The Case for Dredging the “Mouth Bar”
A “mouth bar” is a sandbar that builds up at the mouth of a river where it meets a standing body of water, such as Lake Houston. The West Fork of the San Jacinto has a world-class whopper of a mouth bar.
How and Why Mouth Bars Form
A mouth bar forms when water in the river slows down as it spreads out in a standing body of water. The lower velocity of the river can no longer suspend particles of sediment. According to academic and petroleum geologists I talked to, this phenomenon exists in rivers everywhere. In fact, mouth bars are an essential element of delta formation.
Sequence of Events in Formation
As a mouth bar grows in height and emerges from the river, it backs water up and slows it down. This causes the river upstream of the mouth bar to gradually fill with sediment, ultimately choking the river and forcing it to seek a new path. At this point, the higher pressure created by the backwater forces the river to seek new channels. At this point, typically the river splits into two (bifurcates). This accounts for the branching structures found in most deltas.
That is exactly what’s happening where the West Fork of the San Jacinto meets Lake Houston as this series of time-lapse images shows. Note the growth of the mouth bar in areas highlighted in white below.
2011 image of the mouth bar where the West Fork of the San Jacinto meets Lake Houston. Note how bar has formed at tip of main channel.
By 2013, the mouth bar had taken on a triangular shape where it was starting to split the main flow of the river.
Image taken on the last day of 2016. The mouth bar grew considerably in the Tax Day and Memorial Day floods in 2015 and 2016, primarily by extending its length.
October 2017. During Hurricane Harvey, the mouth bar doubled in size. It definitely splits the flow of the river now.
On 9/14/17, the bar looked like this from a helicopter.
Approximately two-thirds of the homes damaged by flooding in the upper Lake Houston area were between this bar and where the U.S. Army Corps of Engineers will stop dredging.
Historical Context: A Lesson in Geomorphology
The growth of this mouth bar was predictable. Brown & Root said in 2000 that it would emerge exactly where it did. What will happen in the future if we don’t dredge it? That, too, is predictable. See this presentation by William Dupré, professor of geosciences at the University of Houston. Professor Dupré’s presentation, given at the Houston Geological Society April conference on flooding, contains excellent illustrations of how rivers migrate laterally over time.
Consequences of Not Dredging
A retired chief geologist for a leading oil company (who specialized in sedimentation) tells me that if this bar is not dredged, we could expect the following consequences. It will, he says:
- Continue to grow in height, width and length.
- Slow down and back up water behind it.
- Force increased sedimentation upstream (including areas soon to be dredged)
- Likely also increase the frequency and magnitude of overbank flooding upstream of the mouth bar
- Jeopardize homes, bridges, pipelines and other infrastructure on both sides of the river as it branches.
Two Options for Dredging
The contract that the Army Corps of Engineers expects to sign with a dredging vendor does NOT currently include this bar in its scope. I wish it did for all the reasons listed above.
The proposed contract includes a clause that allows expansion of scope if both the Corps and Contractor agree on it. That would be the most cost efficient way to address this problem. Dredges will already be on the river. Millions of dollars of mobilization costs for second dredging project could be avoided and the issue could be addressed sooner.
However, if expanding the scope of the Corps project is not possible, I believe residents of the Lake Houston area should insist that the County covers it in the upcoming flood bond referendum.
Posted by Bob Rehak on 6/23/2018
298 Days since Hurricane Harvey
