Earth Week Part 4: Slope of Sand Mine Dikes, Riparian Vegetation and Cost Offsets
Yesterday, I posted about how greater setbacks from rivers could improve safety for sand mines and downstream residents. Setbacks reduce the potential for erosion, sedimentation and consequent flooding. Here’s a related post that shows what happens when you try to build too close to rivers.
First, understand that the closer you mine to the river, the steeper the slope of dikes must be. At a certain point, the slope becomes so steep that:
- Grasses and trees can’t take root in it.
- The loose soil becomes prone to erosion.
- During floods, water in the river rises faster than in the pit.
- It exerts pressure on the dike.
- The dike can collapse through one of more of several mechanisms (piping, erosion, overtopping, sloughing, etc.)
- The river invades the pit.
- Depending on the depth of the pit, the volume of sediment in it, and the force of the flood, sediment could be carried downstream.
Another factor leading to dike collapse in the photo above is the road built on top of it. Running heavy equipment over the sandy soil causes it to compact and push outward. Vehicle traffic also keeps vegetation that could bind the soil from growing.
Accidents Waiting to Happen
It doesn’t take a Harvey-scale flood to breach these loose dikes. The unmemorable July 4th flood of last year breached the dike shown above.
Another flood on December 7th last year breached a dike in another sand mine downstream from the first one in three places!
Here’s another breach at the same mine that hadn’t yet healed when I photographed it on September 28th last year.
When such breaches happen on both sides of a point bar, the river will “capture” the pit by rerouting through it – the shortest distance between two points.
West Fork vs. East Fork and Value of Riparian Vegetation
Almost all of these problems could be solved by greater setbacks from rivers. That would retain more natural riparian vegetation and allow lower, more gradual slopes on dikes. It would also allow additional re-vegetation to take hold.
Imagine 131,000 cubic feet per second ripping through a channel like this. That’s how much came down this portion of the West Fork at the peak of Harvey. It’s easy to see how the river could erode these dikes and invade the mines.
That’s why we need greater setbacks. It will allow more conveyance through the normal channel. And if we just leave native negation in place, it should help hold the dikes in place.
Now contrast the images above with this one taken on a portion of the East Fork where there are no sand mines.
Offsetting Opportunity Costs with Conservation Easements
Mother Nature’s solution to sedimentation is free. If we could only just learn to respect the river and its flood plains. Yes, there would still be some sedimentation to deal with, but not nearly as much.
The loss of sand close to the river is an opportunity cost, not an out-of-pocket cost. Groups like the Bayou Land Conservancy can help offset some of that opportunity cost by providing income in exchange for conservation easements. I wish miners would explore this option more…for everyone’s benefit including theirs. It certainly might reduce their legal costs.
Posted by Bob Rehak on April 27, 2019
606 Days after Hurricane Harvey