Buffalo Bayou Case Study

Geomorphic Setting: River floodplain; bayou with high, erodible streambanks.

Treatment Objectives: To repair and stabilize a high riverbank slope that had failed as a result of frequent flood discharges. To demonstrate that a VMSE system was capable of withstanding severe scour erosion and lateral bank instability in lieu of a conventional vertical retaining wall or hard armor bank protection.

Nature of Problem: The combination of natural flooding and controlled releases resulted in the abrupt rise and fall of water level in the bayou in addition to prolonged periods of high water. These hydrologic conditions, combined with sandy and silty soils with little cohesion, resulted in serious erosion and streambank slope failures.

Treatment Considerations: Previous treatments, consisting of bank armoring with stacked cement filled bags tied together with steel reinforcing bars, were not only unsightly but also ineffective in preventing slope failures at this site. Successful treatment required redirecting the stream away from the toe of the bank, rebuilding and securing a toe berm, and stabilizing the slope face. Bank return flows during rapid drawdown after bankfull flows caused significant seepage erosion and slumping of the bank face. Accordingly, the interception and diversion of seepage water from the bank was also essential to successful stabilization.

Selected Treatment: A multi-pronged approach was adopted to deal with the various site conditions and requirements. A fill slope with a grade of 1V:0.5H was reconstructed upon a foundation of wrapped concrete rubble installed in a 2 m (7 ft) deep toe trench (see Figure 2). The fill was constructed in 0.6 m (2 ft) lifts wrapped with a synthetic geogrid (see Figure 3). Burlap was placed at the front face of the lifts between the soil and geogrids to prevent soil from raveling out through the geogrid openings. Live brushlayers approximately 15 cm (6 in) thick and long enough to extend from the undisturbed soil at the bank and project several feet beyond the reconstructed fill face were placed on each wrapped soil layer. Because continued seepage and saturation of the fill would have substantially reduced the slope factor of safety, it was necessary to install suitable drainage. Accordingly, vertical strip or chimney drains were installed at the rear of the fill on 1.5 m (5 ft) centers that intercepted and collected the seepage water and conducted it into a gravel drain beneath the fill. Water collected in the trench drain in turn was discharged to the bayou via pipes located at the downstream end of the project.

Live Fascines were placed on a midslope bench of reinforced fill. Live Staking was installed on edges, sides, and selected areas. A Live Boom (spur dike) was used to redirect the stream away form the bank.

Figure 1. Appearance of site before stabilization. Slope failures and stream-bank erosion resulted in near vertical scarps in high bank and substantial recession of crest.

Figure 2. Geogrid roll being spread out prior to placing lifts of compacted soil.

Placement of cut brushlayer atop geogrid wrapped soil lifts.

Slope protection system under construction. Vegetated geogrid reinforced fill was placed on top of rubble filled trench at base
(from Gray and Sotir, 1996).

VMSE project site on bayou immediately after construction. Buttress fill reinforced with vegetated geogrids (from Gray and Sotir, 1996).

VMSE site a year after construction. Bank is stable, well vegetated, and has survived major bankfull flows (from Gray and Sotir, 1996).

5 years after restoration.
Photo courtesy of Robbin Sotir.

Acknowledgment: This case study was adapted from Gray, D. H. and Sotir, R. (1996). Biotechnical and Soil Bioengineering Slope Stabilization, John Wiley & Sons, New York , NY.