Analysis of Wedge Stability Using Different Methods

Summary The stability problem of a rock slope containing a wedge resting on two intersecting discontinuities is of great interest in rock slope engineering. It is a statistically indeterminate problem with two extra unknowns according to the force (stress) equilibrium analysis. The widely used limit equilibrium methods in practice assume that the directions of the shear forces acting on the two discontinuities are parallel to their line of intersection. The validity of this assumption, however, has not been verified theoretically. This paper presents a general limit equilibrium method that determines the directions of the shear forces by using Pans Maximum principle and an upper bound method that applies the classic upper bound theorem of limit analysis to avoid making extra assumptions. The formulations of the two methods are derived. A non-symmetric wedge and a symmetric wedge are analyzed using the two derived methods. To further explore the influence on stability due to the direction of the shear force acting on the two discontinuities, three-dimensional finite-element analyses are also conducted. The results are compared and discussed.     

Fine-Grained Aluminosiliciclastic Rocks of the Middle Riphean Stratotype Section in the Southern Urals: Formation Conditions,Composition and Provenance Evolution

General trends of the formation of Middle Riphean fine-grained aluminosiliciclastic rocks in the Bashkir Meganticlinorium are considered. It is shown that Yurmatinian shales do not contain any significant pyroclastic admixture. Judging from the relatively constant Th/Cr ratio throughout the Yurmatinian section, the tectonic regime in the study territory during the early Middle Riphean is suggested to be rather stable. The main paleoclimatic indices and indicators of the pelitic material maturity (CIA, CIW, IVC, PIA, and Ce/Y) suggest that paleodrainage systems in the early Middle Riphean were dominated by humid climate that gave way to the arid or semiarid type in the middle Yurmatinian. The low Mo/Mn ratio and some other indicators of redox conditions in shales from all Yurmatinian lithostratigraphic units show that no explicit reducing conditions existed in the basin during the early Middle Riphean. The shales were characterized by the increase in K₂O/Al₂O₃ ratio, gradual enrichment in REE, and growth of LREE/HREE and La_N/Yb_N ratios toward the middle Yurmatinian, indicating the gain of an appreciable amount of slightly weathered arkosic aluminosiliciclastic material in the sedimentary basin about 1220–1200 Ma ago. The REE distribution and the UCC- and AUC-normalized shale compositions suggest that the eroded upper crust was compositionally close to the UCC. The occurrence of mafic and ultramafic rocks is also inferred. Data points of Yurmatinian shales plotted in the Cr–Ni, Eu/Eu*–Gd_N/Yb_N, and (La/YB)_N–Yb_N diagrams are localized between the fields of Upper Archean and post-Archean rocks or within the latter field. Hence, post-Archean igneous and metamorphic complexes prevailed in paleodrainage systems of the early Middle Riphean. This is also confirmed by the model Nd ages.     

Flow splitting at the bifurcation between two valleys: idealized simulations in comparison with Mesoscale Alpine Programme observations

Summary This paper investigates the characteristics of channelled airflow in the vicinity of a junction of three idealized valleys (one valley carrying the incoming flow and two tributaries carrying the outflow), using a two-dimensional single-layer shallow water model. Particular attention is given to the flow splitting occurring at the junction. Nondimensionalized, the model depends on the valley geometry, the Reynolds number, which is related to the eddy viscosity, and on the difference of the hydrostatic pressure imposed at the exit of the tributaries. At the spatial scale considered in this study, the Rossby number relating the inertial and Coriolis forces is always larger than 1, implying that the effect of earth rotation can be neglected to a first approximation. The analysis of the flow structure within the three valleys as well as the calculation of the split ratio (fraction of the air flow diverted into one of the two downstream valleys with respect to the total mass flux in the upstream valley) show that (i) the flow pattern depends strongly on the Reynolds number while the split ratio is comparatively insensitive; (ii) the valley geometry and the difference between the upstream and downstream hydrostatic pressures affect the flow pattern, the location of the split point and the split ratio; (iii) the relative contribution of flow deflection by the sidewalls and the blocking/splitting mechanism differs between the settings of a “Y-shape” valley and a “T-shape” valley. Quantitative comparison of the present results with numerical simulations of realistic cases and with observations collected in the region of the Rhine and Seez valleys (Switzerland) (“Y-shape” valley) and in the region of the Inn and Wipp valleys (Austria) (“T-shape” valley) during the Mesoscale Alpine Programme (MAP) field experiment shows good agreement provided that the normalized valley depth NΔH/U_u significantly exceeds 1, i.e., when “flow around” is expected. A structural disagreement between the idealized simulations and the observed wind field is found only when NΔH/U_u ≃ 1, that is, in the “flow over” regime. This shows that the dimensionless valley depth is indeed a good indicator for flow splitting, implying that the stratification is a key player in reality.     

Variation in tree mortality and regeneration affect forest carbon recovery following fuel treatments and wildfire in the Lake Tahoe Basin, California, USA

Background

Forest fuel treatments have been proposed as tools to stabilize carbon stocks in fire-prone forests in the Western U.S.A. Although fuel treatments such as thinning and burning are known to immediately reduce forest carbon stocks, there are suggestions that these losses may be paid back over the long-term if treatments sufficiently reduce future wildfire severity, or prevent deforestation. Although fire severity and post-fire tree regeneration have been indicated as important influences on long-term carbon dynamics, it remains unclear how natural variability in these processes might affect the ability of fuel treatments to protect forest carbon resources. We surveyed a wildfire where fuel treatments were put in place before fire and estimated the short-term impact of treatment and wildfire on aboveground carbon stocks at our study site. We then used a common vegetation growth simulator in conjunction with sensitivity analysis techniques to assess how predicted timescales of carbon recovery after fire are sensitive to variation in rates of fire-related tree mortality, and post-fire tree regeneration.

Results

We found that fuel reduction treatments were successful at ameliorating fire severity at our study site by removing an estimated 36% of aboveground biomass. Treated and untreated stands stored similar amounts of carbon three years after wildfire, but differences in fire severity were such that untreated stands maintained only 7% of aboveground carbon as live trees, versus 51% in treated stands. Over the long-term, our simulations suggest that treated stands in our study area will recover baseline carbon storage 10?C35?years more quickly than untreated stands. Our sensitivity analysis found that rates of fire-related tree mortality strongly influence estimates of post-fire carbon recovery. Rates of regeneration were less influential on recovery timing, except when fire severity was high.

Conclusions

Our ability to predict the response of forest carbon resources to anthropogenic and natural disturbances requires models that incorporate uncertainty in processes important to long-term forest carbon dynamics. To the extent that fuel treatments are able to ameliorate tree mortality rates or prevent deforestation resulting from wildfire, our results suggest that treatments may be a viable strategy to stabilize existing forest carbon stocks.     

Numerical Study of the Response of an Atmospheric Surface Layer to a Spatially Nonuniform Plant Canopy

High-accuracy large-eddy simulations of neutral atmospheric surface-layer flow over a gapped plant canopy strip have been performed. Subgrid-scale (SGS) motions are parameterized by the Sagaut mixed length SGS model, with a modification to compute the SGS characteristic length self-adaptively. Shaw’s plant canopy model, taking the vertical variation of leaf area density into account, is applied to study the response of the atmospheric surface layer to the gapped dense forest strip. Differences in the region far away from the gap and in the middle of the gap are investigated, according to the instantaneous velocity magnitude, the zero-plane displacement, the potential temperature and the streamlines. The large-scale vortex structure, in the form of a roll vortex, is revealed in the region far away from the gap. The nonuniform spatial distribution of plants appears to cause the formation of the coherent structure. The roll vortex starts in the wake of the canopy, and results in strong fluctuations throughout the entire canopy region. Wind sweeps and ejections in the plant canopy are also attributed to the large vortex structure.     

Joint PDF Modelling of Turbulent Flow and Dispersion in an Urban Street Canyon

The joint probability density function (PDF) of turbulent velocity and concentration of a passive scalar in an urban street canyon is computed using a newly developed particle-in-cell Monte Carlo method. Compared to moment closures, the PDF methodology provides the full one-point one-time PDF of the underlying fields containing all higher moments and correlations. The small-scale mixing of the scalar released from a concentrated source at the street level is modelled by the interaction by exchange with the conditional mean (IECM) model, with a micro-mixing time scale designed for geometrically complex settings. The boundary layer along no-slip walls (building sides and tops) is fully resolved using an elliptic relaxation technique, which captures the high anisotropy and inhomogeneity of the Reynolds stress tensor in these regions. A less computationally intensive technique based on wall functions to represent the boundary layers and its effect on the solution are also explored. The calculated statistics are compared to experimental data and large-eddy simulation. The present work can be considered as the first example of computation of the full joint PDF of velocity and a transported passive scalar in an urban setting. The methodology proves successful in providing high level statistical information on the turbulence and pollutant concentration fields in complex urban scenarios.     

Cosmic‐ray rejection for single spectroscopic CCD images by means of template matching

This article describes a novel algorithm for cosmic‐ray rejection in single spectroscopic CCD images. This algorithm is based on a variation of template matching. It focuses on identifying those pixels belong to spectra, while other conventional algorithms tried to locate the cosmic‐ray hits directly. The main principle is applying template matching to find suspicious blocks, which is followed by surface patching to locate the legitimate pixels accurately. Therefore, the rest pixels are the ones corrupted by cosmic‐ray hits. Meanwhile, almost all the parameters are automatically extracted from the images. Examples of its performance are given for both simulated and observed images. It shows an advantage of significantly low false alarm rate with relatively high detection rate (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)