Discussion: View Thread

Rainfall/Streamflow in Central Texas

  • 1.  Rainfall/Streamflow in Central Texas

    Posted 08-29-2017 03:26 PM
    ​​Austin, Texas got 6-8 inches of rain from Hurricane Harvey, which is easily tolerated here as long as it falls over days as it did rather than hours.  Three-day rainfalls of 28+ inches were recorded 70 miles Southeast of Austin.  These rainfalls over a relatively small area have generated massive flows (142,000 cfs and rising) in the Colorado River.  If you're interested check out the Lower Colorado River Authority (LCRA) rainfall data at https://hydromet.lcra.org/  Those wishing to monitor rainfall in the Houston Area should consult Harris County Flood Warning System which indicates widespread rainfall of 30 to 51 inches

    I'm looking forward to some assessments of this storm at upcoming EWRI meetings including the EWRI Congress in Minneapolis.


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    Eric Loucks P.E., D.WRE, M.ASCE
    Austin TX
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  • 2.  RE: Rainfall/Streamflow in Central Texas

    Posted 08-30-2017 08:54 AM
    Thanks for the info. I went to Weather Underground and the Houston Airport didn't have data from the 18th onward. The amount of rain some of these gauges are showing exceeds our annual average in NY, which is not a particularly dry area. Unbelievable.

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    Timothy Groninger P.E., M.ASCE
    White Plains NY
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  • 3.  RE: Rainfall/Streamflow in Central Texas

    Posted 08-30-2017 04:25 PM
    It exceeds Houston's annual average too.  What is wild about this, is that we have exceeded the 0.2% (500-year) 4-day rainfall totals of 18-19" by more than double for over more than 20,000 square miles in only 3 days.  This was not a small event in either geographic extent or total rainfall. 

    And on top of that, the remnants dropped close to 1% (100-year) rainfall volumes on Cities like Beaumont which are almost 100 miles away from the center of Houston.

    Clean-up costs are going to be in excess of $10 billion.  With almost 1 in 3 homes in the Houston/Harris County areas damaged, there will be 300,000 to 500,000 building permits requested within the next year.  

    Things are almost too big to grasp at this time.  But like the children's joke, "How do you eat an elephant? One bite at a time."  We are going to have to take on this elephant one project at a time until we have worked together to bring Houston, and the surrounding 7 million people, back to some sort of normality.  

    The good news, is that after watching the people from Houston for almost 30 years, I know that we will be working together as long as we have to in order to take care of our City and the people within it.


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



  • 4.  RE: Rainfall/Streamflow in Central Texas

    Posted 08-30-2017 06:39 PM
    Edited by Lindsay O'Leary 08-31-2017 11:38 AM
    Eric,

    Thank you for starting this conversation and sharing LCRA's site. I've been tracking rainfall and river gauge data since Friday evening thanks to continuous monitoring provided by the USGS, Lower Colorado River, Brazos River, and San Jacinto River Authorities. Screen capture from the USGS and these authorities are shown below, reporting rivers rising more than 20 feet above their respective flood stages within the time span of one to two days. San Jacinto River discharge rates, near Conroe, Texas, increased by several orders of magnitude, peaking at more than 100,000 cfs. The SJRA's database indicates the river's median daily discharge, over the past 44 years, is slightly more than 20 cfs. I repeat, discharge rates increased from 20 cfs to more than 100,000 cfs in a matter of days. 

    The state of Texas is still in search and rescue mode. When the floodwaters subside, I know ASCE members from across the state will flock to impacted areas – assessing damage, assisting with clean-up efforts, and rebuilding. While Harvey continues to cause historic damage, the resilient community of Texas only becomes stronger.

    How can you HELP? 
    'Amazing' Recovery Effort Bolsters Houston in the Face of Hurricane Harvey Flooding
     

    Discharge Rates for the San Jacinto River During Hurricane Harvey Aug. 2017. Source: San Jacinto River Authority Discharge and stage data for the Colorado River at Smithville during Hurricane Harvey Aug. 2017. Source: LCRADischarge and stage data for the Colorado River at La Grange during Hurricane Harvey Aug. 2017. Source: LCRA



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    Lindsay O'Leary P.E., LEED AP, M.ASCE
    Executive Director
    ASCE Texas Section
    Austin TX
    loleary@...
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  • 5.  RE: Rainfall/Streamflow in Central Texas

    Posted 08-31-2017 12:22 PM
    Harvey is truly a "nightmare scenario", coastal storm surge flooding with hurricane force winds followed by intense rainfall and river flooding.

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    Ralph LaMoglia P.E., M.ASCE
    Civil Engineer
    Wantagh NY
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  • 6.  RE: Rainfall/Streamflow in Central Texas

    Posted 08-31-2017 12:43 PM
    ​Sounds like Houston rainfall went beyond a frequency that can be statistically determined from perhaps only 100 years of rainfall data, but instead approached the Probable Maximum Precipitation which is an amount determined to be the most possible rain during a given time period for the meteorological conditions of the region. Do you know what the 24, 48 or 72 PMP is for Houston?

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    Todd Wagner P.E., D.WRE, M.ASCE
    City Utilities of Springfield
    Springfield MO
    ------------------------------



  • 7.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-01-2017 09:18 AM
    As a homesick Texan in Washington, my heart goes out to everyone in the greater Houston area. I still have many family members there and I am grateful none of them were significantly affected.  From a Water Resources perspective, Hurricane Harvey has been amazing to witness and discuss in my Hydrology class.  Many topics, from storm energy and precipitation formation to land management and city planning were relevant in light of the storm.  Fittingly, I saw this figure tonight in my text book (Environmental Hydrology, Ward et. al. 2016). It shows the probable maximum precipitation for a 6-hour duration over the Gulf Coast counties in Texas at 32 inches.  So your question, Todd Wagner, is spot on.



    ------------------------------
    Natalie Martinkus P.E., Ph.D. A.M.ASCE
    Director, Center for a New Washington
    Heritage University
    Yakima WA
    ------------------------------



  • 8.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-01-2017 09:18 AM

    Todd, I was looking into this earlier this week when I was trying to place the flooding into context.  From http://nws.noaa.gov/oh/hdsc/studies/pmp.html, I looked at HMR 51 and 53.  In HMR 51, the contours for the maps dont exactly get all the way t the coast, likely as a tip of the hat to what Hurricane/Tropical Storm systems can dump on the coast.  If you look approximately 50/100 miles inland, you see that the last contour for the Texas coastal plain area has all-season 24 hour PMP for 10 mi2 basin is greater than 47.1 inches, the 48 hour is greater than 51.8 inches, and the 72 hour is >55.7 inches.   I was looking at some past tropical system rainfall and as impressive as Hurricane Harvey is with >50 inches over the course of the storm (>72 hours), I found one reference that had the 1979 Tropical Storm Claudette dumping 43 inches at Alvin, TX in 24 hour period.....Have not confirmed the veracity of that data point, but Harvey did  not approach that amount of rainfall in any 24 hour period from the rainfall data I have seen so far.                                                                                                                                                         



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    Robert Holmes Ph.D., P.E., D.WRE, F.EWRI, F.ASCE
    National Flood Hazard Coordinator
    US Geological Survey

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  • 9.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-01-2017 12:45 PM
    ​Bob, good to hear from an old friend and one of our true national experts on the subject! Of course statistics of this type are multi-dimensional and perhaps the most impressive aspect to this event is the area covered by such an incredible total storm volume, creating literally feet of standing water over much of the urban areas. The PMP maps were developed with an assumption of 10sqmi storm coverage and if that were the case, this would be a minor story because only a relative few would be affected. But when you take these rainfall depths and put them over hundreds or even thousands of square miles and then locate it over a major metropolitan area, you have an event that perhaps we can't currently capture statistically with the available data and statistical tools! Todd

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    Todd Wagner P.E., D.WRE, M.ASCE
    City Utilities of Springfield
    Springfield MO
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  • 10.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-01-2017 09:19 AM
    The 72-hour, 10 square mile PMP is given as 55.7 inches in HMR51 (NOAA/USACE, 1978).  Given the number of 15-, 20- and 25-inch storms we've experienced in the east half of Texas in the past 17 years, I suggest designers look at how their projects might fare in a 0.5 or 0.8 times PMP scenario.  Beaumont received 26 inches in 24 hours similar to a storm in Houston in June 2001.  Due to Houston's prior experience, they were a bit more prepared for this type of rainfall.  The 24-hour PMP is 47 inches and the US 24-hour rainfall record (43 inches, Alvin Texas, July, 1979) still stands.

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    Eric Loucks P.E., D.WRE, M.ASCE
    Austin TX
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  • 11.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-01-2017 12:44 PM
    As other posters have noted, perhaps traditional historical data may not provide the insight needed to accurately predict frequency of rainfall events.

    Return period predictions are typically take a "static" approach and do not account for systematic changes. I want to point your attention to a recent publication by Call et al. (doi: 10.1002/2016WR020277, featured in the AGU-EOS newsletter) which questions the use of such systematic approaches to flood frequency predictions. The researchers included systematic changes (climate change, water management practices, and river channel adjustment) into their inundation model. 

    Surely, such a massive storm event, like Harvey, has altered the landscape and hydrology of SE Texas and neighboring states. Although landscape changes may not directly result in changes in rainfall frequency, flood frequency predictions should certainly be altered to reflect new drainage characteristics.

    As is the case with inundation models, physically based meteorological models, which should include systematic changes, can bridge the gap between traditional statistical predictions of rainfall frequency and observations. Computational cost of such models are relatively high, but consideration of the value of "better" predictions of rainfall frequency and inundation may offset the cost of more robust modeling, in particular when considering cases of critical infrastructure, densely populated areas, or vulnerable environments.


    ------------------------------
    Hassan Ismail EIT, A.M.ASCE
    Ph.D. Candidate
    University of South Carolina
    Columbia, SC
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  • 12.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-01-2017 12:45 PM
    Intersting PMP question Todd.

    Not sure that traditional PMP methods  will be overly informative for this type of event.

    Traditional  storm transposition methods are implicitly "single storm" transpositions that naturally decrease the estimated mean precip with area.

    For example,  NWS Hydrometeorlogical Report 51 on PMP shows a 72 hour contour of just over 55 inches for a 10 sq. mi area, but only  ~22 inches for 20,000 sq mi area.

    ------------------------------
    Stuart Schwartz A.M.ASCE
    Sr. Research Scientist
    University of Maryland Baltimore County
    Baltimore MD
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  • 13.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-01-2017 05:03 PM

    Tropical Storm Harvey Rainfall in Eastern Texas


    I'm an old-school civil engineer who studied and designed stormwater and flood control systems over 45 years. Since I retired in 2006, I've found other interests. But since the recent hydrometeorological events in Texas, my interests have been awakened. I dusted off my old Weather Bureau technical reports to see how the Harvey rainfall totals compared with probable maximum precipitation (PMP) estimates made in the 1960s and 1970s.


    It's difficult to obtain accurate rainfall totals from media reports since they typically report only the maximum values for shock value. So, we probably won't know <g class="gr_ gr_40 gr-alert gr_spell gr_inline_cards gr_run_anim ContextualSpelling ins-del" id="40" data-gr-id="40">areal</g> average precip values until all data are compiled and analyzed.


    This said, I will make this observation: the total precipitation over several days appears to be in line with PMP estimates published years ago. The following PMP estimates from T.P. No. 51 are for Eastern Texas described as the All Season, 72-hr PMP for the following areal coverages:


    10 sq miles ............55.7"

    200 sq miles.......... 48.8"

    1,000 sq miles....... 41.3"

    5,000 sq miles....... 30.7"

    10,000 sq miles..... 26.5"

    20,000 sq miles..... 22.0"


    Of course, when all the data are in, we might have to rethink the whole PMP procedure. Since it's been 40 years since T.P. 41 came out, and considering all the data collected since then, maybe it's time to recalculate. The agencies probably will say they don't have the money for it. My suggestion would be to add these words to the study proposal, "This study is to update the analysis to account for man-made effects of GLOBAL WARMING." Then there'll be plenty of money for the study.


    At first, I was concerned about potential upstream dam breaches. But, this should not be of concern for any major dam built in the past 40 or 50 years since they should have, by law, been designed for the PMP. This assumes of course that they were properly designed structurally.


    The media goes on about how some dams are "dumping" excess water and causing increased flooding. This is nonsense. In all cases, the stage-storage-discharge relationship in reservoirs results in flood peak attenuation, not increases. Only timing-of-peak considerations might prove to be problematic, but that would be difficult to determine.


    Just some thoughts from an old engineer.



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    Timothy Doyle P.E., F.ASCE
    Consultant
    Bluffton SC
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  • 14.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-05-2017 11:14 AM
    Timothy:
    Thanks for the perspective, and the numbers for PMP event.

    A dam will attenuate the water downstream, but only at the cost of flooding the area upstream.  One of the things that is happening in Houston is the need to release the water downstream because there are many homes impacted in the developments upstream of the reservoirs.  The government owned land only consists of a portion of the land (I think around 50%) that is flooded during events like this one. 

    Houston's reservoir system was tested to its limit.  The Corps is releasing 8 times the design spillway flow in order to; 1. prevent the spillway from breaching; (this is a partial success, the spillway breached, but didn't fail) 2. lessen the impact to the thousands of homes built at elevations below the spillway elevation upstream of the spillway; (there are a lot of homes still flooded, so I do not know if this can be considered a success) 3. hoping that the downstream impacts are less than the upstream impacts. (this will have to be evaluated after the flows stop.)

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    Dwayne Culp, Ph.D., P.E., P.Eng, M.ASCE
    Culp Engineering, LLC
    Richmond TX
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  • 15.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-06-2017 11:09 AM
    Umm, what? This doesn't seem reasonable, am I missing something?

    2. lessen the impact to the thousands of homes built at elevations below the spillway elevation upstream of the spillway

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    Edward Dickson P.E., M.ASCE
    AECOM
    Charlotte NC
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  • 16.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-07-2017 05:19 PM
    Edward, and Theodore:

    Yet it is true.   The elevation of the spillway is higher than the 100-year water surface, so the property owners were allowed to develop.  Since it is not technically in the 100-year floodplain, the home builders were not even required to elevate the structures unless it was convenient to do so.  Fortunately most, but not all of the homes were elevated because it was convenient to design without a lot of extra cut in the streets.

    The reason that they had to open up the spillways to higher than anticipated flow rates was to protect the homes in the non government owned lands.  This caused some homes downstream of the spillway, which had not flooded in the previous 3 days to experience higher water surfaces, and significant water damage.   The article by the Dallas Morning News has a link to a report that discusses the whole concept that was written in 1996.  It also includes a number of exhibits that show how the spillways impact the community.

    As Houston grew, officials ignored 'once-in-a-lifetime' chance to spare thousands from flooding





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    Dwayne Culp, Ph.D., P.E., P.Eng, M.ASCE
    Culp Engineering, LLC
    Richmond TX
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  • 17.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-11-2017 01:46 PM
    Great link Mr. Culp!  It clearly demonstrates the cost of not keeping the flood charts up to date.  The COE policy of only considering increased runoff from platted subdivisions is not helpful, especially in Metropolitan areas where development is predicted to take place over a very short time frame.  I was unable, when working for the City of Fort Worth, to impose slab elevation levels more than 1 ft. above the 100 year plain even though I knew the upstream development would shortly make those levels inadequate.

    James R. Anderson, P.E.
    Principal, Anderson Consulting
     





  • 18.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-06-2017 11:10 AM
    Why were homes allowed to be built below the emergency spillway elevation upstream of the dam?  I'm guessing they were built up to the 100 yr elevation.  I wonder if the homeowners were properly educated on the flood risk of their properties?

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    Theodore Ragsdale P.E., M.ASCE
    Sr. Project Engineer
    Riverstone Group, Inc.
    Moline IL
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  • 19.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-06-2017 11:56 AM

    Thanks all for the great discussion, Harvey has been an extraordinary event for the people of Texas and western Louisiana.

    I wanted to make those on this thread aware of the completion of a statewide Probable Maximum Precipitation (PMP) study for Texas and immediately surrounding watershed regions in New Mexico and Mexico. The study was completed in September 2016 by Applied Weather Associates, LLC for the Texas Commission on Environmental Quality(TCEQ). The PMP values from this study replace those provided in Hydrometeorological Reports (HMRs) 51 and 55A. 

    Links to the study and associated PMP GIS tool can be found at the following link to the TCEQ Dam Safety Program page under Probable Maximum Precipitation (PMP) Study.

    http://www.tceq.texas.gov/compliance/investigation/damsafetyprog.html 

    The TCEQ Dam Safety Program also held workshops in 2016 covering this study. The presentation can be found further down on the page under "Session 3 - Probable Maximum Precipitation (PMP) Study for Texas (PDF)"



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    Nathan Gullo P.E., M.ASCE
    Principal Engineer
    Lower Colorado River Authority
    Austin TX
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  • 20.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-07-2017 11:10 AM
    Dwayne:

    Good to see you active on this page after Hurricane Harvey. I don't think you put enough emphasis on your first your first point. I am quite certain that protecting the levee integrity was the primary, if not only, decision criteria the Corps used to establish the extreme discharge rate. There were some highly sleep deprived Corps personnel during the Tax Day flooding last year when similar impound levels occurred.

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    Richard Ruchhoeft P.E., M.ASCE
    PROJECT MANAGER
    Port of Houston Authority
    Houston TX
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  • 21.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-11-2017 01:55 PM
    I think it's about time for the Corps to start putting on a project to update and develop newer HMR's to suit the 21st-century climate. The landscapes have changed dramatically over the past 40 odd years. Not only Global warming but rapid constructional developments have pushed the runoff estimates from areas beyond their imaginable extent. I think we should develop empirical equations suitable just for Houston.

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    Saurabh Nomula, MS.E.E., E.I.T.,
    Stormwater Design Engineer
    Harris Engineering, Inc.
    Raleigh, NC
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  • 22.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-12-2017 05:36 PM
    ​I believe we engineers need to accept partial responsibility for not being strong enough in our warnings to the public. I believe we have mislead them by labeling rainfall events by year, instead of percentage or probability. Perception becomes reality, and public perception is that "100 year event" means an event that occurs only once in 100 years. I believe that if we said "1% chance of this water surface" or "1 in 100 chance of flooding" people would perceive the risk of flooding a proposed structure as much higher than they currently do.

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    David Booth P.E., M.ASCE
    Civil Engineer
    CLSI
    Westminster MD
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  • 23.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-13-2017 01:48 PM
    ​actually, Federal agency's official language is to refer to probabilistic events in term of Annual Exceedance Probability (AEP) or Annual Chance Event (ACE) as in the 1-percent AEP or ACE;  usage among the public and engineering community, however, persists in using the return period notation, i.e., the 100-year event.  I don't think it helps much, though, as folks that understand 100-year event to mean exactly one in 100 years won't understand 1% chance every year any better.


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    Benjamin Pope P.E., M.ASCE
    Senior Associate
    AECOM
    Raleigh NC
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  • 24.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-13-2017 01:51 PM
    ​No one seems to talk about the concept of an event happening over a time period.  Your chance of dying every year is not so large, but your chance of dying over 100 years is almost 100%. 

    Should we be surprised that we see 100-yr events occurring during our lifetime?

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    Sheila Baran P.E., M.ASCE
    Assistant Professor
    Cassville NY
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  • 25.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-13-2017 03:52 PM

    Many interesting discussions on this topic. Too much water in too short time – perhaps that is how one can characterize the effects of Hurricane Harvey; and perhaps more is likely to come in the changing climate. Water as rainfall, water as storm surge, the reduced capacity of gravity drainage flow because of high sea level, and perhaps an ill-conceived flood control reservoir concept that did not consider the interests of all – all these factors on the setting of a relatively flat topography of Houston. 

    Perhaps modern societies (not only in Houston but everywhere) are too addicted to put concrete everywhere – on natural and reclaimed coastal low lands without giving in-depth thoughts to keeping enough rooms for water to flow – for water to dissipate its energy. Perhaps it is important that engineers and other professionals stop telling people that they can solve any problem given the money and time – without adequately highlighting the consequences. Innocent people – home and property owners rely on the judgment of professionals – it is too important to respect that trust.

    Was there no improvement from the past? Was there any lack of sincerity on the part of engineers, planners and politicians in understanding the problems as well as in generating alternatives to solve it? Perhaps not – yet one may ask why such problems continue to persist despite the fact that there have been many such recurring events.   

    Funding seems not a problem. What then one may ask? I leave it at this.

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    Dr. Dilip Barua, Ph.D, P.Eng, M. ASCE
    Consultant - Coastal, Port and Marine Engineering
    Vancouver, Canada
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  • 26.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-13-2017 06:09 PM
    David Booth, Benjamin Pope, and Sheila Baran have made an interesting observation that we as hydrologists may have forgotten.  If you cannot explain it to someone not in the field, do we really understand it?  We explain things as 100-year event or 1% event.  This is part of our secret language as hydrologists.  I do not know if it is any more explicative than a 30% chance of rain is to most of us. 

    I wonder if we would get the same results if we had to explain that they have a 28% chance of experiencing a 100-year flood over the course of a mortgage.  Imagine the shock on people's faces, if they had to sign a document at closing that said that because the are in the 10% floodplain, they had a 95% chance of experiencing a flood event in their home over the course of the mortgage. 

    We have chosen language that confuses the general public, perhaps not intentionally.  I hope that we can start lifting this veil to reveal the true ugly truth about the risk associated with placing homes, businesses, and people in a floodplain. 

    I know people in Houston are looking for answers, and some of the people speaking up have good solutions, but some of them have very bad ones too.  Perhaps its time to take the management of flood control away from those in local government, and make them independent of the 5 elected positions in a County Court building, so that their budget is not revoked every time we have a crisis in the hospitals.  Maybe it's time to put an agency in place that does not have to respond to a potential flood control project with, "That's not in our County, so we can't do it, even if it will reduce the risk to 100,000 homes in our County."

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



  • 27.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-14-2017 11:18 AM
    A lot of interesting discussion in this thread from many angles of the events in Houston.

    I heard an interesting talk on the traits of a successful engineer given by a past ASCE president. He made a very compelling analogy explaining return period and understanding risk. It goes something like this...

    If you have a 30 year mortgage. A 100-year flood has about a 30% chance of occurring while you are paying off your home. That is a 2/6 chance: like playing Russian Roulette with two bullets in the revolver.
    If we, engineers and policy makers, decide to take steps to mitigate the risk of flooding in a certain area through perhaps improved drainage or retention, thus cutting in half the likelyhood of the 100-year event actually causing flooding. The likelyhood of flooding during our 30 year mortgage is now about 15%. That is like playing Russian Roulette with one bullet in the revolver. Would you still play? Do you and the public understand the risk that still exists?

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    Hassan Ismail EIT, A.M.ASCE
    Ph.D. Candidate
    University of South Carolina
    Columbia SC
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  • 28.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-14-2017 05:10 PM
    I make it a point to explain to clients that the 100-year event has a probability of about 65% of occurring at least once in that 100 years.  That often puts a different light on the subject for them.

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    J Richard Weggel Ph.D., P.E., D.CE, F.ASCE
    Professor Emeritus
    Blue Bell PA
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  • 29.  RE: Rainfall/Streamflow in Central Texas

    Posted 09-14-2017 11:38 PM
    Just to try to clear up a few items mentioned in this thread. 

    1.  The likelihood of seeing one or more floods over a period of time can be computed using the binomial distribution (see Bulletin 17B Page 10-1 for the equation and Page 10-3 for the computed values).   For 1% annual chance exceedance event it is 26% over 30 years and 63% over 100 years.  As a retired water resources engineer from USACE we have tried to convert from using 100-year to these values since we began uncertainty calculations in the late 1990's but it has been very tough to do both inside and outside of the agency.   

    2.  Some of the discussion has been about Addicks and Barker Dams operated by USACE and they are not levees.  I believe the district was evacuating water from the dams using the outlets works and only at one location was a spillway flow seen.    Originally these dams (built in the 1940's) did not have gated outlets so at that time the real estate requirements were likely quite different. I believe the Houston Chronicle has a pretty good article on these dams from September 2016 if you want more on their history including they were part of a larger project including downstream improvements that were never constructed.  Link:   http://www.houstonchronicle.com/news/houston-texas/houston/article/Houston-dams-are-old-beat-up-and-a-vital-line-of-9199908.php

    3.   I am not aware of the COE (USACE) policy Mr. Anderson is referring to when discussing "..only considering increased runoff from platted subdivision.."   When designing and doing updated dam safety conditions USACE would at a minimum look at existing runoff conditions. 

    4.  Mr. Nomula.  The National Weather Service was historically the lead on updating HMR's (it has typically involved several agencies) and it has been recognized for some time that storms (many in Texas) that have occurred since HMR 51 (June 1978) that would require an update.  Because of funding and personnel constraints the updates have not happened.   In addition, there is a movement within the dam safety community to no longer rely on the PMP/PMF process in dam safety studies.  

    5.  The issue of communicating risk and uncertainty to the public and even some engineers still remains.  It can be difficult and confusing and there are some papers on this issue.  It remains a challenge for those trying to communicate it to the pubic.   

    6.  For specific information on the Addicks and Barker Dams and their operation during and after the hurricane I recommend visiting the Galveston District web site or Facebook page.  I have seen a number of updates on the dams via Facebook.  The following link is to the press releases from the District: http://www.swg.usace.army.mil/Media/News-Releases/Article/1291369/corps-releases-at-addicks-and-barker-dams-to-begin/

    Hope this information was informative.  




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    Jeffrey McClenathan P.E., M.ASCE
    Civil Engineer Hydraulics
    Omaha NE
    ------------------------------