Search

Experiments were conducted to examine soil erosion by headcut development and migration in concentrated flows typical of upland areas. In a laboratory channel, packed sandy loam to sandy clay loam soil beds with preformed headcuts were subjected to simulated rain followed by overland flow. The rainfall produced a well-developed surface seal that minimized surface soil detachment. During overland flow, soil erosion occurred exclusively at the headcut, and after a short period of time, a steady state condition was reached where the headcut migrated at a constant rate, the scour hole morphology remained unchanged, and sediment yield remained constant. A fourfold increase in flow discharge resulted in larger scour holes, yet aspect ratio was conserved. A sediment bed was deposited downstream of the migrating headcut, and its slope depended weakly on flow discharge.

Experiments were conducted to examine soil erosion by headcut development and upstream migration in rills typical of upland areas. Soil material, simulated rain, overland flow discharge, and initial headcut height were held constant in each experiment, but initial slope of the bed varied from 1% to 10%. Air-dried, crushed, and sieved sandy loam to sandy clay loam soil was incrementally packed into a laboratory channel 2 m long and 0.165 m wide to a depth of 0.25 m. Soil bulk density was 1425 kg m−3 in each experiment. A pre-formed headcut 25-mm high was constructed 1.5 m downstream of the entrance of the flume. Simulated rain, applied at 21 mm h−1 for 4 h, produced a well-developed surface seal that minimized the detachment of the surface soil. Following the rainfall, overland flow at a rate of 52 l min−1 was released onto the bed, soil erosion occurred at the pre-formed headcut overfall, and a scour hole developed, enlarged, and migrated upstream. The rate of headcut migration was constant within each experiment, but higher slopes of the bed generally resulted in lower rates of migration. At slopes on the bed of 2% and smaller, the overfall nappe at the headcut brinkpoint remained submerged, and a steady-state condition was achieved: sediment yield and geometry of the scour hole remained constant as the headcut migrated upstream. For slopes on the bed of 3% and greater, the overfall nappe became aerated, and as the headcut migrated upstream, the depth of scour increased. Higher slopes on the bed resulted in deeper scour holes. Mechanisms of soil erosion included the formation of tension cracks and seal removal at the headcut brinkpoint, soil washout along the aerated headcut face, and plunge-pool scour. The slope of the sediment deposit downstream of the migrating headcut was 2.2% for all experiments, and suggests that flow discharge and not the initial slope of the bed controlled transport capacity and downstream adjustment of the bed.

Gravity currents are of considerable environmental and industrial importance as hazards and as agents of sediment transport, and the deposits of ancient turbidity currents form some significantly large hydrocarbon reservoirs. Prediction of the behavior of these currents and the nature and distribution of their deposits require an understanding of their turbulent structure. To this end, a series of experiments was conducted with turbulent, subcritical, brine underflows in a rectangular lock-exchange tank. Laser-Doppler anemometry was used to construct a two-dimensional picture of the velocity structure. The velocity maximum within the gravity current occurs at y/d ≈ 0.2. The shape of the velocity profile is governed by the differing and interfering effects of the lower (rigid) and upper (diffuse) boundaries and can be approximated with the law of the wall up to the velocity maximum and a cumulative Gaussian distribution from the velocity maximum to the ambient interface. Mean motion within the head consists of a single large vortex and an overall motion of fluid away from the bed, and this largely undiluted fluid becomes rapidly mixed with ambient fluid in the wake region. The distribution of turbulence within the current is heterogeneous and controlled by the location of large eddies that dominate the turbulent energy spectrum and scale with flow thickness. Turbulent kinetic energy reaches a maximum in the shear layer at the upper boundary of the flow where the large eddies are generated and is at a minimum near the velocity maximum where fluid shear is low.

There are distinct microenvironmental abnormalities of thymic architecture in several murine models of SLE defined using immunohistochemistry and a panel of mAb dissected at thymic epithelial markers. To address the issue of the relationship between the thymic microenvironment and autoimmunity, we studied backcross (NZB × NZW) F1 × NZW mice in which 50% of offspring develop nephritis associated with proteinuria and anti-DNA antibodies. We reasoned that if thymic abnormalities are associated with development of disease, the correlation of abnormalities with lupus-like disease in individual backcross mice will form the foundation for identification of the mechanisms involved. In parallel, we directed a genetic linkage analysis, using markers previously shown to be linked to nephritis and IgG autoantibody production, to determine if such loci were similarly associated with microenvironmental changes. Our data demonstrate that all (NZB × NZW) F1 × NZW backcross mice with disease have microenvironmental defects. Although the microenvironmental defects are not sufficient for development of autoimmune disease, the severity of thymic abnormalities correlates with titers of IgG autoantibodies to DNA and with proteinuria. Consistent with past studies of (NZB × NZW) F1 × NZW mice, genetic markers on proximal chromosome 17 (near MHC) and distal chromosome 4 showed trends for linkage with nephritis. Although the markers chosen only covered about 10–15% of the genome, the results demonstrated trends for linkage with thymic medullary abnormalities for loci on distal chromosome 4 and distal chromosome 1. We believe it will be important to define the biochemical nature of the molecules recognized by these mAbs to understand the relationships between thymic architecture and immunopathology.

The asteroid gene of Drosophila was found to lie within 189 bp of Star. Asteroid cDNA clones were isolated and sequenced and a single putative open reading frame was identified that encodes a novel protein of 815 amino acids with a calculated molecular mass of 93 kilodaltons. Using cDNA probes, asteroid transcripts were localized to the proliferative tissues of embryos and to the mitotically active tissue anterior to the morphogenetic furrow in eye imaginal discs. Ribonuclease protection assays identified a mutation of asteroid that acts as a dominant enhancer of Star mutations and also enhances the Ellipse mutation, EgfrE1. Based on these data, a model for asteroid gene function in EGF receptor signaling is presented.Key words: Drosophila, asteroid, Star, EGF receptor, eye development.

To understand more fully the fluid and sediment dynamics of upper stage plane beds, laboratory experiments were conducted using mobile and fixed beds where turbulent motions of fluid and sediment were measured using laser anemometry. Bed-elevation fluctuations on mobile upper stage plane beds reveal millimeter-high bed waves. Vertical profiles of flow velocity, mixing length, and eddy viscosity (diffusivity) are represented well by the law of the wall. For the mobile bed, von Karman/s κ ≈ 0.33 and equivalent sand roughness to mean bed-grain size varies from 9 to 17 because of the presence of bed load and low-relief bed waves. For fixed beds with no sediment transport, κ ≈ 0.41 and equivalent sand roughness is equal to the mean bedgrain size. The decrease in κ for mobile beds is related to the relative motion of grains and fluid. Mobile-bed turbulence intensities are greater than those for sediment-free fixed beds because of enhanced wake formation from the lee side of near-bed grains and low-relief bed waves. Sediment diffusivities es calculated in a similar way to fluid diffusivities e indicate that es≈e. Sediment diffusivities calculated using the equilibrium balance between upward diffusion and downward settling of sediment are similar to e in near-bed regions (y/d 0.3, suggesting that larger, more energetic turbulent eddies are responsible for sediment suspension higher in the flow.

The presence of sand moving at low transport rates over a flat bed modulates the production of turbulence when compared to clearwater flow at similar mean flow conditions. Phase Doppler anemometry is used to discriminate the turbulence characteristics of the carrier fluid from the sediment grains (0.22 mm diameter) and shows that the presence of mobile sediment increases the near-wall velocity gradient and shear velocity when compared with the clearwater values. This increased shear velocity is associated with a greater bed roughness height and near-bed turbulence intensities and smaller mixing lengths. Quantification of slip velocities between the fluid and sediment phases reveals particle Reynolds numbers that range from 1 to 30. Turbulence enhancement is shown to occur at lower values of both the Stokes number and ratio of the particle size-to-turbulent length scale than in past work. Several mechanisms of turbulence modulation may be invoked to explain these changes, including increased bed roughness, eddy shedding from large grains, grain inertial effects, and particle-coherent structure interactions. These mechanisms may be significantly influenced by both particle-particle and particle-wall interactions. Since mobile sediment modulates the carrier fluid turbulence, there is a need for modification of existing theories of sediment suspension and for caution when interpreting velocity profiles that are obtained without discriminating the fluid and sediment phases.

Laser Doppler anemometry (LDA) was used to measure the instantaneous downstream and vertical velocities in a series of simple and reflected saline density currents in a lock-exchange flume tank. All the currents were turbulent and subcritical. Mean downstream fluid velocities were in excess of the head velocity by up to 30%, and instantaneous velocities were greater by up to 50%. Turbulence intensities were highest within the head, and generally greatest in the middle part of the current, but did not correspond with the level of highest mean velocities. The maximum Reynolds stress also occurred within the head; large negative values were associated with shear along the upper boundary of the current. Peaks of turbulence, Reynolds stress and shear velocity occurred in association with the arrival of reflections. In large-scale turbidity currents, such reflections would be capable of re-entraining and resuspending sediment deposited by the forward current. Some reflections take the form of solitary waves within a residual flow with a velocity vector in the opposite direction. In nature, these could produce symmetrical ripples in environments below storm-wave base.

Experimental research has played an integral role in the development of foundational knowledge on which the physical sciences and engineering are based. Motivated by the control, precision, and reproducibility that it affords, as well as by its ability to adapt to the changing needs of the discipline, geomorphologists now embrace the utility of laboratory-based inquiry. The formative years for the use of hydraulic flumes, river modeling flumes, and wind tunnels are reviewed in this article, as these apparatuses and their appurtenant equipment provide unrivalled opportunities for education and research, commonly producing transformative results that have markedly improved the understanding of Earth surface processes. Laboratory-based geomorphology is entirely complementary to field- and numerical-based approaches.

The residents surrounding the Love Canal environmental disaster would not have met New York State’s criteria for an environmental justice community and its working-class residents would not have received any benefits under the state’s current policy. In this paper, we examine how states are defining the environmental justice community and consider the missed opportunities of non-EJ communities. Specifically, we look to nineteen existing state policies and identify the criteria and methodologies used for defining the environmental justice community as well as the benefits conferred upon these communities as a result of their designation. We then discuss the implications of the pervasive demographic threshold approach and its corresponding potential for misaligning environmental justice benefits with communities at risk.

A series of experiments were performed in a mixing box in order: (1) to investigate the applicability of phase Doppler anemometry (PDA) to discriminate fluid and sediment particle sizes and velocities in sediment-laden turbulent flows; and (2) to relate the size and amount of sediment in suspension to the grid-generated turbulence. Natural impurities within the water provide excellent “seeding” to represent the fluid and can be easily discriminated from spherical glass beads (75-355 μm) used as sediment. Slight asphericity in the glass beads results in larger grain size ranges determined by PDA compared to the nominal sieved sizes. The mean, root-mean-square and skewness of the vertical fluid velocities increase at higher grid oscillation frequencies but decrease with distance from the grid. Similarly, the size and amount of suspended sediment increase with grid oscillation frequency and decrease with distance from the grid. The suspension of sediment is shown to be dependent on the magnitude and anisotropy of the fluctuating vertical component of velocity. Phase Doppler anemometry offers a unique methodology to investigate the complex links between the transport of sediment and the turbulent flow field.

Detailed measurements of flow velocity and its turbulent fluctuation were obtained over fixed, two-dimensional dunes in a laboratory channel. Laser Doppler anemometry was used to measure the downstream and vertical components of velocity at more than 1800 points over one dune wavelength. The density of the sampling grid allowed construction of a unique set of contour maps for all mean flow and turbulence parameters, which are assessed using higher moment measures and quadrant analysis. These flow field maps illustrate that: (1) the time-averaged downstream and vertical velocities agree well with previous studies of quasi-equilibrium flow over fixed and mobile bedforms and show a remarkable symmetry from crest to crest; (2) the maximum root-mean-square (RMS) of the downstream velocity values occur at and just downstream of flow reattachment and within the flow separation cell; (3) the maximum vertical RMS values occur within and above the zone of flow separation along the shear layer and this zone advects and diffuses downstream, extending almost to the next crest; (4) positive downstream skewness values occur within the separation cell, whereas positive vertical skewness values are restricted to the shear layer; (5) the highest Reynolds stresses are located within the zone of flow separation and along the shear layer; (6) high-magnitude, high-frequency quadrant-2 events (‘ejections’) are concentrated along the shear layer (Kelvin-Helmholtz instabilities) and dominate the contribution to the local Reynolds stress; and (7) high-magnitude, high-frequency quadrant-4 events occur bounding the separation zone, near reattachment and close to the dune crest, and are significant contributors to the local Reynolds stress at each location. These data demonstrate that the turbulence structure associated with dunes is controlled intrinsically by the formation, magnitude and downstream extent of the flow separation zone and resultant shear layer. Furthermore, the origin of dune-related macroturbulence lies in the dynamics of the shear layer rather than classical turbulent boundary layer bursting. The fluid dynamic distinction between dunes and ripples is reasoned to be linked to the velocity differential across the shear layer and hence the magnitude of the Kelvin-Helmholtz instabilities, which are both greater for dunes than ripples. These instabilities control the local flow and turbulence structure and dictate the modes of sediment entrainment and their transport rates.

The nature of flow, sediment transport and bed texture and topography was studied in a laboratory flume using a mixed size-density sediment under equilibrium and non-equilibrium (aggradational, degradational) conditions and compared with theoretical models. During each experiment, water depth, bed and water surface elevation, flow velocity, bed shear stress, bedload transport and bed state were continuously monitored. Equilibrium, uniform flow was established with a discharge of about 0.05 m3 s−1, a flow depth of about 0.01 m, a flow velocity of about 0.81–0.88 m s−1, a spatially averaged bed shear stress of about 1.7–2.2 Pa and a sediment transport rate of about 0.005–0.013 kg m−1 s−1 (i.e. close to the threshold of sediment transport). Such equilibrium flow conditions were established prior to and at the end of each aggradation or degradation experiment. Pebble clusters, bedload sheets and low-lying bars were ubiquitous in the experiments. Heavy minerals were relatively immobile and occurred locally in high concentrations on the bed surface as lag deposits. Aggradation was induced by (1) increasing the downstream flow depth (flume tilting) and (2) sediment overloading. Tilt-induced aggradation resulted in rapid deposition in the downstream half of the flume of a cross-stratified deposit with downstream dipping pebbles (pseudo-imbricated). and caused a slight decrease in the equilibrium mean water surface slope and total bedload transport rate. These differences between pre- and post-aggradation equilibrium flow conditions are due to a decrease in the local grain roughness of the bed. Sediment overloading produced a downstream fining and thinning wedge of sediment with upstream dipping pebbles (imbricated), whereas the equilibrium flow and sediment transport conditions remained relatively unchanged. Degradation was induced by (1) decreasing the downstream flow depth (flume tilting) and (2) cutting off the sediment feed. Tilt-induced degradation produced rapid downstream erosion and upstream deposition due to flow convergence with little change to the equilibrium flow and sediment transport conditions. The cessation of sediment feed produced degradation and armour development, a reduction in the mean water surface slope and flow velocity, an increase in flow depth, and an exponential decrease in bedload transport rate as erosion proceeded. A bedload transport model predicted total and fractional transport rates extremely well when the coarse-grained (or bedform trough) areas of the bed are used to define the sediment available to be transported. A sediment routing model, MIDAS, also reproduced the equilibrium and non-equilibrium flow conditions, total and fractional bedload transport rates and changes in bed topography and texture very well.

Functional recovery in animal models of nervous system disorders commonly involves behavioural compensation, in which animals alter the use of their limbs after injury, making it difficult to distinguish ‘true’ recovery from substitution of novel movements. The purpose of this study is to investigate how abnormal movements are produced by using biomechanical assessment of limb joint motion, an approach commonly used to diagnose human pathological gait. Rats were trained to cross a runway whilst kinetic (ground reaction forces) and kinematic (limb segment positions) data were synchronously recorded. Inverse dynamic analysis was used to calculate limb joint moments, or torques, and joint mechanical power throughout the stride for major joints of the forelimbs and hindlimbs, both before and after denervation of a major ankle extensor muscle. Before surgery, rats moved with joint moment and power profiles comparable to other quadrupeds, with differences attributable to species variation in limb posture. After surgery, rats trotted asymmetrically, with a near plantigrade stance of the left hindlimb. Surprisingly, ankle joint moments and power were largely preserved, with dramatic reductions in range of motion and joint moments at the proximal joints of the affected limb. Stiffening of the proximal limb compensated for increased compliance at the ankle but decreased the total mechanical work done by the injured limb. In turn, more work was done by the opposite, i.e. uninjured, hindlimb. This is the first study to quantify the biomechanical adjustments made within and between limbs in laboratory rodents after nervous system injury.

Soil erosion, in its myriad forms, devastates arable land and infrastructure and strains the balance between economic stability and viability. Gullies may form in existing channels or where no previous channel drainage existed. Typically, gullies are a result of a disequilibrium between the eroding force exerted by concentrated flowing water and the resistance of the earth materials in which it is flowing, caused by either an increase in erosional forces (related to a concentration of flow, constriction of flow, an increase in discharge, or decrease in sediment load) or decreased erosional resistance (related to a decrease in cover or some surface disturbance causing decreased cohesion). A gully is a complicated system as its evolution is controlled by water erosion at the gully head and bed, which triggers gravitational mass-movement on gully sidewalls. Gully erosion usually, but not always, includes one or more headcuts that migrate upslope over time. These are step changes in bed surface elevation where intense, localized erosion takes place, and thus are commonly associated with significant increases in sediment load. Reported experimental data shows that actively migrating gully headcuts display a self-similar organization with migration rates dependent on upstream flow depth and discharge, tailwater depth, and soil properties. The depth of gullies is often limited by the presence of a non-erodible or impervious soil layer. When erosion reaches such a layer, the gully typically widens, creating a wide shallow cross section. Once a gully is initiated, transport and deposition of the eroded soil and widening of the gully channel, further govern its evolution. Our knowledge of these processes in shallow concentrated flows within agricultural soils, however, is still quite limited and largely scaled down from river hydraulics. Experiments were conducted to examine channel sidewall expansion due to overland flow discharge. Packed soil beds were subjected to simulated rainstorms and clear-water overland flow. During overland flow, the flow rate was systematically increased to induce widening within the channel. Within these channels, equilibrium must be maintained between potential scour depth and potential channel width. Channel expansion and peaks in sediment discharge occurred episodically, linked directly to increases in upstream discharge.

The Star/asteroid (S/ast) region of Drosophila melanogaster has been cloned by P element transposon tagging using the snw chromosome as a source of defective P elements. In each mutation examined, the element integrated into the region was a 0.5-kb element from a region proximal to sn and not one of the head-to-head elements from the sn locus. Previously described spontaneous and X-ray induced mutations of S and ast were located on the molecular map by Southern analysis and restriction endonuclease mapping of genomic clones. S mutations are either large deletions of the cloned region or DNA breaks located near the P element insertions that cause ast mutations. Both S and ast mutations reduce the steady-state amounts of a 3.4-kb RNA. The molecular data, together with the phenotypic interactions observed for S and ast alleles, are consistent with the interpretation that S and ast mutations are lesions within the same gene or within genes that are functionally related.Key words: Drosophila, Star, asteroid, P elements.

[1] Headcut erosion can severely accelerate soil loss in upland concentrated flows and lead to significant soil degradation in agricultural areas. Previous experimental work has demonstrated that actively migrating headcuts display systematic morphodynamic behavior and that impinging jet theory can provide an excellent analytic foundation for predicting these erosional phenomena. This research sought to examine systematically the effect of an upstream sediment load on the morphodynamics of actively migrating headcuts in upland concentrated flows. Using a specially designed experimental facility, actively migrating headcuts in packed soil beds were allowed to develop freely when subjected to concentrated, clear-water flows similar to rills, crop furrows, and ephemeral gullies. After such headcut development and without altering the boundary conditions of the experiment, a sediment load composed of relatively coarser grained sand was introduced into the flow well upstream of the erosional feature. At relatively low upstream sediment loadings, the morphodynamics and sediment discharge of the actively migrating headcuts remained unchanged in time and space. As upstream sediment loadings increased, the size and migration rate of the headcuts decreased markedly, yet sediment discharge was not strongly affected as its sediment texture shifted from headcut-controlled flux to sediment-feed flux. The headcut erosion process was completely arrested when the upstream sediment feed rate exceeded a specific magnitude. Analytic arguments are used to explain the reaction time and morphodynamic response of an actively migrating headcut to this upstream sediment loading. This research shows that headcut erosion can be strongly modulated by an upstream sediment load, further complicating the prediction of soil erosion on upland areas dissected by sediment-laden concentrated flows.

[1] Since the early 1950s, the U.S. Soil Conservation Service (SCS) and later the U.S. Department of Agriculture Natural Resources Conservation Service has built over 11,000 flood control reservoirs (FCR) in 47 states. FCR built in Texas and Oklahoma in the early 1950s to mid-1950s were impounded during the most severe drought on record in the region. In this study, the sediment trapped in FCR is used to reconstruct the variation in sediment yield through the drought years to the present. New sediment surveys of four FCR in McCulloch County, Texas, are combined with three previous surveys by the SCS. The new surveys are conducted using acoustic profiling to map water depth and sediment thickness in submerged areas of the reservoirs and real-time kinematic GPS in the dry areas. Sediment coring is used to determine sediment dry bulk density. The survey results are used to construct a composite history of the normalized sediment yield for the study area. Normalized sediment yield is the annual sediment yield normalized by the soil erodability factor K and the combined slope length and steepness factor LS of the watershed. The results indicate that sediment yield was lowest during the relatively drought-free period from 1971 to 2007, averaging 4.2 t/ha/yr/unit K/unit LS and over 70 times higher during the early part of the 1950s drought from 1951 to 1953, averaging 300.3 t/ha/yr/unit K/unit LS. These results have important implications for predicting the remaining useful life of FCR in the region and planning for future droughts.

A model for the entrainment and bed load transport of sediment grains of different sizes, shapes and densities by a unidirectional turbulent flow is developed in terms of (1) sediment types available for transport; (2) the mean and turbulent fluctuating values of fluid forces acting upon the sediment grains; and (3) the nature of the interaction between turbulent fluid forces and available sediment, resulting in entrainment and transport of grains as bed load or in suspension. The behavior of the model is explored extensively, and compared with natural data from flumes and rivers. The predicted threshold of entrainment of individual size fractions within a mixed-size bed agrees well with observations as long as the pivoting angle is specified appropriately as a function of grain size. The rate and size distribution of bed load transport generally agrees well with natural data as long as effective bed shear stress in the presence of bed forms can be defined, bed load transport measurements are reliable, and the size distribution of available sediment is accurately specified.

[1] Experiments were conducted to examine the effect of vertical stratification in soil erodibility on the development and migration of steady state headcut scour holes in upland concentrated flows typical of agricultural fields. Packed soil beds with a preformed step were subjected to identical simulated rainstorms and clear-water overland flow rates, which resulted in predictable, actively migrating headcut scour holes with nearly identical characteristics. When an erosion-resistant layer was incorporated into the packed soil bed at a depth that exceeded this expected plunge pool scour depth, the erosion and hydraulic processes of the migrating headcuts remained unchanged. When the erosion-resistant layer was placed so as to intersect this potential headcut scour depth, the erosivity of the reattached wall jet was unable to erode this layer, and the depth of scour, the nappe entry angle, and sediment efflux all were reduced inversely proportional to the relative depth of the resistant layer. These data were successfully predicted using modified jet impingement theory for headcut scour holes and demonstrate further the effects of soil management and tillage practices on total soil losses from agricultural fields.

Sediments impounded within flood control reservoirs are potentially important archives of environmental and geomorphic processes occurring within drainage basins. The concentrations of select sediment-associated trace elements were assessed within the impoundment of Grenada Lake, a relatively large flood control reservoir in Mississippi with a history of contaminant bioaccumulation in fish. The post-construction sediments (after 1954) are discriminated from the pre-construction sediments (before 1954) based on depth variations in sediment texture and 137 Cs emissions. The concentrations of select trace elements of the post-1954 sediments all are statistically greater than the pre-1954 sediments, and these same sediments also are enriched in clay. Once these concentrations are normalized by clay content, all trace elements in the post-1954 sediments are lower in concentration than the pre-1954 normalized sediments. Moreover, the trace elements when normalized by clay or Al content show virtually no change vertically (over time) within the reservoir impoundment. This suggests that the sources of these sediment-associated trace elements within Grenada Lake, whether natural or anthropogenic, have not changed appreciably over the lifespan of the reservoir and that the degradation of sedimentologic and ecologic indices within the lake are due to the sequestration of clay or clay-sized materials.

[1] Recent experimental results suggest there are at least two distinct bed form initiation processes. Bed forms may be generated from local bed defects that are propagated down and across stream by flow separation processes when sediment transport is patchy and sporadic. Alternatively, bed form development may occur over the whole bed at once when sediment transport is general and widespread. Herein, we critically test a simple model for this latter bed form initiation mode that was originally presented by H.-K. Liu in 1957 but not tested because of the complex nature of the measurements required. The theory is based on the idea that a moving sand bed might be likened to a dense fluid and that a hydrodynamic instability develops at the interface between the sediment transport layer and the near-bed fluid, leading to the organization of the transport system into laterally extended, discrete bed forms. Our predicted initial bed form lengths are within 10% of the lengths observed in a series of flume experiments, suggesting that transverse waveforms on sand beds sheared by unidirectional currents can be initiated as an interfacial hydrodynamic instability of Kelvin-Helmholtz type when the current is sufficiently vigorous to produce general sediment movement, that is, to create a sediment transport layer that acts as a pseudofluid with density composed of solid and fluid components.

[1] The kinematics and morphodynamics of low-amplitude, small-scale sand waves developed over migrating dunes are examined using data drawn from laboratory experiments. We refer to the superimposed features as “sand sheets,” a general descriptive term for low-amplitude bed waves that are not easily classified as ripples, dunes, or bars. Within the experiments, the sheets formed downstream of the reattachment point at a distance that was invariant with dune size. Some sheets lacked slip faces composed of sand grains avalanching down a slope near the angle of repose. Over equilibrium dunes, three to four sand sheets were observed per 100 s. Sheet thickness was 10% of the height of the dune upon which they were superimposed; they migrated at 8 to 10 times the dune rate; they had nearly constant lengths over the full range of dune lengths and flow conditions; and they had aspect ratios of ∼0.025. Dunes and sand sheets represent distinct scales of sediment transport with different migration rates. However, sediment transport rates, calculated from the sand sheet and dune morphologies, are nearly identical. For transport equivalence to occur, sand sheets migrating at 10 times the dune rate must be 0.1 times the size, which is consistent with the morphological observations. Superimposed bed waves on dunes are often considered simply as additional roughness elements, but these results indicate that such bed waves are the agency by which the dune bed form itself moves downstream.

[1] The development and migration of headcuts in rills and gullies can adversely impact soil resources in agricultural areas and accelerate landscape degradation worldwide. Analytical treatments of this erosion process have been based on a turbulent impinging jet analogue, though partially validated by field and laboratory data. Experiments were conducted using fixed headcut models to define the mean flow structure within these erosional features and to assess the applicability of similarity arguments for jets to characterize this flow domain. These data show that (1) flow within headcut scour holes is analogous to plane turbulent reattached wall jets and (2) similarity arguments used to define the limits, length scales, and velocity distributions in classical free jets and wall jets are applicable within discrete regions of the headcut scour hole domain but are affected by the ventilation characteristics of the overfall. This study contributes new knowledge on the hydrodynamics within headcut scour holes, and it confirms the applicability of jet theory for analyzing such soil erosion processes.

[1] A depth-averaged two-dimensional numerical model has been developed to simulate flow, sediment transport, and bed topography in river channels with emergent and submerged rigid vegetation and large woody debris. The effect of helical flow in bends is considered by adopting an algebraic model for the dispersion terms in the depth-averaged two-dimensional momentum and suspended-sediment transport equations and by adjusting the bed load transport angle. The governing equations are discretized using the finite volume method on a nonstaggered, curvilinear grid. Model validity has been assessed using experimental data observed in both fixed- and movable-bed laboratory flumes and a natural channel with submerged and emergent rigid vegetation. In general, mean flow velocities, sediment transport rates, and changes in bed topography predicted by the model agree well with the experimental observations. For laboratory and field cases, root-mean-square relative errors for velocities were less than about 13% and 44%, respectively, and about 50% of errors for changes in bed topography were less than 14.5% and 8% of the flow depth, respectively.

[1] While more than 75,000 dams nationwide have served a multitude of purposes over the past 50 years, current emphasis is being placed on decommissioning those aged infrastructures considered environmentally harmful. Grenada Lake, a relatively large flood control reservoir in Mississippi, is located in a highly erodible region of the United States and is fed by historically unstable stream channels. Through a variety of stratigraphic analyses, postimpoundment sediments are identified in the reservoir, showing that the lake has lost a nominal 3% of its flood storage capacity since its inception in 1954. A simple sediment budget constructed for the Yalobusha River's contribution of channel-derived sediments shows that about 76% of this sediment remains stored upstream, about 16% is stored in the reservoir, and about 8% has exited the lake. Sediment delivery ratios, known to be low for relatively large watersheds, appear to be unaffected by upstream channel instabilities and aggressive channelization projects.

[1] Bed form initiation in unidirectional flow is examined on a flat bed composed of a homogeneous 0.5 mm sand. Velocity profiles taken prior to bed form development indicate that the examined flows are typical of fully turbulent, uniform, open channel flows. Under these conditions, two separate modes of bed form initiation are observed: defect and instantaneous initiation. Defect initiation occurs at lower flow stages, where sediment transport is sporadic and patchy, and is characterized by defect propagation associated with flow separation. Instantaneous initiation occurs at larger flow strengths, where sediment transport is general and widespread. This form of bed form initiation begins with the imprinting of a cross-hatch pattern on the flat sediment bed, which leads to chevron-shaped forms that migrate independently of the initial pattern. The chevrons eventually align to form incipient crest lines. This mode of bed form initiation does not appear to be linked to turbulent structures, but integral scales derived from velocity measurements prior to bed form development are similar to the initial bed form length scales.

There is considerable evidence of a key role for CD4+ T cells in the pathogenesis of rheumatoid arthritis. Several attractive candidate antigens, mostly joint-specific, have been studied, but information regarding T cell responses to these antigens in patients is limited and occasionally contradictory. Novel reagents (such as major histocompatibility complex and peptide tetramers) and sensitive techniques (such as intracellular cytokine staining) will aid in future studies to identify antigen-specific T cells. In addition, a new animal model of inflammatory arthritis has recently provided new perspective to the study of rheumatoid arthritis by drawing attention to systemic self-antigens as targets of autoimmunity and anti-self antibodies as markers of T cell activity and effectors of disease.

T cell receptor engagement with CD28 costimulation is generally required for naive T cell activation, whereas reactivation of memory cells is less dependent on CD28 costimulation. We studied this process in chronic beryllium disease, in which the frequency of antigen-specific CD4+ T cells in the lung is large and circulating antigen-specific cells are also detectable. In the lung, a large fraction of CD4+ T cells stopped expressing CD28 mRNA and protein, and this change in phenotype correlated with lung inflammation. In the presence of concentrations of CTLA-4Ig that inhibited the CD28-B7 interaction, beryllium-specific CD4+ T cells in lung were still able to proliferate and secrete IFN-gamma in response to beryllium in culture. This functional independence of CD28 costimulation included lung CD28+ effector cells. Although lung CD4+CD28- cells retained the ability to secrete Th1-type cytokines in response to beryllium, they showed less proliferative capacity and were more susceptible to cell death compared with CD28+ T cells. In contrast to lung cells, inhibition of the CD28-B7 interaction markedly reduced responses of beryllium-specific T cells in blood. Taken together, these findings suggest transition within memory CD4+ T cells from CD28 dependence in central memory cells to functional independence and then loss of CD28 expression in effector cells.

The establishment of Total Maximum Daily Loads (TMDL) for contaminants entering water bodies requires the development of suitable methods for contaminant measurement. In an effort to assess quantities of suspended-sediment transport, an acoustic device is being developed in a collaborative effort between the University of Mississippi's National Center for Physical Acoustics and the Department of Civil Engineering along with the USDA-ARS National Sedimentation Laboratory with support from the National Science Foundation. The acoustic device will be non-intrusive, automatic, and provide suspended-sediment concentration and particle-size information at many points throughout the flow depth in a short time period. In order to test the new acoustic device, validation against an established measurement method is necessary. Due to the properties of the acoustic technique, this validation cannot be performed by sampling directly beneath the device. Towards this end, a laboratory flume with a sediment-starved bed was used to establish whether laterally separated samples collected simultaneously at like elevations within the flow would have similar concentrations. It was found that the average error, after a constant correction was applied, was 6%. This was deemed acceptable for use in instrument testing. Starved bed results are compared with previously collected data in dune and upper-stage plane bed regimes, demonstrating the profound effect of a mobile bed on the accuracy of suspended-sediment sampling.

The geometry and dynamics of low-relief bed forms (pebble clusters, bed-load sheets, and bars) developed in a mixed-size and mixed-density sediment (sandy gravel) were studied in a laboratory channel. In most runs, equilibrium uniform flow was established with discharge 0.05 m3/s, mean flow depth 0.1 m, mean flow velocity 0.82-0.87 m/s, mean bed shear stress 1.6-2.2 Pa, dimensionless bed shear stress 0.04-0.07, and mean bed-load transport rate 0.009-0.013 kg/m-s. Low-relief bed forms were observed directly and in bed-height records and bed profiles measured with a high-resolution, ultrasonic bed profiler. Pebble clusters are groups of relatively large, temporarily immobile clasts with lee side and stoss side accumulations of finer sediment, and with adjacent scoured zones w th heavy-mineral concentrations. Pebble clusters are concentrated in relatively coarse areas of the bed such as in the troughs of bed-load sheets and bars. Asymmetric, sinuous-crested bed-load sheets ranged from one to two grain diameters up to 12 mm in height and 0.6-1.2 m in length. Migration rates determined from bed-form spacings and periods were typically 1-1.5 mm/s. These bed forms had relatively fine-grained crests and coarse-grained troughs and a well-mixed grain texture along the stoss side. Bed-load sheets had little associated form drag. Low-relief bars were 10-17 mm high and 1.7-2.4 m long, and typical migration rates were 0.4 mm/s. The migration of these bars was associated with fluctuations in the water-surface slope and bed-load transport rate with periods of about two hou s. Low-relief bed forms in heterogeneous sediments appear to be ubiquitous features of lower-stage "plane" beds and those transitional to dune-covered beds where the bed is not completely mobile.

[1] Soil erosion due to head cut development and migration can devastate agricultural lands, yet current prediction technology does not address this important erosion process. Here an analytical model of this erosional phenomenon is presented. Realistic, physically based approximations to the laws governing mass, momentum, and energy transfer in the neighborhood of the scour hole result in closed-form predictive algorithms for the magnitude of the plunge pool erosion and the rate of head cut migration. The model introduces a special treatment of nonventilated overfall conditions, is limited to homogeneous, unbounded soil layers, and is validated by available experimental measurements.