The intensity of slope and fluvial processes after a catastrophic windthrow event in small catchments in the Tatra Mountains

Strong wind events frequently result in creating large areas of windthrow, which causes abrupt environmental changes. Bare soil surfaces within pits and root plates potentially expose soil to erosion. Absence of forest may alter the dynamics of water circulation. In this study we attempt to answer the question of whether extensive windthrows influence the magnitude of geomorphic processes in 6 small second-to third-order catchments with area ranging from 0.09 km^(2) to 0.8 km^(2). Three of the catchments were significantly affected by a windthrow which occurred in December 2013 in the Polish part of the Tatra Mountains, and the other three catchments were mostly forested and served as control catchments. We mapped the pits created by the windthrow and the linear scars created by salvage logging operations in search of any signs of erosion within them. We also mapped all post-windthrow landslides created in the windthrow-affected catchments. The impact of the windthrow on the fluvial system was investigated by measuring a set of channel characteristics and determining bedload transport intensity using painted tracers in all the windthrow-affected and control catchments. Both pits and linear scars created by harvesting tend to become overgrown by vegetation in the first several years after the windthrow. The only signs of erosion were observed in 10% of the pits located on convergent slopes. During the period from the windthrow event in 2013 until 2019, 5 very small(total area <100 m^(2)) shallow landslides were created. The mean distance of bedload transport was similar(t-test, p=0.05) in most of the windthrow-affected and control catchments. The mapping of channels revealed many cases of root plates fallen into a channel and pits created near a channel. A significant amount of woody debris delivered into the channels influenced the activity of fluvial processes by creating alternating zones of erosion and accumulation.

Fluvial Processes in the Meandering Reach of the Lower Wei River During the Course of Degradation

This paper presents an analysis of the changes of the longitudinal and lateral profiles in the meander- ing reach of the Lower Wei River over the period from October 1973 to October 1976 during the course of degradation.Analysis results indicated that retrogressive erosion and subsequent downstream erosion occurred in the reach due to the lowering in the Tongguan elevation and the inflowing water carrying low sediment con- centrations.At the end of the degradation,the main channel widths of the majority ...

Multiple time scales of fluvial processes—theory and applications

Fluvial processes comprise water flow,sediment transport and bed evolution,which normally feature distinct time scales.The time scales of sediment transport and bed deformation relative to the flow essentially measure how fast sediment transport adapts to capacity region in line with local flow scenario and the bed deforms in comparison with the flow,which literally dictates if a capacity based and/or decoupled model is justified.This paper synthesizes the recently developed multiscale theory for sediment-laden flows over erodible bed,with bed load and suspended load transport,respectively.It is unravelled that bed load transport can adapt to capacity sufficiently rapidly even under highly unsteady flows and thus a capacity model is mostly applicable,whereas a non-capacity model is critical for suspended sediment because of the lower rate of adaptation to capacity.Physically coupled modelling is critical for fluvial processes characterized by rapid bed variation.Applications are outlined on very active bed load sediment transported by flash floods and landslide dam break floods.

Fluvial processes and their impact on the finless porpoise's habitat after the Three Gorges Project became operational

Since the filling of the reservoir of the Three Gorges Project(TGP)dam in the Yangtze River in 2003,erosion downstream from the dam site has affected the finless porpoise’s habitat.In this study,a one-dimensional(1D)fluvial process mathematical model is used to calculate flow and sediment transport in the middle and lower reaches of the Yangtze River,including the finless porpoise’s habitat.By analyzing the calculation results for the water resources,suspended load,and bed materials in the finless porpoise’s habitat after the riverbed deformation,we evaluated the possible impact on this rare Yangtze River aquatic animal.The results show that,with the erosion of riverbed over the next 20 years,the water quantity comprising the habitat will decrease to half of its present amount,and the bed materials will be eroded to coarse grading,such that the reserve will become a gradually disappearing stream.Effective engineering measures should be used to decrease the erosion in the main channel,in case the overall erosion cannot be stemmed and controlled,to ensure an adequate water volume flows into the finless porpoise’s habitat.

Accumulation phenomena in fluvial processes and the corresponding stochastic model

Accumulation occurs widely in fluvial processes.Accurately accounting for the effects of previous water and sediment conditions on accumulation is essential for studying riverbed evolution.In this study,to reveal the physical mechanisms of accumulation,various geometric observations of both the upstream and downstream reaches of dams on several typical fluvial channels were analyzed.The changes in water and sediment conditions were defined as external disturbances.Assuming that the probability of an external disturbance conforms to a Poisson distribution,and that the response intensity induced by an individual disturbance decays exponentially over time,a mathematical description of the accumulation of internal responses to external disturbances is given.Furthermore,a corresponding theoretical model for simulating the spatiotemporal readjustments of characteristic river variables is proposed based on stochastic theory.The proposed models are then applied to investigate spatiotemporal readjustment in the upper and lower reaches of dams following their construction.The results indicate that temporally,the vertical,lateral,and overall readjustment rates of the reaches are relatively fast in the early period following dam construction but then decrease rapidly over time.Accumulated riverbed degradation,channel width,and sedimentation continuously increase until a new dynamic equilibrium is reached.These phenomena reflect the representative accumulation characteristics of fluvial processes.Spatially,the erosion intensities in downstream reaches decrease nonlinearly along the channel until eventually diminishing.The unbalanced spatial distribution of erosion intensity arises from the system response characterized by propagation in space but decay over time,which is characteristic of accumulation phenomena after disturbances.The results of the developed model show that the spatiotemporal readjustments of the studied cross-sections and channel reaches can be accurately described by the unified theoretical formula derived herein.The model predictions show good agreement with observed field data with determination coefficients of 0.92,0.93,0.76,and 0.95 for vertical,lateral,longitudinal,and overall readjustments,respectively.The proposed theoretical models account for both the accumulation characteristics of fluvial processes and their spatial distributions.In demonstrating the proposed ap-proach,this study provides a theoretical basis and new calculation method for quantitatively describing the spatiotemporal readjustments of non-equilibrium fluvial channels following external disturbances.

Analysis of Morphological Processes in a Disturbed Gravel-Bed River （Piave River）： Integration of LiDAR Data and Colour Bathymetry

The magnitude of river morphological changes are better analyzed through the use of quantitative approaches, wherein resolution accuracy and uncertainty assessment are treated as crucial key-factors. In this sense, the creation of precise DEMs （Digital Elevation Models） of rivers represents an affordable tool to analyze geomorphic variations and budgets, except for wetted areas, where reliable channel digitalization can normally be obtained only using expensive bathymetric surveys. The proposed work aims at improving channel surface models without having available bathymetric sensors, by deriving dry areas elevations from LiDAR data and water depth of wetted areas from aerial photos through a predictive depth-colour relationship. The methodology was applied to two different sub-reaches of the Piave River, a gravel-bed river which suffered severe flood events in 2010. Erosion and deposition patterns were identified through DEM differencing, showing a predominance of scour processes which can lead to channel instability situations. The bathymetric output was compared to other previously-derived models confirming the accuracy of the in-channel elevation estimates. Finally, a discussion on the role played by longitudinal protections during the studied flood events is proposed, focusing the attention on the incidence of two major bank erosions that removed significant volumes of stable areas.

Impact of hydroelectric projects on river environment: Analysis of water quality changes in Ningxia Reach of Yellow River

In this paper, the relationship between hydroelectric projects and the river environment is analyzed. Recently, the large-scale regulation of runoff by large hydroelectric projects in the Ningxia Reach of the Yellow River has altered natural runoff processes, causing an increase in the probability of low discharge and an overall adjustment of riverbed evolution and river characteristics. During low-flow years, the combined effects of these two changes can weaken the self-purification capacity and reduce the water environmental capacity of the river. This is one of the main reasons for the recent decrease of water quality in the Ningxia Reach. This research shows that it is necessary to implement river training projects to maintain stable flow paths, not only for adjusting river regimes and for flood control, but also for increasing the self-purification capacity and the water environmental capacity of the river. Methods and proposals for coordinating the operation of hydroelectric projects with the protection of the river environment are presented in the interest of promoting sustainable development.

Non-capacity transport of non-uniform bed load sediment in alluvial rivers

Rivers often witness non-uniform bed load sedim ent transport. For a long tim e, non-uniform bed load transport has been assum ed to be at capacity regime determined exclusively by local flow. Yet whether the capacity assumption for non-uniform bed load transport is justified remains poorly understood. Here, the relative time scale of non-uniform bed load transport is evaluated and non-capacity and capacity models are compared for both aggradation and degradation cases with observed data. As characterized by its relative time scale, the adaptation of non-uniform bed load to capacity regime should be fulfilled quickly. However, changes in the flow and sedim ent inputs from upstream or tributaries hinder the adaptation. Also, the adaptation to capacity regime is size dependent, the finer the sediment size the slower the adaptation is, and vice versa. It is shown that the capacity model may entail considerable errors compared to the non-capacity model. For modelling of non-uniform bed load, non-capacity modelling is recommended, in which the temporal and spatial scales required for adaptation are explicitly appreciated.

Riverine microplastics and their interaction with freshwater fish

This paper outlines the nature of microplastic contamination in rivers and the risks to freshwater fishes.We discuss how input sources influence the concentration and composition of microplastics and examine factors that subsequently influence their spatiotemporal dynamics in a river system.We then discuss how the distributions and assemblages of microplastics can impact the risk of interactions with fishes,and the processes associated with the internalisation of microplastic into the body and across the organs and tissues.Finally,we examine the physical and toxicological effects of microplastic exposure in fish species,with special attention directed towards impacts at environmentally relevant concentrations.This review integrates expertise in fluvial geomorphological processes and how they influence the movement and storage of microplastics in river channel environments at a range of scales.We combine this knowledge with expertise in fish ecology and biology to set out a new and integrated analysis of microplastic dynamics in rivers and how these microplastics interact with fish.The integration of knowledge from these fields allows us also to comment upon the microplastic risk to fish and other biota in river environments.