Hurricane Florence

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Flood frequency

  • 1.  Flood frequency

    Posted 09-19-2018 10:12
    I agree that the storm forecasting that only uses hurricane categories is woefully inadequate to warn of the potential for high rainfall totals and the resulting flooding.  However, as hydrologists, we must also recognize that with global warming and increased urbanization, the rainfall statistics that we use to determine various magnitude flood events need updating.  The concept of increased rainfall intensity and the resulting "flood inflation" for various magnitude storm events needs to be addressed.  With increased rainfall statistics, stream and river stage measurements across the country, we should be able to re-evaluate and update the decades old data used to predict rainfall/runoff amounts that are used to design and size critical storm water infrastructure.  Whether we can agree that climate change is real and its causes, is not the important issue.  Let's allow the data to drive the science and implement the findings in our storm water management programs.

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    [Barry ] [Nelson] [LG, Aff. Member ASCE]
    [Chief Hydrologist]
    [Northwest Geoscience, PC]
    [High Point] [NC]
    [336 885 4381]
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  • 2.  RE: Flood frequency

    Posted 09-25-2018 10:31
    I agree completely about the need to up-date rainfall frequency numbers. ​I have been asking when Technical Paper 40 is going to be revised to reflect the actual rainfall data? Intensity, duration, frequency curves....Designing a culvert for the 10-yr, 24-hr storm frequency will likely change as will detention basins and runoff volumes for development. I have gotten no answers. ASCE Water Resources should take a proactive role in getting this discussion on the table and moving it forward. If there is anyone who is interested, please let me know.

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    Fredric Royal M.ASCE
    Town Engineer
    Wilmington NC
    (919)369-8397
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  • 3.  RE: Flood frequency

    Posted 09-25-2018 22:57
    Dear Frederic,

    That update to TP40 is out there - it's called NOAA Atlas 14 and the National Weather Service has been issuing it in pieces across the US. For North Carolina, the product is NOAA Atlas 14, Volume 2. Please see:

    HDSC/OWP
    Noaa remove preview
    HDSC/OWP
    National Weather Service (NWS) has provided probable maximum precipitation (PMP) guidance and studies since the late 1940s at the request of various federal agencies and with funding provided by those agencies. Probable maximum precipitation activities were discontinued in 1999 due to lack of funding, but copies of NWS PMP documents can be found on this site.
    View this on Noaa >



    You can use this site many different ways to get annual maxima/partial duration, intensity or depth, at observed stations or at a location you simply click on. You can also download the data and incorporate them as a layer within a GIS environment.

    These data are based on historical records, so they are not going to capture climate change, but they are certainly an improvement over TP40, a publication from 1961, both because of the better analytical tools used in their development and for the longer and broader precipitation records used.

     - Glenn

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    Glenn Moglen Ph.D., P.E., F.EWRI, F.ASCE
    Agricultural Research Service
    Beltsville, MD 20110
    (301)504-8745
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  • 4.  RE: Flood frequency

    Posted 09-27-2018 22:47
    Hello Everyone,

    I responded a couple days ago about NOAA Atlas 14 coming out in increments across the country. As it happens, I found this notice from Dr. Sanja Perica at the National Weather Service in my email today:

    Hydrometeorological Design Studies Center of the NOAA's Office of Water Prediction has released updated precipitation frequency estimates for Texas. They are published as  NOAA Atlas 14 Volume 11: Precipitation-Frequency Atlas of the United States, Texas.
    The NOAA Atlas 14 Volume 11 precipitation frequency estimates supersede the estimates published in the following publications:  NWS HYDRO-35 (1977), Technical Paper No. 40 (1961) and Technical Paper No. 49 (1964).
    Precipitation frequency estimates with supplemental information are available for download through the   Precipitation Frequency Data Server - PFDS.  Accompanying documentation, describing the data used in this project and project methodology, will be published in December 2018.
    Please note that if you have visited the PFDS recently, you may need to refresh your browser.  For more information please see  https://www.weather.gov/wrh/wrh_faq_clear_cache_browser_howto.

    --

    Sanja Perica, Ph.D.
    Chief, Hydrometeorological Design Studies Center

    Office of Water Prediction, NWS, NOAA, W/OWP
    1325 East-West Highway, Silver Spring, MD 20910

    Phone: 301-427-9552



    ------------------------------
    Glenn Moglen Ph.D., P.E., F.EWRI, F.ASCE
    Agricultural Research Service
    Beltsville, MD 20110
    (301)504-8745
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  • 5.  RE: Flood frequency

    Posted 09-27-2018 23:08
    Thanks, Glenn, for your post. I've been teaching from NOAA Atlas 14 for the past 2 - 3 years as the updates are there for PA.

    To the others on this thread who didn't know about the updates, I'm working with a committee that is trying to determine how to best let our community know when major updates are available, especially those where EWRI members are contributors and where EWRI technical committees are working. Several of us were surprised at the initial thread and we realized that what we thought had been widely announced, had not reached everyone who needs to know. I'd love to know what you think is the best way we can reach you with this new information. Please post or send me an email with any thoughts on where we should post information on new products, etc. Is it Collaborate? Currents? Email? Scheduling webinars? Short courses? Others?

    Thanks!

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    Shirley Clark P.E., D.WRE, M.ASCE
    Professor
    Penn State Harrisburg
    Middletown PA
    (717) 948-6127
    Penn State HarrisburgProfessor
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  • 6.  RE: Flood frequency

    Posted 09-25-2018 22:56
    Dear Alan,

    There are studies in the peer reviewed literature that have sought to use models of future climate to forecast future precipitation frequency. With such information, it's possible to estimate how stormwater infrastructure should be sized for future conditions. I did exactly this in the paper that I published with Eliana Rios in the ASCE Journal of Hydrologic Engineering back in 2014. Please see:

    G.E. Moglen and G.E. Rios Vidal (2014). "Climate Change and Stormwater Infrastructure in the US Mid-Atlantic Region: A Design Mismatch?" Journal of Hydrologic Engineering, ASCE. doi: 10.1061/(ASCE)HE.1943-5584.0000967.

    There are certainly other publications that approach the climate change - non-stationary precipitation frequency question that you could search on, but this one might get you started.

    - Glenn

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    Glenn Moglen Ph.D., P.E., F.EWRI, F.ASCE
    Agricultural Research Service
    Beltsville, MD 20110
    (301)504-8745
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  • 7.  RE: Flood frequency

    Posted 09-26-2018 16:11

    I remember discussing the hurricane rainfall and flood frequency issues just about a year ago when Hurricane Harvey nearly drowned Houston. Too much water in too short time – water as rainfall, water as storm surge, the reduced capacity of gravity drainage flow because of high sea level, etc. Or water for an extended period of time. And one should not be surprised if the scenario becomes a recurring problem – in years to come in all storm/hurricane prone countries.

    If one goes into some gov websites, such as: https://msc.fema.gov/, https://www.usgs.gov/ and https://www.fhwa.dot.gov/ one would find many assisting literature/advices on storm, flood zoning, frequency, vulnerability, etc. But somehow there remains a gap between the scientific understanding and caution, and the actual implementation of housing and other civil engineering projects. If such gap exists what are the reasons? Perhaps one may look into different factors like:

    • Lack of adequate and realistic zoning, local codes and regulations – or enforcing them.
    • Lack of public education and convincing, or in not taking account of their genuine concerns and experience (they have the first-hand knowledge of suffering and what happens to them).
    • Lack of understanding of developers, civil bureaucrats, politicians and governing bodies – and applying their powers to override or veto scientific and engineering concerns, advices and cautions.
    • Inadequacy of conventional flood and rainfall frequency estimate routines in light of the probable enhanced storminess and rainfalls due to climate change.
    • Role of insurance companies in the decision-making processes.
    • Incompetence of engineers in understanding the problem – and in taking care of all different factors during the planning, designing and implementation processes.


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    Dr. Dilip Barua, Ph.D, P.Eng, M. ASCE
    Vancouver, BC, Canada
    Website: https://widecanvas.weebly.com
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  • 8.  RE: Flood frequency

    Posted 09-27-2018 10:44
    ​This is a great discussion. One item I want to mention is storm water drainage system maintenance. Storms like Florence and Harvey will always cause flooding issues due to storm surge and rainfall amounts, but one item that can help is proper maintenance. This would not be a cure for these problems, but when storm water drainage systems are not maintained and cleaned out regularly a situation is created that can cause flooding during smaller storm events. We have flooding issues in Charleston and the surrounding area and many of these issues would be fixed or at least the flooding reduced if the drainage system was not full of silt. We certainly need to look at all the other parts of this problem that have been brought forward for discussion, but if we are not maintaining the built systems it does not matter what infrastructure improvements are made to handle events like Florence; ten years after it is built when it has not been properly maintained for 5+ years it certainly will not operate as designed and be overwhelmed by events that it should be able to handle.

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    James Wilson P.E., M.ASCE
    P.E.
    Charleston Water System
    Charleston SC
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  • 9.  RE: Flood frequency

    Posted 09-28-2018 14:37
    James,
    I am fairly new to the Low Country. I was wondering if there are any regulation, development requirements or statues in the Charleston area requiring developers to mitigate drainage, water control on their development site? I saw this in action in Arizona. The developers were required to manage and maintain the proper water flow and drainage on the property their were developing. They are responsible to detain rainwater on the property for up to 48 hours. It made them include detention areas that would contain the water until it evaporated or seeped into the soil. These areas are typically used for playgrounds or parks with landscaping that will assist in the dissipation of the water. The results are that each Development then is responsible for any flooding on their site and not passing the issue down the road or stream if you will.
    Just a thought, maybe this is already in place.


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    Peter Rubino P.E., M.ASCE
    Retired
    Intel Corp
    Johns Island SC
    (480)861-5523
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  • 10.  RE: Flood frequency

    Posted 09-27-2018 22:49
    I totally agree to all and this is a very great discussion indeed. I'm learning a lot and enjoing the exchange of knowledge.

    There must be a huge gap between scientist and specialist on the one side and decision makers on the other side. The science is able to forecast heavy rainfall and give warnings. Radar data gives accurate information about precipitation and the hydrologists are able to forecast floods with high precision. All these is known and published very well.

    At the other hand are so much people affected losing their homes. Mostly of them twice in short time. No one can be surprised by this. And this must be nothing new, that tropical cyclones carrying dangerous rainfall. In summer I visited Schyler VA and read in their museum about a hurricane in the 1940th which caused terrible floods and landslides with lots of fatalities.

    I'm in flood protection planning in Germany. We determine the highest possible flood and boundaring out the flooded areas with laserscan data. These areas will be declared to flooding zones by state law. In further future it will be prohibited to build residental buildings in theses zones by federal and state law. Often is a levee the boundary of the zone. We are estimating the potential damage in these areas and getting diiferent damage functions. With that we are able to figure out the best and most effencial way to reduce costs at all.
    Insurance companies are zoning without levee boundaries. If people are living in this zone it will be impossible for them to get a flood insurance.

    Maintainance of the drainage system is a very big issue in Germany. Environmentalists are trying to renature the river systems to improve the quality. The embankments shall be natural and grown with trees and bushes. But in my opinion this can be dangerous in case of flood in rivers with levee protection. The velocity is going down and the water rises unexpected higher.

    I'm not very familiar with the local law in the US. I recognized that the codes for house building are much more stricter in Germany than in the US. Maybe thats the key issue. By the way, the german laws and codes are having other issues and they also struggeling with an increase of major flood events.

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    Steffen Krei S.E., M.ASCE
    Dipl. Ing. (FH)
    Neuruppin
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  • 11.  RE: Flood frequency

    Posted 09-28-2018 12:19
    Certainly all of what Dilip Kumar Barua has stated is true. I might add that in the U.S. there is callous disregard of science by planners, politicians, policy makers. During my professional career, my colleagues have been well-aware of these issues. We should not allow new construction in floodplains and U.S. Federal subsidies of flood insurance should end, particularly in coastal regions. Conditions will only worsen with global warming and sea level rise. Some engineering consultants act unethically by supporting development in floodplains, simply for financial gain.

    Responding to Alan Nelson, my professional career has been mostly spent in real-time operational flood forecasting with the U.S. National Weather Service. With respect to "the rainfall statistics that we use to determine various magnitude flood events need updating", U.S. NWS is doing this at the Hydrometeorological Design Studies Center:HDSC/OWP
    Noaa remove preview
    HDSC/OWP
    National Weather Service (NWS) has provided probable maximum precipitation (PMP) guidance and studies since the late 1940s at the request of various federal agencies and with funding provided by those agencies. Probable maximum precipitation activities were discontinued in 1999 due to lack of funding, but copies of NWS PMP documents can be found on this site.
    View this on Noaa >

    .However, for individual flood events, explicit flood predictions are made that are not dependent on rainfall statistics, but instead rely on observed and predicted precipitation (and more). Please see: NOAA - National Weather Service - Water and Meteorological Model Ensemble River Forecasts
    Weather remove preview
    Meteorological Model Ensemble River Forecasts
    A sweeping cold front with a fall-like air mass will deliver below average temperatures for much of the northern tier of the country this upcoming weekend. High temperatures will range 5 to 15 degrees below average. Hurricane Rosa is likely to make landfall over Mexico early next week.
    View this on Weather >



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    Thomas Adams M.ASCE, PhD
    Terrapredictions
    Blacksburg VA
    (513) 739-9512
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  • 12.  RE: Flood frequency

    Posted 09-27-2018 23:09
    Following up on what James Wilson said. Maintenance is an issue, but where I live and where my students are working on a small regional project of modeling the existing drainage system, we are also looking at trying to locate and capture locations for additional temporary storage. We are noticing that, in some of the upper watershed (we are a small watershed - only a few square miles), the pipes are oversized due to minimum requirements for pipe sizing. Basin outlets are oversized on some basins or the drainage area was overestimated because even in cloudburst storms (our most recent problem) of several inches in an hour, basins are never filling up. We are not using the capacity we have. While this would not prevent flooding, it may reduce the impact for some who are on the fringes of the flash flood zones and potentially in the river flooding areas. So can we identify and better use the existing storage in the system? That's the first question we are trying to answer.

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    Shirley Clark P.E., D.WRE, M.ASCE
    Professor
    Penn State Harrisburg
    Middletown PA
    (717) 948-6127
    Penn State HarrisburgProfessor
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  • 13.  RE: Flood frequency

    Posted 01-29-2019 17:43
    https://acwi.gov/hydrology/Frequency/b17c/​


    I assume folks have seen the new Guidelines For Determining Flood Flow Frequency Bulletin 17C issued in March of last year.



    Mark W. Killgore, P.E.*, D.WRE, F.ASCE

    Dam Safety Regional Engineer

    Division of Dam Safety & Floodplain Management

    Virginia Department of Conservation & Recreation

    600 East Main Street, 4th Floor

    Richmond, VA 23219

    Office: 804-786-1359

    Fax: 804-371-2630

    Mobile: 804-396-5346

    Email: mark.killgore@...



    *Professional Engineer licensed in VA & WA

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  • 14.  RE: Flood frequency

    Posted 01-30-2019 11:19
    If memory serves me right, the flood frequency analysis using the Probability Distribution Fundtion in Bulletin 17C, or any other probability distribution function, you need a relatively stable period of record.  That would mean that, IF, climate change was occurring and magnitudes of peak discharges are increasing over time, you could not use any probability distribution analysis.  You could not even use a flood of record and known precipitation amounts and distribution to calculate a Unit Hydrograph and than apply precipitation amounts versus frequency from say TP 20 or TP40 (if they still use these) because the precipitation amounts and frequency are changing over time.

    The real question is, "How do we compute, say the 1% chance exceedance frequency discharge given any climate change?"

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    Bradford Price P.E., F.ASCE
    RETIRED
    Buena Vista VA
    (540)226-0180
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