Self‐adjustment of stream bed roughness and flow velocity in a steep mountain channel

Abstract

Understanding how channel bed morphology affects flow conditions (and vice versa) is important for a wide range of fluvial processes and practical applications. We investigated interactions between bed roughness and flow velocity in a steep, glacier-fed mountain stream (Riedbach, Ct. Valais, Switzerland) with almost flume-like boundary conditions. Bed gradient increases along the 1 km study reach by roughly 1 order of magnitude (S 5 3-41%), with a corresponding increase in streambed roughness, while flow discharge and width remain approximately constant due to the glacial runoff regime. Streambed roughness was characterized by semivariograms and standard deviations of point clouds derived from terrestrial laser scanning. Reach-averaged flow velocity was derived from dye tracer breakthrough curves measured by 10 fluorometers installed along the channel. Commonly used flow resistance approaches (Darcy-Weisbach equation and dimensionless hydraulic geometry) were used to relate the measured bulk velocity to bed characteristics. As a roughness measure, D 84 yielded comparable results to more laborious measures derived from point clouds. Flow resistance behavior across this large range of steep slopes agreed with patterns established in previous studies for both lower-gradient and steep reaches, regardless of which roughness measures were used. We linked empirical critical shear stress approaches to the variable power equation for flow resistance to investigate the change of bed roughness with channel slope. The predicted increase in D 84 with increasing channel slope was in good agreement with field observations.

Steep Bedload‐Laden Flows: Near Critical?

“…Several friction laws relating the velocity to flow depths, slope, and roughness ( D 84 or σ Ks ) were tested on the reference points. The Ferguson () friction law proved to be the most relevant for such very shallow flows with substantial roughness, consistently with several other studies (e.g., Rickenmann & Recking, ; Schneider et al, ): with d the water depth (m) and g gravitational acceleration (m/s 2 ). To extend the friction law for all areas of flow, a modified Ferguson () friction law was used by introducing equation into equation : …”

“…Several direct field measurement campaigns implemented in mountain rivers during mean flows seldom reported highly supercritical Froude numbers (e.g., Comiti et al, , ; Lenzi, ; Magirl et al, ; Nitsche et al, ; Recking et al, ; Schneider et al, ; Tinkler, ; Zimmermann & Church, ). Interestingly, most peak‐flow reconstructions after moderate‐ to extreme‐magnitude flood events were also subcritical or near critical (Hauer & Habersack, ; Lumbroso & Gaume, ; Rickenmann & Koschni, ; Ruiz‐Villanueva et al, ).…”

Geschiebehaushalt in kleinen Hochgebirgsbächen der Nordtiroler Zentralalpen

“…Eine wesentliche Erkenntnis dieser Arbeiten war der erhebliche Unterschied zwischen der Kornverteilung des über einen gewissen Zeitraum transportierten Geschiebes und jener der Bachsohle. Die Kornverteilung der Bachsohle von Hochgebirgsbächen ist in der Regel sehr grob und das Mittelkorn wie auch andere charakteristische Korngrößen stehen häufig in einem positiven Zusammenhang mit dem lokalen Bachgefälle (Kammerlander 2017;Schneider et al 2015a). Die Grobkomponenten der Bachsohle stellen maßgebende Rauheitselemente dar und beeinflussen somit den Fließwiderstand im Gerinne.…”

Controls of channel morphology and sediment concentration on flow resistance in a large sand-bed river: A case study of the lower Yellow River

“…Among the variables included in the empirical analyses, mean flow depth and/or water surface slope (stream bed slope) are generally considered as the two most important. Some investigations have demonstrated that unit discharge or dimensionless unit discharge exerts an important influence on flow resistance (Rickenmann, 1991;Bjerklie et al, 2005a;Comiti et al, 2007;Ferguson, 2007;David et al, 2010;Rickenmann and Recking, 2011;D'Agostino and Michelini, 2015;Schneider et al, 2015). In fact, unit discharge or dimensionless unit discharge is physically equal to the product of mean flow depth and water surface slope in that it is the function of flow depth and water surface and usually preferable to the use of boundary shear stress for estimating incipient sediment motion in steep streams (Comiti et al, 2007).…”

“…Investigations on flow resistance have survived for more than a century (Manning, 1891;Keulegan, 1938;Vanoni, 1941Vanoni, , 1946Vanoni and Brooks, 1957;Qian et al, 1959;Peterson and Mohanty, 1960;Simons and Richardson, 1960;Vanoni and Nomicos, 1960;Simons et al, 1963;Qian and Zhou, 1965;Rouse, 1965;Golubtsov, 1969;Limerinos, 1970;Judd and Peterson, 1969;Bathurst, 1978Bathurst, , 1985Bathurst, , 2002Bray, 1979;Hey, 1979;Davis and Sutherland, 1980;Griffiths, 1981;Jarret, 1984;Aguirre-Pe and Fuentes, 1990;Bennett, 1995;Dingman and Sharma, 1997;Nikora et al, 1998;Lee and Ferguson, 2002;Ferro, 2003;Ferguson, 2007Ferguson, , 2010López et al, 2007;Recking et al, 2008;Reid and Hickin, 2008;Comiti et al, 2009;David et al, 2010;Robert, 2011;Ferreira et al, 2012;Nitsche et al, 2012;Schneider et al, 2015). Among these studies, the methods used for quantifying flow resistance can be generally categorized into two major groups: (i) a characteristic particle size approach (or a semiempirical approach) …”