Mountain River Channels Research Articles

Experiments on flow velocity profiles in mountain river channels

ABSTRACT Research on the vertical profiles of flow velocity in mountainous river channels is limited, particularly in scenarios where complex bed geometries are absent. Due to the coarse roughness and seepage flow on streambeds composed of gravel, the conventional formulae for flow velocity profiles derived from fluvial river channels do not apply to mountainous river channels. Based on flume experiments with a bed packed with natural gravel and a slope ranging from 0.006 to 0.16, we derived a theoretical formula for flow velocity profiles. This new formula integrates the influence of the subsurface flow and velocity reduction near the water surface, demonstrating a strong alignment with measurements. Our findings indicate that for shallow water flow over rough bed surfaces, the turbulence intensity diminishes along the vertical direction in the near-bed region while remaining relatively constant in the upper water body. Contrary to conventional theories which attribute the increase in flow resistance and the decrease in sediment transport rates in mountainous river channels to form drag, our study emphasizes that the subsurface flow plays a significant role in the overall flow resistance of mountainous river channels and should not be overlooked.

Litter Selfie: A Citizen Science Guide for Photorecording Macroplastic Deposition along Mountain Rivers Using a Smartphone

Macroplastic pollution in mountain rivers can threaten water resources, biodiversity, and the recreational values provided by them. The first step towards evaluating and then mitigating these risks is the systematic collection of reliable and spatially uniform data on the amount and type of macroplastics deposited in different land covers occurring in a mountain river channel. To maximise the opportunity for the large-scale collection of such data using the citizen science approach, we propose in this study an illustrated step-by-step guide to sample the macroplastic deposited along mountain rivers and to record the collected information using a photo taken by smartphone and a simple online form. Our guide includes three steps: (i) the location of sampling plots across 3–4 predefined surface covers occurring in mountain rivers of temperate climate, (ii) the hand collection of macroplastic deposited in them, and (iii) the photorecording and archiving of information on macroplastics collected using a smartphone and an online form. The proposed guide can allow for the low-cost collection of data on macroplastic deposition in mountain rivers on regional and global scales. The collected data can be further analysed by environmental scientists to quantify the amount and types of macroplastic deposited and to evaluate the resulting risks. They can be also used as illustrative materials to increase the awareness of local communities about the plastic pollution problem.

Physical Modelling of Roll and Pitch Motions of Travelling or Stationary Ship in Large-Scale Landslide-Generated Waves

Large-scale landslides often occur in river-type reservoirs, and landslide-generated waves affect navigation channels and the navigation of ships. Thus, such waves cause widespread regional disasters. This study establishes a mechanical model of landslide-generated waves via field investigations and data collection, reveals the mechanism and process of landslide-generated waves, and investigates the propagation characteristics of landslide-generated waves along a sloping wave. The feasibility of the model is verified via (i) regularity analysis, (ii) comparative analysis of the effect of landslide-generated waves of mountain river channel reservoirs on the movement characteristics of navigation vessels and stationary vessels, (iii) deviation from the equilibrium position, and (iv) an in-depth study of the influence of large-scale landslide-generated waves on ships in different navigation positions in a river channel. Countermeasures are proposed for a sailing ship to tackle a sudden landslide-generated wave; these measures can provide a theoretical basis for ships to sail safely through large-scale landslide-generated waves.

The impact of extreme hydro-meteorological events on the transformation of mountain river channels (Polish Flysch Carpathians)

The impact of extreme hydro-meteorological events on the transformation of mountain river channels (Polish Flysch Carpathians)

Climate regulates the erosional carbon export from the terrestrial biosphere

Erosion drives the export of particulate organic carbon from the terrestrial biosphere (POCbiosphere) and its delivery to rivers. The carbon transfer is globally significant and can result in drawdown of atmospheric carbon dioxide (CO2) if the eroded POCbiosphere escapes degradation during river transfer and sedimentary deposition. Despite this recognition, we lack a global perspective on how the tectonic and climatic factors which govern physical erosion regulate POCbiosphere discharge, obscuring linkages between mountain building, climate, and CO2 drawdown. To fill this deficit, geochemical (δ13C, 14C and C/N), hydrometric (water discharge, suspended sediment concentration) and geomorphic (slope) measurements are combined from 33 globally-distributed forested mountain catchments. Radiocarbon activity is used to account for rock-derived organic carbon and reveals that POCbiosphere eroded from mountain forests is mostly <1300 14C years old. Annual POCbiosphere yields are positively correlated with suspended sediment yields, confirming results from Taiwan and a recent global analysis, and are high in catchments with the steepest slopes. Based on these relationships and the global distribution of slope angles (3-arc-second), it is suggested that topography steeper than 10° (16% of the continental area) may contribute ~40% of global POCbiosphere erosional flux.Climate is shown to regulate POCbiosphere discharge by mountain rivers, by controlling hydrologically-driven erosion processes. In catchments where discharge measurements are available (8 of the 33) a significant relationship exists between daily runoff (mmday−1) and POCbiosphere concentration (mgL−1) (r=0.53, P<0.0001). The relationship can be described by a single power law and suggests a high connectivity between forested hillslopes and mountain river channels. As a result, annual POCbiosphere yields are significantly correlated with mean annual runoff (r=0.64, P<0.0001). A shear-stress POCbiosphere erosion model is proposed which can explain the patterns in the data. The model allows the climate sensitivity of this carbon flux to be assessed for the first time. For a 1% increase in annual runoff, POCbiosphere discharge is predicted to increase by ~4%. In steeper catchments, POCbiosphere discharge increases more rapidly with an increase in annual runoff. For comparison, a 1% increase in annual runoff is predicted to increase carbon transfers by silicate weathering solute fluxes in mountains by 0.4–0.7%. Depending on the fate of the eroded POCbiosphere, river export of POCbiosphere from mountains may act as an important negative feedback on rising atmospheric CO2 and increased global temperature. Erosion of carbon from the terrestrial biosphere links mountain building and climate to the geological evolution of atmospheric CO2, while the carbon fluxes are sensitive to predicted changes in runoff over the coming century.

Strength and erosion mechanism of soft bedrock under high shear stresses

Bedrock erosion plays a critical role in mountain river channel evolution, especially, where there are hydro-projects with high velocity flow conditions. In this study, laboratory experiments are carried out to investigate strength and erosion mechanism of soft bedrock under high shear stresses by high velocity open channel flows. Prototype bedrock from the upper reaches of Yangtze River is employed as testing samples. New observed dataset is generated concerning the flow, bedrock strength and evolution of bedrock erosion, which can be exploited to evaluate bedrock erosion rate and test mathematical bedrock erosion models.

Environment-friendly reduction of flood risk and infrastructure damage in a mountain river: Case study of the Czarny Dunajec

Migration of a mountain river channel may cause erosional risk to infrastructure or settlements on the valley floor. Following a flood of 2010, a cutbank in one of the bends of the main channel of the Czarny Dunajec, Polish Carpathians, approached a local road by 50m. To arrest the erosion of the laterally migrating channel, water authorities planned construction of a ditch cutting the forested neck of the bend, reinforcement of the ditch banks, and damming the main channel with a boulder groyne. In order to avoid channelization of the highly valued, multithread river reach that would deteriorate its ecological status and cause increased flood risk to downstream reaches, an alternative approach to prevent bank erosion was proposed. The new scheme, applied in 2011, included opening of the inlets to inactive side braids located by the neck of the bend of the main channel. This solution reestablished the flow in the steeper low-flow channels, allowing us to expect a cutoff and abandonment of the main channel during subsequent floods. Gravelly deflectors were constructed directly below the inlets to the reactivated side channels to divert the flow into the channels and prevent the water from entering the main channel. Hydraulic measurements performed before and after the implementation of the scheme confirmed that it enabled shifting the main water current, with the highest average velocity and bed shear stress, from the braid closest to the road to the most distant braid. Similar surveys of fish and benthic macroinvertebrate communities indicated that flow reactivation in the side channels was beneficial for these groups of river biota, increasing their abundance and taxonomic richness in the reach. Not only was the implemented solution significantly less expensive, but it also enhanced ecological functions of the multithread channel and the variability of physical habitat conditions and maintained the role of the reach as a wood debris trap. However, avulsion of the main channel in the river bend immediately upstream during the flood in May 2014 again caused erosional risk to the road, although at another location. This indicates that with the highly unstable, multithread channel pattern of the Czarny Dunajec, the best practice of river maintenance in a relatively unmanaged valley reach would be allowing free channel migration within the floodplain area and reinforcing, where necessary, the boundary between the erodible river corridor and the managed terrace.

Effects of environmental changes and human impact on the functioning of mountain river channels, Carpathians, southern Poland

Abstract In the northern slope of the Carpathian Mountains and in their foreland, river and stream channels have been significantly transformed by human impact. These transformations result from changing land use in river basins and direct interference with river channels (alluvia extraction, engineering infrastructure, channel straightening). Anthropogenic impacts cause significant changes in the channel system patterns leading to increased impact of erosion. This mainly leads to the channelling of the fluvial system. This article reviews studies of structure and dynamics of Carpathian river channels conducted based on the methodology of collection of data on channel systems, developed in the Department of Geomorphology of the Institute of Geography and Spatial Management, Jagiellonian University.

Geomorphic Responses to a Large Check‐Dam Removal on a Mountain River in Taiwan

AbstractThe ability to understand and predict the impacts of dam removal in river systems is important, especially as dam decommissioning is becoming increasingly popular. In this study, we document the morphological and sediment impact of the removal of Chijiawan Check Dam in May 2011; a 13‐m‐high dam located on a coarse‐grained, steep mountain river channel in Taiwan. An estimated 0.2 million m3 of sediment had accumulated within the impoundment before its removal. Longitudinal and bankfull cross‐sectional surveys and a detailed sediment textural survey were undertaken along a 3.2‐km study reach of the Chijiawan Creek between 2010 and 2012. A rotating knickpoint with migration rates of up to 22 m/day was observed along the study reach, following dam removal. The rate and character of channel change, associated with the dam removal, appear to be driven as much by channel morphology and distance from the dam as by the hydrology variability. Our results suggested that relatively small amounts of sediment were eroded during the first 3 weeks following dam removal because of low discharge conditions. However, after 1 and 15 months, 10 and 75% of the sediment that had accumulated within former impounded was eroded, respectively. Sites near the former dam had a sediment texture that reflected the transport of released sediment, and this suggested that basin‐wide sediment processes exerted a strong influence. The removal of Chijiawan Dam offers unique insight on how sediment processes can drive river channel responses to sediment pulses may vary with discharge and sediment load, in areas subject to remarkably high flows and sediment loads. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Tying bulk watershed properties to mountain river channel evolution

Rivers evolve over tens to thousands of years, winding and drifting and branching from single channels to braided, complex flows. Understanding exactly how and why rivers change course is a complex problem that depends on the erosion of the landscape and incorporates a range of variables, from local environmental conditions, such as the precipitation rate, to the topography, water chemistry, and properties of the rocks in the watershed. At smaller scales, channel evolution depends on the physical properties of, and interactions between, individual grains of sediment. Correlating across this entire range of scales is incredibly difficult, but using a set of comparable observations from 83 watersheds in the U.S. Rocky Mountains, Mueller and Pitlick tie climatic, topographic, and geologic features to sediment processes and river evolution.

A nonlocal theory of sediment buffering and bedrock channel evolution

Bedrock erosion in mountain river channels ultimately sets the erosion rate of the surrounding hillslopes and the rate of sediment supply to the channels. The supply of coarse bed sediment acts as a dampening effect on further erosion by depositing an alluvial cover that temporarily obscures the bedrock. For landscapes where the residence time of the alluvial bed cover is comparable to the timescale of bedrock incision, coarse sediment supply and transport generate a strong negative feedback on fluvial downcutting and the coupled process of hillslope‐channel erosion is inherently self‐buffering. Here we study a simple model of self‐buffered bedrock channel erosion that incorporates the spreading of bed sediment cover downstream in a way that allows for a broad‐tailed, power law probability distribution of transport velocities of bed sediment over the long‐term. This leads us to consider a nonlocal transport law (fractional advection) parameterized by a scaling exponent 0 ≤ α &lt; 1 which collapses to local advection for α → 1. For strong sediment buffering, we find that nonlocality 1 − α has a direct control on the power law scaling of channel slope S with upstream area A, giving S ∼ A−(1−α)/2 at steady state. Empirical observations of slope‐area scaling are consistent with α &lt; 1 and nonlocal transport. In general, the model predicts linear, logarithmic, or power law stream profiles depending on the extent of buffering, the degree of nonlocality, and the scaling of the bedrock erosion law. It also predicts, somewhat counterintuitively, that bed cover should thicken with distance x downstream slower than linearly as xα, i.e., the more nonlocal the bed sediment spreading process (α → 0), the slower the bed cover increases downstream. We deduce that long‐range, heterogeneous transport of coarse sediment in mixed bedrock‐alluvial rivers may be a key element of landscape scaling and an important factor in landscape dynamics.

Air concentrations of submerged and unsubmerged hydraulic jumps in a bedrock step‐pool channel

The classical hydraulic jump, which is a hydraulic jump formed in a horizontal, smooth, and prismatic laboratory flume, has often been used for estimating hydraulic and morphologic characteristics of mountain river channels. However, most studies utilizing classical hydraulic jump relationships have neglected the role of aerated flows and jump types prevalent in natural systems. By developing a methodology to survey, digitize, and visualize air concentrations for jumps in bedrock rivers, we present new data for submerged jumps and sloping jumps in a bedrock step‐pool channel. As with laboratory research, our results indicated peak, mean, and local air concentrations of natural jumps were skewed by submergence of the jump roller beneath the free surface. However, unlike laboratory research, cross‐stream variations were significant for both jumps, varying up to 20% across the jump width. Traces through each jump thalweg also indicated the classical hydraulic jump was a poor analogue for estimating surficial air concentrations.

A Channelization Model of Landscape Evolution

The geomorphic evolution of many landscapes is fundamentally deter- mined by the evolution of the river channels and their interactions with hillslopes. Consequently, models of landscape evolution ought to track the evolution of the channel geometry so as to quantify the rate of erosion of channel bottoms and to follow the changes in hillslope-channel coupling over time. Unfortunately, the spatial resolution required to describe channel morphology adequately is computationally impractical. It is also beyond the resolution of most digital elevation data. What is required is a parameterization scheme for approximating fine scale channel morphol- ogy at a coarse pixel scale. Such a parameterization is already implicitly employed in most models by assuming channel equilibrium, which ties the width and depth of a model channel to the square root of discharge through a pixel. Channel fluxes are thereby predictable, and a closed form of the governing equations is attained. In reality, mountain river channels do not take a simple equilibrium form and show great spatial variability and evident disequilibrium geometry. Since the time scales of changes in channel geometry, bedrock channel erosion, and hillslope response are all closely related, it is reasonable to infer that the spatio-temporal development of the landscape is determined by their interaction and that channel disequilibrium is a fundamental factor in the dynamics of landscape evolution. If this is the case, we need an alternative sub-grid scale parameterization that aggregates channel properties such as surface morphology, roughness, cross-sectional geometry, so that the time depen- dent behavior of these properties can be estimated at the coarse pixel scale. We introduce such a parameterization measure, which we term channelization, after extensive investigation of the pixel resolution dependence of topographic relief. We focus in particular on the effect of coarse graining on digital elevation data for derived measures such as channel slope and upstream area and demonstrate that we can approximately correct for this effect. We show that a very simple geomorphic model can be constructed around the channelization parameter and the resolution-invariant topographic measures. This model demonstrates that channel disequilibrium may play a significant role in mountain landscape dynamics. It also shows how geomorphic models in general could be modified to incorporate such sub-pixel scale complexities and to better model these dynamics.