts allowing runoff to enter.  Anyone have any suggestions on spacing between cuts, dimensions of cuts, concerns regarding erosion at inlet, etc.?  Thanks

    Wes Arola, RLA

    You could go completely curbless and sheet flow into planting which will reduce the concern for erosion through certain strategically placed “inlets” unless you are trying to hold grade in some areas…Another interesting thing to think about it a plant list or materials as far as a detail for infiltration and treating the run off. Good luck! Interesting discussion

    Geoffrey Campbell

    State Highway has a detail for curb cuts, with a concrete flume -ish thing. But this, as said before, will concentrate the flow and then you have to look at putting in rip rap or some other energy dissipation device, which may not be attractive. A depressed curb, will allow sheet flow and still maintain edging for pavement. On parking spaces though you’ll need wheel stops. How do you plan to transition from pavement to biofiltration surface? 3:1 slope, mini-walls? That always seems to be the trickiest part because of the surface area you need vs. free-board and wall ht.? 3:1 slope is gonna eat up lots of real-estate that could have been used for surface infiltration while a curb wall could be a safety hazard and depending on how tall it is could require an ugly guard rail. The 3:1 slope however is good for pre-treatment if its’ turf or something comparable. Love to hear how it goes. Good luck.


    I know Portland, OR has installed several different types of curb cuts, both as a part of new construction and as retrofits. Someone there could be a great resource for dimensions, etc. I have attached a few photos of one of their projects. Good luck!

    Andrew Garulay, RLA

    Those are interesting inlets. Thanks for sharing the pictures.

    Chris Whitis

    Portland does a great job of creating individual cells that daisy chain together with each cell having an inlet and an outlet. Most of their examples are on pretty shallow grade, though and street in the St. John’s area outside Portland was a bit steeper and you can get a feel for the spacing of the inlets. The most typical inlet erosion protection we’ve seen has been large (2-3″ or larger) pebbles. We’ve tried a paver baffle that has the pavers raised at differing heights to try and break the flow as our MPW was not receptive to loose stone from a vandalism standpoint.

    Portland’s BES has a good management manual with curb inlet details. Check out Appendix G for the download. Haven’t looked into it to thoroughly to see if they’re recommending any minimum spacing requirements. Spacing would be dependent on how much area you’re trying to drain and I’d err on the side of more inlets than not.

    Portland BES Stormwater Manual

    Here are some additional images of various green streets via
    Green Streets

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