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Title: Critical Evaluation of How the Rosgen Classification and Associated "Natural Channel Design" Methods Fail to Integrate and Quantify Fluvial Processes and Channel Response

Author
item Simon, Andrew
item DOYLE, MARTIN - UNIV OF NORTH CAROLINA
item KONDOLF, MATHIAS - UC, BERKELEY
item Shields Jr, Fletcher
item RHOADS, BRUCE - UNIV OF ILLINOIS
item MCPHILLIPS, MUNSELL - INTUITION & LOGIC INC

Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2007
Publication Date: 10/1/2007
Citation: Simon, A., Doyle, M., Kondolf, M., Shields Jr., F.D., Rhoads, B., McPhillips, M. 2007. Critical Evaluation of How the Rosgen Classification and Associated "Natural Channel Design" Methods Fail to Integrate and Quantify Fluvial Processes and Channel Response. Journal of the American Water Resources Association (JAWRA) 43(5): 1117-1131. DOI: 10.1111/j.1752-1688.2007.00091.x

Interpretive Summary: While there are several reasons for limiting the use of classifications in restoration design, there are equally important reasons for maximizing the use of physically-based analyses in restoration design. The foremost advantage of the process-based approach is that it is well established in the scientific and engineering literature. For decades, geomorphologists and hydraulic engineers have been quantifying river processes and developing models that have been tested and refined over time. Developing a design using this rich literature leverages off of a substantial scientific background, and thus provides a critical foundation from which to defend the design approach. Such literature and historical precedence is lacking for the classification approach. The physics of erosion, transport, and deposition are the same regardless of the hydro-physiographic province or stream type because of the uniformity of physical laws. Channel adjustment is driven by the imbalance between driving and resisting forces, sediment supply and sediment-transporting capacity. Determining rates and magnitudes of adjustment, sediment-transport rates and ultimate channel forms are a matter of defining those spatially- and temporally-varying forces and variables. The physics of erosion, transport, and deposition are the same regardless of the hydro-physiographic province or stream type because of the uniformity of physical laws. Channel adjustment is driven by the imbalance between driving and resisting forces, sediment supply and sediment-transporting capacity. Determining rates and magnitudes of adjustment, sediment-transport rates and ultimate channel forms are a matter of defining those spatially- and temporally-varying forces and variables.

Technical Abstract: Over the past 10 years the Rosgen classification system and its associated methods of “natural channel design” have become synonymous to some with the term “stream restoration” and the science of fluvial geomorphology. Since the mid 1990s, this classification approach has become widely adopted by governmental agencies, particularly those funding restoration projects. Problems with the use of the classification are encountered with identifying bankfull discharge, particularly in incising channels and with the mixing of bed and bank sediment into a single population. A C-type channel with gravel bed and silt-clay banks would classify the same way as a C-channel with sand bed and sand banks (C-5). The use of the form-based classification for engineering design is shown to be flawed by ignoring processes governed by force and resistance, and the imbalance between sediment supply and transporting power. Examples of the failure of typical reach-based solutions in unstable systems are provided. The Rosgen classification is probably best applied as a communication tool to describe channel form but in combination with “natural channel design” techniques are not diagnostic of how to mitigate channel instability. For this, physically-based, mechanistic approaches that rely on quantifying the driving and resisting forces that control active processes and ultimate channel morphology, are better suited. The physics of erosion, transport, and deposition are the same regardless of the hydro-physiographic province or stream type because of the uniformity of physical laws. Channel adjustment is driven by the imbalance between driving and resisting forces, sediment supply and sediment-transporting capacity. Determining rates and magnitudes of adjustment, sediment-transport rates and ultimate channel forms are a matter of defining those spatially- and temporally-varying forces and variables.