Origin of Hysteresis in Bed Form Response to Unsteady Flows

Field and laboratory studies indicate that changes in riverbed morphology often lag changes in water discharge. This lagged response produces hysteresis in the relationship between water discharge and bed form geometry. To understand these phenomena, we performed flume experiments to observe the response of a sand bed to step increases and decreases in water discharge. For an abrupt rise in discharge, we observed that bed forms grew rapidly by collision and merger of bed forms migrating with different celerities. Growth rate slowed as bed forms approached equilibrium with the higher discharge regime. After an abrupt discharge drop, bed form decay occurred through formation of smaller secondary bed forms, in equilibrium with the lower discharge, which cannibalized the original, relict features. We present a simple model framework to quantitatively predict time scales of bed form adjustment to flow changes, based on equilibrium bed form heights, lengths, and celerities at low and high flows. For rising discharge, the model assumes that all bed form collisions result in irreversible merger, due to a dispersion of initial celerities. For falling discharge, we derive a diffusion model for the decay of relict high-stage features. Our models predict the form and time scale of experimental bed form adjustments. Additional experiments applying slow and fast triangular flood waves show that bed form hysteresis occurs only when the time scale of flow change is faster than the modeled (and measured) bed form adjustment time. We show that our predicted adjustment time scales can also be used to predict the occurrence of bed form hysteresis in natural floods

The Physical Basis for Anomalous Diffusion in Bed Load Transport

Recent studies have observed deviation from normal (Fickian) diffusion in sediment tracer dispersion that violates the assumption of statistical convergence to a Gaussian. Nikora et al. (2002) hypothesized that particle motion at short time scales is superdiffusive because of inertia, while long-time subdiffusion results from heavy-tailed rest durations between particle motions. Here we test this hypothesis with laboratory experiments that trace the motion of individual gravels under near-threshold intermittent bed load transport (0.027 \u3c τ* \u3c 0.087). Particle behavior consists of two independent states: a mobile phase, in which indeed we find superdiffusive behavior, and an immobile phase, in which gravels distrained from the fluid remain stationary for long durations. Correlated grain motion can account for some but not all of the superdiffusive behavior for the mobile phase; invoking heterogeneity of grain size provides a plausible explanation for the rest. Grains that become immobile appear to stay at rest until the bed scours down to an elevation that exposes them to the flow. The return time distribution for bed scour is similar to the distribution of rest durations, and both have power law tails. Results provide a physical basis for scaling regimes of anomalous dispersion and the time scales that separate these regimes

Sorting Out Abrasion in a Gypsum Dune Field

Grain size distributions in eolian settings are the result of both sorting and abrasion of grains by saltation. The two are tightly coupled because mobility of particles determines abrasion rate, while abrasion affects the mobility of particles by changing their mass and shape; few field studies have examined this quantitatively. We measured grain size and shape over a 9 km transect downwind of a line sediment source at White Sands National Monument, a gypsum dune field. The sediment source is composed of rodlike (elongate), coarse particles whose shapes appear to reflect the crystalline structure of gypsum. Dispersion in grain size decreases rapidly from the source. Coarse particles gradually become less elongate, while an enrichment of smaller, more elongate grains is observed along the transect. Transport calculations confirm that White Sands is a threshold sand sea in which (1) the predominant particle diameter reflects grains transported in saltation under the dune-forming wind velocity and (2) smaller, elongate grains move in suspension under this dominant wind. Size-selective transport explains first-order trends in grain size; however, abrasion changes the shape of saltating grains and produces elongate, smaller grains that are spallation and breaking products of larger particles. Both shape and size changes saturate 5–6 km downwind of the source. As large particles become more equant, abrasion rates slow down because protruding regions have been removed. Such asymptotic behavior of shape and abrasion rate has been observed in theory and experiment and is likely a generic result of the abrasion process in any environment

Impulse Framework for Unsteady Flows Reveals Superdiffusive Bed Load Transport

Sediment transport is an intrinsically stochastic process, and measurement of bed load in the environment is further complicated by the unsteady nature of river flooding. Here we present a methodology for analyzing sediment tracer data with unsteady forcing. We define a dimensionless impulse by integrating the cumulative excess shear velocity for the duration of measurement, normalized by grain size. We analyze the dispersion of a plume of cobble tracers in a very flashy stream over two years. The mean and variance of transport distance collapse onto well-defined linear and power-law relations, respectively, when plotted against cumulative dimensionless impulse. Data suggest that the asymptotic limit of bed load tracer dispersion is superdiffusive, in line with a broad class of geophysical flows exhibiting strong directional asymmetry (advection), thin-tailed step lengths and heavy-tailed waiting times. The impulse framework justifies the use of quasi-steady flow approximations for long-term river evolution modeling

Sedimentary Bed Evolution as a Mean-Reverting Random Walk: Implications for Tracer Statistics

Sediment tracers are increasingly employed to estimate bed load transport and landscape evolution rates. Tracer trajectories are dominated by periods of immobility (“waiting times”) as they are buried and reexcavated in the stochastically evolving river bed. Here we model bed evolution as a random walk with mean-reverting tendency (Ornstein-Uhlenbeck process) originating from the restoring effect of erosion and deposition. The Ornstein-Uhlenbeck model contains two parameters, a and b, related to the particle feed rate and range of bed elevation fluctuations, respectively. Observations of bed evolution in flume experiments agree with model predictions; in particular, the model reproduces the asymptotic t−1 tail in the tracer waiting time exceedance probability distribution. This waiting time distribution is similar to that inferred for tracers in natural gravel streams and avalanching rice piles, indicating applicability of the Ornstein-Uhlenbeck mean-reverting model to many disordered transport systems with tracer burial and excavation

Timescale Dependence of Aeolian Sand Flux Observations Under Atmospheric Turbulence

The transport of sand in saltation is driven by the persistently unsteady stresses exerted by turbulent winds. Based on coupled high-frequency observations of wind velocity and sand flux on a desert dune during intermittent saltation, we show here how observations of saltation by natural winds depend significantly on the timescale and method used for determining shear stress and sand flux. The correlation between sand flux and excess shear stress (stress above a threshold value) systematically improves for longer averaging timescale, T, and is better for stress determined by the law-of-the-wall versus the Reynolds stress method. Fitting parameters for the stress-flux relationship do not converge with increasing T, which may be explained by the nonstationary nature of wind velocity statistics. We show how it may be possible, based on the scale-dependent statistics of stress fluctuations, to rescale saltation flux predictions for wind observations made at different timescales. However, our observations indicate hysteresis and time lags in thresholds for initiation and cessation of saltation, which complicate threshold-based approaches to predicting sediment transport at different timescales

Barchan-Parabolic Dune Pattern Transition From Vegetation Stability Threshold

Many dune fields exhibit a downwind transition from forward-pointing barchan dunes to stabilized, backward-pointing parabolic dunes, accompanied by an increase in vegetation. A recent model predicts this pattern transition occurs when dune surface erosion/deposition rates decrease below a threshold of half the vegetation growth rate. We provide a direct test using a unique data set of repeat topographic surveys across White Sands Dune Field and find strong quantitative support for the model threshold. We also show the threshold hypothesis applied to a barchan dune results naturally in its curvature inversion, as the point of threshold crossing progresses from the horns to the crest. This simple, general threshold framework can be an extremely useful tool for predicting the response of dune landscapes to changes in wind speed, sediment supply, or vegetation growth rate. Near the threshold, a small environmental change could result in a drastic change in dune pattern and activity

Fractal Patterns in Riverbed Morphology Produce Fractal Scaling of Water Storage Times

River topography is famously fractal, and the fractality of the sediment bed surface can produce scaling in solute residence time distributions. Empirical evidence showing the relationship between fractal bed topography and scaling of hyporheic travel times is still lacking. We performed experiments to make high-resolution observations of streambed topography and solute transport over naturally formed sand bedforms in a large laboratory flume. We analyzed the results using both numerical and theoretical models. We found that fractal properties of the bed topography do indeed affect solute residence time distributions. Overall, our experimental, numerical, and theoretical results provide evidence for a coupling between the sand-bed topography and the anomalous transport scaling in rivers. Larger bedforms induced greater hyporheic exchange and faster pore water turnover relative to smaller bedforms, suggesting that the structure of legacy morphology may be more important to solute and contaminant transport in streams and rivers than previously recognized

Concussion-associated gene variants and history of concussion in elite male rugby athletes

Occurrence of and outcomes following a concussion are probably affected by the interaction of multiple genes in a polygenic manner [1,2]. This study investigated whether suspected concussion-associated polygenic profiles of elite rugby athletes with a history of previous concussion (RAC) differed from rugby athletes with no history of previous concussion (RANC). We hypothesised that concussion-associated risk genotypes would be underrepresented in RANC compared to RAC. Participants were from the RugbyGene project, comprising elite male rugby athletes (RA) (185 white males; mean (standard deviation) height 1.86 (0.07) m, mass 102 (12.6) kg, age 26.4 (5.1) yr) competing at an elite level in rugby union (n = 165) and league (n = 20) in the UK, Ireland, Italy and South Africa. Concussion history was collected using a self-reported concussion history questionnaire. PCR of genomic DNA was used to determine genotypes using TaqMan probes, and total genotype scores (TGS) were calculated, then groups were compared using χ2 and odds ratio (OR) statistics. In addition, multifactor dimensionality reduction (MDR) was used to identify genetic interactions. Seventy-eight percent of RA reported a history of sustaining at least one concussion and 54% of RA reported sustaining multiple (≥2) concussions from rugby. For BDNF-AS rs6265, the GG genotype was more common in RAC compared to RANC (69.7% vs 61.0%, P = 0.006, OR = 9.90, 95% CI = 01.81-54.06) (Fig. 1). The GG genotype of BDNF-AS rs6265 was more common in RAC compared to RANC (70.7% vs. 61.0%, P = 0.041, OR 4.44, 95% CI = 1.04-120.97) (Fig. 1). However, TGS did not differ between RANC and RAC (Fig. 2A) recovery duration and family history of neurological conditions (P > 0.05). Receiver operating characteristic curve (ROC) and area under the curve (AUC) analysis confirmed the TGS algorithm could not identify concussion history (AUC = 0.436; 95% CI = 0.338-0.534; P = 0.218; Fig. 2B). MDR could not identify a model to predict concussion history, recovery duration and family history of neurological conditions with a sufficiently powerful cross-validation statistic (P ≤ 0.05). These findings support the growing evidence that incidence and recovery from concussion could be influenced by an athlete’s genetic predisposition. Such knowledge could be used in the future and when additional relevant variants have been identified, to inform individualised management strategies for athletes in possession of risk genotypes.Peer reviewe

Tendon and ligament-associated gene variants and history of soft tissue injury in elite male rugby athletes

There is a genetic component to tendon and ligament injuries which is highly likely to be polygenic in nature (1). Elite rugby has one of the highest reported injury incidences of any professional sport with some of the most severe injuries affecting tendons and ligaments (1). Thus, this study investigated if suspected tendon and ligament injury-associated polygenic profiles of elite rugby athletes (RA) with a history of prior tendon and ligament injury differed from RA with no history of injury. We hypothesised that tendon and ligament injury-associated genotypes and polygenic profiles would be overrepresented in RA with a history of soft tissue injury compared to RA with no history of injury. Participants were from the RugbyGene project, comprising elite male RA (185 white males; mean (standard deviation) height 1.86 (0.07) m, mass 102 (12.6) kg, age 26.4 (5.1) yr) competing at an elite level in rugby union (n = 165) and league (n = 20) in the UK, Ireland, Italy and South Africa. Soft-tissue injury history was collected using a self-reported injury history questionnaire. PCR of genomic DNA was used to determine genotypes using TaqMan probes, and total genotype scores (TGS) from 13 polymorphisms were calculated, then groups were compared using χ2 and odds ratio (OR) statistics. In addition, multifactor dimensionality reduction (MDR) and inferred haplotype analysis were used to identify genetic interactions. For MMP3 rs679620, the C allele was more common in the tendinopathy group (TD) compared to the non-injured tendon group (NIT) (63.5% vs 50.0%, P = 0.02, OR = 1.62, 95% CI = 01.00-2.60). However, the C allele was more common in the non-injured ligament group (NIL) compared to the ligament rupture (LR) group (63.7% v 47.9%, P = 0.02, OR = 1.91, 95% CI = 1.09-3.35). For COL5A1 rs12722 the TT genotype was more common in NIT compared to the tendon rupture group (TR) (25.0% vs. 3.8%, P = 0.006, OR 4.35, 95% CI = 0.49-37.01). TGS differed between NIL and the ligament sprain group (LS) (U=1868.50;P = 0.02). Receiver operating characteristic curve (ROC) and area under the curve (AUC) analysis confirmed the TGS algorithm could identify LS (AUC = 0.61; 95% CI = 0.52-0.72; P = 0.02) . The T-C inferred haplotype frequency of COL5A1 rs12722 and COL5A1 rs3196378 respectively, was higher in TR, LS and the all-injured athlete groups compared to NIT, NIL and the all-non-injured group (P < 0.01) (Fig. 3). MDR could not identify a model to predict any of the injury groups with a sufficiently powerful cross-validation statistic. The current data suggests musculoskeletal soft-tissue injury could be influenced by an athlete’s genetic predisposition. This study provides further insight into the detailed aetiology of musculoskeletal soft tissue injuries within elite rugby and may, in future, be worthy of consideration for managing the interindividual variability of injury risk in rugbyPeer reviewe