High-resolution radar measurements of snow avalanches

Two snow avalanches that occurred in the winter 2010-2011 at Vallée de la Sionne, Switzerland, are studied using a new phased array FMCW radar system with unprecedented spatial resolution. The 5.3 GHz radar penetrates through the powder cloud and reflects off the underlying denser core. Data are recorded at 50 Hz and have a range resolution better than 1 m over the entire avalanche track. We are able to demonstrate good agreement between the radar results and existing measurement systems that record at particular points on the avalanche track. The radar data reveal a wealth of structure in the avalanche and allow the tracking of individual fronts and surges down the slope for the first time. Key Points Validation between our radar results and existing point measurement systems High-resolution radar allows tracking of fronts and surges from start to finish Velocity linked with topography may be used to measure rheology of snow ©2013. American Geophysical Union. All Rights Reserved

Looking inside an avalanche using a novel radar system

Snow avalanches are a significant natural hazard in alpine regions and their flow dynamics have similarities to pyroclastic flows and other geological mass movements. However, the potential for artificial release and the temporary nature of their deposits makes them somewhat easier to study. This article explains recent developments in radar technology for imaging these flows. These new data mean that, for the first time, we are seeing the whole flow averaged over spatial scales that are dynamically relevant. This provides an opportunity to properly test existing models for the dynamics used in risk applications and to gain knowledge of the flow physics, which will guide the next generation of model formulations

The importance of non-normal contributions to velocity gradient tensor dynamics for spatially developing, inhomogeneous, turbulent flows

We investigate the properties of the velocity gradient tensor for spatially evolving turbulent flows (a near-wake, two axisymmetric jets and a planar mixing layer). Emphasis is placed on the study of the normal and non-normal parts of the tensor. Non-normality plays a greater role in the dynamics than is observed for HIT and does so for all spatial locations examined. This implies a greater role for the deviatoric part of the pressure Hessian. Results for the wake flow, where we isolate the coherent part of the dynamics using a modal decomposition, clarify how these competing effects operate. Previous studies have shown the shape of the Q–R diagram (formed by the second and third invariants of the characteristic equation for the tensor) is approximately universal at small-scales for different flows. The non-normal dynamics are neglected in the Q–R approach but appear to differ significantly between flows

Robust classification for the joint velocity-intermittency structure of turbulent flow over fixed and mobile bedforms

Two datasets of turbulence velocities collected over different bedform types under contrasting experimental conditions show similarity in terms of velocity-intermittency characteristics and suggest a universality to the velocity-intermittency structure for flow over bedforms. One dataset was obtained by sampling flow over static bedforms in different locations, and the other was based on a static position but mobile bedforms. A flow classification based on the velocity-intermittency behaviour is shown to reveal some differences from that based on an analysis of Reynolds stresses, boundary layer correlation and turbulent kinetic energy. This may be attributed to the intermittency variable, which captures the local effect of individual turbulent flow structures. Locations in the wake region or the outer layer of the flow are both shown to have a velocity-intermittency behaviour that departs from that for idealized wakes or outer layer flow because of the superposition of localized flow structures generated by bedforms. The combined effect of this yields a velocity-intermittency structure unique to bedform flow. The use of a time series of a single velocity component highlights the potential power of our approach for field, numerical and laboratory studies. The further validation of the velocity-intermittency method for non-idealized flows undertaken here suggests that this technique can be used for flow classification purposes in geomorphology, hydraulics, meteorology and environmental fluid mechanics. © 2014 The Authors

FMCW phased array radar for automatically triggered measurements of snow avalanches

Radar has emerged as a means of measuring avalanche dynamics for validating models to predict avalanche behaviour for improved calculation of risk zones. However, current radar measurements do not provide a true representation of an entire avalanche flow. An FMCW, multi-chirp, 8 receiver channel phased array radar has been developed in attempt to provide 2-D field measurements in unparalleled detail. In this paper, we summarise the design of the radar and discuss further additions to the radar design since a previous report. The radar is deployed in a bunker at well-equipped avalanche test site where it has been automatically recording naturally occurring avalanches. Several avalanches have been recorded including a large wet snow avalanche which occurred on the 7th December 2010 has already been recorded. We present an encouraging MTI image of this avalanche from one of the receiver channels which reveals a wealth of information embedded within the data. In the future we hope to cohere the 8 channels of data to provide cross-range information, and also resolve the range and Doppler of various components of the avalanche

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

Integrated Assessment of the Anthropic Pressure Level on Natural Water Bodies: The Case Study of the Noce River (Basilicata, Italy)

Fragmentation is a phenomenon that involves the transformation of large patches of natural habitats into smaller ones (fragments) that tend to be isolated from the originals. In this case, the degree of environmental fragmentation of the Noce River in the Basilicata region (Italy) will be analysed. Following the installation of hydroelectric plants, the river has undergone such alterations that it has been classified as a Heavily Modified Water Body (HMWB). Environmental fragmentation is caused not only by soil sealing, which causes the loss and subsequent fragmentation of natural patches, but can also be caused by major changes in natural patches. In the case of a territory crossed by a watercourse, these patches may be subject to changes in the natural course of the river or in the vegetation present close to it. The aim of this work is to calculate, through GIS applications, the level of fragmentation of the adjacent area surrounding the water body along which there are several hydroelectric plants. Through a change detection in 2006, 2013 and 2018, metric and biodiversity indicators will be calculated to define the level of anthropic pressure of the water body. The results reveal that the variation of the calculated indices, both for landscape metrics and diversity indices, concerned "natural" land use classes, whose variation caused fragmentation of natural patches by changing the shape of the water body