Slumping on the Bayou
The instability of riverbanks is generally divided into two types: banks that collapse vertically and banks that are worn away laterally or horizontally by the erosive force of the water flowing downstream.
Vertical collapse is called slumping or sloughing. The face of the bank slides or rotates away, often leaving a concave scar or scarp in the bank and a clump of sediment at the base. Sometimes the slump takes trees and vegetation with it. The vegetation can continue to grow in the collapsed sediment, which creates a sort of terrace or beach at the foot of the high bank.
According to a lengthy analysis by geologists working with Save Buffalo Bayou, slumping is the primary type of bank collapse on Buffalo Bayou.
What is Slumping?
Slumping is different from erosion or shear stress, although slumping is frequently mistaken for erosion.
Watch this slide show created by geologist Bill Heins explaining and documenting rotational slumping on Buffalo Bayou.
In this type of slumping on Buffalo Bayou, water—mainly from overbank flooding after big storms but also from heavy lawn watering or even heavy rains—seeps from above and saturates the loose, permeable sandy-clay layers below, causing them to become unstable. This loose sandy clay sits on top of an impermeable layer of hard, red clay of the very old Beaumont formation. When it hits the hard Beaumont clay, water percolating down from above travels sideways into the bayou, wetting the hard clay. Water running underground laterally (groundwater) on top of the hard clay likewise loosens the unstable sandy clay and makes the hard layer slick.
When wet this very old layer of hard clay serves as a glide plane. The mucky, unstable bank loses its footing, slips down, and slides out.
In Buffalo Bayou it is easy to see the groundwater seeping out of the bank above the red clay layer. Numerous seeps or springs of this type, also identified as wetlands, visibly emerge from the banks through the muddy terraces of material slumped from the high banks throughout the project area and elsewhere. (Note the escarpment in the bank in the background—demonstrating the slumping we are talking about.)
This hard Beaumont clay dates from the Pleistocene era of some 250,000 years ago.
Watch this slide show of graphics and photographs by geologist Tom Helm that also explain this slumping dynamic so prevalent throughout the bayou.
One of the features of this type of vertical slumping, as mentioned above, is that the top and face of the bank slides down taking living trees and brush and their clumps of roots with it. The bank sediment generally stays in place at the foot of the bank and the trees and vegetation re-root and continue to grow.
The high bluff of the Hogg Bird Sanctuary at the downstream limit of the project area is an example of this. It is an excellent example of (vertical) slumping and rebuilding.
Here is a slide show of the south-facing Hogg Bluff that slumped after the Memorial Day 2015 storm and subsequently restored itself. Notice the vegetation that glided intact from the top and remained at the foot of the bank, collecting sediment, helping to rebuild the bank.
What Can You Do About It?
Unfortunately there is simply no remedy for slumping here or elsewhere—other than to move back and out of the way. Do not build on top of the bank, don’t cut down the trees, don’t water or mow the grass or even think about putting in short-rooted grass. Plant a strip of native grasses. Do control runoff, although native trees and vegetation help with that. Most experts recommend a vegetated buffer for urban streams at least as wide as the stream, but not less than 15 feet, with some recommending a minimum of 100 feet. (p. 4)
But if your bank has already slumped away, leave in place any trees or vegetation that slid down with it. They can continue to live, reroot, provide structure and collect sediment to rebuild the bank. This is nature’s way, and it’s been happening for a very long time.
You can also try to mimic this natural process by building brush mattresses and anchoring them into the bank, or even bringing in a tree or two with root balls, known as large woody debris, if you’ve already lost all your trees, and placing them parallel or at a slight angle to the bank with the root ball facing upstream, possibly anchoring that to the bank also to prevent it from floating downstream.
Tossing some native riparian plant seed on the banks will help nature reinforce your banks. Consider buying some smartweed seed or Riparian Recovery Mix. Consulting Your Remarkable Riparian Owner’s Manual will help explain best practices and nature’s landscaping on Texas riverbanks.
Riprap and other hardening methods are specifically rejected as erosion control methods by the Corps of Engineers due to their “lack of shear stress reduction, lack of habitat creation, lack of natural occurrence [in Buffalo Bayou], and erosive energy transfer [they] place on the opposite bank of the bayou at a downstream location.” (p. 23) The weight of riprap used on a slumping riverbank can actually accelerate slumping.
Some further reading: