A recent mouth bar survey shows that dangers still likely lurk just beneath the surface. Here’s why they will only get worse if we don’t take action.
Corps Survey Shows Average Depth South of Mouth Bar
An Army Corps survey of the West Fork Mouth Bar area conducted on 9/4/2019 shows that the average depth after dredging is a bit more than five and a half feet. And that’s only because they surveyed some areas pretty far downstream from the mouth bar.
The survey contains 21,766 sampling points. You can review them all here, in this 507-page PDF. Unfortunately, my web host does not allow posting of Excel files. So you can’t explore the data as conveniently as you might like.
The Corps did not measure the mouth bar itself, which appears to be about four feet high in places. So that also skewed the data. And I am sure that the shallow areas near the bar prevented the survey boat from acquiring information there to. For instance, there are no recorded depths less that one foot.
Scattergram of All Soundings
This graph helps visualize where the bulk of the values lie as well as the extremes and range.
Distribution of Values
A bar graph is another way to visualize the distribution of values. On the vertical axis, we can see the number of soundings. On the horizontal axis, we can see the depth in feet. About half the survey points fell within the 5-6 foot range. That’s not surprising for a survey with an average depth of about 5.5 feet.
Sedimentation from Imelda Not Included
It’s important to realize that the Corps conducted this survey AFTER dredging, but BEFORE Imelda. We saw how much sediment came down the East Fork – enough to raise the bottom depth from 18 feet to 3 or 4 feet between Luce Bayou and Royal Shores, according to boater Josh Alberson.
West Fork Flooding
Of course, Imelda did not dump as much rain on the West Fork. So flooding there was less severe. But it still ranked as a “major flood.”
At US59, the flood peaked at about 8PM on September 19th as you can see in the graph below from the Harris County Flood Warning System.
This map from Jeff Lindner, meteorologist for Harris County Flood Control, shows the distribution of rainfall across Harris and souther Montgomery Counties, plus streams that flooded. The purple color applied to the West Fork indicates it experienced a major flood. Note: these are 48-HOUR rain totals.
In fact, I have heard reports of the river flooding streets in Atasocita Point and Fosters Mill, long after the peak of the storm. Other areas may also have flooded.
Imelda’s floodwaters also ruptured a number of sand mine dikes on the West Fork. They then carried a significant amount of sediment downstream. I took the shot below on 10/2, almost two weeks after the storm. It shows the convergence of Spring Creek and the West Fork, just west of the US59 bridge. Note the huge difference in the color of water between the two streams.
That’s, in part, thanks to breaches like the one below in West Fork sand mines that were still open long after the flood.
You could see the impact downstream.
What does all this mean?
The Corps has shortened the ramp between where it stopped dredging in Phase I and where it started dredging the mouth bar area. While they increased the conveyance by a minor amount, water will hit an underwater wall when it gets to the mouth bar. It’s like herding water into a box canyon. If the City were to lower the lake by 5.5 feet, you would see a sediment dam about a half mile long.
Despite dredging three feet, the mouth bar in its current state will still contribute to flooding…in my opinion.
The City still has not announced any plans for additional dredging despite the availability of more than $40 million earmarked for that by the State and County.
The Great Lakes Dredge has been anchored at the mouth bar for a month and a half. Recently, a crane showed up at the Army Corps command post opposite Marina Drive in Forest Cove. One can only wonder whether Great Lakes will pull their dredge out of the river like Callan did last month.
The Value of Self-Reliance
If dredger(s) have to remobilize at a later date, the cost could eat up close to half of the money earmarked for dredging by the State and County. Mobilization in Phase I cost approximately $18 million.
FEMA and the Corps have refused to dredge more than the 500,000 cubic yards that they already have. Nevertheless, Stephen Costello, the mayor’s flood czar, is making one more plea for reconsideration.
This whole episode reminds one of the wisdom of self-reliance.
The City needs to put up some of its own money out of that billion dollars we’ve paid in drainage fees to:
- Redo the post-dredging survey to see how much sediment Imelda deposited.
- Model the effects of various flood scenarios.
- Share the results with the world.
- Take appropriate action to restore public confidence.
I find it incredible that after two years of arguing about this, no one has yet modeled the effects of the mouth bar on flooding and shared it with the world.
Posted by Bob Rehak on 10/10/2019
772 Days after Hurricane Harvey and 21 since Imelda
The thoughts expressed in this post represent my opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.