Stochastic 1D site response analysis at a site in central Italy

Site response analysis is strongly influenced by the uncertainty associated to the definition of soil properties and model parameters. Deterministic, or even parametric analyses are unable to systematically assess such uncertainty, since the site characterisation can hardly be sufficiently accurate for a deterministic prediction of site response and alternative approaches are hence needed. A fully stochastic procedure for estimating the site amplification of ground motion is proposed and applied to a case study in central Italy. The methodology allows to take into account the record-to-record variability in an input ground motion and the uncertainty in dynamic soil properties and in the definition of the soil model. In particular, their effect on response spectra at the ground surface is evaluated.     

Evaluation of a probabilistic approach to simulation of alkalinity and electrical conductivity at a river bank filtration site

In river bank filtration, impurities present in the river water travel with the bank filtrate towards the pumping well. During this passage, certain types of impurities, such as turbidity, total coliform, and so forth, may get attenuated; however, it is interesting to note that some of the instant raw river water quality parameters, such as alkalinity and electrical conductivity, increase after the passage of water through the porous medium. This occurs because water, when passing through the soil pores, absorbs many of the solutes that cause an increase in alkalinity and electrical conductivity. Measurements at a river bank filtration site for a year showed that alkalinity of 116–32 mg l^?1 in river water increased to 222.4–159.9 mg l^?1 in the river bank filtered water. Likewise, the electrical conductivity increased from 280–131 μS cm^?1 to 462–409.6 μS cm^?1. This study uses a probabilistic approach for investigating the variation of alkalinity and electrical conductivity of source water that varies with the natural logarithm of the concentration of influent water. The probabilistic approach has the potential of being used in simulating the variation of alkalinity and electrical conductivity in river bank filtrate. Copyright © 2011 John Wiley & Sons, Ltd.     

Sediment hydrocarbon contamination at the site of the Baie Verte,Newfoundland, oil spill: Results of a Four Year Study

Sediments samples were taken at 3, 15, 27, and 39 months following a spill of diesel oil at Baie Verte, Newfoundland, to determine the effects of weathering on diesel oil trapped in the sediments and to attempt to develop a suitable method for extraction and analysis.Extraction was carried out using hexane, concentration was effected by a procedure modified from Junk et al. (1974) and analysis was completed within 18.0 min by high performance liquid chromatography (HPLC) using a bonded amino silane-(NH₂) column (5 μm, 5 mm × 25 cm) with hexane as mobile phase. A backflush cycle of 22.0 min using hexane and methl-t-butyl ether (90:10%) was performed to elute any polar compounds from the column.This class analysis by HPLC indicated a decrease in the amount of naphthalene, fluorene, phenanthrene and benzo[a]pyrene equivalents over time and at the conclusion of the study (39 months) no PAHs except for fluorene were found in the sediments from Baie Verte.     

A chronology of post‐glacial landslides suggests that slight increases in precipitation could trigger a disproportionate geomorphic response

Large, deep‐seated landslides are common throughout the south‐eastern San Juan Mountains of Colorado and New Mexico, but their timing and initiation are not well understood. Determining when the landslides occurred would aid in clarifying the mechanisms for initiating landslides in the region and would help us to understand post‐glacial landscape evolution. We studied seven pre‐historic landslides located within the Tertiary volcanic rocks of the San Juan Volcanic Field. The landslides range in area from ~0.8 km² to ~11.3 km² and most are located in areas that were previously mapped as having been ice‐covered during the last glaciation. Landslide deposits were dated using a variety of methods including surface exposure dating (chlorine‐36, ³⁶Cl), radiocarbon dating of basal bog sediments and organic material buried in soils, and relative soil development. The resulting limiting ages range from approximately 14 ka to 2 ka and show that deep‐seated landsliding has occurred throughout the post‐glacial period. This broad range in ages is inconsistent with our initial hypothesis, which proposed that landslides were likely the result of debuttressing of glacial walls during glacial retreat. Furthermore, the timing of landslides does not seem to correlate with documented post‐glacial climatic shifts. Therefore, we conclude that landsliding in the region was the result of wetter than normal periods lasting months to years acting on weak bedrock preconditioned to failure and prepared by glacial debuttressing. Our findings suggest that the study area is likely still susceptible to deep‐seated landsliding and may become even more prone to large‐scale slope failure if future climate change increases precipitation in the San Juan Mountains. Copyright © 2017 John Wiley & Sons, Ltd.     

Using a non-integer moment of the impulse response to estimate the half-space conductivity

The nth-order moments of the electromagnetic impulse response are useful for interpreting electromagnetic data. We have derived an analytic expression for the half-order moment of a conductive half-space. By inverting this expression, the measured half-order moment can be used to estimate an apparent conductivity of the ground. The first-order moment can also be used to estimate the half-space conductivity. A sensitivity analysis indicates that for an airborne EM configuration, the half-order moment will be most sensitive to material in the top 26–48 m, while the first-order moment will be sensitive to deeper material (down to depths between 66 and 127 m).     

Parametric analysis of the seismic response of a 2D sedimentary valley: implications for code implementations of complex site effects

A detailed 2D model has been constructed and validated for Euroseistest valley, in northern Greece. We take advantage of this model to investigate what parameters, in addition to surface soil conditions (obviously the most important parameter), can be used to correctly characterize site response in a 2D structure. Through a parametric analysis using 2D numerical simulations for SH waves, we explore the differences between the computed ground motion for different simplifications of the valley's structure. We consider variations in the velocity structure within the sediments, and variations of the shape between sediments and bedrock. We also compare the results from different 1D models reflecting current approaches to the determination of site response. Our results show clearly that, in the case of Euroseistest, site response owes fundamentally to its closed basin shape because it is largely controlled by locally generated surface waves. Thus, in terms of predicting site response, a rough idea of its shape ratio and of the average mechanical properties of the sediments are better than a very detailed 1D profile at the central site. Although the details of ground motion may vary significantly between the models, the relative amount of surface waves generated in the 2D models seems to be relatively constant. Moreover, if we quantify the additional amplification caused by the lateral heterogeneity in terms of the ‘aggravation factor’ introduced by Chávez-García & Faccioli [7], a roughly constant factor between 2 and 3 seems to appropriately take into account the effects of lateral heterogeneity. Of course, a correct estimate of the overall impedance contrast is necessary to correctly predict the maximum amplification, a caveat that also applies to 1D models. In this sense, Euroseistest rings an alarm bell. In this valley the more significant impedance contrast lies at about 200 m depth, and it is missed both by consideration of the average shear wave velocity of the first 30 m (the V_s30 criterion) or using the detailed velocity profile down to a depth where a shear wave velocity larger than 750 m/s is found. Our conclusions indicate that, in order to improve current schemes to take into account site effects in building codes, the more to be gained comes from consideration of lateral heterogeneity, at least in the case of shallow alluvial valleys, where locally generated surface waves are likely to be important.     

Impact of fractional probability distributions on statistics of hydraulic conductivity,dynamics of groundwater flow and solute transport at a low-permeability site

The fractional Brownian motion (fBm) and fractional Lévy motion (fLm) can easily describe the geometry and the statistical structure of hydraulic conductivity (K) for real-world. However, the fBm and fLm models have not been systematically evaluated when building the K field for a low-permeability site. In this study, both the fBm and fLm are used to simulate the low-K field at NingCheGu (NCG), Tianjin, China. Groundwater flow and solute transport are then computed using MODFLOW and MT3DMS, respectively, and the influence of the fBm/fLm models for K on groundwater flow and solute transport is discussed. Results show that the fLm fits better the statistics of the low-K medium than fBm, and the random logarithmic K (LnK) field generated by fLm is more stable because the resultant LnK field captures more of the measured properties at the field site than that generated by fBm. In contrast, the LnK generated by fBm is more likely to form both high-K channels and low-K barriers. The fBm therefore predicts more extreme behaviours in flow and transport, including the preferential flow, low-concentration blocks and solute retention. The overall groundwater renewal period and solute travel time for the fLm simulation are slightly shorter than those for fBm. The impacts of the fLm and fBm models on the statistics of the resultant LnK fields and the dynamics of groundwater flow and solute transport revealed by this study shed light on the selection and evaluation of the fractional probability distribution models in capturing the K fields for low-K media.     

Evidence of karst from behaviour of the Asmari limestone aquifer at the Khersan 3 Dam site,southern Iran

Abstract

The Asmari limestone formation is the major aquifer system at the Khersan 3 Dam site, Zagros, Iran. Characterization of the aquifer system and study of karst development are essential for forecasting leakage potential and to plan remediation works. The aquifer functioning and karst structure were evaluated by geology, well hydrodynamics and natural tracing studies, showing one unconfined and three artesian sub-aquifers, the last being characterized by rapid flow, with contributing old recharge water, and a recharge at higher elevation than the unconfined sub-aquifer. The anticline structure outcrops the artesian sub-aquifers downstream of the site. The confining layers disappear and the aquifer system discharges as a mix of all groundwater. Artesian groundwater is estimated to contribute about 80% (dry season) and 50% (wet season) of flow in the mixing zone. Very few karst features are observed in cores and galleries at the site, despite some karst landforms in the reservoir area. However the aquifer exhibits fast flow of karst type in the artesian sub-aquifers.     

Seasonal water sources and response to precipitation events of Calligonum mongolicum at the oasis margin in the Hexi River Corridor

Calligonum mongolicum is often planted as a windbreak and for sand stabilization on mobile and semi-mobile sand dunes in extremely arid regions. However, water availability remains a key limiting factor for its survival and population expansion, and water use strategies and responses to precipitation events of this species are unknown. Here, we determined water use strategy of C. mongolicum under extreme arid conditions by measuring the oxygen stable isotopes (δD and δ¹⁸O) in xylem water and in potential water sources (precipitation, soil water, and groundwater). We used the IsoSource model to determine the relative contributions of different water sources to water utilization by C. mongolicum. Our results showed that: (1) water sources used by C. mongolicum exhibited seasonal variability, with shallow soil water accounting for 42% of utilization during early spring (April), and deep soil water and groundwater being predominantly used during the summer and autumn and accounting for 61%–84% of utilization, (2) C. mongolicum did not respond to small precipitation events, but responded significantly to large precipitation events. C. mongolicum maintained the utilization of soil water in all layers at 74%–81% of deep soil water and groundwater before a 5.8 mm precipitation event. A precipitation event of 18.8 mm increased the contribution of surface water from 4% before to 17% after precipitation, indicating that C. mongolicum has a strong capacity for self-regulation and adaptation; namely, C. mongolicum is capable of developing an optimal phenotype through self-regulation, thereby maximizing water acquisition.     

Carbonatitic volcanism in the Kaiserstuhl alkaline complex: Evidence for highly fluid carbonatitic melts at the earth's surface

Extrusive carbonatites are described from the Miocene alkaline complex of the Kaiserstuhl, Rhinegraben, Western Germany. Agglutinated carbonatitic lapilli form pyroclastic rocks in which all components show forms acquired when a highly fluid melt was sprayed into the air by an explosive eruption: droplets, spherical and elliptical lapilli, rods, dumbbell and pear-shaped forms.Complete morphological analogies suggest a mechanism similar to the formation of “Pele's tears”, basaltic droplets formed by the eruption of the most fluid Hawaiian basaltic magmas. Evidence is provided by this example that CaCO₃-carbonatitic magmas can exist in nature under surface conditions displaying extremely low viscosity.     

Snowmelt energetics at a shrub tundra site in the western Canadian Arctic

Snow accumulation and melt were observed at shrub tundra and tundra sites in the western Canadian Arctic. End of winter snow water equivalent (SWE) was higher at the shrub tundra site than the tundra site, but lower than total winter snowfall because snow was removed by blowing snow, and a component was also lost to sublimation. Removal of snow from the shrub site was larger than expected because the shrubs were bent over and covered by snow during much of the winter. Although SWE was higher at the shrub site, the snow disappeared at a similar time at both sites, suggesting enhanced melt at the shrub site. The Canadian Land Surface Scheme (CLASS) was used to explore the processes controlling this enhanced melt. The spring‐up of the shrubs during melt had a large effect on snowmelt energetics, with similar turbulent fluxes and radiation above the canopy at both sites before shrub emergence and after the snowmelt. However, when the shrubs were emerging, conditions were considerably different at the two sites. Above the shrub canopy, outgoing shortwave radiation was reduced, outgoing longwave radiation was increased, sensible heat flux was increased and latent flux was similar to that at the tundra site. Above the snow surface at this site, incoming shortwave radiation was reduced, incoming longwave radiation was increased and sensible heat flux was decreased. These differences were caused by the lower albedo of the shrubs, shading of the snow, increased longwave emission by the shrub stems and decreased wind speed below the shrub canopy. The overall result was increased snowmelt at the shrub site. Although this article details the impact of shrubs on snow accumulation and melt, and energy exchanges, additional research is required to consider the effect of shrub proliferation on both regional hydrology and climate. Copyright 2010 John Wiley & Sons Ltd and Crown in the right of Canada.     

Nonlinear identification of the soil response at Dahan downhole array site during the 1999 Chi–Chi earthquake

Records of the 1999 Chi–Chi earthquake, provided by the Dahan downhole array were analyzed, and nonlinear hysteretic stress–strain relations in the soil layers were estimated by the method previously used for studying the response of soils during the 1995 Kobe and the 2000 Tottori earthquakes. The obtained models of the soil behavior were applied for evaluating changes of the shear moduli in the soil layers and for the nonlinear identification of the soil behavior at Dahan site during the Chi–Chi earthquake. We found that reduction of the shear moduli in the soil layers did not exceed 5%, and the soil response was virtually linear. The content of the nonlinear components in the soil response was about 5% of the intensity of the response, and it was mostly due to the odd-order nonlinearities. A similarity in the stress–strain relations describing the behavior of soils during the 1995 Kobe earthquake, the 2000 Tottori earthquake, and the 1999 Chi–Chi earthquake was found, which indicates the possibility to describe the behavior of similar types of soils at different sites by similar stress–strain relations and predict soil behavior in future earthquakes.     

Evaluation of the North American Regional Reanalysis (NARR) precipitation fields in a topographically complex domain

ABSTRACT

The North American Regional Reanalysis (NARR) precipitation product was evaluated using station observations and catchment water yield in British Columbia (BC), Canada, at inter-annual, monthly, and daily time scales. A structural break occurred in 2003, associated with exclusion of Canadian precipitation gauge data from NARR’s data assimilation process beginning in that year. The NARR product under-predicted precipitation in mountainous regions, over-predicted in the northern region of BC’s Interior Plateau, and catchment-averaged NARR precipitation was less than observed water yield in coastal BC. The product was unable to reproduce even the seasonal pattern at three stations. This study highlights uncertainties associated with the NARR precipitation product, and presents a cautionary tale that is likely relevant not only to the application of NARR in BC, but may be relevant for other re-analysis products and other regions with complex topography and sparse station networks.     

The reference spectral noise ratio method to evaluate the seismic response of a site

The seismic response of several sites in the Durban area of South Africa has been investigated using a refined version of the spectral ratio (H/V) generated by cultural seismic noise. Two samples of noise separated by an interval of several minutes for the vertical and the two orthogonal horizontal components were taken at each site. Though the two samples differed noticeably in the time as well as in the frequency domain, when the components are considered separately, the ratios of the horizontal to the vertical spectra were very similar for most of the pairs. This similitude was obvious not only in the frequencies of the peaks, but also in the range of amplifications involved. These observations have been used to introduce a refinement to the Nakamura Method. This refinement consists in using as a reference the H/V spectral ratio of a well-known, hard rock site. All the other soft sites of interest are then compared with this standard reference site. This comparison provides valuable information on the frequencies and amplification factors introduced by the critical soft sediments. This refined method will be referred here as the ‘Reference spectral noise ratio’.This method, is compared to a similar one devised to investigate the response of horizontal layers to longitudinal seismic waves, using the ratio of the spectra generated by long- period waves.     

Spatial dependence of the hydraulic conductivity in a well‐type configuration at the mesoscale

The spatial distribution of the hydraulic conductivity κ is modelled by a power law, and we present a methodological approach to quantify the exponent (crowding index) of such a law as detected within a well‐type flow configuration. Based upon the outcome of several pumping tests conducted into a caisson (mesoscale), we identify the crowding index as function of the volumetric flow rate. Hence, we develop a simple (although approximated) procedure to assess whether the spatial distribution of κ can be characterized by a power law. We demonstrate that, even at the mesoscale, the conductivity κ can not be regarded as a formation's property (nonlocality), in agreement with the recent developments on the theory of flows into radial configurations.     

Evidence for a hypogene paleohydrogeological event at the prospective nuclear waste disposal site Yucca Mountain,Nevada, USA,revealed by the isotope composition of fluid-inclusion water

Secondary calcite residing in open cavities in the unsaturated zone of Yucca Mountain has long been interpreted as the result of downward infiltration of meteoric water through open fractures. In order to obtain information on the isotopic composition (δD and δ¹⁸O) of the mineral-forming water we studied fluid inclusions from this calcite. Water was extracted from inclusions by heated crushing and the δD values were measured using a continuous-flow isotope-ratio mass spectrometry method. The δ¹⁸O values were calculated from the δ¹⁸O values of the host calcite assuming isotopic equilibrium at the temperature of formation determined by fluid-inclusion microthermometry.The δD values measured in all samples range between ? 110 and ? 90‰, similar to Holocene meteoric water. Coupled δ¹⁸O–δD values plot significantly, 2 to 8‰, to the right of the meteoric water line. Among the various processes operating at the topographic surface and/or in the unsaturated zone only two processes, evaporation and water–rock exchange, could alter the isotope composition of percolating water. Our analysis indicates, however, that none of these processes could produce the observed large positive δ¹⁸O-shifts. The latter require isotopic interaction between mineral-forming fluid and host rock at elevated temperature (>100 °C), which is only possible in the deep-seated hydrothermal environment. The stable isotope data are difficult to reconcile with a meteoric origin of the water from which the secondary minerals at Yucca Mountain precipitated; instead they point to the deep-seated provenance of the mineral-forming waters and their introduction into the unsaturated zone from below, i.e. a hypogene origin.     

Modelling transport dynamics of contaminated sediments in the headwater of a hydropower plant at the Upper Rhine River

Sediments are an essential habitat compartment in rivers, which is a subject to dynamic transport processes. In many rivers, the fine deposited sediments are contaminated with heavy metals and organic compounds. Contaminated deposits are considered as potential hot spots because of the risk of the mobilization under erosive hydraulic conditions. Numerical models for particulate contaminant transport are then necessary and can be applied to estimate and predict the potential impact of mobilized contaminants as an important contribution to sediment management. This paper focuses on the quantification of the amount of contaminated sediments resuspended during the extreme flood event in 1999 and the prediction of deposition one year after the flood event. To assess such erosive flood event, a 2D numerical transport model was developed to analyse the dynamics of erosion and sedimentation processes in the headwater of a cross dam at the Upper Rhine River. The dam consists of a weir, a hydropower plant, and a navigation lock. As the weir is operating only for flood management, a huge amount of sediment highly contaminated with the hexachlorobenzene (HCB) was deposited in the weir zone. Therefore, numerical simulations were performed to determine the spatial and temporal distribution of deposited contaminated sediments as depending on the river discharge and its distribution to the hydraulic structures. The numerical investigation presented here is taken as a retrospective analysis of the contaminated sediment dynamics in the headwater to improve future sediment management.     

Field observations and numerical model experiments for the snowmelt process at a field site

There are several levels of models for the snowmelt process in terms of the snow thermal structure: isothermal, bi-layered and multi-layered models. However, it is difficult to choose the appropriate level of complexity for application because the number of unknown variables is crucial in model handling. One of the major issues in energy balance snow models is the shape of the snow temperature vertical profile. This profile, if taken as a specified function, would simplify a snowmelt model calibration and computation significantly. In this study, in order to determine the appropriate representative snow vertical thermal profile, snow temperature measurements have been performed using five snow thermocouples placed vertically along an observation tower with insulating arms. Also, as a field experimental study of an energy balance snow model, the net radiation, air temperature, relative humidity and wind speed along with the vertical one dimensional snow temperature profile have been observed at a field site in Lake Tahoe Basin. The computational results correspond with the measured snow temperature profile and snow water equivalent reasonably well. It is illustrated that the temperature in the snow near surface (called the “active layer”) varies daily, and the lower snow layer (called the “inactive layer”) is barely affected by the atmosphere. The results of field observations and the numerical experiments show that the vertical temperature distributions in the active layer, which is the upper layer affected by energy exchange with the atmosphere, generally have an exponential shape during night time under cold weather, while snow pack stays around 0 °C during daytime. Both of the results indicate that not only the snow temperature in the top active layer, but also the thickness of snow active layer fluctuates during the snowmelt process. The observation results show that the thickness of the active layer may reach about 60 cm in Sierra Nevada, California. These results provide significant information for the development of appropriate approximations in physically based snowmelt modeling.     

Measurement of shallow shear wave velocities at a rock site using the ReMi technique

Shallow shear wave velocities beneath a rock site are characterized using the refraction microtremor (ReMi) technique developed by Louie [Faster, better: shear-wave velocity to 100 m depth from ReMi arrays. Bull Seism Soc Am 2001; 91: 347–64]. Ground motion from a passing train enabled capture of energy propagating parallel to the recording array. This allowed evaluation of the variation of the minimum phase-velocity of the dispersion curve envelope and better estimation of the true minimum velocity beneath the site. We use a new method to image and evaluate the dispersion curve envelope via power–slowness profiles through the slowness–frequency plots introduced by Louie [Faster, better: shear-wave velocity to 100 m depth from ReMi arrays. Bull Seism Soc Am 2001; 91: 347–64]. Data illustrated the frequency dependency of dispersion curve uncertainties, with greater uncertainty occurring at low frequencies. These uncertainties map directly into uncertainty of the inverted velocity–depth profile. Above 100 m depth velocities are well constrained with 10% variability. Variability is greatly reduced when the energy propagation is along the geophone array. Greater velocity variation is observed below 100 m depth.