Erosion can sometimes be sudden. It’s not always a slow process of water grinding away at dirt and dissolving it, or wearing down rocks. This post will examine several examples around us and look at their implications. I intend it as a continuation of yesterday’s post about ditch maintenance.
There are four main types of erosion.
- Hydraulic action – When rapidly moving water churns against river banks and scours or undermines them.
- Abrasion – Caused by small pebbles moving along a river bank or bed and knocking other particles loose. Think of sandpaper.
- Attrition – When rocks carried by the river knock against each other. They break apart to become smaller and more rounded. This is how boulders turn into gravel.
- Solution – When water dissolves certain types of rocks, for example limestone. We often see this in Florida, where sinkholes frequently develop.
Most of these processes happen slowly. But the first can be sudden. One storm. One flood. And boom. That river bank where you used to sit and quietly contemplate nature is gone.
Now You See It; Now You Don’t
Sometimes large slabs of a river bank or ditch suddenly slump into a river, almost like mini landslides. One flood expert commented on the picture above; he said “The owners of those new homes may suddenly find the ditch in their backyards.”
At other times, the size of a flood forces a river to widen. We saw this during Harvey and Imelda. The relentless pounding of flood waters carries away everything in their path. Cutbanks (the outside of a river bend) are especially vulnerable. Water slams directly into them like a firehose and washes them away. This action actually changes the course of a river over time.
Most of the time, it happens so slowly, we barely notice it. But during large floods, it’s sometimes sudden, large, and devastating to homeowners or businesses near rivers.
Three More Examples of Hydraulic Action
Example A: East End Park
Example B: Balcom House and River Migration
Example C: River Aggregate Mine on West Fork in Porter
The third example comes from the abandoned River Aggregates sand mine beyond the new development in the first picture above. It’s a spectacular example of river migration.
In this case, the San Jacinto West Fork migrated 258 feet toward the mine’s dike in 23 years. When I first photographed the dike after Harvey, the river had eaten away an average of 12.4 feet per year. At the time, the dike was only 38 feet wide, and I predicted it could soon fail. It did. Within approximately a year.
Wait a minute, you say! What happened to the pond. After the river bank collapsed, the pond drained, exposing sediment already within it. And the action of draining concentrated more sediment in it, like all the remnants of food trapped in your sink drain after you’re done washing dishes.
History of Pond
The missing, shallow pond in the foreground above used to be the settling pond for River Aggregates.
Here’s how it looks today from a helicopter.
Lessons of Life Near a River
Most people never live long enough to see massive changes such as these in rivers. In most places, river change happens on a geologic time scale. But along the Gulf Coast, hurricanes can create floods that make rivers change on a human time scale, as these examples have shown.
What can we deduce from this?
- Around here, we need to give rivers room to roam. Parks, green spaces, and golf courses, often represent the highest and best use of land near a river, bayou or ditch.
- Building too close to rivers, bayous and drainage ditches can be costly. Disturbing wetlands and topsoil accelerates erosion. That, in turn, can threaten everything in its path. Be prepared to maintain anything you build near a watercourse, including the watercourse itself. And be prepared to fight what ultimately becomes a losing battle.
- We need greater separation between mines and the San Jacinto river. Keep mines out of the meander belt. They worsen downstream sedimentation. And as we have seen, that can contribute to sediment build ups that require public money to remove. The alternative, leaving them in place, contributes to flooding.
Posted by Bob Rehak on 5/18/2020
993 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.