Tag Archive for: storage capacity

TACA Now Claims Sand Mines Helped to Prevent Flooding Downstream

David Perkins, CEO of TACA (the Texas Aggregate and Concrete Association) now claims that sand mines helped to prevent flooding downstream. The claim appeared in the current issue of Mining Technology in an article titled “Did Sand Mining Exacerbate Flooding During Hurricane Harvey?”

Perkins is quoted as saying, “…what [these sand mines] actually do – in contrast to what people were stating – is that they stored quite a bit of that floodwater and helped to prevent additional flooding …”

He explained how.  “… For example, one 60-acre pit that’s 100 feet deep holds 6,000 acre feet of water. We’ve got some great potential capacity for off-channel storage that we could incorporate into our mining activities.” [Emphasis added twice.]

Did Mines Really Help Prevent Downstream Flooding?

Did they prevent flooding or could they have the potential to do so? Perhaps, but only if they followed best management practices common in other states and countries. More on that later.

Let’s suspend disbelief for a moment and examine Mr. Perkins’ theory.

The pits are already filled with water, so you only have a tiny portion of those acre feet available for storage during a flood! Not as much as he implies. Is the amount significant?

The difference between the top of the water and the top of the dikes is usually 10 to 20 feet at best. So let’s be generous to Mr. Perkins and assume 20. And let’s use the pit below as a test case. It’s 160 acres – about the size of Kingwood’s East End Park so people will have a reference.

Here’s what the same pit looks like from ground level.

Peak flow from Harvey would have filled this pit in less than 8 minutes.

How 8 Minutes was Calculated

The pit above is approximately 160 acres.

  • Assume 20 vertical feet are available to store water.
  • 20 feet x 160 acres = 3200 acre feet of potential storage.
  • Now let’s calculate how fast Harvey would fill that up.
  • Approximately 150,000 cubic feet per second (cfs) flowed through this area.
  • There are 43,560 cubic feet in an acre foot. To find the fill rate…
  • 150,000 cubic feet per second/43,560 = approximately 3.5 acre feet per second
  • To convert seconds to minutes, 3.5 x 60 = 210 acre feet per minute.
  • 1600 acre feet of extra capacity/210 acre feet per minute = 7.6 minutes.
  • Thus, Harvey could have filled 20 feet in this pit in less than 8 minutes.

Did It Delay, Reduce, or Prevent Flooding?

So this pit really wouldn’t have done much by itself to prevent flooding. It delayed flooding on the West Fork for 8 minutes in a 4-day flood. It reduced flow by 3,200 acre feet. But it did little prevent flooding as Mr. Perkins claims. To put things into perspective, the total amount of flow going into Lake Houston would have filled up NRG stadium in 3.5 minutes (see page 7).

Contribution to Flood Reduction if You Put All Mines Together

However, if you consider all the pits on the West Fork together (20 square miles), they might have delayed  flooding by 10 hours according to the same calculations. That’s starting to sound like a significant contribution. Barker and Addicks Reservoirs together store a total of 410,000 acre feet. West Fork sand mines occupy 12,800 acres. If every acre had 20 vertical feet of storage available, you would have 256,000 acre feet of storage. That might not stop Harvey but it could certainly help reduce flooding – especially in smaller floods.

Two Problems Need to Be Addressed with Perkins’ Theory

Mr. Perkins’ theory has two major flaws that would need to be addressed before it could be taken seriously by residents.

That would mean following best management practices common in other states and countries.

Ignoring Best Practices Contributes to Flooding

Regardless of storage POTENTIAL, if mines fail to follow best management practices, they are likely to do more harm than good.

Texas does not enforce best management practices common in other states, such as setbacks from rivers, sloping of dikes, and strengthening of dikes.

In Texas, we locate mines in floodways. And dikes are so thin that they often fail. Case in point: the mine we are talking about. There, dikes have been breached repeatedly. The river has cut through the pit and carried sediment downstream. That sediment then helped clog the river and create floods, not prevent them.

Rather than trying to deny what happened and change the debate, TACA should acknowledge what happened and work with citizen groups and government to create new regulations that protect the public as well as themselves.

What Really Happened in West Fork Mines During Harvey

Here’s what it looked like. This series of photos shows West Fork sand mines during Harvey on 8/30/17, one day after the peak. The river was flowing at only one-third of its peak on this day, according to the San Jacinto River Authority.

West Fork Sand Mine complex inundated by Harvey. Two of the three stockpiles in this photo were decimated by the flood. Sediment from the pits was also picked up by currents within them and carried downstream. Mine used in illustration above is in center. 

Following are several close ups that show water breaching dikes, entering the mine and eroding areas within it and then carrying sediment downstream. All images taken on 8/30/17, courtesy of Google Earth and NASA.

Floodwater broke dikes, captured the sand pit and flowed straight through it.

Rapids within the mine.

Force of floodwater washed out road INSIDE mine. 

Floodwater rushing out of forest into sand mine. It then flowed through and over dikes on the opposite side.

Exit Point for Floodwaters in this Mine

This next image shows the floodwaters exiting the mine on the far side after they scooped up sediment.

Floodwaters exit mine during Harvey. Photo taken on 8/30/2017.

Harvey was not the only flood when this has happened. The Memorial Day flood in 2015 and the Tax Day Flood in 2016 also saw breaches of the dikes. Those floods were much smaller and still caused problems, underscoring the need to modify permitting, regulations and best practices.

Exit point when river captured mine during Memorial Day flood in 2015.

Exit point two months later, on 7/31/2015. Breach was still unprepared and mine was leaking sediment into river.

By March of 2016, the breach had been repaired, but you can also see how sand was building up against it.

Less than a year later, when the flood waters had subsided, we can see the growth and orientation of the sand bars within the mine on the upper left in this 2017 image. This indicates that current within the mines during river capture was forcing sediment out of the mines.

I fail to see how this particular sand mine can prevent flooding. These pictures tell a different story. If the operator and TACA supported best management practices common in other states and countries, they might be able to help prevent flooding. But until that happens, I’m going to call Mr. Perkins’ claim a perfect-world generalization that has real-world limitations and exceptions.

And until TACA and the mines acknowledge the role they have played in flooding, residents will have a hard time emotionally accepting their presence in the community.

As always, these are my opinions on a matter of public interest. 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 11, 2018

347 Days since Hurricane Harvey