Increasing Levee Protection from Wave Overtopping Erosion

Linda Benedict  |  9/14/2015 11:43:02 PM

Jeffrey Beasley, Christopher Thornton, Steven Hughes and Edward Bush

Starting this year the U.S. Army Corps of Engineers in New Orleans will be installing high-performance turf reinforcement mats on levees to reduce erosion and failure associated with wave overtopping. High-performance turf reinforcement mats are erosion products that gain part of their strength to withstand strong hydrodynamic forces through the establishment and interaction with vegetation.

The application of the reinforcement mats is a part of the continued effort by the Army Corps to provide protection to the residents of the Greater New Orleans Metropolitan Area from future flooding. During Hurricane Katrina, which struck Louisiana in 2005, several levees were eroded because of waves overtopping levees, a process that involves wave action in which water flows down the protected side of the levee. Over time, the force of the flowing water can erode the back slope of a levee to the point of failure.

Through tests conducted at the Colorado State University Hydraulic Laboratory in Fort Collins, which contains the world’s largest Wave Overtopping Simulator, the role of vegetation both alone and as a component of the reinforcement mats was investigated. Early tests showed levees constructed from clay differed in their ability to handle varying rates of wave overtopping depending on several factors. Bare soil failed shortly after being exposed to large waves generated by the Wave Overtopping Simulator, whereas vegetated soils increased wave overtopping erosion resistance, and vegetated reinforcement mats provided further resistance than vegetation alone. However, it was noted through additional testing that the performance of the reinforcement mats was influenced by changes in vegetative growth.

Trays with only grass and grass-vegetated reinforcement mats, which had been previously tested and did not fail in the Wave Overtopping Simulator, were exposed to the harsh Colorado winter before being re-tested the following spring. The cold temperatures stressed the grass so that parameters such as plant density and rooting were reduced. When the tests were re-administered, the grasses by themselves failed at the previously tested overtopping rates whereas a grass- vegetated reinforcement mat did not. This clearly demonstrated the synergy of vegetation in conjunction with reinforcement mats provides greater wave overtopping erosion resiliency.

Colorado State University and LSU AgCenter scientists were able to show rooting of the grasses had decreased greater than 30 percent for all treatments (grass alone and reinforcement mats with grass) after winter exposure. This showed changes in vegetation significantly affected wave overtopping erosion performance. Vegetation with higher root architectural parameters such as root length and surface area, often associated with denser, healthier grass swards, provided greater wave overtopping erosion resistance compared to less dense grass swards. The interaction of the roots with the soil appears to provide a cohesive factor in retaining soil when subjected to dynamic hydrodynamic forces. This relationship of rooting architecture to erosion performance has also been confirmed for dikes constructed from sand in Florida. In tests conducted for the Army Corps for reinforcement mat application at Lake Okeechobee, differing grass ground coverages and reinforcement mats were evaluated. Again changes in rooting architecture influenced vegetative erosion resistance to wave overtopping and reinforcement mat performance.

From this research the scientists found that reinforcement mats can provide greater protection against erosion from wave overtopping and that managing vegetation may be an important factor in maintaining reinforcement mat performance.

Jeffrey Beasley and Edward Bush are both associate professors in the School of Plant, Environmental and Soil Sciences. Christopher Thornton and Steven Hughes are with the Engineering Research Center at Colorado State University, Fort Collins, Colorado.

The summer 2015 issue of Louisiana Agriculture magazine includes articles on a variety of topics that affect Louisiana’s agriculture industry and the environment – water management at Catahoula Lake, 4-H youth wetland programs, artificial reefs for water conservation, corn nitrogen management in saturated soil conditions, and more. 36 pages

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