All River Levels Falling, Flood Danger Over, But…
May 21, 2024 – After a second round of May floods (and the third this year), all river levels are falling today. The East Fork, West Fork and main stem of the San Jacinto River are finally falling this morning. So are the water levels in Lake Conroe and Lake Houston.
But sedimentation will likely be a problem when floodwaters totally recede.
Gage Readings For Lake Conroe
The gates at Lake Conroe are still open, but only releasing 1,599 Cubic Feet per Second (CFS). That’s about a twelfth of what the San Jacinto River Authority released at the peak of this second wave of May floods.
The graph below shows that the lake level is slowly approaching its normal conservation pool of 201 feet.
All Gage Readings on West Fork Falling
In response, the West Fork at US59 fell below flood stage yesterday evening and continues to fall.
River Grove Park is draining, but the soccer fields are not yet playable.
At West Lake Houston Parkway, the West Fork is well within its banks and falling.
Readings on East Fork
Upstream on the East Fork at SH105, the East Fork crest has passed and the river continues to fall.
At FM2090, the East Fork is 4 feet below flood stage and falling.
The East Fork fell below flood stage yesterday afternoon at FM1485 and continues to fall.
Lake Houston and Below
At the FM1960 bridge over Lake Houston, the level continues to fall well below flood stage, but is still about a foot above normal.
At the Lake Houston dam, water levels are also falling, but the lake is also about a foot above the spillway.
Floodwaters are still being released from the lake via both the gates and spillway.
The gates can release 10,000 CFS. That means about another 7,500 CFS are going over the spillway.
Downstream, on the main stem of the San Jacinto at Highway 90, the river is well below flood stage and continuing to fall.
West Fork Still Flowing Through Abandoned Sand Pit
That’s all great news. But the West Fork is still flowing through an abandoned sand pit near the Hallett Mine. You can see the impact at the confluence of the West Fork and Spring Creek.
Can Water Moving at 5 MPH Transport Sand?
I measured debris coming out of the pit. It was moving at approximately 5 miles per hour (MPH).
Despite what some miners claim, 5 MPH should be more than enough to carry sand downriver.
Below is an industry-standard graph that shows the speed necessary to erode, transport and deposit sand/sediment of different particle sizes. Hydrologists and geologists call it a Hjulström curve, named after Filip Hjulström (1902–1982), the man who developed it.
After converting centimeters per second to miles per hour, I superimposed the speed of the river as a blue line over the graph.
The scientific Unified Soil Classification System defines sand as particles with a diameter of 0.074 to 4.75 millimeters. I rendered that range in red at the bottom of the chart.
Floodwater moving at 5 MPH can transport the entire range of sand sizes according to the Hjulström curve. You can see it in the photo above.
The abandoned pit captured by the river is about a mile long and a half-mile wide.
The river will need to recede before we can see exactly how much moved down to the West Fork between Humble and Kingwood, or settled at the mouth of the river near Lake Houston.
In fairness, some of the sediment deposited downstream came from natural erosion from riverbanks. But there was also unnatural erosion from development and (I have heard) other mines. It is impossible to apportion responsibility precisely.
What we can safely say is that sand mining practices have increased sedimentation downstream and few people seem eager to fight the industry … even as we get ready to launch another round of dredging that will cost taxpayers $34 million.
Posted by Bob Rehak on 5/21/24
2457 Days since Hurricane Harvey
The thoughts expressed in this post represent 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.
