Review of step-pool hydrodynamics in mountain streams:
24 Jun 2019-Vol. 43, Iss: 5, pp 607-626
TL;DR: In this paper, steppools are one of the major types of bed morphology prevalent in mountain streams and they have a unique flow structure as compared to low-gradient streams, in terms of large boundary elements and...
Abstract: Step-pools are one of the major types of bed morphology prevalent in mountain streams. They have a unique flow structure as compared to low-gradient streams, in terms of large boundary elements and...
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TL;DR: In this paper, the authors review progress on five key questions related to how boulders influence the evolution of unglaciated, eroding landscapes: 1) What factors control boulder production on eroding hillslopes and the subsequent downslope evolution of the boulder size distribution.
27 citations
Journal Article•
TL;DR: In this paper, an experiment was conducted to study the maximum flow resistance of step pool streams and the morphology of the steps formed from clastic materials, and the step pool formation was qualitatively simulated to analyze numerically the formation process.
Abstract: An experiment was conducted to study the maximum flow resistance of step pool streams and the morphology of the steps formed from clastic materials. The step pool formation was qualitatively simulated to analyze numerically the formation process. A flume 4.88 mm long and .15 m wide was used and flow velocity measurements were done by electronically timing passage of salt plume down the flume. Observations showed that the natural step pool streams arranged the morphology to maximize flow resistance.
9 citations
TL;DR: In this paper, three-dimensional numerical simulations were performed for different flow rates and various geometrical parameters of step-pools in steep open channels to gain insight into the occurrence of energy loss and its dependence on the flow structure.
Abstract: Three-dimensional numerical simulations were performed for different flow rates and various geometrical parameters of step-pools in steep open channels to gain insight into the occurrence of energy loss and its dependence on the flow structure. For a given channel with step-pools, energy loss varied only marginally with increasing flow rate in the nappe and transition flow regimes, while it increased in the skimming regime. Energy loss is positively correlated with the size of the recirculation zone, velocity in the recirculation zone and the vorticity. For the same flow rate, energy loss increased by 31.6% when the horizontal face inclination increased from 2° to 10°, while it decreased by 58.6% when the vertical face inclination increased from 40° to 70°. In a channel with several step-pools, cumulative energy loss is linearly related to the number of step-pools, for nappe and transition flows. However, it is a nonlinear function for skimming flows.
9 citations
01 Jan 2000
TL;DR: In this article, the authors identify the flow conditions under which stable bedforms exist; provide the geometric characteristics of these bedforms; measure the magnitude of the streamwise velocity and energy dissipation factor; and determine the friction factor under various flow conditions and gravel sizes.
Abstract: The goal of this study was to identify the flow conditions under which stable bedforms exist; provide the geometric characteristics of these bedforms; measure the magnitude of the streamwise velocity and energy dissipation factor; and determine the friction factor under various flow conditions and gravel sizes. Design criteria and recommendations for stable bedforms were provided upon the termination of this research. Stable bedforms are defined as those bedforms of which the spatial characteristics (height and spacing) do not change with time. The focus of this study was on streams with slopes greater than 3%, as clear design requirements for bed geomorphologic stability are lacking for these cases, and they are of particular interest in the design and retrofit of culverts for both anadromous and resident migratory fish passage.
2 citations
TL;DR: In this paper , the authors used a double-averaging approach to estimate the width, reach, and doubleaveraged variables for steady, uniform flow in an open channel, using an empirical model for the coefficient of form drag, and a flow resistance model that compares well to a large data set of flow velocity in natural channels.
Abstract: Flow resistance in open channels determines the average flow velocity in a river, with important implications for downstream and at-a-station hydraulic geometry and sediment transport in natural channels. However, flow resistance in steep mountain channels is challenging to understand and predict due to heterogeneous channel morphology and spatially variable flow. Using a double-averaging approach appropriate for very rough channels, the width, reach, and double-averaged variables for steady, uniform flow in an open channel are derived. Using an empirical model for the coefficient of form drag, a flow resistance model is derived that compares well to a large data set of flow velocity in natural channels. Several parameters that together describe bed morphology are observed to be relatively consistent in the data set used and compare well to previous estimates of their values. This implies a degree of self-organization in bed morphology that may simplify the challenging problem of predicting flow resistance in steep mountain rivers.
1 citations
References
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TL;DR: In this paper, a simple technique for predicting the Darcy-Weisbach friction factor for river channels with varying amounts of large woody debris was developed based on measurement or visual estimation of cross-sectional areas of debris formations in the plane perpendicular to flow.
Abstract: A simple technique for predicting the Darcy-Weisbach friction factor for river channels with varying amounts of large woody debris was developed. First, debris density is determined based on measurement or visual estimation of cross-sectional areas of debris formations in the plane perpendicular to flow. The Darcy-Weisbach friction factor is then computed using debris density, channel geometry, mean flow velocity, and the debris drag coefficient. The debris drag coefficient may be computed fom a power function with experimentally determined coefficients. For verification of the proposed procedure, debris density and friction factors were measured in river reaches in western Tennessee, and southeastern New South Wales, Australia. Friction factors computed using the procedure were within 30% of measured values for straight, sand-bed reaches and within 38% of measured values for sinuous, gravel-bed reaches. The computational procedure explained 84% of the variance in observed values.
190 citations
TL;DR: In this article, the evolution of the steepness parameter c = (H/Ls)/S was measured in the field from 1992 to 1998 and the results indicated that maximum resistance conditions are gradually reached at the end of a series of ordinary flood events.
Abstract: The principle that formative events, punctuated by periods of evolution, recovery or temporary periods of steady-state conditions, control the development of the step–pool morphology, has been applied to the evolution of the Rio Cordon stream bed. The Rio Cordon is a small catchment (5 km2) within the Dolomites wherein hydraulic parameters of floods and the coarse bedload are recorded. Detailed field surveys of the step–pool structures carried out before and after the September 1994 and October 1998 floods have served to illustrate the control on step–pool changes by these floods. Floods were grouped into two categories. The first includes ‘ordinary’ events which are characterized by peak discharges with a return time of one to five years (1·8–5·15 m3 s−1) and by an hourly bedload rate not exceeding 20 m3 h−1. The second refers to ‘exceptional’ events with a return time of 30–50 years. A flood of this latter type occurred on 14 September 1994, with a peak discharge of 10·4 m3 s−1 and average hourly bedload rate of 324 m3 h−1. Step–pool features were characterized primarily by a steepness parameter c = (H/Ls)/S. The evolution of the steepness parameter was measured in the field from 1992 to 1998. The results indicate that maximum resistance conditions are gradually reached at the end of a series of ordinary flood events. During this period, bed armouring dominate the sediment transport response. However, following an extraordinary flood and unlimited sediment supply conditions, the steepness factor can suddenly decrease as a result of sediment trapped in the pools and a lengthening of step spacing. The analogy of step spacing with antidune wavelength and the main destruction and transformation mechanism of the steps are also discussed. Copyright © 2001 John Wiley & Sons, Ltd.
186 citations
"Review of step-pool hydrodynamics i..." refers background in this paper
...Mountain streams with slopes greater than 3–7% constitute step-pool morphology (Aberle and Smart, 2003; Lenzi, 2001; Montgomery and Buffington, 1993; Papanicolaou and Maxwell, 2000; Whittaker, 1987b)....
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...Step-pool morphology can be found in the 3–20% slope range of mountain streams (Aberle and Smart, 2003; Lenzi, 2001; Montgomery and Buffington, 1993; Papanicolaou and Maxwell, 2000; Whittaker, 1987b) and has a typical arrangement, as shown in Figure 1....
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TL;DR: In the Erlenbach stream, a pre-alpine steep channel in Switzerland, sediment transport has been continuously monitored with piezoelectric bedload impact and geophones since 1986 as mentioned in this paper.
Abstract: In the Erlenbach stream, a pre-alpine steep channel in Switzerland, sediment transport has been monitored for more than 25 years. Near the confluence with the main valley river, stream flow is monitored and sediment is collected in a retention basin with a capacity of about 2000 m3. The basin is surveyed at regular intervals and after large flood events. In addition, sediment transport has been continuously monitored with piezoelectric bedload impact and geophone sensors since 1986. In 2008–2009, the measuring system in the Erlenbach stream was enhanced by installing an automatic system to obtain bedload samples. Movable metal baskets are mounted on a rail at the downstream wall of the large check dam above the retention basin, and they can be moved automatically into the flow to take bedload transport samples. The wire mesh of the baskets has a spacing of 10 mm to sample all sediment particles coarser than this size (which is about the limiting grain size detected by the geophones). The upgraded measuring system permits to obtain bedload samples over short sampling periods and to measure the grain size distribution of the transported material and its variation over time and with discharge. The analysis of calibration relationships for the geophone measuring system confirms findings from very similar measurements which were performed until 1999 with piezoelectric bedload impact sensors; there is a linear relationship between impulse counts and bedload mass passing over the sensors. Findings from flume experiments are used to discuss the most important factors which affect the calibration of the geophone signal. The bedload transport rates as measured by the moving baskets are among the highest measured in natural streams, with values of the order of several kilograms per meter per second. Copyright © 2012 John Wiley & Sons, Ltd.
182 citations
"Review of step-pool hydrodynamics i..." refers background in this paper
...…equations developed for steep streams differ from the observed magnitudes up to an order of three, mostly due to the sediment-supplylimited conditions of mountain streams (Kammerlander et al., 2017; Nelson et al., 2009; Rickenmann et al., 2012; Schneider et al., 2016; Yager et al., 2012a)....
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...However, it is essential to understand that bedload transport equations developed for steep streams differ from the observed magnitudes up to an order of three, mostly due to the sediment-supplylimited conditions of mountain streams (Kammerlander et al., 2017; Nelson et al., 2009; Rickenmann et al., 2012; Schneider et al., 2016; Yager et al., 2012a)....
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TL;DR: In this paper, the authors compared the geometry, step characteristics, and flow hydraulics of 20 step-pool reaches without large woody debris (LWD) to 20 steppool reaches with LWD.
Abstract: [1] Step-pool streams dissipate flow energy primarily through spill resistance. We compared the geometry, step characteristics, and flow hydraulics of 20 step-pool reaches without large woody debris (LWD) to 20 step-pool reaches with LWD. Non-LWD streams exhibited significantly shallower flows, lower steps, shorter step spacings, greater percentages of water-surface drop created by steps, larger grain sizes, and smaller Darcy-Weisbach friction factors. Grain resistance was negligible in both stream types. Form resistance created by irregularities in the channel shape associated with steps contributed more to the total flow resistance in LWD reaches. Although both stream types showed poor correlation between step height and flow resistance, the significant positive correlation between flow resistance and step height/length ratio in the non-LWD reaches demonstrates the increasing effect of spill resistance with increasing step height. The lack of such a trend in the LWD-loaded reaches suggests that spill resistance was highly influenced by a few large log steps in these reaches. LWD creates deep pools and increases flow resistance along step-pool streams. It thus stabilizes channels and stores sediment in steep headwater streams recently scoured by debris flows.
175 citations
"Review of step-pool hydrodynamics i..." refers background in this paper
...However, the contribution of grain and form resistance is less than 20%, and about 80– 90% of the water surface drop is accounted to the presence of steps (Comiti et al., 2007; Curran and Wohl, 2003; MacFarlane and Wohl, 2003; Wilcox et al., 2006)....
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TL;DR: Ohtsu et al. as mentioned in this paper performed a systematic study of skimming flows on stepped channels with different slopes and step heights and provided a solid database that might lead to better and improved design criteria.
Abstract: Ohtsu et al. must be congratulated for their systematic study of skimming flows on stepped channels with different slopes and step heights. Their experimental data provide a solid database that might lead to better and improved design criteria. Important outcomes include some estimate of friction factors for skimming flows and a design flow chart for stepped canals. Despite these outstanding results, the writers would like to comment constructively on further design criteria for skimming flows, and to provide additional flow resistance results and a discussion on the physical processes. It is hoped that the present contribution will add to safer stepped chute design. The writers would like to point out that the energy dissipation performances of stepped canals with moderate slopes are far from being totally understood and further experimental research is needed.
175 citations
"Review of step-pool hydrodynamics i..." refers background in this paper
...It is observed that the average flow velocity (uavg) from point measurements is 20% higher than the average flow velocity (ueq) obtained from a continuity equation (Ohtsu et al., 2004), which leads to inaccurate results....
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