Hurricane Florence

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  • 1.  Update

    Posted 09-15-2018 22:29
    I live in Holly Springs which can be regarded as a satellite city to Rally. We began feeling the  effects of the hurricane late Thursday Sep 13. Friday and today Saturday, all what we have is continuous moderate rain and wind.

    My main concern is the devastation left at the coastal cities, especially flooding that covered streets and inside houses. I really wonder to which criterion of water quantity were the storm drainage system designed? and what were the provisions for capacity improvement? It would be ironic and illogical to have Texas flooded by Harvey, which literally means the insufficiency of the existing storm drainage system.

    Ala Al-Kazzaz C.Eng, M.ASCE
    Procurement Consultant
    World Bank
    Holly Springs NC

  • 2.  RE: Update

    Posted 09-16-2018 17:04
    The devastation along the coast during the early part of the storm was due mainly to storm surge and wind driven tides. With the subsequent heavy rain, there will be significant damage along riverine systems that flood due to upstream rainfall. The design criteria for local drainage systems immediately along the coast are not significant causes of damage when compared to the other factors, but will be a factor in nuisance flooding. There is certainly a need for discussion of what design criteria are appropriate with the extreme rainfall events that seem almost commonplace now.

    Larry Sneeden P.E., M.ASCE
    Coastal Stormwater Services, Inc
    Wilmington NC
    (910) 799-9822

  • 3.  RE: Update

    Posted 09-24-2018 19:13
    To start a discussion about appropriate design criteria, I can only convey the security level of German protection systems. Flood protection is usually realized with levees. The height of it depends on the security level of the objects to be protected.
    Residential buildings usually have a security level with an annuity of 100 years. The levees are built at minimum 1m higher for wave height and others.
    The protection level is determined with historical gage values (sometimes water height and sometimes runoff). This can be problematic when the climate changes rapidly and many extreme events occur in a row. We had at the river Elbe between 2002 an 2013 five major floods with an annuality above 50 years. So the secutity level rose for about 2 feet.
    Germany is spending now billions to secure these levees.

    I calculated the annuity for the Lumber River gage in Lumberton NC with the German estimation method. Until 2015 the 100y-flood level was at 7,600 ft3/s. 2016 hit Hurricane Matthew the region and caused a flood with 14,600 ft3/s runoff (>1,000y). With the Hurricane Matthew and Florence event in the statistic, the 100y-flood is now at 17,100 ft3/s. This means that the levee height has to be increased 3.5 ft now to keep the residents safe.

    To lower the impacts, there are some single pilot projects in Europe. In the Rhine valley they lowering the embankments between levee and river. I dont know if it was done yet. Someone suggested to pave the embankment the acclerate the water in case of flood. We are actually involved in relocation projects to lower the flood level. In any case it's all very expensive.

    Steffen Krei S.E., M.ASCE
    Dipl. Ing. (FH)