ReduceFlooding.com has obtained a copy of the study withheld by the Army Corps that the Corps used to justify dredging only 500,000 cubic yards from the mouth bar of the San Jacinto West Fork. The Corps refused to supply it in response to my Freedom of Information Act (FOIA) request in June. However, the City of Houston did supply the Corps document in response to a similar FOIA request. Now, thanks to Council Member Dave Martin, the public has an opportunity to compare the two studies side by side for the first time.
Kings River resident near mouth bar wading in knee deep water almost to West Fork channel marker. Caution: do not let children attempt this. Pockets of deeper water may exist that could cause drowning. Picture taken eight days ago. The island being excavated in the distance has since been removed; see last image in post.
After reviewing the Corps document, I can see why the Corps refused to supply it. It has more holes in it than a West Texas stop sign.
History of Controversy
For almost a year, the City and the Army Corps have argued over how much sediment was deposited in the mouth bar of the San Jacinto river by Hurricane Harvey. That determines how much dredging FEMA will fund. Initially, the City recommended working with two Texas Water Development Board sedimentation surveys conducted in 2011 and 2018. But no measurements exist from the period immediately BEFORE Harvey – only AFTER. So the Corps rejected that idea.
The Stockton Protocol combines ultra-high-resolution CHIRP seismic data with core sampling. The seismic identifies layer thickness and the core sampling identifies layer composition. (Note: the process is somewhat like the oil field practice of confirming seismic with core samples from exploratory wells.) The hope: that by analyzing changes in sediment composition (such as color, grain size, roundness, hardness, etc.), researchers can differentiate Harvey sediment from other floods and then measure it accurately.
Core sample from Tetra Tech Study. Different colors and consistencies indicate sediment came from different floods.
The Army Corps recommended a Texas A&M Galveston professor, Dr. Timothy Dellapenna, to do the research. However, the City of Houston and A&M could not agree on contract terms. Therefore, the City hired Tetra Tech, to perform the research that Dr. Dellapena outlined.
At the end of the day, even with 500,000 cubic yards, those two estimates still vary by almost 3X. According to Houston City Council Member Dave Martin, the Corps never explained why they rejected the Tetra Tech analysis.
The Corps simply accepted its own results and started dredging without public explanation or input. The Corps document raises many questions that may or may not have valid answers.
The USGS gauge used by the Corps for its analysis stopped working during the peak of Harveywhen most sediment would have been moving. The Corps report did not acknowledge this.
Corps Analysis Requires Explanations Never Supplied
Why did the Corps:
Base its analysis on a gage at US59 that stopped functioning during the peak of Harvey, when most sediment was moving?
Assume Harvey distributed sediment in the same patterns over the same distances as lesser storms?
Ignore build up of sediment from Tax Day and Memorial Day storms at the mouth bar as a factor that could have increased the percentage of sediment falling out of suspension during Harvey?
Not consider bank erosion downstream from the gage, relying instead on standard charts for “bed-load transport” for sandy rivers?
Ignore approximately 20 square miles of sand mines in the West Fork floodway where loose sand and silt were inundated by 131,000 cubic feet of water per second, unlike previous storms?
Use a 1-D instead of a 2- or 3-D model for this complex environment?
Not publicly disclose model inputs/outputs and data for peer review and validation?
Initially reject the use of two TWDB surveys, then reverse course and base all of their findings on them – without explaining why?
Exclude extreme data from their study, even though Harvey was one of the most extreme rainfall events in U.S. history?
Mislabel all charts, graphs and photos in its report?
Refuse to disclose their report in response to a FOIA request, contrary to official Army policy?
Omit the organization’s name and the author’s name from the report?
Treat the volume that Tetra Tech found related to Harvey in the mouth bar area alone as if it represented the total volume deposited in the entire West Fork by Harvey?
Corps Rejects Use of TWDB Surveys, Then Bases Own Analysis On Them
To estimate Harvey-related volume, the City initially proposed analyzing two Texas Water Development Board sedimentation surveys from 2011 and 2018.
The Corps rejected that idea, suggested the Stockton Protocol, rejected those findings, then based its own analysis on the two TWDB surveys it rejected earlier. This is like following a Three-Card Monte game!
Basically, the Corps tried to estimate the amount of sediment that Harvey’s flow could theoretically carry. That would depend on velocity and sediment size/weight. But the gage at US 59 stopped recording at the peak of Harvey. So they also had to estimate the discharge (volume of flow in cubic feet per second [cfs]). Then they used industry-standard curves to estimate sediment transport based on estimated discharge. But they discarded rates over 45,000 CFS because they produced unexpectedly high values.
They also ignored the presence of mile-wide sand mines upstream. The river ruptured the dikes of those mines and captured the pits during Harvey.
West Fork Sand Mine Complex inundated by Harvey. This reach of the river is normally about 150 feet wide.On this day, the day AFTER Harvey’s peak, the flow was more than a mile wide.
When the industry-standard sediment transport curves yielded unacceptably high results, the Corps resorted to a simple 1-D model (developed earlier for another purpose) to calculate the sediment load, because flows beyond 45,000 cubic feet per second “produced sediment loads far beyond a reasonable range.”
Corps Assumes Harvey Transported Same Percentage To Mouth Bar as Other Storms
One potentially fatal assumption: The Corps assumes that Harvey transported the same percentage of its sediment load to the mouth bar as all other storms between 2011 and 2018. Said another way, they assume that Harvey behaved LIKE all other storms. Yet not all those floods inundated sand mines.
Moreover, had the Corps measured river bank erosion at intervals between 2011 and 2018, they would have found that virtually all of it occurred during Harvey and very little occurred during Tax Day, Memorial Day and other storms.
Quantum Leap in Erosion Not Factored In
Harvey’s erosive power was NOT proportional to other storms, as the photos below show. River banks eroded more than a hundred feet during Harvey in many places. Yet the Corps report never even mentions erosion.
In 2011, the distance from the ridgeline of this home on Riverbend Drive to the West Fork was 326 feet.
On 1/23/2017, after the Tax and Memorial Day Floods, the distance had decreased only 2 feet.
This shows how much shoreline Harvey ALONE eroded. The yellow line is exactly the same length as after the 2016 floods.
After Harvey, the new distance to the river bank was 216 feet – 108 feet less.
The Tax and Memorial Day Floods combined eroded this river bank by 2 feet. Harvey alone eroded it another 108 feet – 50 times more!
Photographic analysis shows similar quantum leaps in erosion related to Harvey elsewhere along the West Fork.
Another home west of River Grove Park lost 27 feet between 2011 and early 2017, but 111 feet in Harvey.
River Grove Park lost 0 feet from 2011 to early 2017, but 74-feet in Harvey.
Romerica lost 62 feet between 2011 and early 2017, but 144 feet in Harvey.
Net: In four days, Harvey eroded from 2X to 75X more sediment than all other storms during the previous six years. It did NOT act proportionally.
The shearing force of 240,000 cubic feet per second coming down the West Fork literally pulled thousands of trees out by their roots and dislodged sediment disproportionately compared to previous floods (see below). The Gallery page of this web site clearly shows the extent of this devastation. It contains 450 images taken from a helicopter on 9/14/2017, two weeks after Harvey.
Hurricane Harvey ripped trees out by their roots to a degree that previous storms did not. This increased erosion exponentially compared to other storms.
Corps Assumes Mouth Bar Growth Did Not Affect Percentage Deposited by Harvey
The Army Corps also assumes that Harvey transported the same fraction of the total sediment load (20%) to the mouth bar that all storms did between 2011 and 2018. That’s a dubious assumption for several reasons:
Previous storms progressively built a wall across the mouth of the West Fork that grew higher and higher during the study period.
As it grew, that wall increasingly slowed water down and likely accelerated the rate of deposition behind it (which helps explain why the Corps had to dredge its way to the mouth bar).
This constant 20% contradicts numerous anecdotal reports from lakeside residents and boaters claiming that Harvey carried vastlymore sediment to the mouth bar (and their yards/docks) than previous storms. The wife of the resident wading across the river in the image above told me that, on a scale of 1 to 5, the Tax and Memorial Days floods deposited sediment in her yard equal to a 1. But Harvey, she said, was a 6. In other words, off the scale.
No wonder the Corps didn’t want the public looking at this!
Taxpayers Deserve Independent Scientific Review
Professionals rarely like to have their conclusions questioned. However, those who have confidence in their conclusions welcome peer and public review. They encourage second opinions and provide all of their data for review. They also welcome the opportunity to explain and defend their results. None of those things happened in this case.
Instead, the Corps concealed its results as if this involved national security, not public safety. Why? That may be the biggest question of all associated with this project.
The Corps has an excellent, hard-earned reputation. This study undermines it.
As mentioned above, the Tetra Tech study may also have flaws, but the Corps never revealed what its concerns were.
Only one thing is certain. Public safety rests on wildly differing studies. Taxpayers deserve an independent scientific review to resolve the differences between these two studies. The City concurs with the findings in this post and also calls for an independent scientific review. The Corps could not be reached for comment; their new public affairs officer does not list her phone number.
Dredging will likely end next week, with the Corps proclaiming it has restored the conveyance of the river to pre-Harvey conditions (when they have no pre-Harvey measurements).
So we need an independent scientific review to happen quickly. Email you Congressmen and Senators immediately.
Corps Plans Still Being Kept from Public
The Corps still has not released its dredging plans, despite a FOIA request made in June when mouth bar dredging started.
If that is an accurate assessment, the Corps would leave a sediment wall under the water approximately 30-35 feet high and 1-2 miles long in the mouth of the West Fork.
Congressman Dan Crenshaw reviews progress of dredging operation on Friday, August 16. Looking southwest towards Atascocita. Notice how the small island in the first image above has now been removed. The mouth bar itself will remain in place, most of it underwater nowwhere it is invisible to the public.
Others Scrambling to Pick Up the Pieces
It may look like the Corps has dredged. But it also looks like the Corps will leave 80-90% of the mouth bar in place. Remember, sand bars are like ice bergs in the sense that what you see above water is small compared to the amount you can’t see below water.
At this point, City, County, State and Federal leaders are scrambling to put together a plan to address the rest of the sediment. Some of that sediment is clearly pre-Harvey. I will discuss options for removal of that portion and maintenance dredging in a future post.
Posted by Bob Rehak on 8/20/2019
721 Days after Hurricane Harvey
As in previous posts on this subject, I promise the Corps that I will print their rebuttal verbatim if they disagree with any of the points in this post.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2019/08/USGS.08069500.140334.00065..20170829.20170830..0..pres_.gif.png?fit=580%2C430&ssl=1430580adminadmin2019-08-20 16:14:592019-08-21 11:07:08Recently Obtained Documents Raise Questions about Amount of Sediment in Mouth Bar Due to Harvey
A November 2018 article appearing in the peer-reviewed scientific journal Nature found that urban growth can intensify both rainfall and runoff from hurricanes. Further, urban growth can increase the risk of flooding and shift the location of flooding. The article specifically studied the effects of Hurricane Harvey on Houston and found that urban growth increased the probability of such an extreme flood across the basin by 21X.
The Nature study looks at two distinct effects of urbanization. The first is the impact of impervious surface on RUNOFF. The second is the impact of the urban landscape’s surface roughness on RAINFALL.
The Runoff Component
Numerous studies have looked at the relationship between percentage of impervious cover, runoff, and flooding – a well documented phenomenon. Impervious cover accelerates transport of rainfall from neighborhoods to rivers. That raises peak flows rather than spreading them out over time. Dr. William Dupre, professor emeritus from the University of Houston visualized the relationship this way.
Effect of Urbanization on Peak Stream Flows” by Dr. William Dupre, professor emeritus from the University of Houston.
Rainfall Component Much Less Studied
However, the effect of urban growth and a city’s surface topography on RAINFALL from hurricanes is much less studied. The authors say in Nature that, “Urbanization led to an amplification of the total rainfall along with a shift in the location of the maximum rainfall.” (Page 386).
“Much less is known regarding the urban effects on the organized tropical rainfall of a hurricane, in particular during one like hurricane Harvey, which stalled for several days.” They continue, “…experiments (with computer models) clearly show a large increase in rainfall arising from urbanization over the eastern part of the Houston area.”
The authors compared present and past urban landscapes and also modeled a scenario in which the entire region was cropland.
Mechanisms Responsible for Increase Rainfall
To understand the physical mechanisms responsible for the heavier rainfall, they analyzed the vertical convergence of winds and wind fields.
Evacuation During Harvey. Photo courtesy of Jay Muscat.
“The enhanced rainfall … and the shift of rainfall … are tied to the storm system’s drag induced by large surface roughness,” say the authors.
Scientific American explains in more detail. Kerry Emanuel, an atmospheric scientist at Massachusetts Institute of Technology, who did not work on the study said, “We know cyclones are sensitive to characteristics of the surface—mountains, streams, marshland. This new twist is that cities have become big enough to tangibly alter the storm.” Said Gabriele Villarini, an environmental engineer at The University of Iowa and an author on the study, “We removed the urban areas from Houston and replaced them with cropland.”
“The presence of urban areas enhanced all the things you need to get heavy precipitation,” Villarini, one of the study’s authors says. “A stronger drag on the storm winds, associated with a larger surface roughness length” contributed to the increased rainfall.
Emanuel explained, “First, the artificial ruggedness of an urban area slows air down. Whenever air slows in a hurricane, he says, it gets shunted toward the center of the storm and up into the sky. That increases rainfall everywhere [in a metropolitan area].” He added, “A storm moves particularly slowly over downtown areas where buildings are tallest, but the winds bearing down from outside the city are still moving quickly. So, [the storm] is piling up on the city.”
Impact on and Implications for Houston
This increase in urban growth in flat terrain creates problems from a flood perspective, despite mitigation measures already in place.
Urbanization has increased the probability of an event like the flooding associated with Hurricane Harvey by about 21 times, say the authors in Nature on page 388.
The authors make several high-level recommendations.
Urban planning must take into account the compounded nature of the risk now recognized.
Flood mitigation strategies must recognize the effect of urbanization on hurricanes.
Weather and climate models must incorporate the effects of urbanization to increase forecast accuracy on local and regional levels.
“It is critical for the next generations of global climate models to be able to resolve the urban areas and their associated processes,” conclude the authors.
About the Authors and Models
The authors are:
Wei Zhang and Gabriele Villarini from the Department of Hydroscience & Engineering, The University of Iowa, Iowa City, IA
Gabriel A. Vecchi from the Department of Geosciences, Princeton University and the Princeton Environmental Institute, of Princeton, NJ
James A. Smith from the Department of Civil and Environmental Engineering, Princeton University, Princeton, NJ.
To date, the role of a city in altering rainfall during tropical cyclones has received very little attention. Houston has had the largest urban growth and the fifth-largest population growth in the United States in the period from 2001–2011. Much of that growth is now on the periphery of the city. The two fastest growing parts of the region are Fort Bend and Montgomery Counties.
Also, the new NOAA Atlas-14 (rainfall measurements updated after Harvey) does not consider forward-looking urban growth effects. The precipitation frequency data in NOAA Atlas 14 was determined by a statistical analysis of historical rainfall, a key input for FEMA Flood Insurance Rate Map (FIRM) modeling. With all that uncertainty, we need to err on the side of caution in flood planning.
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2019/02/KingwoodGreens-e1551452236612.jpg?fit=1500%2C1038&ssl=110381500adminadmin2019-08-18 20:39:572019-08-19 02:39:01Study Suggests Large Cities Like Houston Can Intensify Rainfall and Runoff From Hurricanes
A developer mysteriously installed two small culverts with grates in the ditch that runs on the west side of Loop 494. I say “mysteriously” because there was no apparent reason to install them. The developer also built a road over the culverts. But the road led nowhere. So there was basically no need for the project.
Picture taken last December. The Porter Dam, an undersized culvert and an unnecessary road, backed water up for blocks.The offending culvert and grate restricted flow in the ditch. Also from last December.The Road to Nowhere crosses a ditch only to end at a detention pond. Photo taken December, 2018.
Residents and businesses speculated that the real purpose of this project was to slow the flow of water from the north to speed up the flow of water from the south where a large shopping center and an apartment complex exist. While the small culvert worked wonders for those developments, it backed water up into businesses and homes for several blocks to the north.
As a result, Mike Eberle, owner of Mike’s Hobby Shop, Lakeside Plumbing and the Gulf Coast Raceway got an unwelcome early Christmas present last year. Here’s what the situation looked like before Christmas, 2018. Not good for a hobby shop!
Photo by Mike Eberle, December 2018.
Photo by Mike Eberle, December 2018.
Photo by Mike Eberle, December 2018.After the flood, Eberle complained to TxDot. Within days, a backhoe began lowering the road over the culvert. By lowering the road, the developer hoped to increase the conveyance of the ditch without removing the culvert.
First Attempt at Porter Dam Mitigation Failed
However, that attempt at mitigating the Porter Dam failed. Within five months, the same area flooded again.
Last week, a reader, Dan Monks, sent me this photo. It looks like the developer is trying a third time with bigger culverts AND a lower road.
Bigger culverts now installed under the Road to Nowhere.Photo 8/14/2019, courtesy of Dan Monks.
Let’s hope this attempt works better than the last and gets things back to normal. These culverts appear to match the size of those farther up the ditch.
Please Help Shine a Public Light on Similar Problems
Hundreds of people, businesses and even a church were damaged by this thoughtless, careless project.
If you see situations that contribute to flooding, please send photos via the submissions page of this web site. By shining a public light on problems, we can encourage people to fix them.
Posted by Bob Rehak on 8/17/2019with photos courtesy of Dan Monksand Mike Eberle
718 Days since Hurricane Harvey
https://i0.wp.com/reduceflooding.com/wp-content/uploads/2019/08/loop494culverts3.jpg?fit=640%2C579&ssl=1579640adminadmin2019-08-17 22:57:532019-08-17 22:58:04Cross Your Fingers: Porter Dam Replaced by Two Large Culverts Under Road to Nowhere
Recently Obtained Documents Raise Questions about Amount of Sediment in Mouth Bar Due to Harvey
ReduceFlooding.com has obtained a copy of the study withheld by the Army Corps that the Corps used to justify dredging only 500,000 cubic yards from the mouth bar of the San Jacinto West Fork. The Corps refused to supply it in response to my Freedom of Information Act (FOIA) request in June. However, the City of Houston did supply the Corps document in response to a similar FOIA request. Now, thanks to Council Member Dave Martin, the public has an opportunity to compare the two studies side by side for the first time.
History of Controversy
For almost a year, the City and the Army Corps have argued over how much sediment was deposited in the mouth bar of the San Jacinto river by Hurricane Harvey. That determines how much dredging FEMA will fund. Initially, the City recommended working with two Texas Water Development Board sedimentation surveys conducted in 2011 and 2018. But no measurements exist from the period immediately BEFORE Harvey – only AFTER. So the Corps rejected that idea.
Corps Demands then Rejects Stockton Protocol
To determine Harvey volume, the Corps then required the City to provide direct measurement of the sediment through something called the Stockton Protocol. (See this memo from Stephen Costello, Houston’s Chief Recovery Officer, outlining this request and the reasons for it.)
The Stockton Protocol combines ultra-high-resolution CHIRP seismic data with core sampling. The seismic identifies layer thickness and the core sampling identifies layer composition. (Note: the process is somewhat like the oil field practice of confirming seismic with core samples from exploratory wells.) The hope: that by analyzing changes in sediment composition (such as color, grain size, roundness, hardness, etc.), researchers can differentiate Harvey sediment from other floods and then measure it accurately.
The Army Corps recommended a Texas A&M Galveston professor, Dr. Timothy Dellapenna, to do the research. However, the City of Houston and A&M could not agree on contract terms. Therefore, the City hired Tetra Tech, to perform the research that Dr. Dellapena outlined.
Corps Produces Own Analysis
At the end of the day, even with 500,000 cubic yards, those two estimates still vary by almost 3X. According to Houston City Council Member Dave Martin, the Corps never explained why they rejected the Tetra Tech analysis.
The Corps simply accepted its own results and started dredging without public explanation or input. The Corps document raises many questions that may or may not have valid answers.
Corps Analysis Requires Explanations Never Supplied
Why did the Corps:
Corps Rejects Use of TWDB Surveys, Then Bases Own Analysis On Them
To estimate Harvey-related volume, the City initially proposed analyzing two Texas Water Development Board sedimentation surveys from 2011 and 2018.
Here is the full text of the Corps’ 4-page unsigned study. We now know that…
Basically, the Corps tried to estimate the amount of sediment that Harvey’s flow could theoretically carry. That would depend on velocity and sediment size/weight. But the gage at US 59 stopped recording at the peak of Harvey. So they also had to estimate the discharge (volume of flow in cubic feet per second [cfs]). Then they used industry-standard curves to estimate sediment transport based on estimated discharge. But they discarded rates over 45,000 CFS because they produced unexpectedly high values.
They also ignored the presence of mile-wide sand mines upstream. The river ruptured the dikes of those mines and captured the pits during Harvey.
Corps Rules Out Extremes for Extreme Event
The Corps says in its report, “there are no measurements above 45,000 cubic feet per second.” Yet the combined peak flows coming from the West Fork, Spring and Cypress Creeks reached approximately 240,000 cubic feet per second during Harvey – five times more. The faster and higher the flow, the more sediment that can be transported downstream and over greater distances.
When the industry-standard sediment transport curves yielded unacceptably high results, the Corps resorted to a simple 1-D model (developed earlier for another purpose) to calculate the sediment load, because flows beyond 45,000 cubic feet per second “produced sediment loads far beyond a reasonable range.”
Corps Assumes Harvey Transported Same Percentage To Mouth Bar as Other Storms
One potentially fatal assumption: The Corps assumes that Harvey transported the same percentage of its sediment load to the mouth bar as all other storms between 2011 and 2018. Said another way, they assume that Harvey behaved LIKE all other storms. Yet not all those floods inundated sand mines.
Quantum Leap in Erosion Not Factored In
Harvey’s erosive power was NOT proportional to other storms, as the photos below show. River banks eroded more than a hundred feet during Harvey in many places. Yet the Corps report never even mentions erosion.
Photographic analysis shows similar quantum leaps in erosion related to Harvey elsewhere along the West Fork.
The shearing force of 240,000 cubic feet per second coming down the West Fork literally pulled thousands of trees out by their roots and dislodged sediment disproportionately compared to previous floods (see below). The Gallery page of this web site clearly shows the extent of this devastation. It contains 450 images taken from a helicopter on 9/14/2017, two weeks after Harvey.
Corps Assumes Mouth Bar Growth Did Not Affect Percentage Deposited by Harvey
The Army Corps also assumes that Harvey transported the same fraction of the total sediment load (20%) to the mouth bar that all storms did between 2011 and 2018. That’s a dubious assumption for several reasons:
Yet the Corps based its estimate on a constant 20%. Page 3 of their report spells out the assumption. Harvey, they say, deposited approximately the same fraction of sediment at the mouth bar as all other storms did during the period between surveys.
This constant 20% contradicts numerous anecdotal reports from lakeside residents and boaters claiming that Harvey carried vastly more sediment to the mouth bar (and their yards/docks) than previous storms. The wife of the resident wading across the river in the image above told me that, on a scale of 1 to 5, the Tax and Memorial Days floods deposited sediment in her yard equal to a 1. But Harvey, she said, was a 6. In other words, off the scale.
Taxpayers Deserve Independent Scientific Review
Professionals rarely like to have their conclusions questioned. However, those who have confidence in their conclusions welcome peer and public review. They encourage second opinions and provide all of their data for review. They also welcome the opportunity to explain and defend their results. None of those things happened in this case.
Instead, the Corps concealed its results as if this involved national security, not public safety. Why? That may be the biggest question of all associated with this project.
As mentioned above, the Tetra Tech study may also have flaws, but the Corps never revealed what its concerns were.
Only one thing is certain. Public safety rests on wildly differing studies. Taxpayers deserve an independent scientific review to resolve the differences between these two studies. The City concurs with the findings in this post and also calls for an independent scientific review. The Corps could not be reached for comment; their new public affairs officer does not list her phone number.
Dredging will likely end next week, with the Corps proclaiming it has restored the conveyance of the river to pre-Harvey conditions (when they have no pre-Harvey measurements).
Corps Plans Still Being Kept from Public
The Corps still has not released its dredging plans, despite a FOIA request made in June when mouth bar dredging started.
Visual observations of the operation suggest that they are dredging a wide area by three feet, to a total depth of about five feet, instead of trying to cut a channel through the mouth bar. That would leave something like an underwater mesa, still blocking the flow and still trapping sediment. Water coming downriver would have to climb a steep hill to get over it.
If that is an accurate assessment, the Corps would leave a sediment wall under the water approximately 30-35 feet high and 1-2 miles long in the mouth of the West Fork.
Others Scrambling to Pick Up the Pieces
It may look like the Corps has dredged. But it also looks like the Corps will leave 80-90% of the mouth bar in place. Remember, sand bars are like ice bergs in the sense that what you see above water is small compared to the amount you can’t see below water.
At this point, City, County, State and Federal leaders are scrambling to put together a plan to address the rest of the sediment. Some of that sediment is clearly pre-Harvey. I will discuss options for removal of that portion and maintenance dredging in a future post.
Posted by Bob Rehak on 8/20/2019
721 Days after Hurricane Harvey
As in previous posts on this subject, I promise the Corps that I will print their rebuttal verbatim if they disagree with any of the points in this post.
Study Suggests Large Cities Like Houston Can Intensify Rainfall and Runoff From Hurricanes
A November 2018 article appearing in the peer-reviewed scientific journal Nature found that urban growth can intensify both rainfall and runoff from hurricanes. Further, urban growth can increase the risk of flooding and shift the location of flooding. The article specifically studied the effects of Hurricane Harvey on Houston and found that urban growth increased the probability of such an extreme flood across the basin by 21X.
A sister publication, Scientific American, reviewed the article the same month and helped explain the findings in Nature.
The Runoff Component
Numerous studies have looked at the relationship between percentage of impervious cover, runoff, and flooding – a well documented phenomenon. Impervious cover accelerates transport of rainfall from neighborhoods to rivers. That raises peak flows rather than spreading them out over time. Dr. William Dupre, professor emeritus from the University of Houston visualized the relationship this way.
Rainfall Component Much Less Studied
However, the effect of urban growth and a city’s surface topography on RAINFALL from hurricanes is much less studied. The authors say in Nature that, “Urbanization led to an amplification of the total rainfall along with a shift in the location of the maximum rainfall.” (Page 386).
“Much less is known regarding the urban effects on the organized tropical rainfall of a hurricane, in particular during one like hurricane Harvey, which stalled for several days.” They continue, “…experiments (with computer models) clearly show a large increase in rainfall arising from urbanization over the eastern part of the Houston area.”
The authors compared present and past urban landscapes and also modeled a scenario in which the entire region was cropland.
Mechanisms Responsible for Increase Rainfall
To understand the physical mechanisms responsible for the heavier rainfall, they analyzed the vertical convergence of winds and wind fields.
“The enhanced rainfall … and the shift of rainfall … are tied to the storm system’s drag induced by large surface roughness,” say the authors.
Scientific American explains in more detail. Kerry Emanuel, an atmospheric scientist at Massachusetts Institute of Technology, who did not work on the study said, “We know cyclones are sensitive to characteristics of the surface—mountains, streams, marshland. This new twist is that cities have become big enough to tangibly alter the storm.” Said Gabriele Villarini, an environmental engineer at The University of Iowa and an author on the study, “We removed the urban areas from Houston and replaced them with cropland.”
“The presence of urban areas enhanced all the things you need to get heavy precipitation,” Villarini, one of the study’s authors says. “A stronger drag on the storm winds, associated with a larger surface roughness length” contributed to the increased rainfall.
Emanuel explained, “First, the artificial ruggedness of an urban area slows air down. Whenever air slows in a hurricane, he says, it gets shunted toward the center of the storm and up into the sky. That increases rainfall everywhere [in a metropolitan area].” He added, “A storm moves particularly slowly over downtown areas where buildings are tallest, but the winds bearing down from outside the city are still moving quickly. So, [the storm] is piling up on the city.”
Impact on and Implications for Houston
This increase in urban growth in flat terrain creates problems from a flood perspective, despite mitigation measures already in place.
The authors make several high-level recommendations.
“It is critical for the next generations of global climate models to be able to resolve the urban areas and their associated processes,” conclude the authors.
About the Authors and Models
The authors are:
This presentation explains the Noah Model that the authors used to calculate air/ground interactions.
Local Questions Raised by Study
To date, the role of a city in altering rainfall during tropical cyclones has received very little attention. Houston has had the largest urban growth and the fifth-largest population growth in the United States in the period from 2001–2011. Much of that growth is now on the periphery of the city. The two fastest growing parts of the region are Fort Bend and Montgomery Counties.
As the city grows, we need mitigation measures that can offset the impact of that growth. That’s why the meeting of the Montgomery County Commissions on August 27th is so important. They will vote on whether to close a loophole that allows developers to avoid building onsite detention ponds. Closing that loophole is important. It will help protect hundreds of thousands of downstream residents as well as those in Montgomery County.
Also, the new NOAA Atlas-14 (rainfall measurements updated after Harvey) does not consider forward-looking urban growth effects. The precipitation frequency data in NOAA Atlas 14 was determined by a statistical analysis of historical rainfall, a key input for FEMA Flood Insurance Rate Map (FIRM) modeling. With all that uncertainty, we need to err on the side of caution in flood planning.
For more about Atlas 14, see this link.
Posted by Bob Rehak on 8/18/2019
618 Days after Hurricane Harvey
Cross Your Fingers: Porter Dam Replaced by Two Large Culverts Under Road to Nowhere
Last year, shortly before Christmas, I reported on flooding between Loop 494 and US59 related to an undersized culvert, nicknamed the Porter Dam.
A developer mysteriously installed two small culverts with grates in the ditch that runs on the west side of Loop 494. I say “mysteriously” because there was no apparent reason to install them. The developer also built a road over the culverts. But the road led nowhere. So there was basically no need for the project.
Residents and businesses speculated that the real purpose of this project was to slow the flow of water from the north to speed up the flow of water from the south where a large shopping center and an apartment complex exist. While the small culvert worked wonders for those developments, it backed water up into businesses and homes for several blocks to the north.
As a result, Mike Eberle, owner of Mike’s Hobby Shop, Lakeside Plumbing and the Gulf Coast Raceway got an unwelcome early Christmas present last year. Here’s what the situation looked like before Christmas, 2018. Not good for a hobby shop!
First Attempt at Porter Dam Mitigation Failed
However, that attempt at mitigating the Porter Dam failed. Within five months, the same area flooded again.
Last week, a reader, Dan Monks, sent me this photo. It looks like the developer is trying a third time with bigger culverts AND a lower road.
Let’s hope this attempt works better than the last and gets things back to normal. These culverts appear to match the size of those farther up the ditch.
Please Help Shine a Public Light on Similar Problems
Hundreds of people, businesses and even a church were damaged by this thoughtless, careless project.
If you see situations that contribute to flooding, please send photos via the submissions page of this web site. By shining a public light on problems, we can encourage people to fix them.
Posted by Bob Rehak on 8/17/2019 with photos courtesy of Dan Monks and Mike Eberle
718 Days since Hurricane Harvey