Abstract. The study of isotopic variation in snowmelt from seasonal snowpacks is useful for understanding snowmelt processes and is important for accurate hydrograph separation of spring runoff. However, the complex and variable nature of processes within a snowpack has precluded a quantitative link between the isotopic composition of the original snow and its melt. This work studies the isotopic composition of new snow and its modification by snow metamorphism and melting. To distinguish individual snowstorms, we applied solutions of rare earth elements to the snow surface between storms. The snowmelt was isotopically less variable than the snowpack, which in turn was less variable than the new snow, reflecting isotopic redistribution during metamorphism and melting.

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Accurate analyses of the hydrogeology of fractured rock require an understanding the flow characteristics of single fractures. It is well known that these flow characteristics are strongly controlled by fracture apertures. Recent investigations on the distribution of apertures in natural fractures suggest that the cubic law can accurately predict the fluid flux through rough‐walled fractures as long as the appropriate average fracture aperture is used. Combining the stochastic cubic law with a simple deformation model results in a nonlinear relationship between fracture hydraulic and mechanical aperture. This relationship is shown to be consistent with published experimental and numerical data above a critical minimum aperture. Below this minimum aperture, the transmissivity of the fracture is approximately constant. Results have implications for the interpretation of laboratory fracture flow data and raise important questions pertaining to the mechanics of fracture deformation below the critical minimum aperture.

Brittle failure limits the compressive strength of rock and ice when rapidly loaded under low to moderate confinement. Higher confinement or slower loading results in ductile failure once the brittle-ductile transition is crossed. Brittle failure begins when primary cracks initiate and slide, creating wing cracks at their tips. Under little to no confinement, wing cracks extend and link together, splitting the material into slender columns which then fail. Under low to moderate confinement, wing crack growth is restricted and terminal failure is controlled by the localization of damage along a narrow band. Early investigations proposed that localization results from either the linkage of wing cracks or the buckling of microcolumns created between adjacent wing cracks. Observations of compressive failure in ice suggest a mechanism whereby localization initiates owing to the bending-induced failure of slender microcolumns created between sets of secondary cracks emanating from one side of a primary crack. Here we analyse this mechanism, and show that it leads to a closed-form, quantitative model that depends only on independently measurable mechanical parameters. Our model predictions for both the brittle compressive strength and the brittle-ductile transition are consistent with data from a variety of crystalline materials, offering quantitative evidence for universal processes in brittle failure and for the broad applicability of the model.

A physically based model for the evolution of a single set of planar, parallel fractures subject to a constant remote stress is presented. The model simulates the mechanical interaction between fractures using a recently developed approximation technique for stress analysis in elastic solids with many fractures. A comparison between experimental and numerical results shows that the model can accurately simulate the development of experimentally generated fracture sets. Once the flaw geometry is specified, only one parameter controls the geometric evolution of the fracture set. This parameter, the velocity exponent, relates fracture propagation velocity to stress concentration at the fracture tip. Monte Carlo sensitivity analyses suggest that this parameter also controls the extent to which fracture growth is concentrated within zones or clusters. Similar analyses suggest that the extent of fracture clustering is less sensitive to the flaw density. Pollard, 1983;Thomas and Pollard, 1993].As the critical role that fractures play in controlling fluid flow has become more apparent, many techniques for simulating the geometry of networks have been developed [e.g., Dershowitz and Einstein, 1988; Dverstorp and Andersson, 1989; La Pointe and Hudson, 1985; Lee et al., 1990; Long and Billaux, 1987]. However, counter to the trend in rock fracture mechanics to base models and interpretations on the physics of rock fracture [DeGraff and Aydin, 1987; Delaney et al., 1986; Olson, 1993; Pollard and Aydin, 1988; Rubin and Pollard, 1987], these techniques rely on descriptive statisticsto stochastically generate realizations of fracture networks which are statistically similar to those observed in nature. This approach can be problematic in that many important geometric aspects of natural fracture networks may not be adequately described by the statistics used to generate the stochastic networks. For example, it has been found that flow through a natural fracture network was consistently greater than flow through statistically similar, stochastically generated networks [Odling and Webman, 1991]. The reason for this discrepancy was attributed to the failure of the stochastic While stochastic simulators can be improved by choosing a better set of statistics to describe natural fracture networks, an alternative to the stochastic approach is to attempt to include the physics of fracture formation explicitly within the fracture network simulator. In so doing, the fracture simulator is improved in the two following ways: (1) the number of possible realizations of the network is restricted to those which are physically reasonable; and (2) information about the geologic history of the region, such as boundary conditions and material properties, can be directly put into the model to further restrict the number of possible realizations. Physically based network simulators have the additional advantage that they can be used to understand how fracture network geometries are related to the conditions of their formation.As a first step toward a ...