Author(s): M. Muste; B. Cikmaz; A. Lamoreux; O. Baydaroglu Yesilkoy; I. Demir; E. Meselhe; C. Bacotiu
Keywords: Manning's n roughness coefficient; Vegetation-induced roughness; Slope-area method
Abstract: Despite extensive efforts to determine the Manning's n roughness coefficient, its estimation continues to be subjective and clumsy being regarded as the outcome of an intuitive or arbitrary process. Especially complex is the evaluation of vegetation-induced roughness, a time-dependent variable that is difficult to quantify even at one instant. Revisiting the protocols for Manning's coefficient estimation is essential not only to improve a key parameter involved in solving all channel-flow related problems but because the current approaches for its estimation are notoriously unreliable and outdated despite that advances in knowledge and measurement techniques are currently available. This paper describes two evidence-based methodologies for determining the Manning's roughness coefficient using information and data acquired in-situ in a small US Midwestern stream. While these methodologies can be applied to any type of roughness, we focus on vegetation-induced roughness as its evaluation is still considered an open challenge. The methods entail the Continuous Slope- Area method complemented by photo-documentation and synoptic surveys. Monitoring of the living roughness associated with the riparian is continuously made over the year to capture its seasonal growth and decay. This monitoring methodology differs from the conventional Manning's n estimation approaches whereby direct measurements are made for specific events and times with estimates provided as average values over ranges of flows and times. The paper describes the settings and measurement protocols associated with the implementation of the hybrid monitoring method and presents preliminary estimations for vegetation-associated roughness during the growing season in spring of 2022.