West Fork Mouth Bar: Going, Going…
It’s not quite gone yet. But the West Fork Mouth Bar, which forms a dam behind the dam, is getting smaller every day. Every time I fly over it, I can see how mechanical dredging has shaved more off of it . Excavators take one row after another. At this point, it appears that about half of the above-water portion of the bar has been removed.
Mouth Bar Photos from Flyover on 4/21/2020
I took this series of photos taken on 4/21/2020.
Goal of Project Still Not Made Public
When contractors remove the last of the island sticking up above water, it’s not clear what the plan will be for the rest of the sand bar below water. Like icebergs, that’s where most of the mass is.
Conveyance improved somewhat last year when the Corps removed 500,000 cubic yards of sediment from the broad area between the mouth bar and Atascocita Point. This project should improve conveyance even more.
But the Corps did not dredge a channel that connected the upstream portion of the West Fork with Lake Houston. And it appears that this effort will not re-establish a channel either.
The end result will be a plateau or mesa 3-5 feet below the surface. The channel immediately upstream where the Corps finished dredging is approximately 25 feet deep. That means water coming downstream in a flood will still hit a wall.
Top Geologist’s Perspective
Tim Garfield was the top geologist for one of the world’s largest oil companies. He lives behind the mouth bar and had this to say about it.
“Although it likely represents some progress, shaving off the subaerial (above water) portion of the mouth bar seems more cosmetic than useful for flood mitigation. The reason: the Corp’s dredging two years ago trenched a channel roughly 400-500’ wide and 20-25’ deep that extends downstream of West Lake Houston bridge. It ends a few hundred yards upstream of the Mouth Bar. This greatly increases flow conveyance in the channel. But that ends abruptly at a sand pile where water depths are less than 5’ deep,” said Garfield.
“That’s a 15’ – 20’ high, underwater sediment dam during floods,” he continued. “That will result in a significant hydraulic backwater effect causing overbank flow and upstream flooding. These points are illustrated in diagram below.”
Conclusion: Hydraulic Model Needed
Garfield concluded, “We really need an extension of the Corps’ dredged channel profile through the 1960 bridge opening to where Lake Houston water depth equals the channel depth of 22’-25’. That work requires an up to date bathymetric map of the area and a hydraulic model to confirm optimal channel configuration and location.”
“That plan should be made public so that we can verify that the vast sums of public funds being spent are effectively reducing the flood risk we are still exposed to,” said Garfield.
Posted by Bob Rehak on 4/27/2020 with thanks to Tim Garfield and RD Kissling
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