Design and Performance of Rootwads in Streambank Restoration
Pennsylvania State University
Basin Program Funds:
Many “natural stream restoration” methods are being applied without the benefits of scientific research to validate their application. Basic engineering principles are often overlooked. The geomorphic behavior of rivers can easily be oversimplified. Stream restoration manuals by several conservation agencies recommend rootwads as a natural stream design method but do not offer any construction guidance other than trial and error experience.
Ballasted rootwads are natural restoration techniques that utilize two natural materials, rock and large woody debris, to stabilize streambanks in a manner that is consistent with the geomorphic time-scale of riverine environments. Like many other natural stream design techniques that were developed by nonengineers primarily to address habitat, rootwads have not been designed for structural stability based on proven physical principals.
This streambank stabilization technique is particularly applicable to two types of streams feeding the Great Lakes:
Without design criteria and trained, professional designers, there is a reluctance of watershed planners and permit agencies to consider rootwads as a viable “soft-engineering” solution to streambank erosion.
- low-gradient streams in fine-grain soils with limited sources of rock for ballast, and
- steep outwashes with glacial deposits available for ballast.
A cooperative study between Pennsylvania State University and the U.S. Department of Agriculture’s Natural Resources Conservation Service (NRCS) designed and installed several rootwad projects in Pennsylvania, developed a computerized design tool for rootwads, and is monitoring and analyzing the stability of rootwad sites in the Mid-Atlantic region.
A stability theory was developed to account for all hydrodynamic, frictional, and gravitational forces acting on the rootwad. Eleven sites were studied in Pennsylvania. Dimensions of each rootwad component were measured, including log diameter and length and rock dimensions. The spatial relationships of all components were measured, including orientation to the bank, location of the cables, and location of the rocks and logs. Also, the depths of the soil strata in the vicinity of the rootwad were obtained. Development of the theory and field portion of this study into the desired design tool was done by a statistical comparison of the theoretically predicted stability with the actual field stability.
The following tasks were completed through this study:
- Recommended scientifically-based safety factors that can be used as criteria for designing ballasted rootwads that are to be used in streambanks
- Developed a design methodology for use with a computer spreadsheet and/or a family of empirical curves that can quantify the amount of ballast required to stabilize a rootwad for a variety of load conditions
- Installed several sites with rootwads for field validation of the criteria
- Offered several workshops that explain rootwad technology, including field trips to one or more of these on-the-land demonstrations
As a result of the Erie County rootwad projects, the band erosion has been reduced along the associated reaches of French Creek and knowledge of the benefits of using rootwads for bank stabilization and habitat has spread to consultants, government agencies, watershed groups, and the general public. The distribution of the final version of the rootwad design tool will be through the NRCS. This project also lead to a new environmental resources management class being developed and taught at Pennsylvania State University.
Contact: Albert R. Jarrett, 814-865-5661