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.
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.
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
Tetra Tech concluded Harvey deposited 1.4 million cubic yards in the mouth bar (although they didn’t state it that clearly). The Corps rejected Tetra Tech’s results and produced its own study. That study concluded Harvey deposited only 283,000 cubic yards in the mouth bar – one fifth as much. However, the Corps authorized 500,000 cubic yards to compensate for the margin of error and additional sediment they would have to dredge just to reach the mouth bar.
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:
- 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!
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.
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.
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.
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).
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.
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.
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.