Anisotropic stress accumulation in cooling lava flows and resulting fracture patterns: Insights from starch-water desiccation experiments

Desiccation of starch-water slurries is a useful analog for the production of polygonal fractures/columnar joints in cooling lava flows. When left to dry completely, a simple mixture of 1:1 starch and water will produce columns that appear remarkably similar to natural columnar joints formed in cooled lava flows. Columns form when the accumulation of isotropic stress exceeds the tensile strength of a material, at which point a fracture forms and advances through the material perpendicular to the desiccating surface. Individual fractures will initially form orthogonal to the desiccation surface but will quickly evolve into a hexagonal fracture network that advances incrementally through the material. However, some fracture patterns found within natural lava flows are not hexagonal (Lodge and Lescinsky, 2009-this issue), but rather have fracture lengths that are much longer than the distance to adjacent fractures. These fractures are commonly found at lava flows that have interacted with glacial ice during emplacement. The purpose of this study is to utilize starch analog experiments to better understand the formation of these fractures and the stress regimes responsible for their non-hexagonal patterns.To simulate anisotropic conditions during cooling, the starch slurry was poured into a container with a movable wall that was attached to a screw-type jack. The jack was then set to slowly extend or retract while the slurry desiccated. This resulted in either a decrease or increase in the chamber cross-sectional area thus creating compressional or extensional regimes. Decreasing chamber area (DCA) experiments resulted in fractures with larger lengths parallel to the direction of wall movement (also direction of compression). It also caused localized thrust faulting and curved column development. Increasing chamber area (ICA) experiments produced a zone of horizontal column development along the expanding margin (produced when the wall detached from the sample). Within this zone vertical fracture traces were observed that extended beyond individual columns.The viscoelastic rheology of both starch-water slurries and cooling lava flows aid in the production of these long and continuous fractures. During desiccation/cooling, the total strain in the material is divided into elastic strain (stress accumulating) and viscous strain (stress relaxing). During isotropic conditions, the viscous component is also isotropic therefore stress is relaxed equally in all directions. However, if there is an existing viscous strain, such as in the DCA and ICA experiments, stress can be preferentially relaxed in a single direction resulting in fracture development with preferred orientations.     

Structures and facies associated with the flow of subaerial basaltic lava into a deep freshwater lake: The Sulphur Creek lava flow, North Cascades, Washington

The ca. 8800 ¹⁴C yrs BP Sulphur Creek lava flowed eastward 12 km from the Schriebers Meadow cinder cone into the Baker River valley, on the southeast flank of Mount Baker volcano. The compositionally-zoned basaltic to basaltic andesite lava entered, crossed and partially filled the 2-km-wide and > 100-m-deep early Holocene remnant of Glacial Lake Baker. The valley is now submerged beneath a reservoir, but seasonal drawdown permits study of the distal entrant lava. As a lava volume that may have been as much as 180 × 10⁶ m³ entered the lake, the flow invaded the lacustrine sequence and extended to the opposite (east) side of the drowned Baker River valley. The volume and mobility of the lava can be attributed to a high flux rate, a prolonged eruption, or both. Basalt exposed below the former level of the remnant glacial lake is glassy or microcrystalline and sparsely vesicular, with pervasive hackly or blocky fractures. Together with pseudopillow fractures, these features reflect fracturing normal to penetrative thermal fronts and quenching by water. A fine-grained hyaloclastite facies was probably formed during quench fragmentation or isolated magma-water explosions. Although the structures closely resemble those developed in lava-ice contact environments, establishing the depositional environment for lava exhibiting similar intense fracturing should be confirmed by geologic evidence rather than by internal structure alone. The lava also invaded the lacustrine sequence, forming varieties of peperite, including sills that are conformable within the invaded strata and resemble volcaniclastic breccias. The peperite is generally fragmental and clast- or matrix-supported; fine-grained and rounded fluidal margins occur locally. The lava formed a thickened subaqueous plug that, as the lake drained in the mid-Holocene, was exposed to erosion. The Baker River then cut a 52-m-deep gorge through the shattered, highly erodible basalt.     

Interpretation of porosity in dacite lava domes as ductile–brittle failure textures

Pores in dacite lavas of three volcanoes in Japan (Unzen, Yakedake and Daisen) have characteristics unlike vesicles. Most of the pores have ragged walls marked by the protrusion of groundmass crystals and phenocrysts. The shapes of these pores range from irregular to planar. Many pores occur in ‘pressure shadows’ adjacent to phenocrysts. Some pores are crack-like and pass around phenocrysts and have blunt terminations. Most of the pores are interpreted as the result of failure of the magma by flow during ductile–brittle transition. This phenomenon, known as cavitation, is well established in hydraulics and the material sciences. Cavitation of the lavas in this study was promoted by steep slope of emplacement, subtle mechanical contrasts between bands, and strain intensification around rigid crystals and lithic fragments. Cavitation in lava can extract volatiles from magma causing premature embrittlement and weakening. Cavitation is inferred to be critical to the initiation of lava-front pyroclastic flows.     

Morphology and composition of spinel in Pu’u ’O’o lava (1996–1998), Kilauea volcano, Hawaii

The morphology and composition of spinel in rapidly quenched Pu’u ’O’o vent and lava tube samples are described. These samples contain glass, olivine phenocrysts (3–5 vol.%) and microphenocrysts of spinel (0.05 vol.%). The spinel surrounded by glass occurs as idiomorphic octahedra 5–50 μm in diameter and as chains of octahedra that are oriented with respect to each other. Spinel enclosed by olivine phenocrysts is sometimes rounded and does not generally form chains. The temperature before quenching was calculated from the MgO content of the glass and ranges from 1150°C to 1180°C. The oxygen fugacity before quenching was calculated by two independent methods and the log f_O2 ranged from −9.2 to −9.9 (delta QFM=−1). The spinel in the Pu’u ’O’o samples has a narrow range in composition with Cr/(Cr+Al)=0.61 to 0.73 and Fe²⁺/(Fe²⁺+Mg)=0.46 to 0.56. The lower the calculated temperature for the samples, the higher the average Fe²⁺/(Fe²⁺+Mg), Fe³⁺ and Ti in the spinel. Most zoned spinel crystals decrease in Cr/(Cr+Al) from core to rim and, in the chains, the Cr/(Cr+Al) is greater in the core of larger crystals than in the core of smaller crystals. The occurrence of chains and hopper crystals and the presence of Cr/(Cr+Al) zoning from core to rim of the spinel suggest diffusion-controlled growth of the crystals. Some of the spinel crystals may have grown rapidly under the turbulent conditions of the summit reservoir and in the flowing lava, and the crystals may have remained in suspension for a considerable period. The rapid growth may have caused very local (μm) gradients of Cr in the melt ahead of the spinel crystal faces. The crystals seem to have retained the Cr/(Cr+Al) ratio that developed during the original growth of the crystal, but the Fe²⁺/(Fe²⁺+Mg) ratio may have equilibrated fairly rapidly with the changing melt composition due to olivine crystallization. Six of the samples were collected on the same day at various locations along a 10-km lava tube and the calculated pre-collection temperatures of the samples show a 5°C drop with distance from the vent. The average Fe²⁺/(Fe²⁺+Mg) of the spinel in these samples shows a weak positive correlation with decreasing MgO in the glass of these samples. The range in Cr₂O₃ (0.041–0.045 wt.%) of the glass for these six samples is too small to distinguish a consistent change along the lava tube. The spinel in the Pu’u ’O’o samples shows a zoning trend in a Cr–Al–Fe³⁺ diagram almost directly away from the Cr apex. This compares with a zoning trend in rapidly quenched MORB samples away from Cr coupled with decreasing Fe³⁺. The trend away from Cr displayed by spinel in rapidly quenched samples is in marked contrast to the trend of increasing Fe³⁺ shown by spinel in slowly cooled lava.     

Comparison of two modeling approaches for groundwater–surface water interactions

An assessment of interactions between groundwater and surface water was carried out by applying two different modeling approaches to a small‐scale study area in the municipality of Havelock, Quebec. The first approach involved a commonly used sequential procedure that consists in determining the daily recharge rate using a quasi 2D infiltration model (HELP), applied in the next step as an imposed flux to a 3D finite‐element groundwater flow model. The flow model was calibrated under steady‐state and transient conditions against measured water levels. The second approach was based on a recently developed physically based, 3D fully coupled groundwater–surface water flow model (CATHY) applied to the entire flow domain in an integrated manner. Implementation, calibration, and results of the simulations for both approaches are presented and discussed. For equal annual precipitation (1038 mm/y) and evapotranspiration (556 mm/y), the second approach computed a recharge rate of 233 mm/y (8.9% higher than the first approach) and a net upward flow from the fractured aquifer (the first approach predicted a net downward flow to the rock). The simulated annual discharge was similar for the two approaches (9.6% difference). Both approaches were found to be useful in understanding the interactions between groundwater and surface water, although limitations are apparent in the sequential procedure's inability to account for surface–subsurface feedbacks, for instance near stream reaches where groundwater discharge is prevalent. The decoupled, two‐model approach provides disaggregated surface, vadose, and aquifer flows, and a simple aperçu at the different components of total discharge. The fully coupled model accounts for continuous water exchanges between the land surface, subsurface, and stream channel in a more complex manner, and produces a better match against observed data. Copyright © 2012 John Wiley & Sons, Ltd.     

Post-emplacement tilting of lava flows inferred from magnetic fabric study: the example of Oligocene lavas in the Jeanne d’Arc Peninsula (Kerguelen Islands)

Statistical analysis of the magnetic fabric of samples from several successive lava flows emplaced under similar conditions can allow determination of the mean flow direction when magnetic fabric data from individual flows do not lead to reliable results. A difference between the obtained flow direction and the present dip direction indicates that the flows were tilted after emplacement. For 2 successive series of flows on the Jeanne d’Arc Peninsula presently NNW dipping, this method shows lava emplacement along a SSW–NNE direction. This indicates a gentle tilting acquired during a period of weak deformation in the whole archipelago. Additionally, the magnetic fabric data allow the reconstruction of the different conditions of emplacement of these two series of lava flows and of formation of the local thick conglomerate interbedded between these series.     

Why permafrost rocks become unstable: a rock–ice‐mechanical model in time and space

In this paper, we develop a mechanical model that relates the destabilization of thawing permafrost rock slopes to temperature‐related effects on both, rock‐ and ice‐mechanics; and laboratory testing of key assumptions is performed. Degrading permafrost is considered to be an important factor for rock–slope failures in alpine and arctic environments, but the mechanics are poorly understood. The destabilization is commonly attributed to changes in ice‐mechanical properties while bedrock friction and fracture propagation have not been considered yet. However, fracture toughness, compressive and tensile strength decrease by up to 50% and more when intact water‐saturated rock thaws. Based on literature and experiments, we develop a modified Mohr–Coulomb failure criterion for ice‐filled rock fractures that incorporates fracturing of rock bridges, friction of rough fracture surfaces, ductile creep of ice and detachment mechanisms along rock–ice interfaces. Novel laboratory setups were developed to assess the temperature dependency of the friction of ice‐free rock–rock interfaces and the shear detachment of rock–ice interfaces. In degrading permafrost, rock‐mechanical properties may control early stages of destabilization and become more important for higher normal stress, i.e. higher magnitudes of rock–slope failure. Ice‐mechanical properties outbalance the importance of rock‐mechanical components after the deformation accelerates and are more relevant for smaller magnitudes. The model explains why all magnitudes of rock–slope failures can be prepared and triggered by permafrost degradation and is capable of conditioning long para‐glacial response times. Here, we present a synoptic rock‐ and ice‐mechanical model that explains the mechanical destabilization processes operating in warming permafrost rocks. Copyright © 2013 John Wiley & Sons, Ltd.     

Geochemistry and Sr–Nd–Pb isotopic constraints on the petrogenesis of Cenozoic lavas from the Pali Aike and Morro Chico area (52°S), southern Patagonia,South America

Geochemical and isotopic analyses (Sr–Nd–Pb) of late Miocene to Quaternary plateau lavas from the Pali Aike and Morro Chico areas (52°S) were undertaken to constrain the melting processes and mantle sources that contributed to magma generation and the geodynamic evolution of southernmost Patagonia, South America. The Pali Aike and Morro Chico lavas are alkaline (Pali Aike, 45–49 wt.% SiO₂; 4.3–5.9 wt.% Na₂O+K₂O) and subalkaline (Morro Chico, 50.5–50.8 wt.% SiO₂; 4.0–4.4 wt.% Na₂O+K₂O), relatively primitive (Pali Aike, 9.5–13.7 wt.% MgO; Morro Chico, 7.6–8.8 wt.% MgO) mafic volcanic rocks that have typical intraplate ocean island basalt‐like signatures. Incompatible trace element ratios and isotopic ratios of the Pali Aike and Morro Chico lavas differ from those of the majority of Neogene southern Patagonian slab window lavas in showing more enriched characteristics and are similar to high‐μ (HIMU)‐like basalts. The rare earth element (REE) modeling to constrain mantle melting percentages suggests that these lavas were produced by low degrees of partial melting (1.0–2.0% for Pali Aike lavas and about 2.6–2.7% for Morro Chico lavas) of a garnet lherzolite mantle source. The major systematic variations of Sr–Nd–Pb isotopes in southern Patagonian lavas are related to geographic location. The Pali Aike and Morro Chico lavas from the southernmost part of Patagonia have lower ⁸⁷Sr/⁸⁶Sr and higher ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁶Pb/²⁰⁴Pb ratios, relative to most of the southern Patagonian lavas erupted north of 49.5°S, pointing to a HIMU‐like signature. An isotopically depleted and HIMU‐like asthenospheric domain may have been the main source of magmas in the southernmost part of Patagonia (e.g. Pali Aike, Morro Chico, and Camusu Aike volcanic field), suggesting the presence of a major discontinuity in the isotopic composition of the asthenosphere in southern Patagonia. On the basis of geochemical and isotope data and the available geological and geotectonic reconstructions, a link between the HIMU asthenospheric mantle domain beneath southernmost Patagonia and the HIMU mega‐province of the southwestern Pacific Ocean is proposed.     

Permian back‐arc extension in central Inner Mongolia,NE China: Elemental and Sr–Nd–Pb–Hf–O isotopic constraints from the Linxi high‐MgO diabase dikes

Early Permian (272 ± 2 Ma) diabase dikes from the Linxi area in central Inner Mongolia of NE China have high MgO (10.4 – 12.3 wt%), Cr (301 – 448 ppm) and Ni (167 – 233 ppm) concentrations, and show enrichments in large ion lithophile element (LILE) and light rare earth elements (REE) but depletions in high field strength element (HFSE, e.g., Nb and Ta), with depleted mantle‐type Sr [⁸⁷Sr/⁸⁶Sr (i) = 0.70315 – 0.70362], Nd [ε_Nd (t) = +6.8 – +7.4], Pb [²⁰⁶Pb/²⁰⁴Pb (i) = 18.10 – 18.16] and zircon Hf [ε_Hf (t) = +14.7 – +19.1] isotopic compositions, but slightly higher zircon δ¹⁸O (5.2 – 6.0 ‰ with an average of 5.7 ‰) than normal mantle. The combined geochemical data indicate their derivation from a depleted mantle metasomatized by recycled crustal component. Elemental and isotopic modeling results suggest that the primary magma was produced through 5 % to 10 % melting of a depleted mantle, which contained approximately 1 % sediment fluid released from the subducted paleo‐Asian Ocean. Considering the widespread distribution of contemporaneous mafic rocks across the central Inner Mongolia, which show REE patterns from E‐MORBs to normal MORBs, we propose a petrogenetic link between the Early Permian mafic magmatism and a back‐arc extension in response to northward subduction of the paleo‐Asian Ocean. The Permian mafic magmatism and the new age constraints from the metamorphic and sedimentary records in this area tend to indicate the ultimate closure of the paleo‐Asian Ocean by the end of Paleozoic.     

Three‐dimensional beam–truss model for reinforced concrete walls and slabs – part 2: modeling approach and validation for slabs and coupled walls

A three‐dimensional beam–truss model (BTM) for reinforced concrete (RC) walls that explicitly models flexure–shear interaction and accurately captures diagonal shear failures was presented in the first part of this two‐paper series. This paper extends the BTM to simulate RC slabs and coupled RC walls through slabs and beams. The inclination angle of the diagonal elements for coupled RC walls is determined, accounting for the geometry of the walls and the level of coupling. Two case studies validate the model: (1) a two‐bay slab–column specimen experimentally tested using cyclic static loading and (2) a five‐story coupled T‐wall–beam–slab specimen subjected to biaxial shake table excitation. The numerically computed lateral force–lateral displacement and strain contours are compared with the experimentally measured response and observed damage. The five‐story specimen is characterized by diagonal shear failure at the bottom story of the walls, which is captured by the BTM. The BTM of the five‐story specimen is used to study the effects of coupling on shear demand for lightly reinforced RC coupled walls. The effect of mesh refinement and bar fracture of non‐ductile transverse reinforcement is studied. Copyright © 2016 John Wiley & Sons, Ltd.     

The challenge of modeling pool–riffle morphologies in channels with different densities of large woody debris and boulders

Studies on pool morphologies include reports of over 80% or 90% of pools being associated with structural controls and large obstructions that include boulders, bedrock outcrops and large woody debris (LWD). A Monte Carlo simulation approach and developmental computer model was created to predict pool formation, spacing and the percentage length covered by pools, riffles, scour holes and runs based on input data that include channel slope, width, the number of small and large boulders, and the number of 10–30 cm, 30–60 cm and >60 cm pieces of wood. The statistical‐empirical model is founded on the idea that boulders, bedrock outcrops and large woody debris provide a physical framework that then controls local water‐surface slopes, velocity patterns and the locations of pools and riffles. The spacing values of individual types and sizes of obstructions are modeled as log‐normal distributions with separate distributions for each obstruction type. Pools are assigned different probabilities of development depending on the obstruction type. Pool and riffle lengths used to create the subsequent morphology follow their own slope‐dependent, log‐normal trends. A minimum distance develops between successive pools because of the backwater and turbulent conditions needed for pool formation. The total number and spacing of pools, riffles and scour holes thus reflects the number and locations of obstructions and characteristics of the pool–riffle couplet. The simulation model accurately captures the number of pools in the modeled data range at 65% of all the verification field sites, and 86% of the verification field sites with a more limited range of width and slope characteristics. Lower levels of prediction capabilities are associated with modeled numbers of scour holes and log jams. The model accurately mimics some statistical attributes of pool spacing, and future versions of the model could be developed to improve overall predictive capabilities. Copyright © 2011 John Wiley & Sons, Ltd.     

Field assessment of surface water–groundwater connectivity in a semi‐arid river basin (Murray–Darling,Australia)

In semi‐arid and arid river basins, understanding the connectivity between rivers and alluvial aquifers is one of the key challenges for the management of groundwater resources. The type of connection present (gaining, losing‐connected, transitional and losing‐disconnected) was assessed at 12 sites along six Murray–Darling Basin river reaches. The assessments were made by measuring the hydraulic head in the riparian zone near the rivers to evaluate if the water tables intersected the riverbeds and by measuring fluid pressure (ψ) in the riverbeds. The rationale for the latter was that ψ will always be greater than or equal to zero under connected conditions (either losing or gaining) and always lesser than or equal to zero under losing‐disconnected conditions. A mixture of losing‐disconnected, losing‐connected and gaining conditions was found among the 12 sites. The losing‐disconnected sites all had a riverbed with a lower hydraulic conductivity than the underlying aquifer, usually in the form of a silty clay or clay unit 0.5–2 m in thickness. The riparian water tables were 6 to 25 m below riverbed level at the losing‐disconnected sites but never lower than 1 m below riverbed level at the losing‐connected ones. The contrast in water table depth between connected and disconnected sites was attributed to the conditions at the time of the study, when a severe regional drought had generated a widespread decline in regional water tables. This decline was apparently compensated near losing‐connected rivers by increased infiltration rates, while the decline could not be compensated at the losing‐disconnected rivers because the infiltration rates were already maximal there. Copyright © 2012 John Wiley & Sons, Ltd.     

Tidally‐averaged flows in the interior Sacramento–San Joaquin River Delta: Trends and change attribution

The hydrology of the San Francisco Bay‐Delta estuary (the Delta) has been significantly modified over the past 150 years to serve a variety of human needs for water supply and food production, albeit with adverse ecological impacts. These adverse impacts, in concert with evolving societal values, have motivated change in the estuary's water management to promote ecosystem restoration goals while continuing to support human uses. Understanding historical flow patterns, as well as the environmental functions provided by these flow patterns, is critical to restoration planning. Building upon previous work on outflow trends from the Delta to San Francisco Bay, this paper evaluates historical trends in key interior Delta flows spanning nine decades (1922–2016) and presents an attribution of these trends to various anthropogenic drivers. We reconstructed historical time series records at four key locations in the interior of the Delta; these time series represent tidally‐averaged flows that are intensively managed and are of great importance to the beneficial uses of water in the region. We derived several scenario‐based flow time series at these same four locations to assist in attributing change to a variety of drivers, including reservoir and export pumping operations, in‐Delta barrier and gate operations, and upstream water diversions. Flow changes were measured relative to 1920‐level land use and water management conditions. We find the four interior Delta locations to have distinct seasonal flow trends and, in general, unique responses to various drivers of change. Our work highlights the complex nature of historical flow changes in a highly‐managed estuarine ecosystem, and the types of modifications that would be necessary to reverse these changes.     

Response analysis of the Higashi–Kobe Bridge and surrounding soil in the 1995 Hyogoken–Nanbu Earthquake

This paper presents results of observation and analysis of the response of one of the longest cable-stayed bridges in the world to the Hyogoken–Nanbu (Kobe) Earthquake of 17 January 1995. It is determined that interaction of the foundations of the bridge towers with the supporting soil plays a decisive role in the overall structural behaviour. The key factor governing the changes of the soil properties at this site is pore-water pressure buildup, which results in liquefaction of the saturated surface soil layers under large dynamic loads. Models of the soil and structure are created and initially validated by accurately simulating the system response to a small earthquake. Soil parameters reflecting the pore-water pressure buildup in the strong earthquake are determined by an advanced non-linear effective stress analysis, combining the Ramberg–Osgood model of stress–strain dependence with a pore pressure model based on shear work concept. They are utilized to investigate and simulate the interaction of the foundation and the supporting soil using the program SASSI with the flexible volume substructuring approach. The results show a good agreement with the observations and have useful implications to the scientific and engineering practice. © 1998 John Wiley & Sons, Ltd.     

Investigation of seismic behavior of fluid–rectangular tank–soil/foundation systems in frequency domain

Main purpose of this study is to evaluate the dynamic behavior of fluid–rectangular tank–soil/foundation system with a simple and fast seismic analysis procedure. In this procedure, interaction effects are presented by Housner's two mass approximations for fluid and the cone model for soil/foundation system. This approach can determine; displacement at the height of the impulsive mass, the sloshing displacement and base forces for the soil/foundation system conditions including embedment and incompressible soil cases. Models and equations for proposed method were briefly explained for different tank–soil/foundation system combinations. By means of changing soil/foundation conditions, some comparisons are made on base forces and sloshing responses for the cases of embedment and no embedment. The results showed that the displacements and base shear forces generally decreased, with decreasing soil stiffness. However, embedment, wall flexibility, and soil–structure interaction (SSI) did not considerably affect the sloshing displacement.     

Modeling surface water–groundwater interaction in New Zealand: Model development and application

Most rivers worldwide have a strong interaction with groundwater when they leave the mountains and flow over alluvial plains before flowing into the seas or disappearing in the deserts, and in New Zealand, typically, rivers lose water to the groundwater in the upper plains and generally gain water from the groundwater in the lower plains. Aiming at simulating surface water–groundwater interaction nationally in New Zealand, we developed a conceptual groundwater module for the national hydrologic model TopNet to simulate surface water–groundwater interaction, groundwater flow, and intercatchment groundwater flow. The developed model was applied to the Pareora catchment in South Island of New Zealand, where there are concurrent spot gauged flows. Results show that the model simulations not only fit quite well to flow measurement but also to concurrent spot gauged flows, and compared to the original TopNet, it has a significant improvement in the low flows. Sensitivity analysis shows river flow is sensitive to the river losing/gaining rate instead of groundwater characteristic, while groundwater storage is sensitive to both river losing/gaining rate and groundwater characteristic. This indicates our conceptual approach is promising for nationwide modeling without the large amount of geology and aquifer data typically required by physically‐based modeling approaches.     

Fluid–structure interaction analysis of 3-D rectangular tanks by a variationally coupled BEM–FEM and comparison with test results

A variationally coupled BEM–FEM is developed which can be used to analyse dynamic response, including free-surface sloshing motion, of 3-D rectangular liquid storage tanks subjected to horizontal ground excitation. The tank structure is modelled by the finite element method and the fluid region by the indirect boundary element method. By minimizing a single Lagrange function defined for the entire system, the governing equation with symmetric coefficient matrices is obtained. To verify the newly developed method, the analysis results are compared with the shaking-table test data of a 3-D rectangular tank model and with the solutions by the direct BEM–FEM. Analytical studies are conducted on the dynamic behaviour of 3-D rectangular tanks using the method developed. In particular, the characteristics of the sloshing response, the effect of the rigidity of adjacent walls on the dynamic response of the tanks and the orthogonal effects are investigated. © 1998 John Wiley & Sons, Ltd.     

Fracture patterns at lava–ice contacts on Kokostick Butte, OR, and Mazama Ridge, Mount Rainier, WA: Implications for flow emplacement and cooling histories

Cooling lava commonly develop polygonal joints that form equant hexagonal columns. Such fractures are formed by thermal contraction resulting in an isotropic tensional stress regime. However, certain linear cooling fracture patterns observed at some lava–ice contacts do not appear to fit the model for formation of cooling fractures and columns because of their preferred orientations. These fracture types include sheet-like (ladder-like rectangular fracture pattern), intermediate (pseudo-aligned individual column-bounding fractures), and pseudopillow (straight to arcuate fractures with perpendicular secondary fractures caused by water infiltration) fractures that form the edges of multiple columns along a single linear fracture. Despite the relatively common occurrence of these types of fractures at lava–ice contacts, their significance and mode of formation have not been fully explored. This study investigates the stress regimes responsible for producing these unique fractures and their significance for interpreting cooling histories at lava–ice contacts.Data was collected at Kokostick Butte dacite flow at South Sister, OR, and Mazama Ridge andesite flow at Mount Rainier, WA. Both of these lava flows have been interpreted as being emplaced into contact with ice and linear fracture types have been observed on their ice-contacted margins. Two different mechanisms are proposed for the formation of linear fracture networks. One possible mechanism for the formation of linear fracture patterns is marginal bulging. Melting of confining ice walls will create voids into which flowing lava can deform resulting in margin-parallel tension causing margin-perpendicular fractures. If viewed from the ice-wall, these fractures would be steeply dipping, linear fractures. Another possible mechanism for the formation of linear fracture types is gravitational settling. Pure shear during compression and settling can result in a tensional environment with similar consequences as marginal inflation. In addition to this, horizontally propagating cooling fractures will be directly influenced by viscous strain caused by the settling of the flow. This would cause preferential opening of fractures horizontally, resulting in vertically oriented fractures.It is important to note that the proposed model for the formation of linear fractures is dependent on contact with and confinement by glacial ice. The influence of flow or movement on cooling fracture patterns has not been extensively discussed in previous modeling of cooling fractures. Rapid cooling of lava by the interaction with water and ice will increase the ability to the capture and preserve perturbations in the stress regime.     

Patterns and dynamics of river–aquifer exchange with variably-saturated flow using a fully-coupled model

The parallel physically-based surface–subsurface model PARFLOW was used to investigate the spatial patterns and temporal dynamics of river–aquifer exchange in a heterogeneous alluvial river–aquifer system with deep water table. Aquifer heterogeneity at two scales was incorporated into the model. The architecture of the alluvial hydrofacies was represented based on conditioned geostatistical indicator simulations. Subscale variability of hydraulic conductivities (K) within hydrofacies bodies was created with a parallel Gaussian simulation. The effects of subscale heterogeneity were investigated in a Monte Carlo framework. Dynamics and patterns of river–aquifer exchange were simulated for a 30-day flow event. Simulation results show the rapid formation of saturated connections between the river channel and the deep water table at preferential flow zones that are characterized by high conductivity hydrofacies. Where the river intersects low conductivity hydrofacies shallow perched saturated zones immediately below the river form, but seepage to the deep water table remains unsaturated and seepage rates are low. Preferential flow zones, although only taking up around 50% of the river channel, account for more than 98% of total seepage. Groundwater recharge is most efficiently realized through these zones. Subscale variability of K_sat slightly increased seepage volumes, but did not change the general seepage patterns (preferential flow zones versus perched zones). Overall it is concluded that typical alluvial heterogeneity (hydrofacies architecture) is an important control of river–aquifer exchange in rivers overlying deep water tables. Simulated patterns and dynamics are in line with field observations and results from previous modeling studies using simpler models. Alluvial heterogeneity results in distinct patterns and dynamics of river–aquifer exchange with implications for groundwater recharge and the management of riparian zones (e.g. river channel-floodplain connectivity via saturated zones).