Christopher J. Keylock

Application of statistical and hydraulic-continuum dense-snow avalanche models to five real European sites

We test the behaviour of two statistical avalanche models and three hydraulic-continuum models of varying dimensionality against five reference events. For the hydraulic models, reference friction coefficients are produced that replicate the runout distance of the historical event. The model sensitivity to friction coefficients, release depth, release area and runout distance is also analysed. The hydraulic-continuum models yield similar reference coefficients on the simplest topography, but diverge for more complex paths, highlighting the importance of boundary conditions on model performance. The Coulomb friction (μ) shows a closer relation to runout distance than the turbulent friction (ξ). For models of this type, the debris deposition pattern is useful for selecting model coefficients, rather than relying purely on runout distances. The results of the sensitivity analysis are site-specific, again highlighting the importance of terrain as a model boundary condition. The release area seems to have less of an influence on model results than the fracture height and ξ. In general, the models are most sensitive to μ. At the end of the paper, we propose a scheme for avalanche hazard zoning that integrates the statistical and dynamic models, such that zoning can be undertaken with some confidence in model output.

The North Atlantic Oscillation and snow avalanching in Iceland

[1] The number of snow avalanche cycles occurring in Iceland is correlated to a major source of variability in climate, the North Atlantic Oscillation. A new index for this oscillation is produced that reflects the cumulative effect of snow build-up in a starting zone. An analysis of the time-series for this index shows that recent avalanche tragedies in Iceland are linked to a rise in this index. Consequently, estimations of avalanche return periods and thus, avalanche hazard zones, may need re-evaluation. Because the oscillation has an impact upon climate throughout the Northern Hemisphere, this may be necessary in a number of other countries.

Quadrant/octant sequencing and the role of coherent structures in bed load sediment entrainment

To permit the tracking of turbulent flow structures in an Eulerian frame from single-point measurements, we make use of a generalization of conventional two-dimensional quadrant analysis to three-dimensional octants. We characterize flow structures using the sequences of these octants and show how significance may be attached to particular sequences using statistical mull models. We analyze an example experiment and show how a particular dominant flow structure can be identified from the conditional probability of octant sequences. The frequency of this structure corresponds to the dominant peak in the velocity spectra and exerts a high proportion of the total shear stress. We link this structure explicitly to the propensity for sediment entrainment and show that greater insight into sediment entrainment can be obtained by disaggregating those octants that occur within the identified macroturbulence structure from those that do not. Hence, this work goes beyond critiques of Reynolds stress approaches to bed load entrainment that highlight the importance of outward interactions, to identifying and prioritizing the quadrants/octants that define particular flow structures.

The flow structure in the wake of a fractal fence and the absence of an "inertial regime"

Recent theoretical work has highlighted the importance of multi-scale forcing of the flow for altering the nature of turbulence energy transfer and dissipation. In particular, fractal types of forcing have been studied. This is potentially of real significance in environmental fluid mechanics where multi-scale forcing is perhaps more common than the excitation of a specific mode. In this paper we report the first results studying the detail of the wake structure behind fences in a boundary layer where, for a constant porosity, we vary the average spacing of the struts and also introduce fractal fences. As expected, to first order, and in the far-wake region, in particular, the response of the fences is governed by their porosity. However, we show that there are some significant differences in the detail of the turbulent structure between the fractal and non-fractal fences and that these override differences in porosity. In the near wake, the structure of the fence dominates porosity effects and a modified wake interaction length seems to have potential for collapsing the data. With regards to the intermittency of the velocities, the fractal fences behave more similarly to homogeneous, isotropic turbulence. In addition, there is a high amount of dissipation for the fractal fences over scales that, based on the energy spectrum, should be dominated by inter-scale transfers. This latter result is consistent with numerical simulations of flow forced at multiple scales and shows that what appears to be an “inertial regime” cannot be as production and dissipation are both high.

FMCW radar imaging of avalanche-like snow movements

High quality field measurements of avalanche flows are required for calibrating computational models which are an essential tool in managing the threat posed by these flows. In this paper we present a new C-band FMCW radar system developed at University College London for gathering highresolution avalanche flow data. The radar employs a full deramp hardware architecture, a diverse set of frequency ramps, and an 8-channel receiver array. We also show initial results of a small-scale field trial carried out using a single channel prototype radar deployed in a snow chute. The results are presented as range-time plots. A simple calculation of the expected flow velocity due to gravity agrees with the estimated experimental value. The results demonstrate the capability of the radar system to record high range resolution microwave images of snow movements. The experiments reported here were carried out as a precursor to full trials of the radar system during which images of full scale avalanche flows will be captured.

A method for characterising the sensitivity of turbulent flow fields to the structure of inlet turbulence

The specification of inlet conditions for eddy-resolving simulations is an important research question in turbulence research. A large number of schemes have been proposed but comparisons to a benchmark simulation are usually undertaken in such a way that it is ambiguous as to whether the source of the improved preservation of the properties of the benchmark case is due to the physics captured in the inlet generation algorithm or to differences in the precise values or spectra generated at a particular position. This article uses gradual wavelet reconstruction to degrade a precursor simulation such that the time series input at each cell has the same histogram and Fourier spectrum as the original precursor case, thereby removing this source of variability. A parameter, ρ, is used to index the nature of the degraded inlets and as ρ increases from 0 to 1 the cross-correlative and nonlinear properties of the precursor are increasingly preserved too. We compare the results of four large-eddy simulations for the flow over a wall-mounted square rib and show that there are clear differences between these simulations for the flow before and over the rib, and in the far-field. However, in the wake, the intense shearing and mixing means that any differences are harder to attribute to the nature of the inlet. Hence, for flows that are mixed strongly, there is less of a need to preserve the cross-correlative structure of the inlet condition as long as the individual velocity components are modelled with an appropriate set of values and Fourier spectrum. By comparing particular inlet generation algorithms in terms of the value for ρ that they can attain, their suitability for modelling flows of particular configurations may be determined.

Evaluation of Topographic Models of Rockfall Travel Distance for Use in Hazard Applications

Characterizing rockfall travel distances over a broad scale is time-consuming and expensive if a detailed numerical model is used that must account for varying boulder size and shape. In this paper four models are developed that utilize simple terrain parameters in order to predict extreme rockfall runout distances. Of these models, three are statistical and the fourth is a simple dynamics model. The results of our model testing suggest that a statistical model is advantageous if one wishes to determine rockfall hazard over a broad area quickly and effectively. In particular, the runout ratio approach is particularly useful at low exceedance probabilities because of the positively skewed distribution used. However, this model would appear to be more sensitive to the underlying data than other techniques. Consequently, the favored model may well depend upon the amount of data available from a given region.

A classification scheme for turbulence based on the velocity‐intermittency structure with an application to near‐wall flow and with implications for bed load transport

Kolmogorovs classic theory for turbulence assumed an independence between velocity increments and the value for the velocity itself. However, recent work has called this assumption in to question, which has implications for the structure of atmospheric, oceanic and fluvial flows. Here we propose a conceptually simple analytical framework for studying velocity-intermittency coupling that is similar in essence to the popular quadrant analysis method for studying near-wall flows. However, we study the dominant (longitudinal) velocity component along with a measure of the roughness of the signal, given mathematically by its series of Holder exponents. Thus, we permit a possible dependence between velocity and intermittency. We compare boundary layer data obtained in a wind tunnel to turbulent jets and wake flows. These flow classes all have distinct characteristics, which cause them to be readily distinguished using our technique and the results are robust to changes in flow Reynolds numbers. Classification of environmental flows is then possible based on their similarities to the idealized flow classes and we demonstrate this using laboratory data for flow in a parallel-channel confluence. Our results have clear implications for sediment transport in a range of geophysical applications as they suggest that the recently proposed impulse-based methods for studying bed load transport are particularly relevant in domains such as gravel bed river flows where the boundary layer is disrupted and wake interactions predominate.

Flow resistance in natural, turbulent channel flows: The need for a fluvial fluid mechanics

In fluvial environments, feedbacks among flow, bed forms, sediment, and macrophytes result in a complex fluid dynamics. The assumptions underpinning standard tools in hydraulics are commonly violated and alternative approaches must be formulated. I argue that we should question the assumption that classical notions in fluid mechanics provide the foundations for the techniques of the future. Recent work on turbulent dissipation, interscale modulation of the dynamics, intermittency, and the role of complex forcings is discussed. An agenda for future work is proposed that involves improving our characterization of complex forcings and developing better understanding of the behavior of the velocity gradient tensor in complex, fluvial environments. This leads to the formulation of modeling tools relevant to fluvial fluid mechanics, rather than a reliance on methods developed elsewhere. One avenue by which such methods might be developed is suggested based on the stretched spiral vortex as a baseline topology. This would result in a nonequilibrium model for turbulence that has greater potential to capture the dynamics in which we are interested. Although these ideas are raised in the context of a future fluvial fluid mechanics, they are applicable to any situation where turbulent flows are forced in complicated ways.

Gradual wavelet reconstruction of the velocity increments for turbulent wakes

This work explores the properties of the velocity increment distributions for wakes of contrasting local Reynolds number and nature of generation (a cylinder wake and a multiscale-forced case, respectively). It makes use of a technique called gradual wavelet reconstruction (GWR) to generate constrained randomizations of the original data, the nature of which is a function of a parameter, ϑ. This controls the proportion of the energy between the Markov-Einstein length (∼ 0.8 Taylor scales) and integral scale that is fixed in place in the synthetic data. The properties of the increments for these synthetic data are then compared to the original data as a function of ϑ. We write a Fokker-Planck equation for the evolution of the velocity increments as a function of spatial scale, r, and, in line with previous work, expand the drift and diffusion terms in terms up to fourth order in the increments and find no terms are relevant beyond the quadratic terms. Only the linear contribution to the expansion of the drift coefficient is non-zero and it exhibits a consistent scaling with ϑ for different flows above a low threshold. For the diffusion coefficient, we find a local Reynolds number independence in the relation between the constant term and ϑ for the multiscale-forced wakes. This term characterizes small scale structure and can be contrasted with the results for the Kolmogorov capacity of the zero-crossings of the velocity signals, which measures structure over all scales and clearly distinguishes between the types of forcing. Using GWR shows that results for the linear and quadratic terms in the expansion of the diffusion coefficient are significant, providing a new means for identifying intermittency and anomalous scaling in turbulence datasets. All our data showed a similar scaling behavior for these parameters irrespective of forcing type or Reynolds number, indicating a degree of universality to the anomalous scaling of turbulence. Hence, these terms are a useful metric for testing the efficacy of synthetic turbulence generation schemes used in large eddy simulation, and we also discuss the implications of our approach for reduced order modeling of the Navier-Stokes equations.