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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Original Message:
Sent: 08-31-2017 08:54
From: Todd Wagner
Subject: Rainfall/Streamflow in Central Texas
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
Original Message:
Sent: 08-30-2017 15:10
From: Dwayne Culp
Subject: Rainfall/Streamflow in Central Texas
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
Original Message:
Sent: 08-30-2017 07:48
From: Timothy Groninger
Subject: Rainfall/Streamflow in Central Texas
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
Original Message:
Sent: 08-29-2017 14:47
From: Eric Loucks
Subject: Rainfall/Streamflow in Central Texas
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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