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Macroroughness and variations in reach‐averaged flow resistance in steep mountain streams
Author(s) -
Nitsche M.,
Rickenmann D.,
Kirchner J. W.,
Turowski J. M.,
Badoux A.
Publication year - 2012
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1029/2012wr012091
Subject(s) - streams , geology , flow (mathematics) , bedform , hydraulic roughness , open channel flow , flow velocity , dimensionless quantity , surface finish , flow conditions , mechanics , hydrology (agriculture) , geotechnical engineering , geomorphology , geometry , materials science , mathematics , sediment , sediment transport , physics , computer network , computer science , composite material
Steep mountain streams typically feature macroroughness elements like large immobile boulders or channel‐spanning bedforms such as step‐pool sequences. The effects of macroroughness on resistance and flow velocity are not well understood and appropriate field parameters for representing macroroughness in flow velocity equations have not been identified. The prediction of flow velocity in rough and steep streams therefore remains challenging. We measured flow velocity and several macroroughness parameters, i.e., boulder concentration, boulder diameter and protrusion, and roughness of longitudinal channel profiles in six reaches of steep mountain streams with plane bed/riffle, step‐pool, and cascade channel morphologies. The between‐site variations in flow resistance can be explained to a large degree by nondimensionalization of discharge and flow velocity using channel slope and a characteristic roughness length. Using any of our roughness parameters as the characteristic roughness length, this nondimensionalization leads to a similarity collapse of the entire data set. The remaining differences in flow resistance among the streams are related to dimensionless measures of macroroughness that describe the concentration of boulders or step density in a reach. Boulder concentration represents the measure best describing the data and is used in a simple regression equation for flow velocity. The predictions were better than predictions by the variable power law equation proposed by Ferguson. Although the regression might not be statistically significant, the observed trends suggest that boulder concentration partly explains the residual variance of between‐site variation of flow resistance.