Tag Archive for: river capture

Rampaging East Fork Floodwaters Cut New Path Through Plum Grove Sand Mine

The sign outside the abandoned Texas Concrete Sand and Gravel Mine in Plum Grove tells readers that an RV resort is coming soon. They might want to rethink that concept. Yesterday, rampaging floodwaters destroyed most of the mine except for a small area near the entry on FM1010.

Classic Example of Pit Capture

The East Fork rerouted itself right through the heart of the mine, sweeping away almost everything in its path. The river swelled to more than half a mile wide and ruptured dikes in at least four places when the river rose 10 feet in 24-hours.

The East Fork at this location rose 15 feet in three days, 10 of those in one day.
Normal course of river is red line on west side of mine. During flood, the river broke through the dike on the north side. Then floodwater filled the mine like a water balloon which burst in multiple places on the south side. Water now follows the yellow line.

This is a classic example of what geologists call pit or river capture. The East Fork entered the northern side of the mine and exited at multiple points on the south. Current coming out of the mine exceeded that in the river itself, carrying mud and muck downstream.

Flooding Based on Less than 10-Year Rain

The gage at this location indicated Plum Grove received only 3.36 inches of rain over a three-day period. However, up to 8 inches fell upstream from here, primarily during a two-day period. Jeff Lindner, Harris County’s meteorologist characterized the rains that produced the flood as, “Generally less than a 10-year event for the 48-hour time period.”

Pictures Taken on 5/3/2021

I took all of the shots below on 5/3/2021, three days after the major portion of the rain fell on April 30.

Looking north at the northernmost portion of the mine. The river appears to have entered the mine in this area. Note the dike in the far distance that’s not visible in the tree-line on the left.
Wider shot, still looking north toward entry point shows white water ripping through mine.
Looking south, you can see that the water in the mine is now higher and faster than the water in the river to the right.
Still looking south toward FM2090, now the only way in and out of Plum Grove. FM1485 is closed due to high water and FM1010 was washed out during Harvey by runoff from Colony Ridge to the southeast.
Looking north across FM2090. Where the water exits the mine, you can see that the force of the main flow is now misaligned with the bridge opening.
The width of the mine is now the width of the river…plus the river. Only the entry of the mine at the upper right remains above water at this time.
Looking east from over FM2090.

Danger of 2090 Washout in Next Big Flood?

Unless someone reroutes the river back to its original course and fixes the dikes, the current through the mine will continue to erode the banks of the roadway at the top of the image above.

These images dramatize the need for real sand-mining reform in Texas. There’s some evidence that Imelda did the same thing to this mine two years ago. But the TCEQ forced the company to repair the dikes. Now that the miners are gone, who will do that?

Plum Grove was lucky that upstream rains only amounted to a ten-year event. A larger storm could have cut the City and Colony Ridge off from the only viable evacuation route. More than 20,000 people would have been affected.

Posted by Bob Rehak on 5/3/2021

1343 Days since Hurricane Harvey and 592 since Imelda

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.

Whose Property Rights are More Important? Yours or Sand Miners’?

In the next two months, I expect to see legislation filed that will strengthen regulations on sand mining. Hopefully, legislation will prevent dangerous practices by the mining industry that have put residents at risk in the past.

Perhaps we can compromise on legislation that lets miners exercise their property rights without harming the property rights of others…or their incomes, safety, and families.

This is another post designed to raise awareness of sand mining problems and how other states have solved them. One of the main problems with sand mining in Texas: virtually all miners locate their mines in floodways. That almost guarantees a phenomenon called pit (or river) capture. Washington State has discovered the following about pit capture.

Causes of Pit Capture

Sand-pit capture happens when pressure from floodwater breaks through dikes and takes a short cut across sand mines instead of following the river’s course. A scientific paper called “Flood Plains, Salmon Habitat, and Sand and Gravel Mining” by David Norman and C. Jeff Cederholm describes the process.  The paper, published in Washington Geology by their Department of Natural Resources, says pit capture is almost inevitable in the long term.

“Regardless of the best planning and intentions, impacts of flood-plain mining may simply be delayed until the river is captured by the … pit,” they say. “While capture may not occur in the next 100-year flood event, it is likely to occur in the future as development and consequent flood magnitude increase. In the long term, stream capture by (sand and) gravel pits is a near certainty.”

Consequences of Pit Capture

The paper cites more than three dozen examples of pit capture. Consequences include:

  • Lowering the river bed upstream and downstream of mining operations
  • River bed erosion and (or) channel incision
  • Bank erosion and collapse
  • Undercutting of levees, roads, bridge supports, pipelines, utility towers and other structures
  • Increasing suspended sediment
  • Deposition of silts

I have described how those consequences happen in several previous posts. Also, the paper describes the processes in detail, so I will not repeat the explanations here.

As in Washington, the Houston area has had many instances of sand-mine pit-capture.

Video Shows Simulation of Pit Capture

This short YouTube video may help you visualize how this process works. A company called Little River made it with funding from the EPA and State of Missouri. Little River specializes in table-top, tank experiments for science classes. This video shows  how pit capture happens and how erosion results.

Depending on the area and depth of the pit, and sediment volume carried by the river, it could take “millennia” to restore the natural environment after pit capture.

Operators’ attempts to prevent pit/river capture by armoring dikes and channelizing rivers often accelerate floodwaters and increase erosion downstream, say the authors (page 13 and figure 17) .

Cures for Pit Capture

The Washington State Department of Ecology Shoreline Management Handbook recommends locating mining activities “outside the shoreline jurisdiction.” They recommend 200 ft. from the floodway or off the 100-year flood plain. The latter coresponds to Texas regulations for the John Graves Scenic Riverway District on the Brazos River.

Immediate Reclamation for Each Segment

The Washington State Department of Natural Resources administers their Surface Mine Reclamation Act (RCW 78.44). It generally requires reclaiming mines immediately after each segment is mined. The 1993 revision of this law requires that most mines in flood-plain environments be reclaimed as beneficial wetlands.

Avoiding pit capture requires thorough and immediate reclamation because of river migration. The longer a pit goes before reclamation, the greater the likelihood that river migration will capture it. We saw an example of that on the San Jacinto. The river is migrating 12 feet per year and is 38 feet away from breaking into a major sand pit.

The immediate reclamation requirement could benefit Texans. Texas law requires sand miners to file a reclamation plan to obtain a permit. However, there is no requirement to execute the plan before leaving the property. Many simply walk away from their obligations, much to the detriment of surrounding property owners and the safety of the public. Requiring miners to reclaim one section of a mine before permitting another would give them a powerful incentive to reclaim land.

Substitution of crushed concrete or quarry rock for gravel also has distinct environmental advantages.

Safeguards for Flood-Plain Mining if Necessary

The authors conclude: “If mine plans call for sites on flood plains, then wide, topographically higher, and thickly vegetated buffers should be considered as a means of reducing the probability of river avulsion in the near term. However, in most instances, buffers only delay the inevitable.:

“Determining an adequate distance between the flood-plain mine pit lake and the river will depend on understanding the rate of river meandering and the risk of avulsion.”

As always, these are my opinions on matters of public policy. They are protected by the First Amendment of the United States Constitution and the Anti-SLAPP statutes of the Great State of Texas.

Posted by Bob Rehak on August 13, 2018

441 Days since Hurricane Harvey

How Floodplain Mining Can Lead to River Capture

In a previous post, I showed satellite and aerial images of water sweeping through a West Fork sand mine complex during Harvey. This is part of a process called river capture. In the photo below you can see the West Fork flowing into and through mines on both sides of the river during Harvey.

Dikes on both sides of the river were breached in the process. In fact, historical images in Google Earth show that they have been breached repeatedly.

During Harvey, floodwaters swept through this complex and breached dikes in multiple locations as the river took a shortcut through the mines. 

Highest Risk Scenarios for River Capture

A review of scientific literature reveals that the risk of stream diversion through pits (river capture) is increased by:

  • Proximity of pits to the river
  • Increased depth of the pit, particularly where the base of the pit is below the lowest part of the river.

The phenomenon seems common and the consequences well documented.

2014 Survey of Scientific Literature Finds 37 Examples

In 2014, Anthony Ladson and Dean Judd, two Australian researchers, found 37 instances of river capture in a review of scientific literature. They published their findings at the Seventh Annual Australian Stream Management Conference in a paper titled A review of the effect of floodplain gravel mining on river stability.

Advanced economies, they say in their introduction, require large amounts of aggregate (sand and gravel) to sustain growth. Aggregate makes up 80% of concrete and 90% of asphalt pavements. 

Many see floodplain mining as a safer way to obtain this aggregate than in-stream mining, but floodplain mining still poses substantial threats to river stability.

Dangers of River Capture

As we saw on the West Fork of the San Jacinto during Harvey, floodwaters take a shortcut through mines that are built on point bars inside meander loops.

Sand bars within sand mine, caused during “river capture” of the mine. These bars prove sand was carried downstream. This photo taken on 10/28/2018 (after Harvey) also shows repairs to mine wall. During floods, the river tries to cut across meanders, runs through the mines and carries sand downstream.

The authors say this can lead to:

  • River bed degradation
  • Bank erosion
  • Channel widening
  • Infrastructure damage or destruction
  • Loss of riparian vegetation
  • Habitat damage
  • Degradation of water quality.

Their review of local, national and international case studies showed that pit capture and subsequent river channel changes, are a common consequence of floodplain mining.

Changing River Environment and Putting Infrastructure at Risk

In their conclusion, Ladson and Judd state, “Although floodplain gravel mining has been considered a safer option than the direct extraction of gravel from a river, substantial risks to river stability and river health values remain. Floodplain gravel mining can cause change in the riverine environment, both locally and distant to the mining site, and in the short and long term.” 

“There are substantial risks to infrastructure if river diversions occur which trigger bed and bank erosion.” 

“There may be a role for river management agencies to influence the amount of mining that is undertaken, and the manner in which it is undertaken, in order to mitigate these threats.”

Mechanisms of River Capture

Another study goes into more detail. This second study, was conducted by Jacobs Engineering in 2015, also in Australia. It describes the processes behind river capture, the risks, and how to reduce them. It is titled Risk_assessment_of_floodplain_mining_pits_in_the_mid-Goulburn_Valley, The Golburn River, like the San Jacinto, meanders through relatively flat land. It also has approximately the same number of sand mines that the San Jacinto has.

Jacobs identified three risk scenarios for river capture:

  1. Lateral migration of river channel into the pit
  2. Sub-surface piping into pits and subsequent failure of pit walls
  3. Flow of water into and through the pit and subsequent erosion of the buffer strip between the channel and the excavated pit.

Some San Jacinto Mines Push Recommended Safety Margins

Jacobs assesses (page 19) that 100 meters is the minimum setback to prevent river capture from occurring. In some places on the San Jacinto, dikes are less than 15 meters wide.

Jacobs also assesses that river capture is almost certain (page 19) where the basement of the pit is more than 5 meters lower than the river. San Jacinto sand miners are mining at more than double that depth.

“The physical processes of pit capture have been well documented from case studies: incision upstream and downstream of the pit are expected, with bed adjustments continuing until the river establishes a new equilibrium and grade,” says Jacobs.

Ways to Mitigate Risk of River Capture

Jacobs identified two main ways (page 47) to reduce this risk:

  • Locate pits out of the 100-year floodplain
  • Implement controls such as levees, grade-control structures, pit setbacks, depth limits, and waterway diversions.

Sadly, all sand pits on the San Jacinto are already in the 100-year flood plain. Worse, all but one are at least partially in the FLOODWAY, which is defined as the main channel of the river during a flood.

Even more sadly, it appears that none of the measures in the second category are being applied to San Jacinto mines either.

Posted by Bob Rehak, July 22, 2018

327 Days since Hurricane Harvey