Whittling Down the San Jacinto West Fork Mouth Bar
The State of Texas, Harris County and the City of Houston are whittling down the mouth bar of the San Jacinto West Fork – teaspoon by teaspoon. Just kidding; it only feels that way.
At the planned rate, the partners will remove approximately one third to one half of the planned 400,000 cubic yards of sediment by the start of hurricane season. But after waiting two and a half years since Hurricane Harvey, any and all progress is welcome! I’m not complaining.
The Mouth Bar Immediately After Harvey
Harvey deposited massive amounts of sediment in the area where the San Jacinto meets Lake Houston.
The Army Corps of Engineers dredged a 600-acre area south of the mouth bar three feet deeper last summer. However, they barely touched the part of the mouth bar above water.
The mouth bar created a sediment dam behind the Lake Houston Dam that contributed to flooding more than 4100 structures in the Humble, Kingwood, and Atascocita areas.
Why Mouth Bar Formed Where It Did
The mouth bar formed where it did because the river water slows down when it meets the lake. The lower velocity causes sediment to drop out of suspension and accumulate faster.
While the Corps used hydraulic dredging to remove 500,000 cubic yards from the mouth bar in three months, the current phase uses mechanical dredging. Partners hope to remove another 400,000 cubic yards in 12 months. The process resembles whittling in that workers remove small chunks at a time.
Big Machines Dwarfed by Size of Job
Mechanical dredging uses large excavators. They load the sediment on pontoons. Tugs then push the pontoons upriver to a placement area. There, skid loaders remove the sediment and put it in trucks. The trucks cart it inland.
The excavators are currently nibbling away at the southern edge of the bar. I took all photos below on 3/6/2020.
Mechanical vs. Hydraulic Dredging
The whole process resembles a five-mile long conveyor belt. It involves excavators, pontoons, tugs, trucks and more. Both mechanical and hydraulic dredging have advantages and disadvantages. Hydraulic dredging takes more time to mobilize, but re-suspends less sediment, and costs less per cubic yard of sediment removed. Mechanical dredging, on the other hand, can mobilize much faster.
At this point, returning to hydraulic dredging feels like a distant dream. No one is commenting on the possibility. But this picture speaks volumes.
It shows the once-bustling, but now-empty Army Corps command post. Just three months ago, it was filled with dredge pipe, spare parts, construction trailers, pontoons, booster pumps, survey boats, and more. Getting all that equipment back will be difficult.
Posted by Bob Rehak on 3/12/2020
926 Days since Hurricane Harvey