In response to the demand for stream bank stabilization measures that provide enhanced aesthetics and more biologically diverse and ecologically compatible environments, numerous documents have been developed to provide conceptual guidance and generic details on a variety of bank protective measures incorporating vegetative and geosynthetic materials in lieu of, or in addition to, rock slope protection. While many of these bank protection measures have been deployed and have survived for a number of years, there remains considerable skepticism within the engineering community regarding performance of these measures when subjected to flood event magnitudes typical of DOT designs for stream bank protection. Further, very little information is available regarding the durability and service life expectations of these measures, nor does the existing guidance provide adequate information on the requirements associated with initial establishment of vegetative components or the long-term maintenance requirements for such measures. Since most DOTs have minimum service life requirements for drainage related infrastructure within the range of 25 to 100 years, such information is crucial to determining whether agency requirements can be achieved. Historically, the limited performance evaluations conducted on these types of measures have primarily focused on the environmental/biologic aspects of the installation, with less attention placed on these more engineering-related elements. The applicability of individual measures to varying stream hydraulic and site conditions, the long-term structural integrity of the measure (and/or the bank the measure was constructed to protect), as well as the anticipated maintenance and inspection costs are all critical elements that must be understood in order to support sound engineering decisions.
The recently completed NCHRP Report 544: Environmentally Sensitive Channel- and Bank-Protection Measures is representative of the current state of the practice. Conceptual design information for 44 separate measures is provided in the document and companion CD. The researchers were able to conduct only relatively limited field investigations for this project, and stated “The need exists for more performance data, such as allowable velocities for some techniques and the amount of vegetative cover required to reach project objectives” and “…research opportunities are identified in the detailed descriptions of each technique.” It is clear that while some data has been collected to date which helps define the performance of these measures, specific data has not typically been made widely available, nor has it been of sufficient quantity to define the full range of measure applicability or long-term performance.
The purpose of this study is to establish a baseline and set of protocols for long-term study of the most common of these biotechnical stream bank protection measures. Site-specific field evaluations as well as data assessments associated with the design, construction, and maintenance of selected facilities will be undertaken such that engineering recommendations can be made on the design and installation limitations and requirements for the selected techniques. In addition, the final product to be delivered at the conclusion of this study will include a database of such configuration that state DOTs can continually add information from their own selected sites and continue to expand the set of information to additional measures as well as augment the initial research data with additional years of evaluations.
The environmental regulatory community across the nation is imposing these nonstructural bank protection devices on DOTs without any evidence that they will adequately protect highway infrastructure. When these measures are designed to act as the primary impediment to washout of stream banks supporting highways or other critical public infrastructure, with the attendant impacts to public safety, such information is critical to ensure that the engineering designer has adequate basis for assessing the risk of the design. Thus, highway agencies are being forced into accepting indeterminate, questionable risks.