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  • 1.  Houston's New Floodplain Regulation

    Posted 04-13-2018 12:26 PM
    As a lot of you are probably aware the Houston Department of Public Works approved new building regulations that all new structures in Houston are to be built two feet above the 500-year floodplain. What frequency does this make their regulations considering the 2 additional feet?

    Alan Shaw

    Alan Shaw P.E., M.ASCE
    Belgrade MT

  • 2.  RE: Houston's New Floodplain Regulation

    Posted 04-14-2018 01:56 PM
    Edited by Morgan Byars 04-15-2018 10:47 AM
    The frequency is only associated with the 500-year event (plus the 2 ft of freeboard) and nothing else.  Are you trying to ask what the recurrence interval would be for a flood elevation that is 2 feet higher than the 500 year?  That is completely location/topography dependent and not terribly relevant.  Regardless it depends on the floodplain relief where wide flat areas would require much greater increases in rainfall (less frequent storm) to result in the same increase in flood depth as compared to a more confined valley.  They implemented these rules advised by the new NOAA Atlas 14 statistics where it just so happens the updated 100-year is close to the existing 500 year.  Using a standard freeboard height is common in design and the amount varies across applications, but it's not as common to associate another storm event with the top of freeboard.  I'm not sure how terribly useful that would be and in addition once you get beyond the 500 year (.002 probability) the statistics become highly volatile and the values less reliable.

    Morgan Byars P.E., M.ASCE
    Consulting Engineer
    Buda TX

  • 3.  RE: Houston's New Floodplain Regulation

    Posted 04-14-2018 08:47 PM
    The reason Houston adopted the new levels, as did Harris County is that NOAA is in the process of finalizing their new rainfall estimates for Texas.

    The old rainfall estimates for Houston for the 24-hour event are 100-year are 12.4-13.5 inches.  The old 500-year estimates are 17.7 to 19.3 inches)

    NOAA is suggesting that the new 24-hour 100-year rainfall event is in the range of 18.5 inches.

    This means that the current 500-year water surfaces, will be in a few years (maybe 4)  approximately the regulatory 100-year water surfaces.  Therefore the new regulation means Houston is a little forward looking, and is currently regulating to the future 100-year event in response to the better data provided by NOAA.  So, Houston is regulating to the future 100-year water surface plus 2 feet of freeboard. 

    I think the math will not totally support the assumption that 18.5 inches using 2001 channel and detention configuration will equal 18.5 inches using 2018 channel and detention configuration, but it will be closer to the future modeling answers  than assuming 13.5 inches using the 2001 channel and detention configuration will equal 18.5 inches of rainfall using the 2018 channel and detention configuration.

    It is politically much easier to increase design criteria after a big event than it is to increase the criteria in the middle of dry period.  The Mayor, and County Judge are both making the assumption that the new water surfaces are increasing as a result of the future 100-year rainfall event data when they get remapped.   And we almost always get remapped after a big storm because mapping money generally comes after a storm.  Its not a bad bet since the Mayor is probably still going to be here when the new maps start getting published in 2020 or so.  Its easier to decrease to the newly flood elevation that is less than we are currently regulating to than it is to increase it during a big dry-snap.

    Dwayne Culp, Ph.D., P.E., P.Eng, M.ASCE
    Culp Engineering, LLC
    Richmond TX

  • 4.  RE: Houston's New Floodplain Regulation

    Posted 04-14-2018 01:56 PM
    The additional two feet does not provide any additional flood frequency capacity. Rather, the regulation protects structures against the 500-year flood with a 2-foot safety factor to account for uncertainties in modeling and debris accumulation. It's like requiring freeboard on a reservoir or levee, it's there for safety and not considered unused capacity. It's encouraging to see this requirement because in my experience water resources engineers often do not apply intentional safety factors to their work unless it's required in regulations, and then we're surprised when our projects don't work as designed.

    John Winchester P.E., M.ASCE
    Boulder CO

  • 5.  RE: Houston's New Floodplain Regulation

    Posted 04-14-2018 01:58 PM
    That stated policy provides 500-year protection with 2 feet of freeboard not a greater level of protection.  When considering wind waves, flowing flow surface turbulence waves, floating debris, and flood plain & channel obstructions 2 feet of freeboard makes sense as a minimum.  I would not assign a higher level of protection for the extra 2 feet.  Just as you don't  give credit for protection at the tippy top of a levee or trapezoidal storm water channel.  I assume that you are referring to riverine flooding only and not coastal - gulf flooding is another risk factor completely.  And there is probably a combined risk issue of coastal storm surge creating a backwater on river and bayou flows where combined frequency analysis is needed in this flat terrain.  Lastly it would be wide to consider a confidence factor related to years of flow records - a shorter record means lower confidence which prompts a family of 500-year flow rates each with a different confidence level.

    Andrew Sienkiewich P.E., M.ASCE
    Long Beach CA

  • 6.  RE: Houston's New Floodplain Regulation

    Posted 04-19-2018 01:43 PM
    ​With 500-yr rainfall estimates easily having +/-25% uncertainty, estimates of 500-yr flood levels can often be even less reliable.  However, some floodplains have features--like the rim of a bucket--which can mean that there are small flood elevation differences for significant increases in extreme flood flow.  So it pays not to overgeneralize!  Unfortunately many communities--Houston included--don't maintain rigorous, updated analyses of Full Spectrum flood hazard. Even more problematic, they don't facilitate property-specific "Real Flood Risk" information for homeowners/businesses, and regional-level information for agencies to optimize flood hazard mitigation measures.  Real Flood Risk is simply the expected average annual cost of flooding (including direct damages and indirect costs) OR equivalent present value, based on the Full-Spectrum hazard and structure depth-damage curve.  Ultimately with flood protection--just like fire protection--agencies need to pursue measures that are justifiable in terms of reducing this "actuarial" cost of flooding.  Our profession will see dramatic changes in the coming decade as we move from an obsolete "NFIP" framework for flood risk (the "false binary" of using some single magic threshold") to the "actuarial" Real Flood Risk framework that has long benefited risk decisions for fire and wind hazards. (Our rapidly evolving state of the art in analyzing flood hazard and risk is playing a big part in driving this change.)   On that note, in some areas requiring elevation 2 ft above the estimated 500-yr flood might make no "actuarial sense."  In the future, as more and more property owners are required by lenders to carry flood insurance--the way they require fire insurance--then actuarially-based flood mitigation decisions will become the norm--just as with building sprinkler systems and neighborhood fire hydrants.  In fact the re-insurance industry will take the place of the NFIP in providing mitigation guidelines, including guidelines for minimizing adverse impact.  The changes will be accelerated as the bond market (see Moody's recent announcement) increasingly looks at how well communities manage their Real Flood Risk.

    Robert Jacobsen P.E., M.ASCE
    Baton Rouge LA