Channel Sediment Transport Research Articles

Fully-coupled hydrologic processes for modeling landscape evolution

Although current landscape evolution models can predict landscapes with specific concave-convex slopes, regolith thicknesses, drainage densities and relief, these models rarely include realistic groundwater and overland flows, and channel-hillslope interactions. To overcome the potential drawbacks, this study couples hydrologic processes with hillslope and channel sediment transport processes to form a new hydrologic-morphodynamic model (LE-PIHM) for regolith formation and landscape evolution. Two scenarios with and without groundwater flow are presented to demonstrate the importance of this coupling. Comparison of the steady state landforms indicates that hillslopes are steeper and relief is higher with groundwater flow. The sensitivity of the solution to mesh geometry is tested and it is shown that model simulations maintain the characteristic features of a landscape over a reasonable range of maximum area and minimum interior angle. To predict long-term landscape change, a morphological acceleration technique is presented and a method for choosing an optimal morphological scale factor is introduced.

Analytical solutions for steady two-dimensional suspended sediment transport in channels with arbitrary advection velocity and eddy diffusivity distributions

ABSTRACTAn analytical solution approach is proposed for the steady two-dimensional suspended sediment transport in channels using the generalized integral transform technique (GITT). Previous analytical solutions are limited by advection velocity distribution, eddy diffusivity distribution, or boundary conditions. The contribution of the new approach is to eliminate these limitations. It solves the suspended sediment transport equation with arbitrary advection velocity and eddy diffusivity distributions. The new solution approach also considers realistic boundary conditions at the free surface and the bed. Compared with previous analytical solutions as well as numerical solutions from a high resolution spectral code, the new approach is of high accuracy. It greatly extends the domain of analytical solutions for suspended sediment transport problems.

Subseasonal changes observed in subglacial channel pressure, size, and sediment transport

AbstractWater that pressurizes the base of glaciers and ice sheets enhances glacier velocities and modulates glacial erosion. Predicting ice flow and erosion therefore requires knowledge of subglacial channel evolution, which remains observationally limited. Here we demonstrate that detailed analysis of seismic ground motion caused by subglacial water flow at Mendenhall Glacier (Alaska) allows for continuous measurement of daily to subseasonal changes in basal water pressure gradient, channel size, and sediment transport. We observe intermittent subglacial water pressure gradient changes during the melt season, at odds with common assumptions of slowly varying, low‐pressure channels. These observations indicate that changes in channel size do not keep pace with changes in discharge. This behavior strongly affects glacier dynamics and subglacial channel erosion at Mendenhall Glacier, where episodic periods of high water pressure gradients enhance glacier surface velocity and channel sediment transport by up to 30% and 50%, respectively. We expect the application of this framework to future seismic observations acquired at glaciers worldwide to improve our understanding of subglacial processes.

Extreme learning machine assessment for estimating sediment transport in open channels

The minimum velocity required to prevent sediment deposition in open channels is examined in this study. The parameters affecting transport are first determined and then categorized into different dimensionless groups, including "movement," "transport," "sediment," "transport mode," and "flow resistance." Six different models are presented to identify the effect of each of these parameters. The feed-forward neural network (FFNN) is used to predict the densimetric Froude number (Fr) and the extreme learning machine (ELM) algorithm is utilized to train it. The results of this algorithm are compared with back propagation (BP), genetic programming (GP) and existing sediment transport equations. The results indicate that FFNN-ELM produced better results than FNN-BP, GP and existing sediment transport methods in both training (RMSE = 0.26 and MARE = 0.052) and testing (RMSE = 0.121 and MARE = 0.023). Moreover, the performance of FFNN-ELM is examined for different pipe diameters.

Addressing the urban stream disturbance regime

AbstractThresholds for particle entrainment and natural disturbance frequency vary across hydrogeomorphic settings, but urbanization increases the rate and extent of channel erosion and sediment transport in alluvial channels. The urban disturbance regime is a change in the frequency, magnitude, and duration of hydrologically induced disturbance on the stream channel and ecosystem that can lead to geomorphic and ecological degradation. To preserve stream stability and ecological function, stormwater management systems should be optimized to maintain the natural disturbance regime of streambed material within the context of societal and environmental goals. Our proposed framework, based on data from 195 sites across 2 continents, shows that the tools of river mechanics can be used to develop relatively simple, regionally appropriate, streambed-specific guidance for stormwater designers, so that engineers can calibrate stormwater facilities that address the urban stream disturbance regime. In the absence of...

Effects of Timber Harvest on Water Quantity and Quality in Small Watersheds in the Piedmont of North Carolina

This paired watershed study tested the effects of timber harvest on water quantity and quality in the North Carolina Piedmont physiographic region. Four headwater watersheds at Hill Demonstration Forest (HF1, HF2, HFW1, and HFW2) and two at Umstead Research Farm (UF1 and UF2) were continuously monitored for discharge and water quality from 2007 to 2013. The HF1 and UF1 watersheds were clearcut (treatment), leaving a 15.2-m vegetated riparian buffer around the streams to protect water quality as described in the North Carolina Neuse River Basin Riparian Buffer Rule. HF2 and UF2 were uncut and used as reference watersheds. Merchantable timber was selectively removed from the riparian buffer, reducing tree basal area by 27% in HF1 and 48% in UF1. HF1 and HF2 were nested within HFW1; thus, HFW1 was considered a partial cut where 33% of the watershed area was harvested, and HFW2 was the reference. We found that discharge in treatment watersheds increased dramatically, averaging 240% in HF1 and 200% in UF1 and 40% in HFW1 during the postharvest period, 2011–2013. Total suspended sediment export in the treatment watersheds also increased significantly in HF1 after harvest, probably due to the increase of discharge and movement of in-channel legacy sediment. Stormflow peak nitrate reached its maximum concentration during the first 2 years after harvest in the treatment watersheds and then declined, corresponding to the rapid regrowth of woody and herbaceous plants in the riparian buffer and uplands. We found that 36% of the UF1 streambank trees were blown down but did not cause a measurable increase in mean daily stormflow total suspended sediment concentration. Most buffer tree blowdown occurred during the first few years after a harvest. Bioclassification of benthic macroinvertebrates indicated that stream water quality remained good/fair to excellent in the treatment watersheds after the harvest. We conclude that the temporary increases in discharge were relatively large for the Piedmont region compared with those for other regions in the southeastern United States. However, the increases in channel sediment transport and nutrient exports associated with the hydrologic change did not have a measurable impact on the indicators of aquatic invertebrate community health or bioclassification rankings. Management and Policy Implications: There are three distinct land provinces across North Carolina: the mountains, Piedmont, and coastal plain. Understanding how region-specific watershed hydrology responds to land management and natural disturbances can provide useful information to land managers as they set flow targets needed to maintain ecological integrity in surface waters or to design riparian buffers for water quality protection. We found that after a clearcut timber harvest with a riparian buffer zone around the streams, the percent increases in annual discharge and mean nitrate concentrations tended to be higher in the Piedmont region than in the other two regions. Our study also found that tree blowdown in the riparian buffer was more likely to occur in the Piedmont Triassic region than in the Piedmont Carolina Slate Belt region. In the Triassic region, additional management activities should be part of the preharvest planning process as they may help mitigate windthrow and uprooting of streambanks and improve the overall riparian buffer functions and flow dynamics. For example, land planners or loggers should assess whether soils have Triassic or Slate Belt characteristics to determine if they need to refrain from creating gaps and retain more windfirm species within the riparian buffer. Although there was variation in blowdown occurrences among regions, Triassic soils comprise about 3.5% of North Carolina's total land area, so any statewide implications would be low and recommendations would be localized.

Temporal variability and memory in sediment transport in an experimental step‐pool channel

AbstractTemporal dynamics of sediment transport in steep channels using two experiments performed in a steep flume (8%) with natural sediment composed of 12 grain sizes are studied. High‐resolution (1 s) time series of sediment transport were measured for individual grain‐size classes at the outlet of the flume for different combinations of sediment input rates and flow discharges. Our aim in this paper is to quantify (a) the relation of discharge and sediment transport and (b) the nature and strength of memory in grain‐size‐dependent transport. None of the simple statistical descriptors of sediment transport (mean, extreme values, and quantiles) display a clear relation with water discharge, in fact a large variability between discharge and sediment transport is observed. Instantaneous transport rates have probability density functions with heavy tails. Bed load bursts have a coarser grain‐size distribution than that of the entire experiment. We quantify the strength and nature of memory in sediment transport rates by estimating the Hurst exponent and the autocorrelation coefficient of the time series for different grain sizes. Our results show the presence of the Hurst phenomenon in transport rates, indicating long‐term memory which is grain‐size dependent. The short‐term memory in coarse grain transport increases with temporal aggregation and this reveals the importance of the sampling duration of bed load transport rates in natural streams, especially for large fractions.

An effective Euler–Lagrange model for suspended sediment transport by open channel flows

An Euler–Lagrange two-phase flow model is developed to study suspended sediment transport by open-channel flows with an Eddy Interaction Model (EIM) applied to consider the effect of fluid turbulence on sediment diffusion. For the continuous phase, the mean fluid velocity, the turbulent kinetic energy and its dissipation rate are directly estimated by well-established empirical formulas. For the dispersed phase, sediment particles are tracked by solving the equation of motion. The EIM is applied to compute the particle fluctuation velocity. Neglecting the effect of particles on flow turbulence as usually suggested for dilute cases in the literature, the Euler–Lagrange model is applied to simulate suspended sediment transport in open channels. Although the numerical results agree well with those by the well-known random walk particle tracking model (RWM) and with the laboratory data for fine sediment cases, it is clearly shown that such an Euler–Lagrange model underestimates the sediment concentration for the medium-sized and coarse sediment cases. To improve the model, a formula is proposed to consider the local fluid turbulence enhancement around a particle due to vortex shedding in the wake. Numerical results of the modified model then agree very well with laboratory data for not only the fine but also the coarse sediment cases.

Sediment transport in ice-covered channels

The existence of ice cover has important effects on sediment transport and channel morphology for rivers in areas with an annual occurrence of an ice season. The interaction of sediment transport and surface ice is poorly understood. In this paper, data from existing flume experiments, each with a limited range of flow and sediment transport conditions, are analyzed. The analysis showed that the bed load transport in ice-covered channels can be described by conventional relationships for the equivalent free-surface flow if the flow strength is expressed in terms of the bed shear stress. A modified Rouse formulation considering the effect of the ice cover on velocity and turbulent diffusion is shown to be applicable for calculating the suspended sediment transport discharge.

The relationship between particle travel distance and channel morphology: Results from physical models of braided rivers

Channel form and sediment transport are closely linked in alluvial rivers, and as such the development of a conceptual framework for the downstream controls on particle mobility and likely deposition sites has immense value in terms of the way we understand and predictively model rivers. Despite the development of conceptual models which frame flood-scale particle transport distance (termed path length) as a function of channel bar locations, an understanding of the controls on such path lengths in braided rivers remains especially elusive, in large part due to the difficulty in explicitly linking morphology and particle transport distances in the field. Here we utilize a series of laboratory flume experiments to link path length distances with channel morphology. Our morphologic characterization is based on ultrahigh-resolution digital elevation models and bar classifications derived from structure-from-motion topography, while we simultaneously capture particle path lengths using fluorescent tracer particles over the course of five physical model simulations. Our findings underscore the importance of channel bars in acting as deposition sites for particles in transport; 81% of recovered tracers were found in association with compound, point, lateral, or diagonal bars. Bar heads (29%) and bar margins (41%) were the most common bar-related deposition surfaces for recovered tracers. Peaks in particle deposition frequency corresponding to channel bars were often noted on path length distributions from tracer data; most tracers were deposited in areas that had experienced shallow (Δz= 0.002 m) deposition. Average path length distance (2.5 m) was closely related to average confluence-diffluence spacing (2.3 m) across all runs. The transferability of this understanding to braided streams has important implications for the development of simplified morphodynamic models which seek to predict braided channel evolution across multiflood timescales.

Anthropogenic activities since the end of the Little Ice Age: a critical factor driving fluvial changes on the Isonzo River (Italy, Slovenia)

The Isonzo is a 140‑km long river that draws its source in the Julian Alps in Slovenia and joins the Gulf of Trieste in the Northern Adriatic, Italy. Its catchment area (~3400 km2) consists of mid‑altitude mountains (70%), a piedmont (22%), and a coastal plain (8%) influenced by Mediterranean climatic conditions. The river is a high‑energy system due to its pronounced hydraulical gradients and with frequent damage to man‑made structures. The objective of this study is to determine the recent functioning of the Isonzo River by (i) a diachronic (from the Little Ice Age), plan‑form analysis of the alluvial plain, using old maps, topographic maps of the IGM and aerial photographs, and (ii) an analysis of channel incision and sediment transport, using data derived from national and regional water services and field observations. Over the last 200 years, the Isonzo River shows a clear tendency to active‑channel narrowing between 15 km and 2 km upstream of the river‑mouth. This phenomenon was accompanied by an incision of the active channel, estimated to be between 0.5 m and 1.5 m at the most vulnerable sites. In the context of climate change since ca. 1860, the narrowing/incision of the active channel increased from the 1880s to 1920s, and prticularly since 1960, because of human impacts (channelisation, embankments, dams) in the catchment, and despite the increase of flood frequencies. The recent hydro‑morpho‑sedimentary functioning of the Isonzo shows that the river is still in a transitional phase towards a physical equilibrium that changes after every human intervention in the active channel.

Sidewall and non-uniformity corrections for flume experiments

ABSTRACTStudying open channel flow and sediment transport in narrow flumes under non-uniform flow conditions, both sidewall and non-uniformity corrections are required for bed-shear stress. This research first reviews conventional predictive methods for bed-shear stress, including the flow-depth method, the hydraulic radius method and Einstein's sidewall correction. It then presents a novel procedure for sidewall and non-uniformity corrections based on a recent cross-sectional velocity distribution model. These methods are compared with data from the log-law under uniform and non-uniform, sub- and supercritical flow conditions, indicating that (i) the flow-depth and the hydraulic radius methods specify the upper and lower bounds for bed-shear stress; (ii) although Einstein's procedure causes a paradox for smooth flumes, it agrees with data from rough beds; and (iii) the proposed is better than Einstein's for subcritical flow, but the latter has advantage for supercritical flow. As an application, sediment inception under non-uniform flow conditions is also discussed.

Some Exact Solutions for Sediment Transport Equations

This paper presents some exact solutions for the nonlinear hyperbolic partial differential equations (PDEs), governing sediment transport in channels and rivers. The infinitesimal transformations for the governing system of PDEs are derived by using Lie group analysis. These are used to reduce the number of independent variables and consequently reduces the original system of PDEs into system of ordinary differential equations (ODEs). Moreover, the reduced system of ODEs are solved and new classes of exact group invariant solutions for the governing PDEs are constructed.

MOŽNOSTI VYUŽITÍ USMĚRŇOVACÍCH STAVEB V RÁMCI REVITALIZACE TOKU

Directing and concentrating structures are used mainly on water coursers with unstable channel and major sediment transport or in areas with major bank erosion. The main purpose of these structures is to redirect water flow from the stressed and very often eroded bank to the centre of the channel or other parts of the channel, where the water flow may be used e.g. for dispersion of unsuitable deposit before its eventual stabilization by natural succession.Another important goal is to achieve a desired change of the stream channel by using the transporting power of the water flow and targeted depositing of sediment. In the area of the deflecting structure (deflector), so-called hydraulic shadow also appears where sediment deposits. The objective is to achieve depositing of sediments at desired places of the stream.It is possible to design a whole range of suitable or less suitable types of flow deflectors. In our article, we will particularly focus on problems of revitalization of a water course; that is design of structures from biological or biotechnical elements for channel stabilization.

Physical context for theoretical approaches to sediment transport magnitude‐frequency analysis in alluvial channels

AbstractTheoretical approaches to magnitude‐frequency analysis (MFA) of sediment transport in channels couple continuous flow probability density functions (PDFs) with power law flow‐sediment transport relations (rating curves) to produce closed‐form equations relating MFA metrics such as the effective discharge, Qeff, and fraction of sediment transported by discharges greater than Qeff, f+, to statistical moments of the flow PDF and rating curve parameters. These approaches have proven useful in understanding the theoretical drivers behind the magnitude and frequency of sediment transport. However, some of their basic assumptions and findings may not apply to natural rivers and streams with more complex flow‐sediment transport relationships or management and design scenarios, which have finite time horizons. We use simple numerical experiments to test the validity of theoretical MFA approaches in predicting the magnitude and frequency of sediment transport. Median values of Qeff and f+ generated from repeated, synthetic, finite flow series diverge from those produced with theoretical approaches using the same underlying flow PDF. The closed‐form relation for f+ is a monotonically increasing function of flow variance. However, using finite flow series, we find that f+ increases with flow variance to a threshold that increases with flow record length. By introducing a sediment entrainment threshold, we present a physical mechanism for the observed diverging relationship between Qeff and flow variance in fine and coarse‐bed channels. Our work shows that through complex and threshold‐driven relationships sediment transport mode, channel morphology, flow variance, and flow record length all interact to influence estimates of what flow frequencies are most responsible for transporting sediment in alluvial channels.

Effects of vegetation on flow and sediment transport: comparative analyses and validation of predicting models

AbstractThe presence of vegetation modifies flow and sediment transport in alluvial channels and hence the morphological evolution of river systems. Plants increase the local roughness, modify flow patterns and provide additional drag, decreasing the bed‐shear stress and enhancing local sediment deposition. For this, it is important to take into account the presence of vegetation in morphodynamic modelling. Models describing the effects of vegetation on water flow and sediment transport already exist, but comparative analyses and validations on extensive datasets are still lacking. In order to provide practical information for modelling purposes, we analysed the performance of a large number of models on flow resistance, vegetation drag, vertical velocity profiles and bed‐shear stresses in vegetated channels. Their assessments and applicability ranges are derived by comparing their predictions with measured values from a large dataset for different types of submerged and emergent vegetation gathered from the literature. The work includes assessing the performance of the sediment transport capacity formulae of Engelund and Hansen and van Rijn in the case of vegetated beds, as well as the value of the drag coefficient to be used for different types of vegetation and hydraulic conditions. The results provide a unique comparative overview of existing models for the assessment of the effects of vegetation on morphodynamics, highlighting their performances and applicability ranges. Copyright © 2014 John Wiley & Sons, Ltd.

Modelling-based assessment of suspended sediment dynamics in a hypertidal estuarine channel

We investigate the dynamics of suspended sediment transport in a hypertidal estuarine channel which displays a vertically sheared exchange flow. We apply a three-dimensional process-based model coupling hydrodynamics, turbulence and sediment transport to the Dee Estuary, in the north-west region of the UK. The numerical model is used to reproduce observations of suspended sediment and to assess physical processes responsible for the observed suspended sediment concentration patterns. The study period focuses on a calm period during which wave-current interactions can reasonably be neglected. Good agreement between model and observations has been obtained. A series of numerical experiments aim to isolate specific processes and confirm that the suspended sediment dynamics result primarily from advection of a longitudinal gradient in concentration during our study period, combined with resuspension and vertical exchange processes. Horizontal advection of sediment presents a strong semi-diurnal variability, while vertical exchange processes (including time-varying settling as a proxy for flocculation) exhibit a quarter-diurnal variability. Sediment input from the river is found to have very little importance, and spatial gradients in suspended concentration are generated by spatial heterogeneity in bed sediment characteristics and spatial variations in turbulence and bed shear stress.

Incipient sediment motion across the river to debris-flow transition

Sediment transport in mountain channels controls the evolution of mountainous terrain in response to climate and tectonics and presents major hazards to life and infrastructure worldwide. Despite its importance, we lack data on when sediment moves in steep channels and whether movement occurs by rivers or debris flows. We address this knowledge gap using laboratory experiments on initial sediment motion that cross the river to debris-flow sediment-transport transition. Results show that initial sediment motion by river processes requires heightened dimensionless bed shear stress (or critical Shields stress) with increasing channel-bed slope by as much as fivefold the conventional criterion established for lowland rivers. Beyond a threshold slope of ∼22°, the channel bed fails, initiating a debris flow prior to any fluvial transport, and the critical Shields stress within the debris-flow regime decreases with increasing channel-bed slope. Combining theories for both fluvial and debris-flow incipient transport results in a new phase space for sediment stability, with implications for predicting fluvial sediment transport rates, mitigating debris-flow hazards, and modeling channel form and landscape evolution.

Semianalytical Solutions for One-Dimensional Unsteady Nonequilibrium Suspended Sediment Transport in Channels with Arbitrary Eddy Viscosity Distributions and Realistic Boundary Conditions

AbstractThis paper presents a new semianalytical approach for the solution of suspended sediment transport in channels under uniform conditions using generalized integral transform technique (GITT). Comparing with previous analytical work, this new approach is not limited by the sediment diffusivity distribution and boundary conditions. Instead, it solves the suspended sediment transport equation with arbitrary distribution of eddy diffusivity. The solutions also consider realistic boundary conditions at the free surface and the bed. The new solutions converge fast and compare well with previous analytical solutions as well as numerical solutions from a high-resolution spectral code. With this new analytical approach, a library of solutions for suspended sediment concentration can be developed. It gives deeper insight into the physical process of the problem and provides benchmarks for the validation of numerical models.

The contribution of aeolian processes to fluvial sediment yield from a desert watershed in the Ordos Plateau, China

In arid zones, many active aeolian dunes terminate at ephemeral and perennial desert rivers. The desert rivers show very high rates of sediment transport that cause deleterious downstream effects on the river system and ecology. High sediment loading has been attributed to severe water erosion of sparsely covered watersheds during infrequent but heavy rainfall. Although aeolian erosion is known to lead to high rates of wind-blown sand transport, direct confirmation of whether the aeolian processes accelerate or inhibit fluvial sediment loss is lacking. Here, we show that an aeolian-fluvial cycling process is responsible for the high rate of suspended sediment transport in a Sudalaer ephemeral desert channel in the Ordos Plateau of China. Frequent aeolian processes, but low frequency (once every 3 years on average) flooding, occur in this region. Wind-blown saltating grains appeared to be unable to cross the desert channel because of interruption of channel-induced recirculating air flow, and therefore tended to settle in the channel during the windy seasons, leading to channel narrowing. During flooding, this narrowed channel was found to yield a threefold increase in suspended sediment loading and a 3.4-fold increase in the weight percentage of the 0.08–0.2 mm sediment fraction on 18 July 2012. Loss of stored aeolian sand due to channel erosion accounted for about half of the total sediment yield in this watershed. These findings show that aeolian processes play an essential role in accelerating the sediment yield from a watershed characterized by aeolian-fluvial interplay and also suggest that the drier the region and the greater the aeolian process, the more the aeolian process contributes to fluvial sediment yield. Copyright © 2013 John Wiley & Sons, Ltd.