Steep Mountain Channels Research Articles

A Comprehensive Review on Sediment Transport, Flow Dynamics, and Hazards in Steep Channels

The hydrological channel networks of the steep mountains are extensively designated and organized geographic systems and are very complicated. They are composed of granular beds that are uneven and are subjected to fluid forces that fluctuate spatially and temporally. The flow and movement of sediment in these streams are significantly shaped by large rocks such as boulders. However, it is difficult to comprehend these mountain streams because significant information is unavailable regarding these channels as compared to plane bed streams. To address this issue, a critical review of the numerical, computational fluid dynamics, and machine learning mechanisms underlying sediment transport across different flow conditions in steep channels is presented while considering the current and foreseeable conditions for various sediment transport phenomena. The present study emphasized to carry out further analysis on these steep channels using advanced available techniques to get an insight into the morphology of these channels. Furthermore, the hazards associated with steep mountain channels are reviewed as they have a significant impact on infrastructure and habitation in mountainous regions.

Confluence morphodynamics in mountain rivers in response to intense tributary bedload input

AbstractCoarse sediment transported by steep mountain tributaries during channelized flash flood events with intense bed load transport poses a significant threat to life, property, and infrastructure. Intense bedload transport occurring in tributary channels and insufficient transport capacity of the main channel flow can provoke flooding at the confluence. Deposition in the confluence can lead to overbank flooding and sedimentation into adjacent settlement areas. Extensive research has been carried out investigating lowland river confluences, where it has been determined that the confluence angle and discharge ratio have the most significant influence on channel morphology and hydraulics. However, there is a lack of information concerning confluences with low width and discharge ratios, high sediment concentrations, and gradients, typically found in steep mountain channels. This study presents results from large‐scale laboratory experiments coupled with numerical modelling with a standardized river confluence geometry. Bedload transport capacities, the shape, and volume of the deposition zone in the confluence, bedload dispersion characteristics, hydraulic and morphological dynamics, and spatial boundaries were analysed for various discharges and sediment concentrations. The confluence angle was 90°, sediment concentration was 5%, 7.5%, and 10%, and the discharge ratio was 0.1. The model was designed to accommodate scale factors of 20–40. With this configuration, a set of experiments based on steady‐state hydraulic conditions was accomplished. Results show that when the discharge ratio and confluence angle are constant, different morphologies occur, indicating that in addition to the confluence angle and the discharge ratio, the sediment concentration, flow velocity, and unit stream power significantly impact both hydraulic and morphologic zones in the confluences of mountain rivers. Additionally, backwater effects upstream of the confluence, and sedimentation in the tributary channel increases with increasing sediment concentration, which not only influences confluence morphodynamics and hydraulics but also the potential for overbanking of both channels.

Flow Resistance in Very Rough Channels

AbstractFlow 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.

Debris‐Flood Hazard Assessments in Steep Streams

AbstractDebris floods most commonly occur in steep mountain channels and on their alluvial fans but can also occur on small gravel bed rivers with watershed areas up to several hundred square kilometers. This became obvious during July 2021 and November 2021 debris floods in northwestern Germany and southwestern British Columbia, Canada. We subdivide debris floods into three categories: those triggered by a supra‐critical bed shear stress ratio, form by dilution from debris flows, and resulting from outbreak floods. This trichotomy challenges traditional hazard assessments; debris floods classify as a fluvial process, yet their destructive mechanics are difficult to characterize. Key hazards interact spatially and temporally in debris floods: inundation, scour, sediment transport and deposition and bank erosion. We describe approaches to quantify hazards by systematically accounting for these processes and introduce a novel approach for hazard quantification and mapping in which flow velocity, depth, and presumed fluid density are combined with the annual event frequency for all event scenarios. The derivative “composite hazard maps” are equally valid for debris floods and debris flows. Isolines of bank erosion based on a probabilistic analysis of a physically based model are added to capture the potential of debris floods to abruptly widen their channels. Substantial challenges remain, specifically in the reliable prediction of sediment transport and progressive bank erosion. Our intention is to homogenize debris‐flood hazard assessments and especially mapping methodologies. This could allow for systematic integration with landuse policies assuring consistency among approaches executed by practitioners acting in this field.

Fixed-Bed and Mobile-Bed Resistance of Channels with Steep Gradients in Mountainous Areas

Flood discharge and sediment transport are closely linked to channel resistance in steep mountain streams. Previous research has mainly focused on the resistance of fixed-bed channels with steep gradients and mobile-bed channels in alluvial rivers. The present study performs an experiment and establishes a calculation method for the fixed-bed resistance of mountain channels. The basic expression of the mobile-bed resistance of steep mountain channels is derived by determining the controlling factors of the bed load movement on the riverbed resistance. The proposed formula can accurately predict the variation of the bed load resistance. The results of the present research improve the understanding of fluid dynamics and sediment transport in steep mountain channels.

Relationship between the accumulation of sediment storage and debris-flow characteristics in a debris-flow initiation zone, Ohya landslide body, Japan

Abstract. Debris flows usually occur in steep mountain channels and can be extremely hazardous as a result of their destructive power, long travel distance, and high velocity. However, their characteristics in the initiation zones, which could possibly be affected by temporal changes in the accumulation conditions of the storage (i.e., channel gradient and volume of storage) associated with sediment supply from hillslopes and the evacuation of sediment by debris flows, are poorly understood. Thus, we studied the relationship between the flow characteristics and the accumulation conditions of the storage in an initiation zone of debris flow at the Ohya landslide body in Japan using a variety of methods, including a physical analysis, a periodical terrestrial laser scanning (TLS) survey, and field monitoring. Our study clarified that both partly and fully saturated debris flows are important hydrogeomorphic processes in the initiation zones of debris flow because of the steep terrain. The predominant type of flow varied temporally and was affected by the volume of storage and rainfall patterns. Fully saturated flow dominated when the total volume of storage was < 10 000 m3, while partly saturated flow dominated when the total volume of the storage was > 15 000 m3. Debris flows form channel topography which reflects the predominant flow types during debris-flow events. Partly saturated debris flow tended to form steeper channel sections (22.2–37.3°), while fully saturated debris flow tended to form gentler channel sections ( < 22.2°). Such relationship between the flow type and the channel gradient could be explained by a simple analysis of the static force at the bottom of the sediment mass.

Bedform development and its effect on bed stabilization and sediment transport based on a flume experiment with non-uniform sediment

Non-uniform sediment deposited in a confined, steep mountain channel can alter the bed surface composition. This study evaluates the contribution of geometric and resistance parameters to bed stabilization and the reduction in sediment transport. Flume experiments were done under various hydraulic conditions with non-uniform bed material and no sediment supply from upstream. Results indicate that flume channels respond in a sequence of coarsening and with the formation of bedform-roughness features such as rapids, cascades, and steps. A bedform development coefficient is introduced and is shown to increase (i.e. vertical sinuosity develops) in response to increasing shear stress during the organization process. The bedform development coefficient also is positively correlated with the critical Shields number and Manning׳s roughness coefficient, suggesting the evolution of flow resistance with increasing bedform development. The sediment transport rate decreases with increasing bed shear stress and bedform development, further illustrating the effect of bed stabilization. An empirical sediment transport model for an equilibrium condition is proposed that uses the bedform development coefficient, relative particle submergence (i.e. the ratio of mean water depth and maximum sediment diameter), modified bed slope, and discharge. The model suggests bedform development can play a primary role in reducing sediment transport (increasing bed stabilization). The model is an extension of Lane׳s (1955) relation specifically adapted for mountain streams. These results explain the significance of bedform development in heightening flow resistance, stabilizing the bed, and reducing sediment transport in coarse, steep channels.

Sediment motion and velocity in a glacier-fed stream

Current understanding of coarse sediment transport (e.g. threshold for motion, travel length and virtual velocity) in mountain rivers is still quite limited, and even less is known about glacial streams. However, the hydrological characteristics of these systems (strong daily discharge fluctuations, high water turbidity) pose challenges to the use of tracers to monitor bed sediment dynamics, as tagged clasts are usually located after bedload events when flow stage has receded, e.g. by means of portable antennas in the case of Passive Integrated Transponders (PIT). The use of stationary antennas, still scarcely in use worldwide, to detect PIT-tagged particles has potential advantages in glacier-fed streams. If water discharge is monitored continuously, a stationary antenna provides real time data on the actual discharge at the moment of tracer particles passage. This study focuses on incipient motion and virtual velocity of bed particles measured by a stationary antennas system in a steep mountain channel (Saldur River, drainage area 18.6km2, Italian Alps) where significant daily discharge fluctuations and bedload transport occur as a result of a nivo-glacial regime. Four stationary antennas were installed 50-m apart in the study reach. A total of 629 PIT-tagged clasts were inserted in the studied reach between 2011 and 2014, ranging in size from 35mm to 580mm, with an overall recovery rate of around 44%. Critical discharge for sediment entrainment was obtained by detecting the movement of tracers placed immediately upstream of antennas. Virtual velocity was derived by knowing distances between the antennas and travel time of tracers. Results on initiation of motion show that the relationship between the size of transported tracers and the discharge measured at the time clasts were passing the stationary antenna is very weak. The influence of antecedent flows on incipient motion was thus investigated by dividing the highest discharge recorded between each PIT deployment and the subsequent entrainment by the actual critical discharge at the time of movement (ratio Qmax/Qc). Results show that approximately 50% of tracers moved at Qmax/Qc≤1.2, and that 73% of tracers moved at Qmax/Qc<1.5. Therefore, about 30% of tracers had to previously experience a discharge substantially greater than the one that actually mobilized them. Also, coarser particles moved at higher Qmax/Qc ratios, suggesting that higher antecedent flows may be needed to destabilize bed clustering. Results on the virtual velocity of the PIT-tagged clasts employed in the field show that the virtual velocity turned out to be highly variable (ranging from 101 to 10−5mmin−1) and weakly related to either particle size or flow discharge. However, virtual velocity was well correlated with the highest flow discharge experienced by each tracer normalized by a percentile of the flow duration curve. This evidence further stresses the importance of flow history on sediment entrainment and transport. Finally, the pros and cons of the deployed monitoring technology are discussed.

Detailed documentation of dynamic changes in flow depth and surface velocity during a large flood in a steep mountain stream

Summary Understanding the discharge capacity of channels and changes in hydraulic properties during large storms is essential for prediction of flash floods. However, such information is limited for steep mountain channels because of their complex nature and the lack of measured data. Thus, we obtained detailed water-level and surface-velocity data during large floods of a steep mountain channel, and documented how complex channel morphology affected water flow during large storms. We installed water-level and surface-velocity sensors at a cascade and at a pool that was 10 m downstream at the Aono Research Forest of the Arboricultural Research Institute of the University of Tokyo Forests in Japan. We successfully obtained 1-min interval data for a major storm with total precipitation of 288 mm that fell over 59 h and a maximum rainfall intensity of 25 mm/h. During the storm, height of the water surface from the deepest point of each cross section ranged from 0.35 to 1.57 m and surface velocity ranged from 0.35 to 4.15 m/s. As expected, the changes in flow depth, surface velocity, and velocity profiles were complex and differed even between the cascade and adjacent pool cross sections. Dramatic changes in flow conditions first occurred at the cascade when discharge increased to a certain point, when water suddenly stagnated at the foot of the cascade and submerged flow might have occurred. Thereafter, the water level increased remarkably but surface velocity and the velocity profile stayed almost constant at the cascade cross section. At the downstream pool, where most rocks were submerged at a mean water depth of 0.7 m, surface velocity suddenly increased dramatically and the velocity profile changed as very slow flow developed in the lower portion of the profile, while water levels increased only slightly. When the rainfall diminished, first, the surface velocity markedly declined, then the velocity profile returned to its original state at the pool, and then submerged flow at the bottom of the cascade ceased. These temporally and spatially detailed flow measurements effectively document and characterize flow conditions during a large flood in a steep mountain channel. These field data confirm the long held hypothesis that marked changes in flow conditions occur when steps become submerged.

Effective Acquisition Protocol of Terrestrial Laser Scanning for Underwater Topography in a Steep Mountain Channel

For better risk management, detailed and quantitative measurement of channel and stream-bed structure is required to understand and predict water and sediment flow in mountain channels. Our previous research demonstrated good performance of green-wavelength Terrestrial laser scanning (TLS) for measurement of submerged stream-bed in a steep mountain channel. This paper examines how the acquisition protocol of TLS affects the accuracy of data collected in the mountain channel. First, it was tested whether varying the scanner height, i.e., incident angle affects the data acquisition in terms of point density and accuracy in the pool unit of step-pool channel. Then, the effect of varying the minimum point spacing on the derived Digital terrain model (DTM) was examined. It was also analyzed whether a combination of multiple TLS data acquired from different directions would improve the accuracy of data compared to data acquired by a single measurement. Furthermore, TLS data were acquired over a cascade unit of the channel and examined whether TLS is capable of capturing reliable underwater data. All the acquired underwater data by TLS were corrected for water refraction and validated using manual surveyed data. The results showed that the accuracy of derived DTM was improved when the scanner height was increased or data was acquired from multiple directions, however, acquiring denser point cloud with a minimum point spacing of 1 mm did not improve the accuracy of the data. Accuracy of TLS measurement in the cascade unit was considerably lower. Special consideration is required for this area.

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

AbstractUnderstanding 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 = 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, D84 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 D84 with increasing channel slope was in good agreement with field observations.

Morphodynamics of steep mountain channels

Morphodynamics of steep mountain channels

Particle friction angles in steep mountain channels

AbstractSediment transport rates in steep mountain channels are typically an order of magnitude lower than predicted by models developed for lowland rivers. One hypothesis for this observation is that particles are more stable in mountain channels due to particle‐particle interlocking or bridging across the channel width. This hypothesis has yet to be tested, however, because we lack direct measurements of particle friction angles in steep mountain channels. Here we address this data gap by directly measuring the minimum force required to dislodge sediment (pebbles to boulders) and the sediment weight in mountain channels using a handheld force gauge. At eight sites in California, with reach‐averaged bed angles ranging from 0.5° to 23° and channel widths ranging from 2 m to 16 m, we show that friction angles in natural streams average 68° and are 16° larger than those typically measured in laboratory experiments, which is likely due to particle interlocking and burial. Results also show that larger grains are disproportionately more stable than predicted by existing models and that grains organized into steps are twice as stable as grains outside of steps. However, the mean particle friction angle does not vary systematically with bed slope. These results do not support systematic increases in friction angle in steeper and narrower channels to explain the observed low sediment transport rates in mountain channels. Instead, the spatial pattern and grain‐size dependence of particle friction angles may indirectly lower transport rates in steep, narrow channels by stabilizing large clasts and channel‐spanning steps, which act as momentum sinks due to form drag.

Scaling relationships between bed load volumes, transport distances, and stream power in steep mountain channels

Bed load transport during storm events is both an agent of geomorphic change and a significant natural hazard in mountain regions. Thus, predicting bed load transport is a central challenge in fluvial geomorphology and natural hazard risk assessment. Bed load transport during storm events depends on the width and depth of bed scour, as well as the transport distances of individual sediment grains. We traced individual gravels in two steep mountain streams, the Erlenbach (Switzerland) and Rio Cordon (Italy), using magnetic and radio frequency identification tags, and measured their bed load transport rates using calibrated geophone bed load sensors in the Erlenbach and a bed load trap in the Rio Cordon. Tracer transport distances and bed load volumes exhibited approximate power law scaling with both the peak stream power and the cumulative stream energy of individual hydrologic events. Bed load volumes scaled much more steeply with peak stream power and cumulative stream energy than tracer transport distances did, and bed load volumes scaled as roughly the third power of transport distances. These observations imply that large bed load transport events become large primarily by scouring the bed deeper and wider, and only secondarily by transporting the mobilized sediment farther. Using the sediment continuity equation, we can estimate the mean effective thickness of the actively transported layer, averaged over the entire channel width and the duration of individual flow events. This active layer thickness also followed approximate power law scaling with peak streampower and cumulative stream energy and ranged up to 0.57min the Erlenbach, broadly consistent with independent measurements.

Green-wavelength terrestrial laser scanning of mountain channel

Abstract. It is crucial to understand and predict how the water and sediment flow in mountain channels for better disaster risk management. Detailed and quantitative measurement of channel and stream-bed structure is in great demand. In this paper, the utility of greenwavelength TLS for measurement of submerged stream-bed is preliminary examined in a steep mountain channel. The ability of through water measurement is evaluated in terms of water depth and flow velocity. Automated extraction of submerged objects is also tested. The results of comparison of water depth derived by TLS and survey displayed significant correlation between them, presenting good performance of green-wavelength TLS for measurement of submerged stream-bed. The analysis using a Generalized Liner Model (GLM) indicated that each of water depth and flow velocity alone does not affect the accuracy of TLS measurement, however, the interaction between water depth and flow velocity may have an influence. The attempt to automatically extract the dimension of submerged objects was partially successful, achieving maximum error of +0.8 cm for two objects.

Riparian indicators of flow frequency in a tropical montane stream network

Summary Many field indicators have been used to approximate the magnitude and frequency of flows in a variety of streams and rivers, yet due to a scarcity of long-term flow records in tropical mountain streams, little to no work has been done to establish such relationships between field features and the flow regime in these environments. Furthermore, the transition between the active channel of a river and the adjacent flood zone (i.e. bankfull) is an important geomorphologic and ecological boundary, but is rarely identifiable in steep mountain channels that lack alluvial flood plains. This study (a) quantifies relationships between field indicators and flow frequency in alluvial and steepland channels in a tropical mountain stream network and (b) identifies a reference active channel boundary in these channels, based on statistically defined combinations of riparian features, that corresponds to the same flow frequency of the bankfull stage and the effective discharge in adjacent alluvial channels. The relative elevation of transitions in riparian vegetation, soil, and substrate characteristics were first surveyed at nine stream gages in and around the Luquillo Experimental Forest in Northeastern Puerto Rico. The corresponding discharge, flow frequency, and recurrence intervals associated with these features was then determined from long-term 15-min discharge records and a partial duration series analysis. Survey data indicate that mosses and short grasses dominate at a stage often inundated by sub-effective flows. Herbaceous vegetation is associated with intermediate discharges that correspond to the threshold for sediment mobilization. Near-channel woody shrubs and trees establish at elevations along the channel margin inundated by a less frequent discharge that is coincident with the effective discharge of bed load sediment transport. Our data demonstrate that in alluvial channels in the study, both the bankfull stage (as marked by a flood plain) and the channel-forming (effective) discharge are associated with the presence of fine-grained substrate and soil, and tall, mature woody vegetation. In montane reaches that lack a flood plain, a boundary that is characterized by the incipient presence of soil, woody shrubs, and trees corresponds to the same flow frequency as the bankfull discharge of nearby alluvial channels. The reference discharge based on these riparian features in steepland sites has an average exceedance probability between 0.09% and 0.30%, and a recurrence interval between 40 and 90 days. We conclude that flows with similar frequencies influence the establishment of riparian vegetation, soil development, and substrate characteristics along channel margins in similar ways. Thus, these riparian features can be used as an indicator of hydrogeomorphic site conditions to identify active channel boundaries that occur at a constant flow frequency throughout the study stream network.

Field measurements of three-dimensional hydraulics in a step-pool channel

We investigated the effects of morphologic position and discharge on flow structure in a steep (0.10 m/m) mountain channel by collecting three-dimensional measurements of time-averaged and turbulent velocity components with a SonTek FlowTracker Handheld ADV (acoustic Doppler velocimeter) on a 30-m reach of a step-pool channel in the Colorado Rockies. Velocity profiles were measured at morphologic positions characteristic of steep channels (above steps, step lips, base of steps, pools, cascades, runs), and at five different discharges. A marked three-dimensionality of flow structure was documented in East St. Louis Creek. Velocities in the streamwise component were the largest contributors to overall velocity vector magnitudes; cross-stream and vertical components contributed averages of 20% and 15%, respectively, to overall vector magnitudes. Turbulence intensities were especially multi-dimensional, however, with large contributions to turbulent kinetic energy from the vertical component of velocity. Analysis of variance indicated that discharge and morphologic position significantly affected mean streamwise velocities, with substantially higher velocities upstream from steps than in pools. Discharge and morphology effects on cross-stream and vertical velocity components, however, were not significant. Discharge and morphologic position also significantly affected turbulence intensities for all flow components, with the greatest turbulence intensities occurring in pools and at high discharges. These results illustrate the strong discharge-dependence of hydraulics in step-pool channels, where relative submergence of bedforms changes rapidly with discharge, and the substantial spatial variation in hydraulics created by step-pool sequences.

The geomorphic significance of step–pools in mountain streams

This paper develops a conceptual model to define the changing role and significance of step–pools as energy dissipators in steep mountain channels. Although energy dissipation is a significant function of step–pools, this role changes over time with variations in discharge. Steps are effective in reducing stream energy at low flows, but their effectiveness diminishes with increasing stage. Accordingly, the manner in which energy dissipation occurs also varies. With increasing flows, spill resistance gives way to a dominance in form and grain resistance. The conceptual model for the changing role of step–pools is illustrated with data from hydraulic analysis and modeling of step–pools in the Santa Monica Mountains of California. The model points to the importance of the size of steps in determining their role in energy dissipation and in their interactions with channel hydraulics. The model offers a new articulation for the geomorphic significance of step–pools in mountain streams, and it serves as a useful template for a more complete understanding of step–pools over a longer time scale.

Defining phases of bedload transport using piecewise regression

AbstractDifferences in the transport rate and size of bedload exist for varying levels of flow in coarse‐grained channels. For gravel‐bed rivers, at least two phases of bedload transport, with notably differing qualities, have been described in the literature. Phase I consists primarily of sand and small gravel moving at relatively low rates over a stable channel surface. Transport rates during Phase II are considerably greater than Phase I and more coarse grains are moved, including material from both the channel surface and subsurface. Transition from Phase I to Phase II indicates initiation and transport of grains comprising the coarse surface layer common in steep mountain channels. While the existence of different phases of transport is generally acknowledged, the threshold between them is often poorly defined. We present the results of the application of a piecewise regression analysis to data on bedload transport collected at 12 gravel‐bed channels in Colorado and Wyoming, USA. The piecewise regression recognizes the existence of different linear relationships over different ranges of discharge. The inflection, where the fitted functions intersect, is interpreted as the point of transition from Phase I to Phase II transport; this is termed breakpoint. A comparison of grain sizes moved during the two phases shows that coarse gravel is rarely trapped in the samplers during Phase I transport, indicating negligible movement of grains in this size range. Gravel larger than about D16 of the channel surface is more consistently trapped during Phase II transport. The persistence of coarse gravel in bedload samples provides good evidence that conditions suitable for coarse grain transport have been reached, even though the size of the sediment approaches the size limits of the sampler (76 mm in all cases). A relative breakpoint (Rbr) was defined by the ratio between the discharge at the breakpoint and the 1·5‐year flow (a surrogate for bankfull discharge) expressed as a percentage. The median value of Rbr was about 80 percent, suggesting that Phase II begins at about 80 percent of the bankfull discharge, though the observed values of Rbr ranged from about 60 to 100 percent. Variation in this value appears to be independent of drainage area, median grain size, sorting of bed materials, and channel gradient, at least for the range of parameters measured in 12 gravel‐bed channels. Published in 2002 by John Wiley &amp; Sons, Ltd.

Channel-bed mobility response to extreme sediment loading at Mount Pinatubo

Since the 1991 eruption of Mount Pinatubo, the specific sediment yields from watersheds draining its slopes have been the highest ever recorded. In spite of this overwhelming sediment load, rivers inundated by pyroclastic-flow deposits delivered almost half of the initial deposits to downslope alluvial and/or debris fans by 1996. Although most of this transport occurred by hyperconcentrated flows and debris flows (lahars), very high sediment transport rates also characterize low flow. Measured flow velocities and depths, channel properties, and the size of both mobile and stable clasts in the Sacobia and Pasig-Potrero Rivers indicate median bed-surface grain sizes of 2 to 8 mm, grain-size-independent dimensionless critical shear stresses ( τ*c) of 0.016 to 0.041 and Manning’s n values of 0.017 to 0.024, values well below those previously reported for steep mountain channels. The dramatic bed-surface fining, bed mobility, and selective transport in these extremely sediment-rich channels indicate that changes in grain size, τ*c values, and bed roughness can increase transport capacity in response to high sediment supply. Our observations also suggest viewing the apparently contradictory concepts of an equal threshold of bed mobility and grain-size‐dependent selective transport as end-member concepts that apply to channels with low (or intermittent) and high (or continuous) sediment supply, respectively.