H51I-0883
Improved statistical characterization of particle-size distributions in sand-bedded rivers
Measured particle-size distributions are commonly reduced to one characteristic value (e.g., median grain diameter) that is used in sediment transport modeling. While convenient, this approach cannot be used to explore the potential influence grain-size distributions may have on sediment transport and deposition. We statistically characterize grain-size distributions in samples of bed-material load, suspended load, and slackwater deposits from the sand-bedded Calamus, North Loup, and Niobrara rivers (Nebraska, USA). Transported sediment samples are best modeled with log-hyperbolic distributions, and slackwater deposits are bi- or multi-modal mixtures. Despite large overlaps in the grain sizes of bed-material-load and suspended-load samples, estimated parameters of fitted log-hyperbolic distributions show consistent differences between these samples across all rivers. Bed-material load samples have higher modes and positive (coarse-grained) asymmetry, whereas suspended load samples have lower modes and weaker asymmetry. In all three rivers, slackwater deposits contain the entire range of grain sizes present in suspended load, but with a significant component of very fine-grained (< 0.02 mm) material that is undetectable in suspended sediment samples. This suggests some degree of fractionated deposition of suspended sediment in areas of near-zero flow velocities. Ultimately, in order to explore the effect of grain-size distributions on sediment transport and river processes, these modeled distributions can be incorporated into a Bayesian hierarchical framework where standard sediment transport equations can be modeled in relation to probability-density particle curves for grain size.
H51I-0884
Examining How Sediment Distributions Affect Sediment Flow Stratification
Recently, the two-way effects of the time varying suppression of turbulence by gradients in suspend sediment concentration under waves have been investigated. The results show that, when this effect is accounted for, the wave coherent component of transport is increased relative to the mean component of transport which can even result in a change of the direction of transport. Comparisons between measured and simulated time series of near-bed sediment concentrations show great coherence (0.95 correlation) and the vertical distribution of net transport rates is more accurately reproduced by the simulations using sediment stratification. However discrepancies in both transport time series and net transport rates are still disappointing. Due to the grain-size dependent nature of the stratification effects, it has been proposed that this model- observation mismatch may be due to the utilization of a single grain size in the simulated sediment load. A modified form of the Generalized Ocean Turbulence Model (GOTM) has been utilized to examine this question. A vertical advection-diffusion model of sediment suspension has been combined with a modified Smith & McLean relation for bottom concentration which distributes the excess bed stress among discrete sediment classes based on their concentration in the bed. The resulting simulations of sediment concentration and transport are compared to field observations as well as simulations using single grain sizes and simulations ignoring the effects of stratification.
H51I-0885
Validating a perturbation approach to the large eddy simulation of wave induced sediment transport
We present a large eddy simulation of wave induced sediment transport. The model uses a perturbation approach where an inviscid and irrotational solution is subtracted from the Navier-Stokes equation allowing for a simple coupling of the large eddy simulation to a fully nonlinear potential flow wave model. We discuss results for oscillating shear flows and wave-induced flows to verify a log boundary layer and the Rouse curve for suspended sediment concentration. A moving bottom algorithm was implemented and the method is being tested for studying scour around partially buried objects near the surf zone (such as mines). It could be later adapted for varying grain sizes.
H51I-0886
Simulation study of the sediment gravity flow using solid – fluid coupling method
We calculate a grains flow along a slope under the water as an example of the sediment gravity flow. The flowing grains shape the bump at the head of the flow. This is called eflow headf. We confirmed the following some features in this simulation. First, the flow velocity of some grains inside the flow head is faster than that at the tip of the flow head. This is because the resistivity from the fluid at the tip of the flow head is higher than that inside the flow head. Second, a few slip surfaces appear inside the flow. The angles of these slip surfaces are the angle of slope +1-3 ° inside the body part (this is the part except the flow head). On the contrary, the angle inside the flow head is much higher than that inside the body part. This is because some lower particles at the flow head are willing to settle down to the slope.The last, the velocity of the flow head is almost constant on the slope, but it decreases to a velocity after the grains reach the flat part. This second velocity lasts until the head part becomes small well. After then, the head velocity decreases linearly and the flow stops. These trends are almost same under the different slope angle and the grain size distribution in our simulation. The feature of the sediment gravity flow varies according to many factors such as grain distribution, grain density, slope angle, flow viscosity and so on. As our results are just performed under the quite limited condition, we need further simulations with more parameters changing. We concluded that the coupling simulation of LBM and DEM can reveal some features of the gravity sediment flow. The further simulations with more parameters changing, however, are required, as our results are just performed under the quite limited condition,
H51I-0887
Experimental observations on the motion of bedload particles
A detailed set of experimental observations on bedload transport was carried out to study the dynamics of sediment particles in a free surface laboratory flume, in the case of low values of the applied Shields stress and on an arbitrarily sloping bed with local values of the longitudinal and transversal inclinations up to 20°. The experimental data concern the average motion characteristics of saltating particles, which are described in terms of the applied Shields stress, the flow velocity profile, the particle velocity profile, the local bedload discharge and the deviation angle, due to the effect of gravity on laterally sloping beds, between the average direction of particle velocity and the fluid bed shear stress. While the effects of gravity on the direction and intensity of bedload transport are presented in Francalanci and Solari (2007); in the present work, results about the average characteristics of the motion of saltating grain are presented. The results show the relation between the average particle velocity within the saltation layer, the average saltation height and the dynamic friction coefficient with the applied Shields stress. In particular, the average particle velocity is shown to be in good agreement with the experimental data from other authors, and appears to increase with the Shields stress; the saltation height, which is directly related to the thickness of the bedload layer, is in the order of 1.5-3 times the particle diameter and slightly increasing with the Shields parameter; the dynamic friction coefficient, estimated through a balance between average resistive and active forces based on the measurements of the velocity profile across the bedload layer of both the fluid and the particles, show a relation not only with the applied Shields stress but also with local bed inclination.
H51I-0888
Mobilisation, Transport and Reactivity of Contaminated Sediments in a High Capacity Estuarine Flume
Experiments have been carried out in a high capacity flume to quantify the transport and reactivity of contaminated estuarine sediments. The study focuses on the parameterisation of the axial re-distribution of metal contaminants within the bed sediment, including the mixing depth, as a function of local bed shear stresses and the sediment-water partitioning of exchangeable metals. The results are directed towards the improvement of coupled hydrodynamic-geochemical models. A high capacity flume, 17 m in length and 1.2 m in width, was modified to accommodate a model funnel-shaped estuary. The flume was filled with contaminant-free sand having particle diameters in the range >63 to <180 µm. A plug of contaminated sediment, from either the Plym or Mersey estuaries, with similar texture was tagged with Rhodium (Rh) and Platinum (Pt) and the chemically labile metals, Ni and Zn and inserted 4.5 m downstream. The vertical velocity profiles of the water were monitored using acoustic Doppler velocimeter. The axial bed level change due to sediment erosion or deposition was recorded regularly during the experiments using an acoustic bed profiler and, simultaneously, water samples were taken near the bed and at 40% of the water column height. Water samples were filtered and the water and suspended particulate matter (SPM) retained for analysis. Sediment cores obtained along the flume, using a liquid nitrogen-cooled corer, were sectioned. Particulate Rh and Pt, and dissolved and particulate Ni and Zn, were determined by inductively coupled plasma-mass spectrometry. The concentrations of SPM near-bed were greater than concentrations near the surface. After 0.5 h, near-bed SPM concentrations had a maximum of 1000 mg L-1, whereas after 8 h they were about 50 mg L-1. The highest concentrations were in the vicinity of the plug, where erosion occurred and they declined within the region of deposition. The concentrations of Rh/Pt in the near-bed SPM varied as function of distance from the plug allowing the mobilisation of contaminated plug sediment to be tracked. Dilution of the Rh/Pt in solids resuspended from the plug by the "background" SPM allowed the formulation of a particle mixing model based on the proportional mixing of the two particle types. Typically, the mixing ratio in the near-bed SPM was <0.2 and in surface SPM <0.1 which may be related to varying bed shear stresses. Determinations of Rh/Pt in the sediment cores showed the migration Rh/Pt downstream of the plug and, importantly, they appeared below the surface layer in the cores suggesting the sediments are vertically mixed, as a consequence of a combination of bed and suspended load transport. Partition coefficients for Ni and Zn, KDs, were quantified and linked to the sediment transport processes. The sediment transport, mixing and partitioning parameters were incorporated into a three-dimensional numerical model comprising sediment and chemical dynamics. The bed shear stresses predicted by the model were used to analyse the transport and mixing of contaminated sediments in the flume.
H51I-0889
Is the critical Shields stress for incipient-sediment motion dependent on channel-bed slope?
A compilation of data from laboratory flumes and natural streams shows that the critical Shields stress, the parameter most often used to assess sediment mobility, increases with bed slope in open-channel flows. This indicates that particles of the same size are more stable in steeper sloping streams, which is contrary to standard models that predict reduced stability with increasing slope due to the added downstream gravitational force. Several effects might explain this discrepancy including increased contributions to form and wall drag, variable friction angles, grain emergence, flow aeration, and variations in the structure of local flow velocity and turbulent fluctuations. A simple 1-D force-balance model is formulated to test these effects. Surprisingly, increased form drag does not appear to be the cause of the slope dependency because both the magnitude and trend of the critical Shields stress are similar for flume experiments and natural streams, and significant variations in form drag in flumes is unlikely. Instead, grain emergence, and changes in the local velocity and turbulent fluctuations due to heightened bed roughness seem to be responsible for the slope dependency. To quantify these effects, a model for the local velocity within the grain roughness layer is proposed based on a 1-D eddy viscosity with wake mixing. In addition, the magnitude of near-bed turbulent fluctuations are shown to scale with the depth-averaged flow velocity, the ratio of flow depth to the characteristic bed-roughness scale, and therefore channel slope (for the same boundary shear stress). Extension of the model to mixed grain sizes indicates that the coarser fraction become increasingly difficult to transport in steep rivers and streams. http://eps.berkeley.edu/~mlamb/
H51I-0890
Predicting Bed Mobility in a Simple River Channel
Prediction of the frequency and spatial pattern of bed mobility in gravel bed rivers is central to a wide range of theoretical and applied interests ranging from sediment transport to the impacts of natural or managed floods on aquatic organisms. Although bed mobility has been investigated in numerous flume and field studies, accurate predictions of grain entrainment and transport in gravel bed rivers remain elusive. Alluvial rivers typically encompass a much wider range of hydraulic and sedimentological conditions than those that have been recreated in laboratory flume studies upon which many grain entrainment and transport models are based. These flume studies are limited to the examination of processes occurring over the short term, commonly with the absence of slower processes such as fine-grain infilling. On the other hand, in field studies key variables can not be controlled and the spatial complexity of processes and conditions complicate data collection and analysis. A unique opportunity currently exists to help bridge this gap between laboratory and field studies: a 3.2 km long, recently constructed, single thread, alternate bar, gravel bed river channel of the Merced River. This channel, constructed for ecosystem restoration purposes, is slowly developing greater complexity, but is still currently defined by a simple plan form and cross-sectional channel geometry compared to most natural gravel bed river channels. This channel can thus be considered a full-scale flume. In the six years since the channel was constructed, a wider range of sedimentological bed conditions have evolved than have been created in a laboratory flume. We are characterizing the bed grain sizes, flow field, grain entrainment, and the sedimentological or bed state conditions in this simple channel. The flow field is modeled using a calibrated, 2D hydrodynamic flow model, MD_SWMS. Grain entrainment is measured with both metal tags inserted into the bed, and painted rock tracers, encompassing a wide range of grain sizes, placed in the bed. Bed state is characterized by measuring the local microtopography, vertical plucking force (used to calculate the degree of grain interlocking), and the bed-parallel force required to roll or slide a grain (used to calculate the grain friction angle). To date these measurements have been made for two flow conditions: a low flow and a 3/4 bankfull flow. Preliminary data show that sedimentological conditions vary greatly from those typically created in laboratory flume studies. For example, coarse grains embedded in and reinforced by fine sediments have significantly higher friction angles than grains in loose beds. During the 3/4 bankfull event, approximately 30% of all tracers were entrained and transported. Areas of both full mobility (all grain sizes become entrained) and partial mobility (some grain sizes become entrained) were measured. Using existing and future data, we plan to develop a calibrated, predictive model of bed mobility based on a range of naturally occurring bed states.
H51I-0891
Mass transport and hydraulics of flow in a high mountain gravel bed stream, the Urumqi river (Chinese Tianshan)
Sediment transport and mass balances in mountain regions remain poorly constrained although they are important issues for understanding mountain erosion and landscape evolution. In order to setup a rational database on fluxes of sediment and hydraulics of flow in a mountain stream, Sediment transport and Hydraulics of flow were surveyd along the Urumqi river a high mountain stream in the tectonically active Tianshan mountains. Survey took place during the summer high flow season of 2004, 2005 and 2006. Results are presented here with special emphasis on sampling procedure, mass budget and fractional transport of grains.
H51I-0892
Measurement of Large-Scale Sediment Transport Dynamics Using Multibeam Sonar
Multibeam Echo-Sounder (MBES) sonar systems have developed rapidly over recent decades and are now routinely deployed to provide high-resolution object detection and bathymetric surveying in a range of aquatic environments, from the deep-sea to lakes and rivers. MBES systems were developed for bottom-detection and measurement of bed morphology, and have previously discarded the received acoustic back-scatter from the water column after the bottom-detection algorithms have been performed. However, modern data handling and storage technologies have facilitated the logging of this large quantity of acoustic intensity and phase information, and commercial MBES systems are now available that provide this capability. This paper develops a novel methodology to exploit this logging capability to quantify the concentration and dynamics of suspended sediment within the water column. This development provides a multi-purpose tool for the holistic surveying of sediment transport by imaging suspended sediment concentration, associated coherent flow structures and providing concurrent high-resolution bathymetry. This paper presents methods of data analysis and results obtained from deployment of the RESON SeaBat 8125 and 7125 MBES systems in the field and during testing in a controlled environment. The field results were obtained from sites on the Paranį river, Argentina, with the aim of examining the dynamics of suspended sediment transport over dune bedforms and in the region of flow mixing between large rivers of significantly different suspended sediment concentration. Controlled testing was performed in a former ship dry-dock by creating flows density currents of known suspended sediment concentration with different types and mixes of sediment. The results demonstrate the capability of the RESON MBES systems to successfully resolve the contrast in suspended sediment concentration, and hence the spatio-temporal monitoring of the associated coherent flow structures. The results demonstrate the ability of MBES systems to obtain large sets of data across a two-dimensional swath: this enables the real-time monitoring of suspended sediment transport and related flow processes on a scale previously unrealisable with single-beam acoustic back-scatter systems.
H51I-0893
Lower Mississippi River: A Mixed Bedrock-alluvial Channel
High-resolution multibeam bathymetric and CHIRP sub-bottom seismic data from the lower most 110 km of the Mississippi River offer a detailed depiction of the bed surface, revealing portions of the channel covered by sediment and areas of exposed substrate. A single line data swath running from Head of Passes to the English Turn bend was collected during low water discharge conditions. The swath zigzags up the river channel, intersecting each bank at an oblique angle of roughly 45 degrees, thereby ensuring that the entire channel bottom is adequately represented by the survey. Local composition of the channel bed is determined based on distinctive bathymetric characteristics, and is corroborated by grab samples collected along the survey path. Dune fields, composed of active and inactive dunes, cover 77% of the channel bottom and are most abundant in straight reach segments. Stagnant dunes, mantled by high-porosity ephemeral mud, cover 7% of the channel bed and tend to be located in shallower water near the river banks (10-15 m depth). Active bedforms cover 70% of the bed and are centered on the channel thalweg at water depths of 15-25 m. The remainder of the river bed is devoid of dunes, and consists of highly-consolidated and stratified sedimentary deposits that are part of the Pleistocene and Holocene substrata that lies beneath the modern Mississippi River (23% coverage). Steeply dipping channel sidewalls (30-80 degrees) are composed of the relict sediments and make up 10% of the bed area. Erosion-resistant substrate covers 13% of the channel bottom, and is frequently exposed in the deepest portions of the thalweg (25-60 m depth). Grooves and flutes are cut into this exposed substrate implying active river incision. Based on the proportion of modern sediment cover versus exposed erosion-resistant substrate on the channel bed, we propose that the lower Mississippi River is best classified as a mixed bedrock-alluvial channel.
H51I-0894
Spatial variation of bed material grain size over a large dune in the Jade Bay tidal channel, German Wadden Sea (preliminary results)
High-resolution bathymetry of a large dune with a height of ~4 m, a length of ~70 m and a cross- channel width of ~400 m was measured in the Jade Bay tidal channel, German Wadden Sea, using a multibeam echosounder (MBES) system. A total of 34 bed material samples were collected along 5 transects (trough-crest-trough) across the dune using a ShipekTM grab sampler. High-resolution flow velocity measurements, by means of an acoustic Doppler current profiler (ADCP), were conducted along 3 of these transects during a tidal cycle. Along the channel, mean grain sizes increase from trough to crest along all 5 transects. This increase is most pronounced in the center of the channel (trough ~340 μm, crest ~610 μm) compared to near the channel sides (trough ~425 μm, crest ~500 μm). Across the channel, mean grain sizes decrease along the crest (center ~610 μm, sides ~500 μm), but increase along the trough (center ~340 μm, sides ~425 μm) from the channel center toward the channel sides. The along-channel increase in mean grain size from trough to crest is coherent with an increase in the maximum near-bed flow velocity (calculated as the average flow velocity in the interval from 1 to 2 m above the bed). Moreover, this increase in near-bed flow velocity from trough to crest is more pronounced in the center of the channel (trough ~0.7 m/s, crest ~1.1 m/s) than near the channel sides (trough ~0.9 m/s, crest ~1.1 m/s), which is in line with the larger gradients in mean grain size in the center than along the sides. The higher flow velocities recorded at the crest relative to the trough is due to flow continuity. The across-track increase in mean grain size in the trough from the channel center toward the channel sides is consistent with an increase in the maximum near-bed flow velocity (center ~0.7 m/s, sides ~0.9 m/s). The decrease in mean grain size on the crest from the channel center toward the channel sides cannot simply be explained by lower near-bed flow velocities near the sides of the channel. Alternatively, this small decrease in mean grain size may be explained by secondary currents oblique to the main current direction generated either by the strong ebb-flood gyre in the channel as an effect of the Coriolis Force or by topographically steering of the large dunes. More detailed measurements of the near-bed hydrodynamics are required to fully understand the complexity of this dune field.
H51I-0895
Suspended and bedload dynamics in a tidally influenced river: the river Dyfi, Wales, UK.
In fluvial-estuarine systems, three main physical processes zones can be identified: a) river-dominated; b) tide- dominated; and c) wave-dominated. In the river-dominated zone, water and sediment transport is directed seaward, whilst in the marine-forced zones, water and sediment fluxes may be bi-directional. Although the spatiotemporal dynamics of water and sediment fluxes in both river and marine-forced systems have been comprehensively studied, the controls, timing and magnitude of material transport through the tidally-influenced zone is only poorly understood. Here, initial results are presented from a field experiment that aims to elucidate the relative role of river and tidal flows on the net pattern and magnitude of sediment transport through a 10 km tidally influenced reach of the River Dyfi in west Wales, UK, which drains into Cardigan Bay and the Irish Sea. Specifically, we report material fluxes under a set of contrasting marine and fluvial forcing scenarios (spring/neap tides and low/flood river flows). The experimental design incorporated measurement of suspended and bedload material fluxes using a combination of time-integrated and stage-dependent sampling, turbidimetry and coarse particle tracing. These measurements were tied to the hydraulic regime through a network of bi-directional flow measurements in the study reach. Additional analysis of material provenance was established through mineralogical and particle size classification. Results from a 3 month field campaign demonstrated a net seaward sediment transport (suspended and bedload) through the tidally-influenced system. Whilst tidal forcing of suspended transport is evident, these fluxes are an order of magnitude lower than those observed in river flood conditions (concentrations vary between 1-100 mg/l in tidal cycles but exceed 3000 mg/l in river floods). Fine to coarse gravels were entrained and transported 1-10 m during bankfull river floods, however, there was little comparable transport in tidal cycles, although very fine gravels were moved during high spring tides. Mineralogical, geochemical and particle size analyses are revealing contrasting sediment provenance dynamics during river floods and tidal cycles. These data will be used to establish the relative frequencies, magnitudes and flux directions of components of the seasonal sediment budget due to tide and river currents.
H51I-0896
Tidal Bedform Morphology and Flow Field Characteristics in the Fraser River, British Columbia
Channel floor morphology was mapped on a reach of the lowermost Fraser River (river km 2-8; 600 m wide single channel) near Steveston, BC using longitudinal profiles of swath bathymetry (Reson 8101 Seabat) and current velocity (RDI 1200 kHz ADCP) and bottom sampling in order to examine the flow field structure over tidal bedforms. Surveys of the channel reach were repeated after 24 h in order to differentiate tidal and riverine discharge controls on bedform evolution. The survey was conducted during a high river discharge phase (22-24 June 2007; 8,800-9,200 cms) ten days after the highest discharge (12,000 cms) observed since 1948. Mean annual flood is 9,600 cms. Preliminary examination of these data indicate reach morphology is a sand bed channel (mean grain size 0.23 mm) with two lateral bars on opposite banks that overlap by about 30/% of length. Downstream thalweg flow velocities were 1.3 m/sec at the surface with no evidence of salt wedge penetration in the reach. Transverse dunes are present on bar surfaces and extending across the entire channel in the overlap zone: irregular (relict?) morphology is present in thalweg pools (19-30 m deep) opposite the bars. Transverse dunes are asymmetrical (downstream slip face) and reach maximum size (1.5-2.5 m height, 40-100 m wavelength) on bar flanks (13-15 m water depth) and in the bar overlap (thalweg) zone. Repeat survey data, collected over a 3 h period centered around slack low semi-diurnal tide both days, showed dune crests had translated 1/5 to 1/10th wavelength downstream, with limited change in individual dune morphologies, suggesting river discharge was dominant in setting sand field morphology at this high discharge and neap (2 m range) tidal phase. This does not conflict with previous investigations in the lower Fraser suggesting that dune morphology lags behind changes in river discharge and tidal phase. These data will be used to calculate bedload transport rates for comparison with previous Fraser rates obtained upstream of this reach and with different methodologies.
H51I-0897
Impedance-Based Measurement of Suspended Sediment Concentrations
A new technique for measuring suspended sediment concentration and range of particle grain sizes is being developed for field use. Suspended sediment flows are multi-phase flows commonly found in river and marine environments. Recent environmental concerns have drawn attention to the mechanism of contaminant transport associated with the erosion and deposition of suspended sediment. Unfortunately existing theoretical models of sediment transport lack solid empirical validation due in part to the difficulty involved in the accurate field measurement and characterization of these multi-phase flows. Of particular interest is the suspended sediment concentration which can vary spatially as well as temporally. While there are several methods available presently to estimate sediment concentration such as: acoustic back scattering, optical back scattering, laser diffraction and remote spectral reflectance, none entirely meet the need for simple, reliable laboratory and field measurement of suspended sediment concentrations. The measurement method proposed herein consists of an electrical based measurement yielding complex impedance as a function of frequency which in turn can be related to a dielectric mixing function for the suspended sediment and water. Prior electrical measurement efforts are limited to scalar conductivity measurements often at a single frequency. The two-phase mixture used for this study is sediment and water, where the grains are held in constant suspension by a mechanical stirring. Variables such as: sediment classification, particle grain size, mixture temperature, and temporal dependence have all been parametrically studied and their effects on the measurement system documented. This data will be used to develop a series of dielectric mixing equations which correlate multi-flow vector impedance measurements to suspended sediment concentration, through electrical properties of the mixture.
H51I-0898
Sand Ripples and Surficial Grain Size on the Florida Continental Shelf Following 2004 Hurricane Ivan
Measurements of ripple dimensions, ripple orientation, and seabed surficial grain size were obtained from the Florida gulf coast (south of Fort Walton Beach) during the ONR RipplesDRI/SAX04 experiment. The data were collected with a ship-tethered tripod containing high resolution acoustic and optical sensors including cameras, projected light, and a scanning sonar. Measurements were collected on a cross-shore transect in water depths ranging from 3 m to 42 m. The measurements were conducted in September, 2004, two weeks after the passage of Hurricane Ivan (the "eye" made landfall 150 km to the west of the study area) and again in November, 2004. Ripples were found throughout the region during both measurement periods, even at the deepest measurement sites. Wave ripples, particularly in depths greater than 30 meters, are presumed to be relict, having been generated across the entire shelf by Hurricane Ivan. The measurements exhibit high spatial variability, with general trends for increasing ripple wavelength, ripple height, and bed sediment grain size with increasing depth and distance offshore.
H51I-0899
Instant Grainification: Real-Time Grain-Size Analysis from Digital Images in the Field
Over the past few years, digital cameras and underwater microscopes have been developed to collect in-situ images of sand-sized bed sediment, and software has been developed to measure grain size from those digital images (Chezar and Rubin, 2004; Rubin, 2004; Rubin et al., 2006). Until now, all image processing and grain- size analysis was done back in the office where images were uploaded from cameras and processed on desktop computers. Computer hardware has become small and rugged enough to process images in the field, which for the first time allows real-time grain-size analysis of sand-sized bed sediment. We present such a system consisting of weatherproof tablet computer, open source image-processing software (autocorrelation code of Rubin, 2004, running under Octave and Cygwin), and digital camera with macro lens. Chezar, H., and Rubin, D., 2004, Underwater microscope system: U.S. Patent and Trademark Office, patent number 6,680,795, January 20, 2004. Rubin, D.M., 2004, A simple autocorrelation algorithm for determining grain size from digital images of sediment: Journal of Sedimentary Research, v. 74, p. 160-165. Rubin, D.M., Chezar, H., Harney, J.N., Topping, D.J., Melis, T.S., and Sherwood, C.R., 2006, Underwater microscope for measuring spatial and temporal changes in bed-sediment grain size: USGS Open-File Report 2006-1360.
H51I-0900
Cobble Cam: Grain-Size Analysis of Gravel Bars from Digital Photographs
Here we report on our efforts to apply the autocorrelation techniques of Rubin (2004) to measure grain-size of fluvial and coastal gravel bars using digital photographs. Digital photos were obtained from approximately 1 m above the ground surface of granule- to cobble-sized sediment. Physical measurements of the intermediate and long dimensions of clasts in the field closely resembled the short and long axes, respectively, measured from the digital photos on a computer. Calibration curves for the autocorrelation technique were generated from a series of the best-sorted samples in the digital photographs. Optimal results occurred when the analyses were discontinued at an autocorrelation threshold of 0.25, for which r-square between the actual grain size and the autocorrelation estimate of grain size was 0.94 and the r.m.s. error was 0.3 phi-units. We also evaluated the effects of wet/dry conditions and illumination on these results. Replicate photos were obtained of dry gravel and the same gravel that was wet artificially. The mean difference in the estimated grain-size from the wet and dry conditions was 4%. Patchy wetness, however, introduced an error on the same length scale of the patchiness. Illumination variability also introduced error (up to 10%) shown by repeat photographs at a single station over the course of a day. These illumination effects can be minimized by the use of shades and a camera flash, although these additional pieces of field equipment may introduce
H51I-0901
Field test of an autocorrelation technique for determining grain size using a digital camera
An extensive field test using Rubin's (2004) autocorrelation technique shows that median and mean grain size can be determined with suitable accuracy using a digital camera and associated autocorrelation when compared to traditional methods such as mechanical sieving and settling-tube analysis. The field test included 205 sediment samples and > 1200 digital images from a variety of beaches on the west coast of the United States, with grain sizes ranging from sand to granules. To test the accuracy of the digital-image grain-size algorithm, we compared results with manual point counts of a large image data set in the Santa Barbara littoral cell. Grain sizes calculated using the autocorrelation algorithm were highly correlated with the point counts of the same images (r2=0.93; n=79) and had an error of only 1%. Although grain sizes calculated from digital images give an accurate result for grains in the image, natural lateral and vertical variability in grain size can cause differences between grain size measured in digital images of the bed surface and grain size measured by sieving a grab sample that includes subsurface sediment. Lateral spatial variability was tested by analyzing the results of up to 100 images taken in a series of 1 m2 sample areas. Comparisons of calculated grain sizes and grain sizes measured from grab samples show small differences between surface sediment and grab samples on high- energy dissipative beaches with well-sorted sediment such as in the Pacific Northwest (r2 > 0.92; n=115). In contrast, on less dissipative, more poorly sorted beaches such as Ocean Beach in San Francisco, differences between surface and subsurface grain size are greater (r2 > 0.70; n=67; within 3% accuracy). In all field tests the autocorrelation method was able to predict the mean and median grain size with ~96% accuracy, which is more than adequate for the majority of sedimentological applications. When properly automated for large numbers of samples, the autocorrelation technique is roughly 2 orders of magnitude faster than traditional grain- size analysis, saving time and money, without sacrificing accuracy in measuring average grain size; however, the technique may not be as accurate in measuring the fine and coarse tails of a size distribution.
H51I-0902
Dominance of Changes in Bed-Sand Grain Size Over Bed-Sand Area in Regulating Suspended-Sand Concentration: Examples From the Colorado River
Sand transport in the Colorado River in Marble and Grand Canyons was naturally limited by the upstream supply of sand. Prior to the 1963 closure of Glen Canyon Dam, the river exhibited the following four effects of sand supply limitation: (1) hysteresis in suspended-sand concentration, (2) hysteresis in suspended-sand grain size coupled to the hysteresis in suspended-sand concentration, (3) production of inversely graded flood deposits, and (4) development or modification of a lag between the time of a flood peak and the time of either maximum or minimum (depending on reach geometry) bed elevation. Construction and operation of the dam have enhanced the degree to which the first two of these four effects are evident, but has not affected the degree to which the last two effects of sand supply limitation are evident in the Colorado River in Marble and Grand Canyons. The first three of these effects involve coupled changes in suspended-sand concentration and grain size that are controlled by changes in the upstream supply of sand. Tributaries downstream from Glen Canyon Dam are now the sole suppliers of sand to the Colorado River in Marble and Grand Canyons. During floods on these tributaries, sand on the bed of the Colorado River fines; this causes the suspended sand to fine and the suspended-sand concentration to increase, even when the discharge of water remains constant. Subsequently, the bed is winnowed of finer sand, the suspended sand coarsens, and the suspended-sand concentration decreases independently of discharge. Also associated with these changes in the upstream sand supply are likely changes in the fraction of the bed that is covered by sand. Thus, suspended-sand concentration in the Colorado River is probably regulated by both changes in bed-sand grain size and changes in bed-sand area. A physically based flow and suspended-sediment transport model has been recently developed, tested, and applied to data from the Colorado River to evaluate the relative importance of changes in bed-sand grain size and changes in bed-sand area in regulating suspended-sand concentration. Although the model was developed using approximations for steady, uniform flow and other simplifications that are not met in the Colorado River, the results nevertheless support the hypothesis that changes in bed-sand grain size are much more important than changes in bed-sand area in regulating the concentration of suspended sand. Because changes in bed-sand volume and bed-sand area are not uniquely related nor necessarily positively correlated, decreases in the upstream supply of sand may actually be associated with increases in the area of the sand on the bed (e.g., as observed during artificial flood tests released from the dam in 1996, 2000, and 2004). Thus, changes in bed- sand grain size may override the effects of changes in bed-sand area in regulating suspended-sand concentration.
H51I-0903
Modified Sediment Rating Curve Approach for Supply-dependent Conditions
Reliable predictions of sediment transport and river morphology in response to driving forces, such as anthropogenic influences, are necessary for river engineering and management. Because engineering and management questions span a wide range of space and time scales, a broad spectrum of modeling approaches has been developed, ranging from sediment transport rating curves to complex three-dimensional, multiple grain-size morphodynamic models. Sediment transport rating curves assume a singular relation between sediment concentration and flow. This approach is attractive for evaluating long-term sediment budgets resulting from changes in flow regimes because it is simple to implement, computationally efficient, and the empirical parameters can be estimated from quantities that are commonly measured in the field (sediment concentration and flow). However, the assumption of a singular relation between sediment concentration and flow contains the following implicit assumptions: 1) that sediment transport is in equilibrium with sediment supply such that the grain-size distribution of the bed sediment is not changing, and 2) that the relation between flow and bed shear stress is constant. These assumptions present limitations that have led to the development of more complex numerical models of flow and morphodynamics. These models rely on momentum and mass conservation for water and sediment and thus have general applicability; however, this comes at a cost in terms of computations as well as the amount of data required for model set-up and testing. We present a hybrid approach that combines aspects of the standard sediment rating curve method and the more complex morphodynamic models. Our approach employs the idea of a shifting rating curve, whereby the relation between sediment concentration and flow changes as a function of the sediment budget in the reach. We have applied this alternative approach to the Colorado River below Glen Canyon Dam. This reach is particularly suited to such an approach because it is substantially sediment supply-limited such that transport rates are dependent on both flow and sediment supply; also, there is a rich dataset available for constraining the empirical parameters and testing the hybrid model. Though more empirical in nature than the morphodynamic models, this modified sediment rating curve approach may have broad potential application because its simplicity allows for relatively rapid evaluation of long-term sediment budgets under a range of flow regimes and sediment supply conditions.
H51I-0904
Effect of Comminution on Bed Sediment Size in a Fluvial System With a Flood Hydrograph
During transport in a fluvial system, sediment grains undergo comminution, thereby modifying the dis-equilibrium sediment transport rates of various size fractions in the bed. It would be useful to predict the relative effects of comminution and differential dis-equlibrium transport on bed composition and slope for steady and variable hydrographs. We have conducted a series of numerical experiments simulating fluvial transport and deposition of sand-sized heterogeneous quartz (Qtz), feldspar (F), and other heavy mineral mixtures (Hm) under varying flood hydrographs with and without comminution. The experimental design consists of a standard steady-state control run to which are compared various runs of simulated floods with discharge (Q) and sediment discharge (Qs) in varying ratios. The sediment supply is a temporally and spatially constant grain size distribution of Qtz:F:Hm equal to 70:25:5 with a feed rate at capacity with Q. The steady state results with constant boundary conditions show that the bed coarsens, the bed slope degrades and bed composition becomes more quartz rich compared to the initial conditions of the uniform feedstock. The results for a flood hydrograph with a matching capacity Qs show that a dynamic steady state is achieved in which the bed alternates between aggradation and degradation. The average bed concavity is greater than the standard case, and the bed composition and texture are more quartz-rich and finer. When grains are allowed to comminute the dis-equilibrium sediment transport rates of the various size fractions increases, leading to a larger deviation about a scenario's given average in the dynamic steady state scenario. The dynamic equilibrium case with comminution is finer and has a larger variance about a mean composition and texture relative to the same setup without comminution.