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Flash flooding

  • 1.  Flash flooding

    Posted 07-24-2019 09:40 PM
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    Washington, DC area had very heavy rains and flash flooding recently.  The photo is of a nature trail in Accotink Bay Wildlife Refuge, where apparently water flowing across the trail floated the porous asphalt pavement.  The sections of displaced asphalt are still in good condition, flexible and with a cushioned walking surface.  I suppose now there should be a specification for anchoring porous asphalt pavement.

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    William Forbes MASCE, PE, ME, BCEE
    Senior Principal Engineer/Vice President of Engineering
    Forensic Analysis & Engineering Corporation
    Virginia Beach, Virginia
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  • 2.  RE: Flash flooding

    Posted 07-25-2019 08:12 AM
    This damage instance would not be specific to porous asphalt pavement, as I've previously observed asphalt roadway pavements, concrete sidewalks, and similarly thin surface features lifted (essentially due to the same process that provides lift across an aerofoil) and shifted downstream during flood events.  In areas of known regular surface flooding of pavements, this might be countered by a lip or curb along the upstream edge to interrupt the acceleration of flow across the flat smooth surface.  In one previous design instance, I specified a thickened pavement toe along that upstream edge; this might be somewhat successful at avoiding damage from the perspective of reinforcing the pavement structure, but (in hindsight) might not address the hydrodynamic lift that could occur.

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    David Thaemert Ph.D.,P.E.
    Senior Lecturer
    University of Hertfordshire
    Hatfield
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  • 3.  RE: Flash flooding

    Posted 07-26-2019 03:07 PM
    I believe to "anchor" the pavement, additional reinforcing in the pavement section is required or it would tear apart from the forces overtopping the pavement, making it cost prohibitive.  In my experience, where there are overtopping flows with high velocities, asphalt does not hold up.  Perhaps the use of a streambank reinforcement as a foundation (e.g. modular concrete blocks) would withstand the forces better and maintenance would be limited to the replacement of the sacrificial layer (e.g. aggregate, sand, etc.).  The structure should be able to remain as a LID BMP.

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    John Wood P.E.,M.ASCE
    Project Engineer
    Pittsburgh PA
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  • 4.  RE: Flash flooding

    Posted 07-28-2019 09:23 AM
    This may be an example of LID with no benefit, or maybe the only benefit being LEED points or the parkland equivalent.  Notice the large gravel beneath the walkway pavement - there is no benefit of porous asphalt on this narrow section when there is plenty of infiltration surface available beneath the asphalt and any water running off the asphalt would reach it easily.  The nearby traditional asphalt, in a thicker section with a sealer applied, was undamaged, although not in as apparent a flow path.

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    William Forbes MASCE, PE, ME, BCEE
    Senior Principal Engineer/Vice President of Engineering
    Forensic Analysis & Engineering Corporation
    Virginia Beach, Virginia
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  • 5.  RE: Flash flooding

    Posted 07-29-2019 10:15 AM
    Use pervious concrete with turn down edges.

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    Robert Hackman P.E.,M.ASCE
    Senior Principal
    Ellicott City MD
    (410)960-4822
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