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An inclusion model, based on the Kuster–Toksoz effective medium theory along with Gassmann theory, is tested to forward model velocities for fluid‐saturated rocks. A simulated annealing algorithm, along with the inclusion model, effectively inverts measured compressional velocity (VP) to achieve an effective pore aspect ratio at each depth in a depth variant manner, continuously along with depth. Early Cretaceous syn‐rift clastic sediments at two different depth intervals from two wells [well A (2160–2274 m) and well B (5222–5303 m)], in the Krishna–Godavari basin, India, are used for this study. Shear velocity (VS) estimated using modelled pore aspect ratio offers a high correlation coefficient (>0.95 for both the wells) with measured data. The modelled pore aspect ratio distribution suggests the decrease in pore aspect ratio for the deeper interval, mainly due to increased effective vertical stress. The pore aspect ratio analysis in relation to total porosity and volume of clay reveals that the clay volume has insignificant influence in shaping the pore geometry in the studied intervals. An approach based on multiple linear regression method effectively predicts velocity as a linear function of total porosity, the volume of clay and the modelled pore‐space aspect ratio of the rock. We achieved a significant match between measured and predicted velocities. The correlation coefficients between measured and modelled velocities are considerably high (approximately 0.85 and 0.8, for VP and VS, respectively). This process indicates the possible influence of pore geometry along with total porosity and volume of clay on velocity.

Excess fine sediment (

A three-dimensional shear velocity model for the crust and upper mantle beneath the central United States is presented by inverting Rayleigh wave phase velocities from 20s to 100s periods. These ph...

The impact of long-term CO2-brine-rock interactions on the frictional properties of faults is one of the main concerns when ensuring safe geological CO2 storage. Mineralogical changes may alter the frictional strength and seismogenic potential of pre-existing faults bounding a storage complex. However, most of these reactions are too slow to be reproduced on laboratory timescales and can only be assessed using geochemical modeling. We combined modeling of CO2-charged formation water and fault gouges (1–1000 years residence time, i.e. 10–106 pore volume flushes) with friction experiments on simulated fault gouges (T = 22–150°C; σn eff = 50 MPa; Pf = 25 MPa; V = 0.2-100 μm/s), having mineralogical compositions as predicted by the models. As an analogue for clay-rich caprocks overlying potential CO2 storage sites in Europe, we used the Opalinus claystone. Our experiments showed that, although significant mineralogical changes occurred, they did not significantly change the frictional behavior of faults. Instead, initial fault-gouge mineralogy imposed a stronger control on clay-rich fault behavior than the extent of CO2-brine-rock interactions, even under chemical conditions allowing for significant reaction. We demonstrated that the impact of mineralogical changes due to CO2-brine-rock interactions on the frictional behavior and seismogenic potential of faults could be assessed using our combination of geochemical modeling and friction experiments. Note that a complete understanding requires evaluation of additional effects, such as that of shear velocity, effective normal stress, and other fault characteristics (maturity, shear strain).

The lithosphere-asthenosphere system is fundamental to our understanding of mantle convection and plate tectonics. Seismic and electromagnetic methods are our primary means of determining its struc...

The topographic parameters and propagation velocity of Aeolian sand ripples reflect complex erosion, transport, and deposition processes of sand on the land surface. In this study, three Nikon cameras located in the windward (0-1 m), middle (4.5-5.5 m), and downwind (9-10 m) zones of a 10-m long sand bed are used to continuously record changes in sand ripples. Based on the data extracted from these images, this study reaches the following conclusions: 1) The initial formation and full development times of sand ripples over a flatbed decrease with wind velocity. 2) The wavelengths of full development sand ripples are approximately twice the wavelengths of initially formed sand ripples. Both wavelengths increase linearly with friction velocity. During the developing stage of sand ripples, the wavelength increases linearly with time. 3) The propagation velocity of full development sand ripples is approximately 0.6 times that of the initially formed sand ripples. Both wavelengths increase as power functions with respect to friction velocity. During the developing stage of sand ripples, the propagation velocity decreases with time following a power law. These results provide new information for understanding the formation and evolution of aeolian sand ripples and help improve numerical simulations.

Summary In this paper, a numerical model to predict flow-induced shear failure along pre-existing fractures is presented. The framework is based on a discrete fracture representation embedded in a continuum describing the damaged matrix. A finite volume method is used to compute both flow and mechanical equilibrium, whereas specifically tailored basis functions are used to account for the physics at discontinuities. The failure criterion is based on a maximum shear strength limit, which changes with varying compressive stress on the fracture manifold. The displacements along fracture manifolds are obtained such that force balance is achieved under conditions, where shear stress of the failing fracture segment is constrained to the maximum shear strength at the segment. Simultaneously, the fluid pressure is computed independently of the shear slip. A relaxation model approach is used to obtain the maximum shear limit on the fracture manifold, which leads to grid convergence.

Subsurface storage of CO 2 into geological formations is considered an important strategy to mitigate increasing atmospheric CO 2 . Time‐lapse seismic monitoring is an integral component of a geological CO 2 sequestration project because the seismic behavior of the rock is a function of both mineralogical composition and pore fluid properties. At the uppermost kilometer of the sedimentary basin, CO 2 can be present at gaseous, liquid, and supercritical states, with the supercritical and liquid states preferred in CO 2 storage operations due to the higher sweep efficiency. In this study, the seismic velocities [both compressional (V p ) and shear (V s ) waves] of two CO 2 ‐saturated sandstone core plugs (Red Wildmoor and Knorringfjellet formations) have been measured under a range of temperatures and pressures in which CO 2 phase transitions occur. The experiments were done using a uniaxial hydrostatic cell equipped with seismic wave transmitting and receiving transducers. The experimental investigation illustrated that seismic velocities (both V p and V s ) decreased until the critical point was reached. Further increases in the CO 2 pressure above the critical point led to a gradual increasing of V p while the V s remained unchanged. The effect of CO 2 on the seismic velocity of the sandstone was compared with the effects of N 2 and distilled water at the same conditions. It was further indicated that the seismic velocity changes were mainly connected to significant changes of CO 2 density and the corresponding bulk rock moduli over the critical point. The observed velocities are in good agreement with Gassmann‐predicted velocities as well as literature data. © 2016 Society of Chemical Industry and John Wiley & Sons, Ltd

Rayleigh wave ellipticity (or ZH ratio) is a function of frequency and is particularly sensitive to shallow crustal structure beneath the seismograph station. Since the depth sensitivity kernels of ZH ratios are different from those of dispersion data, the ZH ratio provides good complementary information for the dispersion-based inversion method. Therefore, we can combine the ZH ratio and dispersion data of Rayleigh wave fundamental mode to better invert for the velocity structure under a specific seismograph station. In this paper, we propose a joint inversion method using the dispersion and ZH ratio data based on the Neighborhood Algorithm. We conduct synthetic tests based on a theoretical model and prove the robustness of the joint inversion method, which can better constrain the shallow crustal structure. Compared to traditional inversion methods that only use dispersion data, the joint inversion can provide a more accurate crustal Vs model as well as Vp/Vs ratios for the layered crust. Finally, we apply the joint inversion technique to real measurements and obtain a more accurate crust shear velocity and Vp/Vs model beneath the station at Kunming (KMI) in southwest China.

The performance of two surface layer parameterizations (MM5-similarity/Eta-similarity schemes) in an Advanced Research Weather Research and Forecasting (ARW) mesoscale model is assessed over the tropical site Kalpakkam in southeast India. The ARW is run with four-nested domains and at a high resolution (1 km) for four 2 day periods covering eight days in each season, 9–17 January, 4–12 April, 4–12 August and 13–21 October 2014 representing winter, summer, the southwest monsoon and the northeast monsoon, respectively. Turbulence observations on winds collected using a fast-response ultrasonic anemometer are used to estimate the surface layer parameters of friction velocity (u*), and Monin–Obukhov length (L), and surface fluxes of heat and moisture using the eddy correlation method. Results of simulations indicate seasonal variation in friction velocity, drag co-efficient, turbulent fluxes of heat and moisture, air temperature, humidity and winds at the study site. Relatively higher u*, drag co-efficients, turbulent fluxes of heat and moisture are found during summer and the southwest monsoon, followed by the northeast monsoon and winter due to the prevalence of more unstable conditions in these seasons. Although the diurnal cycle of various parameters is well produced by both the schemes, the Eta-similarity scheme produced higher values of surface layer variables relative to the MM5-similarity scheme. Although both schemes depict a systematic bias in various surface layer parameters, the predicted values with MM5-similarity scheme are in better agreement with observational estimates as indicated by the qualitative and quantitative comparisons of results. The differences in the results of the two schemes are attributed to the variation in the empirical stability correction functions and the stability regimes used in the two schemes.

A 6-year climatology of radiation fog has been compiled at two sites: the Research Centre for the Lower Atmosphere (CIBA, Spain) and the Cabauw Experimental Site for Atmospheric Research (CESAR, The Netherlands). These sites are contrasting in terms of geographical situation, climate zone, altitude, humidity and soil water availability. Therefore, several climatological differences in fog abundance, onset, dissipation and duration have been quantified between both sites. The more humid site (CESAR) is characterised by relatively short radiation-fog events distributed throughout the year. However, radiation fog at the drier site (CIBA) is more persistent and appears during late-autumn/winter months. In general, its formation requires more time after sunset (~ 2 h more), since further cooling is required to reach saturation. The forecast of these fog events has been evaluated through two different approaches. On the one hand, we extend the statistical method presented by [Menut ~al.(2014)Menut, Mailler, Dupont, Haeffelin and Elias] (M14). This method uses statistics to define threshold values on key variables for fog formation (pre-fog) and verifies its predictability using observations and numerical model output. We present some of the most appropriate threshold values for the forecasting of pre-fog periods at both sites, which differ from those presented in M14 and depend on the optimisation of the hit-rate or the false-alarm rate. Additionally, we also extend M14 suggesting other variables as potential predictors for fog formation (friction velocity and visibility tendency). Finally, we focus on the fog simulation by the Weather Research and Forecasting (WRF) model in terms of liquid water content. The WRF model was able to simulate radiation fog when configured with sophisticated physical options and high resolution. However it failed simulating the onset, dissipation and vertical extension of fog (overestimated). The model results were extremely sensitive to the spin-up time.

The main difficulty in seismic full waveform inversion (FWI) is the strong nonlinearity, which is caused by the complexity of seismic wave propagation. Different components of elastic parameters result in different characteristics in seismic data. Meanwhile, different inversion quality is required in different stages of exploration or exploitation. So, there is no need to pursue matching all the seismic information during the inversion. Some problems can be solved by matching part of seismic information and the strong nonlinearity can also be avoided by this way. According to this consideration, a generalized FWI strategy and method based on seismic data subsets is presented. For different seismic data subsets, the gradients of the misfits have the same form and can be calculated by two times of seismic wave forward modeling just as traditional FWI. The only difference in calculating the gradients is the adjoint source. During the waveform inversion based on seismic data subsets, the responses of seismic data to different scales of perturbation on different media parameters should be analyzed intensively. Different seismic data subsets should be used in different stages of full waveform inversion. And then the residual of this seismic data subset is back-projected along the reasonable sub-kernels to decide where and which component of the model parameter need to be updated. As examples, envelope and reflection data subsets are used in FWI with synthetic and practical data to prove the validity and effectiveness of our presented FWI strategy and method. Especially, in the absence of low frequency contents, reasonable misfits can be used to recover the background compressional and shear velocity models using these FWI methods.

We have previously applied three-dimensional acoustic, anisotropic, full-waveform inversion to a shallow-water, wide-angle, ocean-bottom-cable dataset to obtain a high-resolution velocity model. This velocity model produced an improved match between synthetic and field data, better flattening of common-image gathers, a closer fit to well logs, and an improvement in the pre-stack depth-migrated image. Nevertheless, close examination reveals that there is a systematic mismatch between the observed and predicted data from this full-waveform inversion model, with the predicted data being consistently delayed in time. We demonstrate that this mismatch cannot be produced by systematic errors in the starting model, by errors in the assumed source wavelet, by incomplete convergence, or by the use of an insufficiently fine finite-difference mesh. Throughout these tests, the mismatch is remarkably robust with the significant exception that we do not see an analogous mismatch when inverting synthetic acoustic data. We suspect therefore that the mismatch arises because of inadequacies in the physics that are used during inversion. For ocean-bottom-cable data in shallow water at low frequency, apparent observed arrival times, in wideangle turning-ray data, result from the characteristics of the detailed interference pattern between primary refractions, surface ghosts, and a large suite of wide-angle multiple reflected and/or multiple refracted arrivals. In these circumstances, the dynamics of individual arrivals can strongly influence the apparent arrival times of the resultant compound waveforms. In acoustic full-waveform inversion, we do not normally know the density of the seabed, and we do not properly account for finite shear velocity, finite attenuation, and fine-scale anisotropy variation, all of which can influence the relative amplitudes of different interfering arrivals, which in their turn influence the apparent kinematics. Here, we demonstrate that the introduction of a non-physical offset-variable water density during acoustic full-waveform inversion of this ocean-bottom-cable field dataset can compensate efficiently and heuristically for these inaccuracies. This approach improves the travel-time match and consequently increases both the accuracy and resolution of the final velocity model that is obtained using purely acoustic full-waveform inversion at minimal additional cost.

Sea state can influence the turbulent air–sea exchanges, especially the momentum flux, by modifying the sea-surface roughness. The high-resolution non-hydrostatic convection-permitting model MESO-NH is used here to investigate the impact of a more realistic representation of the waves on heavy precipitation during the Intense Observation Period (IOP) 16a of the first HyMeX Special Observation Period (SOP1). Several quasi-stationary mesoscale convective systems developed over the western Mediterranean region, two of them over the sea, and resulted in heavy precipitation on the French and Italian coasts on 26 October 2012. Three different bulk parametrizations are tested in this study: a reference case (NOWAV) without any wave effect, a parametrization taking into account theoretical wave effects (WAV) and a last one with realistic wave characteristics from the MFWAM analyses (WAM). Using a realistic wave representation in WAM significantly increases the roughness length and the friction velocity with respect to NOWAV and WAV. The three MESO-NH sensitivity experiments of the IOP16a show that this surface-roughness increase in WAM generates higher momentum fluxes and directly impacts the low-level dynamics of the atmosphere, with a slowdown of the 10 m wind, when and where the wind speed exceeds 10 m s−1 and the sea state differs from the idealized one. The turbulent heat fluxes are not significantly influenced by the waves, these fluxes being controlled by the moisture content rather than by the wind speed in the simulations. Although the convective activity is globally well reproduced by all the simulations, the difference in the low-level dynamics of the atmosphere influences the localization of the simulated heavy precipitation. Objective evaluation of the daily rainfall amount and of the 10 m wind speed against the observations confirms the positive impact of the realistic wave representation on this simulation of heavy precipitation.

This paper presents a comparison between two methods for estimating shear stress in an atmospheric internal boundary layer over a beach surface under optimum conditions, using wind velocities measured synchronously at 13 heights over a 1.7 m vertical array using ultrasonic anemometry. The Reynolds decomposition technique determines at-a-point shear stresses at each measurement height, while the Law-of-the-Wall yields a single boundary layer estimate based on fitting a logarithmic velocity profile through the array data. Analysis reveals significant inconsistencies between estimates derived from the two methods, on both a whole-event basis and as time-series. Despite a near-perfect fit of the Law-of-the-Wall, the point estimates of Reynolds shear stress vary greatly between heights, calling into question the assumed presence of a constant stress layer. A comparison with simultaneously measured sediment transport finds no relationship between transport activity and the discrepancies in shear stress estimates. Results do show, however, that Reynolds shear stress measured nearer the bed exhibits slightly better correlation with sand transport rate. The findings serve as a major cautionary message to the interpretation and application of single-height measurements of Reynolds shear stress and their equivalence to Law-of-the-Wall derived estimates, and these concerns apply widely to boundary layer flows in general. © 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

In this work, we study mixed layer lateral dispersion that is enhanced by near-inertial internal Poincare waves in the offshore region of a large stratified lake, Lake Michigan. We examine the hypothesis that the vertical shear created by near-inertial internal Poincare waves is not only an energy source for vertical mixing in the thermocline and mixed layer, but also enhances horizontal dispersion via an unsteady shear flow dispersion mechanism. Complex empirical orthogonal function analysis reveals that the dominant shear structure is observed to mirror the thermal structure, with the location of maximum shear gradually lowered as the mixed layer deepens. This changing structure of shear and vertical mixing produces different characteristics in shear flow dispersion between the early and later stratified periods. The estimated depth-averaged surface layer vertical turbulent diffusivity grows from 10−5 m2s−1 to 10−3 m2s−1 over the stratified period, and the associated lateral dispersion coefficients are estimated as 0.1 –40 m2s−1. The Poincare waves are found to enhance greatly lateral dispersion for times less than the inertial period following release. In contrast, sub-inertial shear is the dominant mechanism responsible for shear dispersion for times greater than the inertial period. A simple approximation of the dispersion coefficient for lateral dispersion is developed, which scales as the product of surface current velocity (or wind friction velocity) and mixed layer depth. The calculated dispersion coefficients agree well with Okubo's diffusion diagram for times up to a week, which suggests that unsteady shear dispersion is a plausible mechanism to explain observed dispersion rates in the mixed layer for early times after release.

Maintenance of habitat for benthic organisms and fish spawning may require flushing of sand from gravel or cobble river beds. An approach is proposed for specifying both the magnitude and duration of flow necessary to scour sand to a desired depth over a specified length of riffle bed. Scour is assumed to begin at the upstream end of the reach to be flushed and to progress downstream. The flow magnitude necessary for scour to an equilibrium depth was found from previously published laboratory data to be related to the median sand grain size and its movability number (the ratio of shear velocity to settling velocity). The required flow duration is determined by the time for scour to progress over the specified distance. The progression rate was determined through new laboratory experiments to be related to the flow velocity and depth, the distance advanced, the sand movability number and the channel slope. Application of the approach is illustrated by example. Copyright © 2015 John Wiley & Sons, Ltd.

Aeolian sand transport results from interactions between the ground surface and airflow. Previous research has focused on the effects on sand entrainment and mass transport of surface features and wind velocity, but the influence of air density, which strongly constrains airflow characteristics and the resulting sand flow, has not been widely considered. In the present study, entrainment, saltation characteristics and transport rates were examined at nine experimental sites ranging in elevation from −154 m below sea-level (Aiding Lake) to 5076 m above sea-level (Tanggula Mountain pass on the Qinghai–Tibetan plateau). At each site, a portable wind tunnel and high-speed camera system were set up, and the friction wind velocity, threshold friction velocity and sand flow structure were observed systematically. For a given volumetric airflow, lower air density increases the wind velocity. Low air density also creates a high threshold friction velocity. The Bagnold wind erosion threshold model remains valid, but the value of empirical parameter A decreased with decreasing air density and ranged from 0·10 to 0·07, the smallest values reported in the literature. For a given wind velocity, increased altitude reduced total sand transport and creeping, but the saltation rate and saltation height increased. The present results provide insights into the fundamental mechanisms of the initiation and transport of sand by wind in regions with an extreme temperature or altitude (for example, alpine deserts and low-lying lake basins) or on other planets, including Mars. These results also provide theoretical support for improved sand-control engineering measures. The data and empirical equations provided in this paper improve the ability to estimate threshold and transport conditions for wind-blown sand.

Because of its non-conformity to Monin – Obukhov Similarity Theory (MOST), the effects of thermal stratification on scaling laws describing the streamwise turbulent intensity σu normalized by the turbulent friction velocity (u∗) continue to draw research attention. A spectral budget method has been developed to assess the variability of σu/u∗ under unstable atmospheric stratification. At least three different length-scales – the distance from the ground (z), the height of the atmospheric boundary layer (δ) and the Obukhov length (L) – are all found to be controlling parameters in the variation of σu/u∗. Analytical models have been developed and supported by experiments for two limiting conditions:z/δ 0.5. Under the first constraint, the turbulent kinetic energy spectrum is predicted to follow three regimes: k 0 , k −1 and k −5/3 , divided in the last two regimes by a break-point at kz = 1 ,w herek denotes the wave number. The quantity σu/u∗ is shown to follow the much discussed logarithmic scaling, reconciled to Townsend’s attached eddy hypothesis σ 2 u /u 2 = B1 − A1 log(z/δ), where the coefficients B1 and A1 are modified by MOST for mildly unstable stratification. Under the second constraint, the turbulent energy spectrum tends to become quasi-inertial, displayingk 0 and k −5/3 with a break-point predicted to occur at 0.3 < kz < 1. The work here brings together

A bulk air–sea flux algorithm couples the ocean and the lower atmosphere through flux boundary conditions and can be used in various analyses and in numerical models. The algorithm described here has two features not present in any other existing bulk flux algorithm. First, it has a new air–sea drag relation. Here, for wind speeds above about 9 m s−1, the friction velocity u✻, which is related to the square root of the surface stress, is linearly related to UN10, the neutral-stability 10 m wind speed. When extrapolated to hurricane-strength winds, this drag relation has better properties than relations formulated in terms of a drag coefficient or a roughness length. The second unique feature of this flux algorithm is that it recognizes two routes by which heat and moisture cross the air–sea interface: one is the interfacial route, which is controlled by molecular processes right at the air–sea interface; the second is the spray-mediated route, which is governed by microphysical processes at the surface of sea spray droplets. Through microphysical theory and our analysis of 4000 sets of eddy-covariance measurements of the scalar fluxes, we separate the measured fluxes into the interfacial and spray contributions and thereby produce the only spray flux algorithm tested and validated against oceanic data. Because all components of our flux algorithm are physics-based and validated with data for winds up to 25 m s−1, one application is extrapolating this algorithm to hurricane-strength winds, where sea spray plays a dominant role in scalar transfer.

Large-scale flow structures (LSFS) in the streamwise direction are important features of gravel-bed river flows, because they may contribute to sediment transport and gas exchange. In the present study, these structures are detected using Huang's empirical mode decomposition and reconstructed with phase-averaging techniques based on a Hilbert transform of the velocity signal. The analysis is based on the fluctuating component of 15 quasi-instantaneous velocity profiles measured with a three-dimensional (3D) acoustic Doppler velocity profiler (ADVP) in an armoured gravel-bed river with a low relative submergence of 2.9 (ratio between flow depth and bed grain diameter). LSFS were identified in most of the measured profiles and consistently showed similar features. We were able to characterize the geometry of these large-scale coherent structures: the front has a vertical linear shift in the time domain and a vertical profile corresponding to a first quarter moon with the apex situated at z/h approximate to 0.4. In the vertical, the front scales with flow depth h, and in the streamwise direction, LSFS scale with three to seven times the mean flow depth. On the bed, the effect of LSFS is a periodic non-linear variation of the friction velocity on average between 0.90 and 1.10 times the mean value. A model for the friction velocity cycle resulting from LSFS oscillation is presented. Copyright (c) 2014 John Wiley & Sons, Ltd.

Although many studies have measured denitrification in stream sediments, few have utilized these data with local water column and sediment measurements to develop a predictive model for NO₃⁻ uptake. In this study, sediment denitrification was measured from cores in five streams under various land uses in south-central Minnesota using denitrification enzyme activity (DEA) assays and amplification of the nirS gene via real-time, quantitative polymerase chain reaction. Hydraulic and environmental variables were measured in the vicinity of the sediment cores to evaluate the influence of fluid flow and chemical variables on denitrification activity. Potential denitrification rates measured using DEA assays ranged from 0.02 to 10.1 mg N m⁻² h⁻¹, and the abundance of the denitrifier gene nirS was positively correlated with these measurements (R² = 0.79, P < 0.001) for most of the streams studied. A predictive model to determine NO₃⁻ uptake via denitrification was derived, implementing dimensional analysis of variables that mediate denitrification in sand-bed streams. The proposed model explained 75% of the variability in DEA rates. The results of this study show that denitrification is most dependent on the distribution of sediment organic matter, interstitial pore space, and stream hydraulic characteristics, including shear velocity at the sediment–water interface and stream depth.

Direct numerical simulations of a turbulent boundary layer flow over a bed of hemispheres of height h are performed using an immersed boundary method for comparison with river biofilm growth experiments performed in a hydraulic flume. Flow statistics above the substrates are shown to be in agreement with measurements performed by laser Doppler velocimetry and particle image velocimetry in the experiments. Numerical simulations give access to flow components inside the roughness sublayer, and biofilm colonization patterns found in the experiments are shown to be associated with low shear stress regions on the hemisphere surface. Two bed configurations, namely staggered and aligned configurations, lead to different colonization patterns because of differences in the local flow topology. Dependence with the Reynolds number of the biofilm distribution and accrual 7 days after inoculum is shown to be associated to local flow topology change and shear stress intensity. In particular, the shear stress τ on the surface of the hemispheres is found to scale as µ(u*/h)Ret0.26 where Ret = u∗h/ν, with u* as the log law friction velocity and ν as the fluid kinematic viscosity. This scaling is due to the development of boundary layers along the hemisphere surface. Associated with a critical shear stress for colonization and early growth, it explains the increasing delay in biomass accrual for increasing flow velocities in the experiments.

It is expected that surface gravity waves play an important role in the dynamics of the ocean surface boundary layer (OSBL), quantified with the turbulent Langmuir number (La-root u(*)/u(s0), where u(*) and u(s0) are the friction velocity and surface Stokes drift, respectively). However, simultaneous measurements of the OSBL dynamics along with accurate measurements of the wave and atmospheric forcing are lacking. Measurements of the turbulent dissipation rate E were collected using the Air-Sea Interaction Profiler (ASIP), a freely rising microstructure profiler. Two definitions for the OSBL depth are used: the mixed layer derived from measurements of density (h(rho)), and the mixing layer (h(epsilon)) determined from direct measurements of epsilon. When surface buoyancy forces are relatively small, epsilon proportional to La-2 only near the surface with no dependency on La at mid-depths of the OSBL when using h(rho) as the turbulent length scale. However, if h(epsilon) is used then the dependence of epsilon with La-2 is more uniform throughout the OSBL. For relatively high destabilizing surface buoyancy forces, epsilon is proportional to the ratio of the OSBL depth against the Langmuir stability length L-L. During destabilizing conditions, the mixed and mixing layer depths are nearly identical, but we have relatively few measurements under these conditions, rather than any physical implications. Observations of epsilon are compared with the OSBL regime diagram of Belcher et al. (2012) and are generally within an order of magnitude, but there is an improved agreement if h(epsilon) is used as the turbulent length scale rather than h(rho).

SUMMARY Proposals are developed to update Tables 11.4-1 and 11.4-2 of Minimum Design Loads for Buildings and Other Structures published as American Society of Civil Engineers Structural Engineering Institute standard 7-10 (ASCE/SEI 7–10). The updates are mean next generation attenuation (NGA) site coefficients inferred directly from the four NGA ground motion prediction equations used to derive the maximum considered earthquake response maps adopted in ASCE/SEI 7–10. Proposals include the recommendation to use straight-line interpolation to infer site coefficients at intermediate values of v¯S(average shear velocity to 30-m depth). The NGA coefficients are shown to agree well with adopted site coefficients at low levels of input motion (0.1 g) and those observed from the Loma Prieta earthquake. For higher levels of input motion, the majority of the adopted values are within the 95% epistemic-uncertainty limits implied by the NGA estimates with the exceptions being the mid-period site coefficient, Fv, for site class D and the short-period coefficient, Fa, for site class C, both of which are slightly less than the corresponding 95% limit. The NGA data base shows that the median value v¯S of 913 m/s for site class B is more typical than 760 m/s as a value to characterize firm to hard rock sites as the uniform ground condition for future maximum considered earthquake response ground motion estimates. Future updates of NGA ground motion prediction equations can be incorporated easily into future adjustments of adopted site coefficients using procedures presented herein. Published 2014. This article is a U.S. Government work and is in the public domain in the USA. Earthquake Engineering & Structural Dynamics published by John Wiley & Sons Ltd.

Summary Larval transport and settlement in benthic invertebrates is theorised to be influenced by bottom roughness and the hydrodynamic forces that roughness generates near the bed. This study of freshwater Dreissena spp. bivalves examined the transport and suspension of pediveliger larvae and larval models in a laboratory flow chamber and larval settlement in Lake Erie. Particle image velocimetry (PIV) measurements in the laboratory, and acoustic Doppler velocimetry (ADV) measurements in the field, were used to estimate small-scale turbulence in the near-bed environment over differing roughness. Quadrant analysis was used to determine the frequency of turbulent sweeps and ejections, and the extent of roughness flow regime (skimming versus wake interference flow) was noted to understand the determinants of larval transport and settlement. Skimming flow generated above bottoms with high mussel densities had significantly lower suspended transport (i.e. suspension off the bottom in the flow chamber; 3.31 ± 1.14%) and lower larval settlement in the field (1526 ± 80 larvae m−2 per day) compared to low mussel densities (6.63 ± 1.54% and 1853 ± 47 larvae m−2 per day). Conversely, wake interference flow indicated by high frequencies of turbulent sweeps and ejections generated by the roughness due to mussel patches resulted in high suspended transport in the flow chamber (i.e. via ejections) and the highest larval settlement in the field (i.e. via sweeps; 1943 ± 59 larvae m−2 per day). The spatial configuration of mussel roughness influenced the creation and magnitude of skimming versus wake interference flow, which can inhibit or enhance larval settlement, respectively.

In situ experiments on sediment resuspension were carried out along a depth transect in shallow polytrophic Lake Langer See, northeast Germany (area = 1.27 km2, Zmax = 3.8 m), in winter under ice and in summer 2006, using a hydrodynamically calibrated erosion chamber (diameter 20 cm). Shear velocity U* was incrementally increased 11 times for 10 min each (0–2.57 cm s−1) to initiate resuspension. Entrainment rates (E) of suspended particulate matter (ESPM) and total P (ETP) were determined by a mass balance. Sandy nearshore sediments at 1–2 m depth showed low ESPM (0.01–6.64 g m−2 h−1) and ETP (0.2–2.96 mg m−2 h−1). There was no difference in critical shear velocity (U*crit) of incipient resuspension between winter and summer. Muddy offshore sediments at 2–3.8 m depth showed higher ESPM (0.09–106.1 g m−2 h−1) and ETP (0.06–532.3 mg m−2 h−1). Under ice, U*crit was 0.9–1.1 cm s−1 higher than in summer, indicating that the wave-unaffected sediment had ample time for consolidation and biostabilization. Wind-wave modeling, using measured ESPM and ETP, revealed that resuspension is primarily (90%) restricted to the nearshore sediments (17% lake area). Our scenarios show that annual entrainment of SPM and TP increased linearly with the duration of ice-free period. Since ice coverage increased shear resistance, climate-driven absence or shortening of ice-cover would reduce the period for settling of particles and their integration into sediments, lowering their biostabilization, thus facilitating resuspension and phytoplankton recruitment in spring after ice thaw.

This study presents the analysis of the velocity fluctuations to describe the conditional statistics of Reynolds shear stress in flow over two-dimensional dunes in the presence of surface waves of varying frequency. The flow velocity measurements over the dunes are made using a 16-MHz 3D acoustic Doppler velocimeter. The joint probability distributions of the normalized stream-wise and vertical velocity fluctuations at different vertical locations are calculated in the trough region of a selected dune in quasi-steady region of the flow. Third-order moments of the stream-wise and vertical velocity components over one dune length are also calculated throughout the flow depth for understanding the effect of surface waves on relative contributions to the Reynolds shear stress due to the four quadrant events. The structure of instantaneous Reynolds stresses is analysed using quadrant analysis technique. It has been shown that the contributions of second and fourth quadrant events to the Reynolds shear stress increase with increase in the frequency of surface waves. In fact, the largest contribution to turbulent stresses comes from the second quadrant. The cumulant discard method is applied to describe the statistical properties of the covariance term u′w′. Conditional statistics and conditional sampling are used to compare the experimental and theoretical relative contributions to the Reynolds shear stress from the four quadrant events. Copyright © 2013 John Wiley & Sons, Ltd.

The present study is focused on the analysis of the mean wall friction velocity on a surface including roughness elements exposed to a turbulent boundary layer. These roughness elements represent non-erodible particles over an erodible surface of an agglomeration of granular material on industrial sites. A first study has proposed a formulation that describes the evolution of the friction velocity as a function of geometrical parameters and cover rate with different uniform roughness distributions. The present simulations deal with non-uniform distributions of particles with a random sampling of diameters, heights, positions and arrangements. The evolution (relative to geometrical parameters of the roughness elements) of the friction velocity for several non-uniform distributions of roughness elements was analysed by the equation proposed in the literature and compared to the results obtained with the numerical simulations. This comparison showed very good agreement. Thus, the formulation developed for uniform particles was found also to be valid for a larger spectrum of particles noted on industrial sites. The present work aims also to investigate in detail the fluid mechanics over several roughness particles. Copyright © 2013 John Wiley & Sons, Ltd.

Asymptotic analysis for small long-wave perturbations of a given stationary shear flow of an ideal fluid with free boundary as is performed. It is shown that small disturbances of the flow are attracted to periodic solution in the case where the governing equations are hyperbolic on the main shear flow solution. A class of shear flows for which Landau damping is realizable, is described. Analytical results obtained are validated by numerical calculations.

* 京都大学工学研究科都市環境工学専攻 〒615-8540 京都市西京区京都大学桂,Dept. Urban and Environment. Eng., Kyoto Univ., Nishikyo-ku, Kyoto, 615-8540 ** 東京理科大学理工学部機械工学科 〒278-8510 野田市山崎,Dept. Mech. Eng., Faculty of Science and Technology, Tokyo Univ. of Sci., Yamazaki, Noda, 278-8510. *** 正 会 員 長崎大学工学部社会開発工学科 〒852-8521 長崎市文教町,Dept. Civil Eng., Nagasaki Univ., Bunkyo-machi, Nagasaki, 852-8521 1 は じ め に 近年,エネルギー・環境問題に対する社会的関心が高 まっており,岩盤工学においては,高レベル放射性廃棄 物の地層処分や二酸化炭素の地中貯留などのプロジェク トが注目を浴びている.特に,高レベル放射性廃棄物の 地層処分においては,地下水シナリオを中心に処分施設 の安全性評価を行うことが重要であると考えられてお り,特に地下空洞掘削に伴う掘削影響領域 (EDZ: Excavation Disturbed Zone)における透水・物質移行特性 の把握が重要な研究テーマの一つとして挙がっている. 岩盤の水理学的挙動は岩盤基質部よりも岩盤中に存 在する潜在的な弱面である不連続面に大きく支配され, 岩盤全体の水理学的挙動の予測には不連続面の透水特 性を把握することが重要となる.また,不連続面は応力 によって垂直・せん断変位,表面凸凹形状や岩石基質部 の変形・破壊などを生じ,不連続面内の空隙構造が変化 する.特に,掘削による応力再配分により,不連続面に 沿ってせん断変位を起こす場合には,ダイレーションの 発現と共にその空隙構造が大きく変化し,それに伴い不 連続面の透水特性も大きく変化するため,岩盤不連続面 内の応力-浸透連成挙動は,結晶質岩内に高レベル放射 性廃棄物の処分施設を建設する場合,特に重要である. 岩盤不連続面内の流体の流動機構を明らかにするた めに,ラフネスを有する粗い不連続面を対象とした流れ の数値シミュレーションが実施されてきたが,一般的に 3次元のナビエ・ストークス (NS)方程式を解く代わりに, 不連続面を局所的に滑らかな 2枚の平行平板としてモデ ル化し,流体の流れを層流と仮定して,レイノルズ方程 式(local cubic law:局所三乗則)が適用されてきた. レイノルズ方程式や局所三乗側は,流れが遅く,レイノ ルズ数が十分に小さい場合,さらに,不連続面の開口幅 の分布が急激に変化しない場合に適用が可能である.1) ただし,一般的に,複雑な空隙構造をもつ岩盤不連続面 の流体の流れは層流ではなく,局所平行平板モデルのよ うな単純なモデルで置き換えることには限界がある.不 連続面の起伏が大きい場合や,流速が速くレイノルズ数 が比較的大きい場合,不連続面内の開口幅方向の流速分 布は放物線とならず,実際の流速場との差が生まれる. このことは,岩盤不連続面内の流体の流れのみならず物 質移行特性にも重大な影響を与えると考えられる.ま た,不連続面に作用するせん断変位は不連続面内部の接 触の状態や空隙構造を変化させ,流体の流れや物質移行 特性をより複雑なものにしている. これまで,不連続面内の流体の流れに関する局所三 乗則,レイノルズ方程式の適用については多くの研究者 によって理論的にあるいは数値解析手法を用いて議論 されてきた.2)~ 4)しかし,不連続面に作用する応力や変 位(特にせん断変位)が内部の接触状況・空隙構造の変 化さらには流体の流れに与える影響について定量的に評 価した事例はあまり見られず,議論も十分されていない. 本研究では,長崎大学で実施した岩盤不連続面のせ ん断-透水同時試験(垂直応力一定下)を以下の 2つの 異なる数値解析モデルを用いてシミュレーションを行 い,室内試験の結果と比較を行った. 論 文

International audience; The local shear rate generated in a cylindrical tank equipped with a Rushton turbine was investigated using particle image velocimetry in a shear-thinning fluid (Carbopol). This non-Newtonian fluid was used in an attempt to mimic fermentation broths. Three Reynolds numbers corresponding to the transition regime were investigated. The hydrodynamics is analyzed, and the velocity field is decomposed by proper orthogonal decomposition into mean flow, organized motion, and turbulence. Then, the contributions of each flow structure to the total dissipation of kinetic energy are presented. The spatial heterogeneity of shear rate is discussed and a new expression is proposed for shear rate. This work shows that the local shear rate is highly heterogeneous in a tank. Future works will need to focus on other types of stirrer and investigate the effect of scaling up reactors on the shear rate heterogeneity.

A quality-controlled data set comprising measurements of aeolian sand transport rates obtained at three disparate field sites is used to evaluate six commonly employed transport rate models (those of Bagnold, Kawamura, Zingg, Owen, Hsu, and Lettau and Lettau) and to recalibrate the empirical constants in those models. Shear velocity estimates were obtained using the von Karman constant and an apparent von Karman parameter. Models were recalibrated using non-linear regression and non-linear regression with least-squares lines forced through axes origins. Recalibration using the apparent von Karman parameter and forced regression reduced the empirical constants for all models. The disparity between the predictions from the different models is reduced from about an order of magnitude to about a quarter of an order of magnitude. The recalibrated Lettau and Lettau model provided the greatest statistical agreement between observed and predicted transport rates, with a coefficient of determination of 0·77. Evaluation of the results suggests that our estimations of threshold shear velocity may be too slow, causing errors in predicted transport rates. Copyright © 2012 John Wiley & Sons, Ltd.

A lack of reliable hydraulic parameters has been a main factor hindering the progress in predicting ice-covered river flows; the predictions need input hydraulic parameters such as the energy, momentum and drag coefficients (α, β and CD). In this paper, a large volume of winter measurements of flow velocity collected from 26 ice-covered rivers is analysed to determine the coefficients. Using cross-sectionally distributed streamwise velocities, α and β are evaluated directly. They are also derived from empirical relationships. For both the riverbed and ice cover, CD is evaluated on the basis of turbulent boundary-layer theory and the quadratic law for friction. The results show that ice-covered river flows feature a number of velocity distributions: a single core of high velocities in the thalweg, a single core of high velocities off the thalweg and multiple cores of relatively high velocities at the cross section. The velocity distributions are significantly non-uniform. Direct evaluations give overall averages of α = 1.23 and β = 1.08. They represent 22% and 8.3% corrections to the literature values (overestimates). An examination of the velocity distributions reveals that the ratio of the maximum velocity to the cross-sectionally averaged velocity equals 1.356. It is recommended that values of CD = 0.004 ± 0.0005 and 0.002 ± 0.0005 be used for the riverbed and ice, respectively. This paper discusses turbulence shear stress and the associated length scale in the boundary layer as well as winter discharges. The results have applications to aquatic ecology, water resources development and flood prevention. Copyright © 2012 John Wiley & Sons, Ltd.

A combination of physical and statistical treatments to post-process numerical weather predictions (NWP) outputs is needed for successful short-term wind power forecasts. One of the most promising and effective approaches for statistical treatment is the Model Output Statistics (MOS) technique. In this study, a MOS based on multiple linear regression is proposed: the model screens the most relevant NWP forecast variables and selects the best predictors to fit a regression equation that minimizes the forecast errors, utilizing wind farm power output measurements as input. The performance of the method is evaluated in two wind farms, located in different topographical areas and with different NWP grid spacing. Because of the high seasonal variability of NWP forecasts, it was considered appropriate to implement monthly stratified MOS. In both wind farms, the first predictors were always wind speeds (at different heights) or friction velocity. When friction velocity is the first predictor, the proposed MOS forecasts resulted to be highly dependent on the friction velocity–wind speed correlation. Negligible improvements were encountered when including more than two predictors in the regression equation. The proposed MOS performed well in both wind farms, and its forecasts compare positively with an actual operative model in use at Riso DTU and other MOS types, showing minimum BIAS and improving NWP power forecast of around 15% in terms of root mean square error. Further improvements could be obtained by the implementation of a more refined MOS stratification, e.g. fitting specific equations in different synoptic situations. Copyright © 2012 John Wiley & Sons, Ltd.

An experimental investigation of the flow behavior of three polypropylene melts with different molecular structures during extrusion through a coat-hanger die is presented. Two linear and one long-chain branched material, rheologically characterized in shear and elongation, were investigated. Using laser–Doppler velocimeter measurements of the velocity profiles across the gap height were performed at five various locations along the die. The uniformity of the velocity distribution along the die has been assessed using the maximum velocities v0 of the corresponding velocity profiles across the gap. The velocity distribution along the die changes with throughput and temperature. Regarding the rheological properties, it was found that the power-law index of the viscosity as a function of shear rate has a decisive influence on the uniformity of flow but that the pronounced strain hardening in elongation typical of the long-chain branched polypropylene is not reflected by the velocity distribution along the die. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers

SUMMARY 1. To manage the environmental flow requirements of sedentary taxa, such as mussels and aquatic insects with fixed retreats, we need a measure of habitat availability over a variety of flows (i.e. a measure of persistent habitat). Habitat suitability measures in current environmental flow assessments are measured on a ‘flow by flow’ basis and thus are not appropriate for these taxa. Here, we present a novel measure of persistent habitat suitability for the dwarf wedgemussel (Alasmidonta heterodon), listed as federally endangered in the U.S.A., in three reaches of the Delaware River. 2. We used a two-dimensional hydrodynamic model to quantify suitable habitat over a range of flows based on modelled depth, velocity, Froude number, shear velocity and shear stress at three scales (individual mussel, mussel bed and reach). Baseline potentially persistent habitat was quantified as the sum of pixels that met all thresholds identified for these variables for flows ‡40 m 3 s )1 , and we calculated the loss of persistently suitable habitat by sequentially summing suitable habitat estimates at lower flows. We estimated the proportion of mussel beds exposed at each flow and the amount of change in the size of the mussel bed for one reach. 3. For two reaches, mussel beds occupied areas with lower velocity, shear velocity, shear stress and Froude number than the reach average at all flows. In the third reach, this was true only at higher flows. Together, these results indicate that beds were possible refuge areas from the effects of these hydrological parameters. Two reaches showed an increase in the amount of exposed mussel beds with decreasing flow. 4. Baseline potentially persistent habitat was less than half the areal extent of potentially suitable habitat, and it decreased with decreasing flow. Actually identified beds and modelled persistent habitat showed good spatial overlap, but identified beds occupied only a portion of the total modelled persistent habitat, indicating either that additional suitable habitat is available or the need to improve habitat criteria. At one site, persistent beds (beds where mussels were routinely collected) were located at sites with stable substratum, whereas marginal beds (beds where mussels were infrequently collected or that were lost following a large flood event) were located in scoured areas. 5. Taken together, these model results support a multifaceted approach, which incorporates the effects of low and high flow stressors, to quantify habitat suitability for mussels and other sedentary taxa. Models of persistent habitat can provide a more holistic environmental flow assessment of rivers.

Similarity theory using local scales was applied to the normalized standard deviation of the vertical wind component, w, and potential temperature, θ, σw/u*loc and σθ/θ*loc, where u*loc and θ*loc are the friction velocity and temperature and ‘loc’ refers to variables that are locally measured. These data were obtained in a tropical city under convective atmospheric conditions within the roughness sublayer. The following parameters were assessed based on the upwind characteristics of the site, denoted as ‘sectors’: the non-dimensional height, z/h (1.16 and 1.20), and the non-dimensional vertical heterogeneity, σh/h (0.56 and 0.32). The results obey a semi-empirical relation of the form σw/u*loc = Φ(−ζloc)1/3 (ζloc = z′/Lloc, where z′ is the effective measurement height, Lloc is the local Obukhov length). The resultant extrapolated near-neutral constants depend on σh/h: (σw/u*loc)neutral = 1.04 and 1.27 for σh/h = 0.56 and 0.32, in the range − 0.01 > ζloc > − 20. Spectral analysis of w reveals a separation of the spectral power at low non-dimensional frequencies (f − 0.6 for σh/h = 0.32. The term σθ/u*loc follows the form Φ(−ζloc)− 1/3 with constants of (σθ/θ*loc)neutral = − 1.31 and − 1.34 for the two sectors, thus being independent of both σh/h and z/h. These findings show that similarity theory using local scales is applicable for determining σw and σθ in the convective roughness sublayer but depends on the vertical heterogeneity of the upwind sectors for σw, which only affects the near-neutral σw/u*loc constants. Copyright © 2012 Royal Meteorological Society