Tag Archive for: Tetra Tech

City Mobilizing for More West Fork Dredging

Mobilization for the next phase of San Jacinto West Fork dredging is underway. The City of Houston and its contractor DRC (a subsidiary of Callan Marine) are already staging equipment in two places on the West Fork.

The program, funded by FEMA, will remove an estimated 800,000 cubic yards of silt and sediment between the original location of the West Fork Mouth Bar and FM1960. The contractor will use primarily hydraulic dredging and the program will take approximately two years, according to District E City Council Member Fred Flickinger.

West Fork Dredging Project Dates Back to Dave Martin Era

Flickinger credits his predecessor, former Council Member Dave Martin, and Chief Recovery Officer Stephen Costello’s tireless efforts in protesting the initial amount proposed for dredging by FEMA back in 2019. FEMA’s initial proposal, based on a four-page, table-top study produced by the Army Corps, called for dredging 283,000 cubic yards.

Martin strongly disagreed with the Corps’ report and appealed it while the City produced its own 94-page technical report. It showed a much higher volume deposited by Harvey. Remember: Harvey funds could not be used to address sediment deposited before Harvey. The City report produced by Tetra Tech relied extensively on core samples. Tetra Tech proved that Harvey laid down the sand in the mouth bar and that the dredging volume should be closer to a million cubic yards.

In August 2020, FEMA and the Corps finally concurred with the City, after extensive discussions and a massive assist from U.S. Representative Dan Crenshaw. Crenshaw and others had been pushing FEMA for years for the additional dredging.

Current Status

The new West Fork dredging program should be ready to go within weeks. DRC is currently bringing in the equipment that they will need.

DRC plans to use primarily hydraulic dredging. They will attack the area between where the mouth bar was (south of Scenic Shores in Kings Point) and the FM1960 Bridge. See map below.

Map from City study showing area of focus.
Hydraulic dredge being assembled at old Army Corps mobilization site south of Forest Cove pool. Photo taken 4/1/24.
DRC is also starting to stockpile mechanical dredging equipment such as these pontoons on Berry Madden’s property south of River Grove Park (top center).

This is good news. The new West Fork dredging will help ensure that water doesn’t back up like it did before. It’s not a guarantee against flooding. Dredging is only one part of a multi-faceted mitigation program that also includes more upstream detention and new floodgates on the Lake Houston dam. More news on those topics to follow.

Posted by Bob Rehak

2407 Days since Hurricane Harvey

Tetra Tech Study Provides Clues To Possible Mouth Bar Dredging Strategies

FEMA has agreed to dredge another million cubic yards from the the area near the San Jacinto West Fork Mouth Bar. A report produced for the City of Houston by Tetra Tech helped convince FEMA. The report relied on sonar, LIDAR, and core sample data to estimate the total volume of sand deposited by Hurricane Harvey in that area: approximately 1.4 million cubic yards.

Need for Ruthless Efficiency

While another million cubic yards may sound like a lot, the area is huge. Dredging the whole 4.3 million square yard area would add only about 8 inches of water depth and leave an underwater mesa between the West Fork and the Lake. According to local geologists Tim Garfield and RD Kissling, who have studied the problem extensively, that would create a sediment trap that accelerates accumulation of sand from future storms.

So, what to do?

Three Strategies Discussed to Date

Those close to the project have discussed several strategies to date.

  • The Corps’ initial strategy: Dredge upstream from the mouth bar. They said 1D modeling showed that would accelerate water flowing into mouth bar and give it the velocity needed to push sand from the mouth bar farther out into the lake.
  • Another strategy: dredge downstream from the mouth bar and let the river push the mouth bar into the dredged area.
  • A third strategy: reconnect the river and the lake with a narrow channel that accelerated the flow of water and carried suspended sediment out into the broader lake south of the 1960 bridge.

2019 Tetra Tech Report

Stephen Costello, the City’s flood czar, says that new survey and modeling work has yet to be completed. That will ultimately determine where new dredging happens. However, he also added that consulting Tetra Tech’s exhibits would help provide clues as to where dredging might be most effective, based on knowledge accumulated to date.

The first chart in Appendix A showed the coring locations and transects (survey lines) of the lake’s bottom profile.
The second chart shows what they found in various coring locations. The feet indicate the thickness of the top layer.

Composition of the core samples provides clues as to what was laid down when. Sand (the yellow dots) is generally laid down during floods which have the energy to transport the heavy particles. However, clay and silt (the green and blue dots) are smaller. So they tend to drop out of suspension when water is calmer.

Finding sand above silt in a core sample indicates that a storm like Harvey likely laid down the sand.

The third chart is the most crucial. It’s a difference map that shows areas of deposition and scour pre- and post-Harvey. This shows two things: where most sediment fell out of suspension and where the main flow of the river tried to churn a path through the mounting muck.

From the difference map above, you can see that the river tried to scour its way through the sediment along a path from LH-16 to LH-21 to LH-23. You can see another area of scour to the far right from LH-15 to LH-25 to LH-26.

Where River Flowed Before Lake Was Impounded

Interestingly, the area of scour to the left follows the river’s relic channel.

San Jacinto River map before Lake Houston was impounded

Note how the West Fork hugged what is now Atascocita Point – the thumb of high land that sticks up in the Tetra Tech illustrations.

Harnessing Natural Energy of the River

From the third and fourth illustrations above, one might conclude that excavating a channel near Atascocita Point represents the best way to harness the natural energy of the river. That’s the shortest channel where scour is deepest.

Given the million cubic yard limit, that path also represents a chance to dig the deepest, widest channel possible within the budget. When technicians compiled the difference map above, most of that path was already at or below its 2011 level.

500,000 square-yard path outlined in yellow would let dredgers excavate six feet. Average bottom depth is already 5.5 feet in that area.

Following that path also lets you funnel future sediment through the FM1960 causeway and disperse it out into the wider, deeper lake.

Next Steps and Timing

At this point, we don’t know what Imelda did to this area. Imelda struck shortly after the Army Corps completed its post-dredging survey in this area last year.

Before the additional dredging can begin, several things must happen.

  • Completion of a new survey
  • Model different scenarios
  • Identify best strategy
  • Locate suitable placement area
  • Compile scope of work
  • Bid job
  • Mobilize

Based on past experience, that could take months to a year or more. It took 13 months after Harvey for the Corps to put equipment in the water for its Emergency West Fork Dredging Project. However, we don’t have as many unknowns this time.

Posted by Bob Rehak on 9/5/2020

1103 Days since Hurricane Harvey

FEMA/Corps To Stop Dredging Mouth Bar Before Finishing Job; What You Can Do

Having barely scratched the surface of the mouth bar of the San Jacinto West Fork, FEMA and the Army Corps will pack up their gear next week and call their job done. Last-ditch pleas by the City of Houston, Harris County and the State of Texas to get the federal government to extend its dredging program have fallen on deaf ears, perhaps because of the shifting of disaster relief funds to the construction of migrant detention facilities.

Mouth bar of the West Fork shortly after start of supplemental dredging. Photo courtesy of BCAeronautics.

Regardless, the bottom line is this: the Corps and FEMA will leave millions of cubic yards of sediment in place without restoring conveyance of the West Fork to a prior good condition.

The pullout caps months of arguments over how much sediment Harvey deposited. The City estimated 1.4 million cubic yards and the Corps 500,000.

According to City Council Member Dave Martin, the Corps agreed Harvey deposited 1.4 million cubic yards of sediment in the river near the mouth bar. The Corps also agreed, said Martin, that there was nothing wrong with the Tetra Tech study that arrived at that total.

Waffling by Corps

As late as last Friday, Martin said, the Corps agreed to write a letter to FEMA, recommending dredging more than the 500,000 cubic yards. The letter would say that almost a million cubic yards of Harvey-related sediment remained in the river and should be removed. However, at a meeting in Austin this Tuesday, the Corps revealed that FEMA told it not to write the letter. The Corps now intends to demobilize equipment as soon as it finishes dredging 500,000 cubic yards from the mouth bar. That should only take until next week.

These developments confirm speculation that the Corps “backed into” the 500,000 cubic yard number for reasons unrelated to Harvey. Mystery still surrounds how they arrived at that number. The Corps refused to release many documents related to their decision. A review of their 4-page analysis obtained from the City found numerous issues, logical flaws, and questionable assumptions – uncharacteristic of the Corps.

Next Steps

With the year-long dredging program now almost complete and perhaps less than a quarter of the sediment removed that is required to restore the natural flow of the river, what will happen next? We have some hope.

  • The Corps has finally approved Berry Madden’s property as a storage site for 500,000 cubic yards. That should be enough to get the next phase of the program started while the City seeks additional storage sites.
  • The City has committed to a maintenance dredging program according to Martin.
  • The State and Harris County have earmarked $30 million and $10 million respectively to continue dredging.
  • Additional funds may become available early next year through SB7.
  • Callan Marine has agreed to remain on site and do the dredging.

Your Help Is Needed

However, to make that money stretch far enough to finish the job, we will need FEMA and the Corps to designate the remaining sediment as Category A. City Council Member Dave Martin is sending this letter to all congressional and senatorial representatives in the area. Designating the sediment as Category A will:

  • Enable reimbursement from FEMA
  • Allow the City of Houston to utilize existing resources and pre-positioned contracts.
  • Save nearly $20 million associated with mobilization.

Please Contact These Officials

Here’s how you can help. Send the letter below to:

Tell them that you support the Category A designation and see the mouth bar removal as crucial to public safety with a letter like the one below.

Sample Letter


Dear _____________: 

Thank you for helping to make dredging of the San  Jacinto West Fork a priority.  It will help reduce flooding, protect property, save lives, and improve public safety.  

However, part of the existing mouth-bar located at the confluence of the San Jacinto River and Lake Houston remains.  

I’m writing to enlist your support in urging the Federal Emergency Management Administration (FEMA) to designate that remaining debris as Category A for reimbursement.  

Category A designation will allow the City of Houston to:  

  • Utilize existing resources and pre-positioned contracts  
  • Save nearly $20 million associated with mobilization  
  • Protect life, property and safety  

Field data collected by the City of Houston and provided to FEMA demonstrates that the remaining debris was directly associated with Hurricane Harvey. As of August 20, 2019, the City of Houston has proactively secured a third United States Army Corps of Engineers permitted disposal site needed for the additional debris.  

Your assistance is crucial to rehabilitate the San Jacinto River to its prior good condition.  Please urge FEMA to grant this Category A designation. It will let the City of Houston continue rebuilding from Harvey.  





Posted by Bob Rehak on 8/30/2019 with drone photo from BCAeronautics

731 Days since Hurricane Harvey

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.

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.

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.

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.

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 barone 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.

The USGS gauge used by the Corps for its analysis stopped working during the peak of Harvey when 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!

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.

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.

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 Harveyfive 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.

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).

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.

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 now where 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.

Local Geologists Develop Way to Estimate Volume of Sediment in Mouth Bar Due to Harvey

Two top geologists, now retired from one of the world’s leading oil companies, have developed a reliable and repeatable way to estimate the volume of sediment deposited in the West Fork mouth bar by Hurricane Harvey. They calculate that Harvey deposited at least 268,000 cubic yards of sediment in that area alone.

Stream mouth bar where the West Fork of the San Jacinto meets Lake Houston creates a sediment dam. It backs water up throughout the entire Hunble/Kingwood area during floods. Water must flow uphill approximately 40 feet to get over this hump.

No Pre-Harvey Measurements Hampered Dredging Program Approval

According to the Army Corps of Engineers and Stephen Costello, the City of Houston’s Chief Resiliency Officer, the lack of a reliable way to estimate the volume due to Harvey was a major stumbling block in funding the dredging effort. The City, FEMA and the Corps have reportedly been arguing about this for at least nine months. The issue has to do with the Stafford Act, FEMA’s enabling legislation. The Stafford Act prohibits FEMA from spending disaster relief funds on cleanup not related to the disaster in question.

Because the City of Houston had no reliable sedimentation survey taken immediately before Harvey, calculating the volume due to Harvey became problematic.

How a Mouth Bar Forms

A mouth bar forms at the mouth of a river where it meets still water (in our case, Lake Houston). As moving water encounters the still water, coarser sediment like sand is deposited. It begins building up and up until sand bars emerge above the surface.

The Insight that Led to a Reliable Way to Estimate Volume

Mainly sand comprises the mouth bar; sand moves only during major floods. The sand above water can only be deposited when floodwater is much higher than the top of the bar. That insight became the key to unlocking the mystery of how much sand Harvey deposited.

Everyone could see from satellite images how much the above-water portion of the bar had grown. But there was no way to tell how much the bar grew below water. Then in October of 2018, Costello presented a “difference map” that showed the sediment buildup between 2011 and 2018.

RD Kissling and Tim Garfield reasoned that if they could calculate the percentage of above-water growth during Harvey from satellite images, they could then apply that same percentage to the total volume of sand deposited below water between 2011 and 2018.

Calculation for visible “above water” growth of the mouth bar during Harvey.

But to determine the total volume added between 2011 and 2018, they first had to:

  • Digitize the difference map
  • Create polygons around the different colors
  • Calculate the area of the polygons
  • Multiply area times thickness for each
  • Add up the results.
Digitized difference map shows boundaries between areas of different thickness.
Multiplying the area of the polygons times the thickness from the difference map yielded volumes for each area.

Tetra Tech Delays May Push Project Past Deadline

According to Costello, the City hired a company called Tetra Tech in early January to calculate the Harvey volume. He said they would do that by harvesting and analyzing core samples. The City expected the results of their study by the end of January. But when I talked to Costello in mid-February, he said Tetra Tech still had not finished harvesting core samples and that he wasn’t expecting results of their analysis until the end of February or early March.

Pro Bono Effort Might Save Taxpayers $18 Million

Kissling and Garfield developed their methodology and donated their time to help save money and to get the mouth bar removed before the start of the next hurricane season. If the current dredging program can be extended before the end of April, taxpayers could save the cost of recommissioning all the equipment. That could total $18 million.

The two geologists reasoned that their methodology would give all parties a basis for allowing the dredging to continue. Then, if Tetra Tech came back later with a different figure, the contract could be adjusted up or down.

Kissling and Garfield emphasize that their methods are conservative, reliable and repeatable. For peer review and public comment, this presentation shows how they arrived at their estimates in a step by step fashion.

I am presenting it here to start a dialog that leads to additional dredging without incurring the cost of remobilizing the massive amount of equipment now on the river.

Estimate is Only a Fraction of What Needs to Be Removed

Kissling and Garfield emphasize that this is just a start. Even if all 268,000 cubic yards were removed, the river would still be up to 20 feet shallower than at the time of impoundment. They hope this leads to a broader discussion of additional dredging which could be financed through other sources, such as the county flood bond and Proposition A. Finally, they point out the need for maintenance dredging after major floods to keep the sediment buildup at sub-critical levels.

Said Garfield, “We are confident this estimate of Harvey-specific sedimentation on the mouth bar is reasonable and should be used to support FEMA funded continuation of dredging. However, removing that volume alone still won’t solve the problem, because the mouth bar is much bigger than that and it remains the largest restriction of flow conveyance to the lake.”

Garfield continued:

“At a minimum, to restore flow and reduce flood risk, a 300-400’ wide channel 20’-25’ deep needs to be dredged to connect the river from the upstream dredging now nearing completion, through the mouth bar, to the FM1960 bridge. That is at least 5 or 6 times as much sediment as our estimate of what FEMA can fund, but without that the recent dredging alone has not solved the flood risk problem in our area.”

Posted by Bob Rehak on February 24, 2019 with help from R.D. Kissling and Tim Garfield

544 Days after Hurricane Harvey