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.

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.

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