Help! The Bank Slid Away

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.

 

  • This graphic of saturation and the geology of Buffalo Bayou was adapted by geologist Bill Heins from the Harris County Flood Control District's own imagery in its 2014 permit application, p. 599.
  • Detailed cross section of a breakaway scarp.
  • A typical outcrop of the clay layer (slip surface) on Buffalo Bayou. Photo taken looking upstream below the Memorial Park boat launch on Woodway with Memorial Park on the right. All photos by Bill Heins.
  • Downstream from the Woodway boat launch in Memorial Park, looking downstream with Memorial Park on the left.
  • Downstream from the Loop 610 Bridge, looking downstream with Memorial Park on the left.
  • Typical slumping behavior. Private property upstream from the project area.
  • A typical breakaway scarp. Photo taken below Pine Hill Lane opposite Memorial Park and upstream of the project area.
  • Typical slumping behavior. Photo taken in the Houston Arboretum looking upstream.
  • Looking downstream in the MPDP project area, Memorial Park on the left.
  • Standing in Memorial Park looking downstream at the middle meander to be filled by the "repair" project, the ancient high bank obliterated. The high bank, an example of slumping, is misidentified by the project engineering consultant as suffering from severe lateral erosion.
  • This photograph was taken in the MPDP project area downstream of the middle meander looking downstream below Rains Way. Note riffles in the water that show the hard clay outcrop across entire stream bed. River Oaks Country Club property on the right.

 

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.

 

Groundwater seeping out of a bank above the hard Beaumont clay layer. Photo taken in Memorial Park on April 29, 2017, by Susan Chadwick

 

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.

 

  • Slumping and downed trees on a high bluff in the Hogg Bird Sanctuary in June 2015 following the Memorial Day flood.
  • Another view of the same bank, same day.
  • May 9, 2016, following the April 18 Tax Day storm, during extended record high water releases from the federal dams.
  • The same high bank three months later on August 4, 2016.
  • The same south-facing high bank nine months later on April 29, 2017, greening with spring growth.
  • South-facing bluff in Hogg Bird Sanctuary on July 11, 2017. Photo by SC.

 

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, to not build on top of the bank, to stop watering the grass, to control runoff, to not cut down the trees or put in short-rooted grass at all, and so on.

First of all, leave in place trees and vegetation that have slid down the bank. 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.

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.