Variability Of Sediment Transport Research Articles

Stage dependent variability in tractive force distribution through a riffle–pool sequence

High resolution data on spatial and temporal variability in flow hydraulics and sediment transport within riffle–pool sequences are required to improve understanding of how fluvial processes maintain these meso-scale bedforms. This paper addresses this issue by providing velocity and boundary shear stress data over a range of discharges from base flow (0.07 m 3 s −1) to just over bankfull (8.52 m 3 s −1), from a sequence of four pools and three riffles in the River Rede, Northumberland. The data supports the reversal hypothesis of Keller [Geol. Soc. Am. Bull. 87 (1971) 753.] as the primary explanation for the maintenance of the riffle–pool sequence, although they also indicate that spatial variability in tractive force is highly stage dependent and complex. Section-averaged velocity data indicate reversal to be evident at four out of six riffle–pool units. An equalisation in velocity was found for the other two riffle–pool units close to bankfull stage. The spatial patterns of tractive force exhibited in the study reach as a result of increased discharge demonstrate that riffle–pool units operate independently of one another. Shear stress reversals were observed in individual riffle–pool units at different river stages during a flood hydrograph, and in some instances, two occurred in the same riffle–pool unit during a single flow event. Pools were characterised by coarser bed sediments and narrower channel widths in comparison to riffles, increasing the likelihood of tractive force reversal in the River Rede. Areas of predicted bed sediment entrainment obtained from τ o− τ c, matched observed channel changes in the upper part of the study reach, but over-estimated change in the middle portion of the reach.

Variability of Sediment Yield from a High Mountain Catchment, Central Spanish Pyrenees

The Izas experimental catchment is located in the high Gállego Valley, Central Spanish Pyrenees, in an environment strongly affected by the presence of snow during at least six months of the year. Discharge and sediment transport variability are closely linked to snowmelt and to intense autumn rainfalls. Following 11 yr of data collection, the seasonal and interannual variability of sediment transport has been estimated, as well as the relative importance of bedload, suspended sediment and solutes. The results obtained show that the total sediment yield is relatively low (between 200 and 320 t km−2 yr−1) and that solutes represent more than 60% of the sediment exported. Bedload, which in other mountain catchments reaches a great quantity, represents here, in general, less than 30% of the total sediment yield. Seasonal and interannual sediment transport variability is very high, mainly depending on the intensity of autumnal rainfalls.

Variability of Sediment Yield from a High Mountain Catchment, Central Spanish Pyrenees

The Izas experimental catchment is located in the high Grillego Valley, Central Spanish Pyrenees, in an environment strongly affected by the presence of snow during at least six months of the year. Discharge and sediment transport variability are closely linked to snowmelt and to intense autumn rainfalls. Following 11 yr of data collection, the seasonal and interannual variability of sediment transport has been estimated, as well as the relative importance of bedload, suspended sediment and solutes. The results obtained show that the total sediment yield is relatively low (between 200 and 320 t kmin 2 yr-1) and that solutes represent more than 60% of the sediment exported. Bedload, which in other mountain catchments reaches a great quantity, represents here, in general, less than 30% of the total sediment yield. Seasonal and interannual sediment transport variability is very high, mainly depending on the intensity of autumnal rainfalls.

Streamflow and Suspended Sediment Transfer to Lake Sophia, Cornwallis Island, Nunavut, Canada

To ascertain the climatic controls on sediment transport to Lake Sophia, Com­ wallis Island, Nunavut, Canada, we made detailed hydrological and meteorolog­ ical measurements in the Sophia River watershed through the 1994 melt season. Streamflow and suspended sediment transport are limited, on an annual time scale, by the supply of snow and sediment in the watershed. Suspended sediment yield from the watershed was only 0.46 t k:m- 2 , which is lower than any previously published yield for a stream in the High Arctic. Snowmelt rgnoff accounted for 88% of the annual suspended sediment load, whereas 6 and 9% were transported in response to a slushflow event and summer rainfall, respectively. These mea­ surements provide no direct evidence that modem-day sediment delivery to Lake Sophia is related to fluctuations in air temperature, which has implications for the paleoenvironmental signal preserved in Lake Sophia's laminated sediments. We suggest that on-site sediment transport studies are necessary to establish the re­ lationships among geology, geography, climate, and hydrology unique to each watershed-lake system and need to be an integral part of any calibration attempt. Additional years of data are needed however to define the interannual variability of streamflow and sediment transport in response to climate.

Streamflow and Suspended Sediment Transfer to Lake Sophia, Cornwallis Island, Nunavut, Canada

To ascertain the climatic controls on sediment transport to Lake Sophia, Cornwallis Island, Nunavut, Canada, we made detailed hydrological and meteorological measurements in the Sophia River watershed through the 1994 melt season. Streamflow and suspended sediment transport are limited, on an annual time scale, by the supply of snow and sediment in the watershed. Suspended sediment yield from the watershed was only 0.46 t km−2, which is lower than any previously published yield for a stream in the High Arctic. Snowmelt runoff accounted for 88% of the annual suspended sediment load, whereas 6 and 9% were transported in response to a slushflow event and summer rainfall, respectively. These measurements provide no direct evidence that modern-day sediment delivery to Lake Sophia is related to fluctuations in air temperature, which has implications for the paleoenvironmental signal preserved in Lake Sophia's laminated sediments. We suggest that on-site sediment transport studies are necessary to establish the relationships among geology, geography, climate, and hydrology unique to each watershed-lake system and need to be an integral part of any calibration attempt. Additional years of data are needed however to define the interannual variability of streamflow and sediment transport in response to climate.

Modeling stratified wave and current bottom boundary layers on the continental shelf

The Glenn and Grant [1987] continental shelf bottom boundary layer model for the flow and suspended sediment concentration profiles in the constant stress layer above a noncohesive movable sediment bed has been updated. The Reynolds fluxes for sediment mass and fluid momentum are closed using a continuous, time‐invariant linear eddy viscosity modified by a continuous stability parameter to represent the influence of suspended sediment‐induced stratification throughout the constant stress region. Glenn and Grant [1987] use a less realistic discontinuous eddy viscosity and neglect the stratification correction in the wave boundary layer. For typical model parameters the two models produce currents above the wave boundary layer that are in better agreement than the suspended sediment concentrations. Within the wave boundary layer the differences are much greater for both the current and the sediment concentration. This leads to significant differences in the sediment transport throughout the constant stress layer. Sensitivities of the updated model were examined on the basis of observed wave and current data acquired during storms on the inner continental shelf. Comparisons between the stratified and neutral versions of the updated model indicate that the stratified version produces a total depth‐integrated sediment transport that can be 2 orders of magnitude less than, and time‐averaged shear velocities that can be nearly half of, that predicted by the neutral version. Sensitivities to grain size distributions indicate that even a small amount of finer sediment can stratify the storm‐driven flows. Sensitivities to closure constants within the range of reported values also produce up to an order of magnitude variation in sediment transport, illustrating the need for dedicated field experiments to refine further estimates of these parameters.

The effect of bedload transport on mean and turbulent flow properties

This paper reports the results of a flume experiment that was designed to investigate the effect of bedload transport on mean and turbulent properties of the flow. The experiment consisted of varying the bedload transport rate for a given hydraulic condition, and of measuring the flow velocity profiles using an Acoustic Doppler Velocimeter (ADV) for each transport rate in order to allow for comparison. Bedload transport was produced by injecting gravel-size particles ( D 50=7.4 mm) with a conveyer belt mounted at the upstream end of the flume. The results indicate that the effect of bedload on flow characteristics is complex. It is shown that bedload transport causes opposite effects on flow velocity depending on the roughness of the bed and the relative magnitude of flow and sediment transport variables. A better understanding of these conflicting results is obtained from the application of an energy budget approach to the analysis of velocity data. This approach demonstrates that bedload affects flow velocity by modifying the rate of dissipation of turbulent kinetic energy. However, the mechanisms responsible for the modification of turbulent dissipation are still unknown.

Process, form and change in dryland rivers: a review of recent research

Many of the world's extensive warm dryland regions support numerous, albeit often infrequently flowing, rivers. Dryland rivers are increasingly a focus of scientific and applied interest but empirical research and fluvial theory for drylands need to be strengthened. Recent research in arid central Australia indicates greater diversity in dryland river process, form and change than has hitherto been appreciated, and highlights the need for a global review assessing the present state of knowledge. This review outlines the distinctive characteristics of dryland fluvial environments (hillslope and channel hydrological and sediment transport processes, river pattern and geometry, temporal and spatial aspects of channel change, sedimentary structures and bedforms), many of which contrast with more humid fluvial environments. Although features common to many dryland fluvial environments can be identified (extreme temporal and spatial variability of rainfall, runoff and sediment transport, poor integration between tributary and trunk channels, importance of large floods as a control on channel morphology, lack of equilibrium between process and form), the fluvial diversity that exists within drylands requires recognition of the limitations to these generalisations. In particular, research in central Australia illustrates the need to understand the rivers of this region using empirical relationships, terms, and concepts additional to those defined by earlier work in drylands. Key deficiencies in dryland fluvial research are identified, and relate to three main areas: limited study of some aspects of modern dryland rivers (floodplain characteristics, influence of vegetation, downstream changes, importance of scale); limited understanding of dryland river behaviour over longer (Cenozoic) timescales; and lack of integration between the results from short-term, process-form studies and studies of the longer term histories of river behaviour. Linking knowledge of past hydrological and channel changes to present-day changes in dryland rivers is suggested as a key research priority. This will help develop a sound theoretical basis for the assessment of future developments in dryland river systems which will contribute to their improved scientific understanding and environmentally sensitive management.

Catchment and lake controls over the formation of varves in monomictic Nicolay Lake, Cornwall Island, Nunavut

On the basis of thin-section sedimentology, 137Cs and 210Pb profiles, and the pronounced seasonality of runoff and sediment delivery, sediments from Nicolay Lake, Cornwall Island, Nunavut (77°46'N, 94°40'W) are interpreted as varves. In thin section, the laminae are conformable, normally graded units of silt and clay. Depending on the location in the lake, the varves frequently contain one or more subannual rhythmites and inclusions of coarse sand and silt grains. Given the unstratified nature of the lake, the rhythmites are interpreted as products of sediment inflow events derived from rainfall, snowmelt, or mass wasting processes. In the most proximal site, these rhythmites may reflect insolation-driven diurnal variations in sediment transport. Isolated coarse grains in the varves are interpreted as eolian sediments washed off the lake ice cover. The lake is currently isothermal, and persistent ice cover and cold inflow prevent the formation of thermal stratification. The high accumulation rate is a critical factor in varve formation and it is probable that increased sediment yield during the past 500 years has led to the formation of varves, compared to the underlying massive mud that accumulated when deposition was focused inland of the lake during higher relative sea level. Evidence in the catchment indicates that high-elevation deglacial deposits have acted as an important fine-grained sediment source throughout the Holocene. These sediments moved progressively downstream through a series of basins by successive degradation and aggradation controlled by glacioisostatic emergence, hence, limiting the progression of this paraglacial sediment wave to areas upstream of the lake until the late Holocene. These results identify the importance of shifting catchment boundary conditions on sediment yield throughout the Holocene, and also indicate the difficulty of interpreting low-frequency yield variations as the direct consequence of changing climate in similar varve records.

Book Review: Material Fluxes on the Surface of the Earth edited for the National Research Council, National Academy Press, Washington, 1994. No. of pages: xiv + 170, Price: £32.95. ISBN 0-309-04745-5; Book Review: Variability in Stream Erosion and Sediment Transport edited by L. J. Olive, R. J. Loughran and J. A. Kesby, International Association of Hydrological Sciences Publication No. 224,

Earth Surface Processes and LandformsVolume 22, Issue 6 p. 607-608 Book Review Book Review: Material Fluxes on the Surface of the Earth edited for the National Research Council, National Academy Press, Washington, 1994. No. of pages: xiv + 170, Price: £32.95. ISBN 0-309-04745-5; Book Review: Variability in Stream Erosion and Sediment Transport edited by L. J. Olive, R. J. Loughran and J. A. Kesby, International Association of Hydrological Sciences Publication No. 224, IAHS Press, Wallingford, 1994. No. of pages: x + 498. Price: £75.00. ISBN 0-947571-19-1 Ian Foster, Ian Foster Coventry University, UKSearch for more papers by this author Ian Foster, Ian Foster Coventry University, UKSearch for more papers by this author First published: 04 December 1998 https://doi.org/10.1002/(SICI)1096-9837(199706)22:6<607::AID-ESP728>3.0.CO;2-FAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume22, Issue6June 1997Pages 607-608 RelatedInformation

Climatic influences on streamflow and sediment flux into Lake C2, northern Ellesmere Island, Canada

Streamflow, suspended sediment transport and meteorological variables at two elevations were measured through the 1990–1992 field seasons at Lake C2, Northern Ellesmere Island, as part of the Taconite Inlet Lakes Project. The objectives were to determine the extent to which suspended sediment flux responded to climatic variability, and to ascertain which meteorological variable was most strongly associated with daily discharge and sediment load. This study provided a partial test of our hypothesis that the annually-laminated sediments of Lake C2 contain a paleoclimate signal. Streamflow to the lake was almost exclusively the result of snowmelt, in response to inputs of atmospheric energy as measured by air temperature at the median watershed elevation (520 m). Sea-level air temperature, global solar and net all-wave irradiance were less clearly associated with discharge. Fluctuations of discharge and suspended sediment concentration were nearly synchronous, and non-linearly related. Daily sediment discharge was therefore linked by streamflow, with a time lag, to the energy available for snowmelt. Mean daily air temperature and cumulative degree-days above 0 °C, at 520 m elevation, were successfully used to predict the daily and seasonal discharge of runoff and sediment to the lake.

Transforms for Runoff and Sediment Transport

Transforms reduce the distributions of surface-runoff and sediment-transport variables to near-exponential distributions typical of rainfall duration and intensity. Two transform parameters are determined from field measurements using either a graphical method or the method of moments. The transforms were tested on both small experimental plots and very large watersheds for the distributions of rainfall depth, runoff volume and discharge, sediment concentration and sediment discharge, as well as concentration and transport of chemicals from surface runoff. As expected from deterministic relationships, the inverse transform exponent bˆ for point rainfall is close to unity, increases to about 1.5 for runoff discharge, and varies between 1.5 and 3 for sediment and chemical transport. Several examples show that the transforms are useful to determine exceedance probability, flow and sediment duration curves, expected sediment load, and sediment-rating curves for poorly correlated concentration and discharge me...

Linking Sediment Transport and Stratigraphy on the Continental Shelf

T H E GOAL OF THE SHELF sediment dynamics component of STRATAFORM is to link sediment transport processes active on the continental shelf to the formation and preservation of beds in shelf sediment deposits. An approach combining shelf sediment-transport models with high-resolution measurements of water-column and properties over periods from several months to several years allows us to make quantitative estimates of modification caused by sediment resuspension during episodic transport events. These modifications include erosion and deposition of material, formation of graded storm beds, and changes in small-scale surface morphology. The characteristics of the resulting event bed (thickness, grading, physical structures) are a function of flow and properties, depending on both temporal and spatial variations in sediment transport. Sediment transport on the continental shelf depends on surface-wave conditions, bottom-boundary-layer currents, fluid stratification, and characteristics, including grain size, density, porosity, and surface roughness. In general, sediment transport rates and depths of reworking are greatest when large, long-period waves occur simultaneously with strong, persistent currents. Thus understanding the magnitude and frequency of transport requires knowledge of the wave and current conditions on the shelf over sufficiently long periods of time to develop statistical characterizations of the wave and current fields and to document the resulting resuspension and modification. This information also provides critical

Environmental controls on blowing dust direction at lubbock, Texas, U.S.A.

AbstractBlowing dust is a common phenomenon at Lubbock, Texas, on the Southern High Plains. The directional variability of blowing dust estimated with the ‘sand rose’ technique, using wind speed and direction data, suggest that dust transport occurs from all directions. An empirical method of determining directional variability using meteorological data on visibility reductions due to blowing dust, however, indicates that most dust comes from the west and south‐west. In addition to wind speed and direction, other environmental factors must be considered in explaining the spatial pattern of dust transport. Soil erodibility is variable in the region, with the most extensive area of highly erodible soils to the west and south‐west of Lubbock. Rangeland dominates land use to the east, while agriculture is extensive to the north, west and south. Local farming techniques leave bare soil during the winter and spring, when most airborne dust is produced and also when strong winds are common from the west and south‐west. Soil moisture is lowest to the southwest of Lubbock, which leads to a decrease in soil structure and an increase in the potential for wind erosion in that direction. Relative humidities affect threshold wind speeds and are lowest during March and April, when winds are common from the west and south‐west. The spatial and seasonal variability and interactions between many factors, both natural and human‐controlled, must therefore considered in explaining the directional variability of aeolian sediment transport at Lubbock.

Controls on bedrock channel incision along nahal paran, Israel

AbstractNahal Paran drains 3600 km2 of Egypt's Sinai peninsula and Israel's Negev Desert. Much of the channel is alluvial, but a canyon 10·5 km long has been incised into Late Cretaceous chert and dolomite in the lower portion of the basin. Slackwater deposits and paleostage indicators preserved within the canyon record approximately 10 floods of 200 to 2500 m3 s−1 over a period of at least 350 years. Step‐backwater simulations of flood‐flow hydraulics indicate extreme variations in stream power per unit area along the length of the canyon, and associated variability in energy expenditure and sediment transport. These variations reflect channel cross‐sectional morphology. The greatest values of stream power occur along the lower half of the study reach, in association with three pronounced knickpoints and an inner channel. The locations of these features reflect the exposure of thick, resistant chert layers along the channel. The presence of several similar, but buried and inactive, knickpoints along the upper study reach indicates that the locus of most active channel incision has shifted with time, probably in response to baselevel changes associated with tectonic activity along the Dead Sea Rift. Thus, the rate and manner of channel incision along the canyon of Nahal Paran are controlled by lithologic variability and tectonic uplift as they influence channel morphology and gradient, which in turn influence hydraulics and sediment transport.

Sediment transport in oscillatory sheet flow

Motivated by an interest in high rates of sediment transport, an experimental investigation was conducted to examine sediment motion in conditions of intense sediment transport. Fluid velocity and sediment concentration were measured simultaneously in a sheet flow regime generated in a u‐shaped oscillatory water tunnel for two types of noncohesive sediments. A probe was developed to measure the instantaneous variations in sediment concentration based on the electrical conductivity of the fluid‐grain mixture. Unlike previous concentration devices, this probe is nonintrusive and is capable of measuring a wide range of sediment concentrations, from extremely dense concentrations within the bed to low‐concentration suspended load well above the moving layer of sediment. Horizontal fluid velocities were measured simultaneously using laser Doppler anemometry in backscatter mode. The sediment concentration measurements provide a comprehensive data set for sediment transport in sheet flow and near sheet flow regimes. The temporal variation of sediment transport over the half cycle of oscillation was examined. Most of the sediment was transported within the moving bed layer, with the total transport occurring between the stationary bed height and a distance about twice the thickness of the moving bed layer. The temporal variation in sediment transport was found to be proportional to cos(σt). The magnitude of the sediment transport increased with increasing bottom shear stress. Fluid velocity and sediment concentration measurements were used to calculate instantaneous and mean rates of sediment transport, which were compared with predictions from existing models.

Sediment transport of the Godavari River basin and its controlling factors

The mean annual water flow and sediment transport of the Godavari River are estimated to be 92 km 3 and 170 × 10 6 ton (t), respectively. In terms of sediment transport and the rate of physical erosion (555 t km −2yr −1), the position of the Godavari would be ninth and fifth respectively among the world rivers. Geology of the basin is the main controlling factor of the sediment transport. The sedimentary rocks located in the lower part of the basin and constituting 7% of the total basin area are responsible for 33% of the sediment load. More than 67% of sediment load is silt and clay. Annual, monthly and daily variations in sediment transport indicate that a few selected days in the hydrological year control the annual sediment budget. Our latest estimates are nearly twice that of our earlier values. The presently available global estimates on sediment transport need to be re-evaluated.

The suspended sediment balance in Changjiang estuary

Circulation and balance of suspended sediment in Changjiang estuary have been analysed. Strong interaction between different waterways and the fact that they are wide and shallow both contributed to the great spatial and temporal variability in circulation and suspended sediment transport. Using a flux decomposition formula with the relative depth as the vertical coordinate, it was shown that although tidal dispersion was the main dispersion mechanism in the turbidity maximum zone, dispersions due to circulation effects were also important.

Experimental Studies on Sedimentation Due to Water Withdrawal

A mathematical model is verified, which predicts aggradation and degradation effects as a result of water withdrawal from rivers or open channels. An experimental study in a large sediment flume, commissioned by the Netherlands' Rijkswaterstaat, was used to analyze the predictive value of the one‐dimensional mathematical model RIVMOR to assess bed‐level changes in the reaches up and downstream of the water intake. Comparison of the results of the mathematical model with the experimental data showed a good agreement with regard to both water‐level and bed‐level changes. It should be realized, however, that the conditions in the sediment flume could be controlled very accurately, facilitating a high level of precision in data collection and in the mathematical model simulation. In a natural river, the variability of discharge, flow resistance, and sediment transport, coupled with the generally poor theoretical description of the latter two aspects will result in less accurate predictions of bed‐level change...

Brahmaputra River, Assam, India: Physiography, Basin Denudation, and Channel Aggradation

The Brahmaputra River in Assam, India, characterized by high seasonal variability in flow, sediment transport, and channel configuration, experienced a secular period of aggradation from 1971 to 1979. The suspended load budget indicates an overall aggradation of the 607‐km Assam reach of the Brahmaputra by about 16 cm during that period, with about 70% of the suspended sediment inflow into the reach being retained in the channel. Expressed as a percentage of the change in storage for the different reaches, computed errors due to sampling variability in sediment discharge generally lie within about 5–15% and do not appear to be large enough to affect the conclusions drawn from the suspended load budget. For a 145‐km reach of the Brahmaputra, an alternative method based on measurement of channel cross sections suggests 21 cm of aggradation, somewhat more than estimated by the suspended load budget. Based on the suspended load carried by trans‐Himalayan rivers, the present rate of denudation of the eastern Himalayas is estimated to be 73–157 cm/103 years. The average rate for the last 2–3 million years, estimated from the volume of alluvial fill in the Brahmaputra valley in Assam, the sediment yield of the Himalayan rivers, and assuming a total yield to deposition ratio of 1.4 (present study), is 3 cm/103 years. The current high rate of denudation of the Himalayas may be attributed mainly to the rapid uplift of the mountain system, recent earthquake activity, and high susceptibility of geologic formations to erosion by running water coupled with the effectiveness of the monsoon rainfall regime.