Field measurements of three-dimensional hydraulics in a step-pool channel

We investigated the effects of morphologic position and discharge on flow structure in a steep (0.10 m/m) mountain channel by collecting three-dimensional measurements of time-averaged and turbulent velocity components with a SonTek FlowTracker Handheld ADV (acoustic Doppler velocimeter) on a 30-m reach of a step-pool channel in the Colorado Rockies. Velocity profiles were measured at morphologic positions characteristic of steep channels (above steps, step lips, base of steps, pools, cascades, runs), and at five different discharges. A marked three-dimensionality of flow structure was documented in East St. Louis Creek. Velocities in the streamwise component were the largest contributors to overall velocity vector magnitudes; cross-stream and vertical components contributed averages of 20% and 15%, respectively, to overall vector magnitudes. Turbulence intensities were especially multi-dimensional, however, with large contributions to turbulent kinetic energy from the vertical component of velocity. Analysis of variance indicated that discharge and morphologic position significantly affected mean streamwise velocities, with substantially higher velocities upstream from steps than in pools. Discharge and morphology effects on cross-stream and vertical velocity components, however, were not significant. Discharge and morphologic position also significantly affected turbulence intensities for all flow components, with the greatest turbulence intensities occurring in pools and at high discharges. These results illustrate the strong discharge-dependence of hydraulics in step-pool channels, where relative submergence of bedforms changes rapidly with discharge, and the substantial spatial variation in hydraulics created by step-pool sequences.     

A roughness-corrected index of relative bed stability for regional stream surveys

Quantitative regional assessments of streambed sedimentation and its likely causes are hampered because field investigations typically lack the requisite sample size, measurements, or precision for sound geomorphic and statistical interpretation. We adapted an index of relative bed stability (RBS) for data calculated from a national stream survey field protocol to enable general evaluation of bed stability and anthropogenic sedimentation in synoptic ecological surveys. RBS is the ratio of bed surface geometric mean particle diameter (D_gm) divided by estimated critical diameter (D_cbf) at bankfull flow, based on a modified Shield's criterion for incipient motion. Application of RBS to adequately depict bed stability in complex natural streams, however, has been limited because typical calculations of RBS do not explicitly account for reductions in bed shear stress that result from channel form roughness. We modified the index (RBS) to incorporate the reduction in bed shear stress available for sediment transport that results from the hydraulic resistance of large wood and longitudinal irregularities in channel dimensions (“form roughness”). Based on dimensional analysis, we derived an adjustment to bankfull shear stress by multiplying the bankfull hydraulic radius (R_bf) by the one-third power of the ratio of particle-derived resistance to total hydraulic resistance (C_p/C_t)^1/3, where both resistances are empirically based calculations. We computed C_p using a Keulegan equation relating resistance to relative submergence of bed particles. We then derived an empirical equation to predict reach-scale hydraulic resistance C_t from thalweg mean depth, thalweg mean residual depth, and large wood volume based on field dye transit studies, in which total hydraulic resistance C_t was measured over a wide range of natural stream channel complexity, including manipulation of large wood volumes. We tested our estimates of C_t and RBS by applying them to data from a summer low flow probability sample of 104 wadeable stream reaches in the Coastal Ecoregion of Oregon and Washington, USA. Stream discharges calculated using these C_t estimates compared favorably with velocity–area measurements of discharge during summer low flow, and with the range of 1 to 2-year recurrence floods (scaled by drainage area) at U.S.Geological Survey gauged sites in the same region. Log [RBS] ranged from − 4.2 to + 0.98 in the survey region. D_gm ranged from silt to boulders, while estimated bankfull critical diameter, D_cbf, ranged from very fine gravel to large boulders. The median value of D_cbf (adjusted for form roughness influences) averaged 40% (inter quartile range 28 to 59%) of the unadjusted estimate D_cbf. Log[RBS] was consistently negatively related to human disturbances likely to produce excess sediment inputs or hydrologic alteration. Log [RBS] ranged from − 1.9 to + 0.5 in the streams within the lower quartile of human disturbance in their basin and riparian areas and was substantially lower (− 4.2 to − 1.1) in streams within the upper quartile of human disturbance. The synoptic survey methods and designs we used appear adequate to evaluate regional patterns in bed stability and sedimentation and their general relationship to human disturbances. Although the RBS concept also shows promise for evaluating sediment and bed stability in individual streams, our approach is relatively coarse, so site-specific assessments using these rapid field methods might prudently be confined to identifying severe cases of sedimentation or channel alteration. Greater confidence to discern subtle differences in site-specific assessments could be gained by calculating RBS using more precise field measurements of channel slope, bed particle size and bankfull dimensions, and by refining our adjustments for energy loss from channel form roughness.     

Tracking of saltating sand trajectories over a flat surface embedded in an atmospheric boundary layer

Saltation is a major mechanism for the transport of soil particles. In the present study, we carried out wind tunnel tests to examine the saltating trajectories of two types of natural sand collected from a beach (diameter, d = 300–500 μm and 200–300 μm respectively) as well as sand from the Taklimakan desert (d = 100–125 μm) in an atmospheric boundary layer. Consecutive images of saltating particles were recorded using a high-speed digital camera at a rate of 2000 fps with a spatial resolution of 1024 × 1024 pixels. The high temporal resolution of the acquired images enabled us to study the particle motion very close to the surface. The saltating particle trajectories were reconstructed from consecutive images, and the physical quantities characterizing the initial and final stages of the particle flight in the windward direction at friction velocities of about 10%–25% above the threshold friction velocity (u / u_t = 1.11–1.26) were analyzed statistically. In addition, the transverse deviation of the saltating particles from the main streamwise direction was evaluated. The results shed new light on the complicated motions involved in sand saltation and should prove useful in the evaluation and formulation of theoretical models.     

The assessment of weathering stages in granites using an EC/pH meter

The sidewall effects of a wind tunnel on aeolian sand transport were investigated experimentally. A wind tunnel was used to conduct the experiments with a given channel height of 120 cm and varying widths (B) of 40, 60, 80, 100 and 120 cm. Both vertical profiles of wind velocity and sand mass flux were measured at different locations across the test section. The results show that the wind velocity with saltation first increases and then decreases to a minimum, from the sidewall to the central line of the wind tunnel. The discrepancy among wind velocities at different locations of the transverse section decreases with decreasing tunnel width. The wind friction velocity across the wind tunnel floor, with the exception of the region closest to the sidewalls, does not deviate strongly in wide wind tunnels from that along the central line, whereas it does vary in narrow tunnels. The sand mass fluxes, with the exception of some near-bed regions, are larger along the central line of the wind tunnel than they are at the quarter width location from the sidewall. Unlikely previously reported results, the dimensionless sand transport rate, Qg / (ρu³) (where Q is the total sand transport rate, g is the gravitational acceleration constant, ρ is the air density, and u is the wind friction velocity), first decreases and then increases with the dimensionless friction velocity, u / u_t (where u_t is the threshold friction velocity). The above differences may be attributed to the sidewall effects of the wind tunnel. A dimensionless parameter, F_B = u / (gB)^1/2, is defined to reflect the sidewall effects on aeolian sand transport. The flows with F_B of 0.33 or less may be free from the sidewall effects of the wind tunnel and can ensure accurate saltation tunnel simulation.     

Sand transport under increased lateral jetting of raindrops induced by wind

Wind tunnel experiments for ‘Raindrop Detachment and Wind-Driven Transport’ (RD–WDT) process were conducted under improved lateral jetting induced by wind velocities of 6.4, 10, and 12 m s^− 1 at nozzle operating pressures of 75, 100, and 150 kPa. Wind-driven rainfalls were also incident on the windward and leeward slopes of 4° and 9° to have a broad variation in the angle of incidence. The objective of this experimental set-up was to distinguish the roles of both impact components of obliquely striking wind-driven raindrops on RD and wind on WDT. Raindrop impact components and reference horizontal wind were quantified by normal (E_tz) and horizontal (E_tx) kinetic energy fluxes and wind shear velocity (u), respectively, to physically model the process of RD–WDT. The results showed, at each level of u, differential sand transport rates by RD–WDT (q_m(RD–WDT)) occurred depending on the magnitude of raindrop impact components, and q_m(RD–WDT) increased as the relative contribution of E_tz increased. Although E_tx was more correlated with q_m(RD–WDT) than E_tz, the extreme increases in E_tx at the expense of E_tz brought about no increases but decreases in q_m(RD–WDT). An RD–WDT model was built under the process of examining the discrete effects of E_tz and E_tx on RD together with u and resulted in a better coefficient of determination (R² = 0.89) than only total kinetic energy (E_t) did alone with u (R² = 0.84). In this study, E_tx was strongly related to u and not to E_tz, which was the principal difference from the previous rainsplash studies, which relied on the compensatory lateral jet development by the compressive pressure build-up at the raindrop–soil interface. Including E_tx in the RD–WDT model both separated the distinct role of each raindrop impact component in RD and improved the performance of u in WDT by better distinguishing its interaction with E_tx, which was not explicitly separated in previous models of RD–WDT.     

The spatial characterization of turbulence around large roughness elements in a gravel-bed river

This study characterizes the flow field above and around multiple instream submerged cobbles, boulders, and pebble clusters in order to obtain a better understanding of the hydrodynamics associated with large roughness elements (LREs) in gravel-bed rivers. Spatially distributed high frequency, three-dimensional velocity measurements were recorded in situ using acoustic Doppler velocimeters at different flow stages. The spatial distributions of turbulent kinetic energy, k_e, longitudinal component integral timescales, ITS_u, and Reynolds shear stresses were characterized and are presented for selected sites. The longitudinal–vertical Reynolds shear stress increased with flow stage more strongly than the longitudinal–lateral or lateral–vertical Reynolds shear stresses and dominate at the highest measured flows. Canonical redundancy analysis was used to relate LRE morphometrics and mean flow conditions to the turbulence parameters estimated in the LRE wakes (i.e., k_e, ITS_u, and Reynolds shear stresses). LRE size and mean unobstructed velocity explained the highest proportion of the variance in the turbulent wake statistics. Multivariate regression models based on LRE width, mean unobstructed longitudinal velocity and flow depth are presented offering a tool to predict LRE wake turbulence.     

The influence of relative sediment supply on riverine habitat heterogeneity

The diversity of aquatic habitats in streams is linked to physical processes that act at various spatial and temporal scales. Two components of many that contribute to creating habitat heterogeneity in streams are the interaction between sediment supply and transport capacity and the presence of local in-stream structures, such as large woody debris and boulders. Data from previously published flume and field studies and a new field study on tributaries to the South Yuba River in Nevada County, California, USA, were used to evaluate the relationship between habitat heterogeneity, local in-stream structural features and relative sediment supply. Habitat heterogeneity was quantified using spatial heterogeneity measures from the field of landscape ecology. Relative sediment supply, as expressed by the sediment supply/transport capacity ratio, which controls channel morphology and substrate textures, two key physical habitat characteristics, was quantified using a dimensionless bedload transport ratio, q. Calculated q values were plotted against an ecologically meaningful heterogeneity index, Shannon's Diversity Index, measured for each study reach, as well as the percent area of in-stream structural elements. The results indicate two potential mechanisms for how relative sediment supply may drive geomorphic diversity in natural river systems at the reach scale. When less mobile structural elements form a small proportion of the reach landscape, the supply/capacity ratio dictates the range of sediment textures and geomorphic features observed within the reach. In these settings, channels with a moderate relative sediment supply exhibit the highest textural and geomorphic diversity. In contrast, when less mobile structural elements are abundant, forced local scour and deposition creates high habitat heterogeneity, even in the presence of high relative sediment supply.     

Minimum Acceptance Criteria for Geostatistical Realizations

Geostatistical simulation is being used increasingly for numerical modeling of natural phenomena. The development of simulation as an alternative to kriging is the result of improved characterization of heterogeneity and a model of joint uncertainty. The popularity of simulation has increased in both mining and petroleum industries. Simulation is widely available in commercial software. Many of these software packages, however, do not necessarily provide the tools for careful checking of the geostatistical realizations prior to their use in decision-making. Moreover, practitioners may not understand all that should be checked. There are some basic checks that should be performed on all geostatistical models. This paper identifies (1) the minimum criteria that should be met by all geostatistical simulation models, and (2) the checks required to verify that these minimum criteria are satisfied. All realizations should honor the input information including the geological interpretation, the data values at their locations, the data distribution, and the correlation structure, within acceptable statistical fluctuations. Moreover, the uncertainty measured by the differences between simulated realizations should be a reasonable measure of uncertainty. A number of different applications are shown to illustrate the various checks. These checks should be an integral part of any simulation modeling work flow.     

Sampling Size Requirements to Delineate Spatial Variability of Soil Properties for Site-Specific Nutrient Management in Rubber Tree Plantations

Site-specific nutrient management is an important strategy to promote sustainable production of rubber trees in order to obtain high yields of natural rubber. Making effective nutrient management decisions for rubber trees depend on knowing the spatial variations of soil fertility properties in advance. In this study the Kriging geostatistical method was used to examine the spatial variability of soil total nitrogen (TN), organic matter (OM), available phosphorus (AP) and available potassium (AK) in a typical hilly rubber tree plantation in Hainan, China. The spatial variability of the soils was small for the TN and OM and had medium variability for the AP and AK variables. Anisotropic semivariograms of all soil properties revealed that elevation and building contour ledge can profoundly affect the spatial variability of soil properties in the plantation, except for the AK variable. Soil samples had to be collected in alignment with the direction of elevation and perpendicular to the direction of building contour ledges, which was needed to obtain more reliable information within the study area in the rubber tree plantation. In formulating a sample scheme for AK, the distribution features of the soil’s parent material should be considered as the influence factor in the study field. The Kriging method used to guide the soil sampling for spatial variability dertermination of soil properties was about 2-5 times more efficient than the classic statistical method.     

Three-dimensional structure of flow at a confluence of river channels with discordant beds

This paper presents three-dimensional data of the mean and turbulent structure of flow collected at a natural confluence of rivers with discordant beds to (1) describe the three-dimensional flow field of a natural junction of channels; (2) assess the role of changes in bed morphology occurring during transport-effective events on the structure of flow at a confluence; and (3) examine how the three-dimensional structure of flow varies with changes in the ratio of momentum flux between the two confluent streams. Three-dimensional measurements of velocity were reconstructed from the measurements obtained with an array of four, two-component electromagnetic current meters. Six detailed velocity profiles were taken at five cross-sections in a wide range of flow conditions. The mean field of flow is characterised by (1) the acceleration of flow in the downstream portion of the post-confluence channel, but by lower velocities upstream in the mixing layer area; (2) a stagnation zone at the apex of the junction; (3) a zone of flow deviation, and strong fluid upwelling, close to the avalanche face and at the margin of the tributary mouth bar; and (4) reduced velocities over the depositional bar at the downstream junction corner. The position and extent of these zones vary with changes in the ratio of momentum flux. Very high intensity of turbulence (peaks up to 50%) and turbulent kinetic energy were observed in the mixing layer region. Distortion of the mixing layer, characteristic of flow where bed discordance is present between the two tributary channels, was evident from mean and turbulent flow data. This field study suggests that the effects of bed discordance on flow, sediment transport, and the resultant bed morphology must be incorporated into conceptual and numeric models of these sites of complex flow.     

A treatise on the use of geostatistics for the characterization of nonrenewable resources

Five decades of geostatistical development are reviewed to summarize the state of the art for spatial interpolation vis-à-vis kriging or a form thereof. Although a search of the literature reveals a variety of kriging methods, there are but two infrastructures for geostatistical interpolation: simple cokriging, for estimating a single variable using two variables, and generalized cokriging, for estimating one or more variables using the same number of variables that are estimated. The many forms of kriging are varieties of these two interpolation infrastructures. This notion is emphasized to aid the selection of an appropriate interpolation model for a nonrenewable resource. These models are discussed, and literature for the models and for applicable software is cited. Additionally, all aspects of spatial interpolation are discussed, including the adequacy of spatial sampling, distribution characteristics of spatial samples, semivariograms, search parameters, and selection of interpolation models in conformance with spatial data characteristics. Finally, the relationship between interpolation and raster-based geographic information systems is emphasized.     

Spatial variability of soil hydrophobicity after wildfires in Montana and Colorado

Increases in runoff and erosion after wildfires are often attributed to the development of hydrophobic soils. The potential for increased overland flow depends on the spatial contiguity of the hydrophobicity as well as its overall strength, but there is limited information on the spatial variability of soil hydrophobicity. We conducted spatially intensive hydrophobicity measurements in 225 m² and 1 m² plots in forested areas of Montana and Colorado burned at moderate to high severity, and in unburned control plots. Both the burned and unburned 225 m² plots contained 10–23 hydrophobic soil patches in which hydrophobicity was strongest at the surface and declined rapidly with depth. The hydrophobic patches were closer together and up to 3 times larger in the burned plots. Consequently, 19% to 76% of the burned plots were hydrophobic compared to just 11% of the unburned plots. In five of the six burned plots, the patches were not laterally connected, suggesting that in most cases Hortonian overland flow generated from hydrophobic patches will infiltrate near its point of origin. The 1 m² plots were smaller than most of the hydrophobic patches, so they did not capture the spatial characteristics of soil hydrophobicity. Characterization of the spatial variability of soil hydrophobicity should be based on measurements conducted at  1 m intervals across areas of > 100 m². Due to the patchiness of soil hydrophobicity at the 10⁰ to 10¹ meter scale, overland flow measurements in small ( 1 m²) plots may overestimate the magnitude and variability of runoff from burned catchments.     

Using landscape characteristics to define an adjusted distance metric for improving kriging interpolations

Interpolation of point measurements using geostatistical techniques such as kriging can be used to estimate values at non-sampled locations in space. Traditional geostatistics are based on the spatial autocorrelation concept that nearby things are more related than distant things. In this study, additional information was used to modify the traditional Euclidean concept of distance into an adjusted distance metric that incorporates similarity in terms of quantifiable landscape characteristics such as topography or land use. This new approach was tested by interpolating soil moisture content, pH and carbon-to-nitrogen (C:N) ratio measured in both the mineral and the organic soil layers at a field site in central Sweden. Semivariograms were created using both the traditional distance metrics and the proposed adjusted distance metrics to carry out ordinary kriging (OK) interpolations between sampling points. In addition, kriging with external drift (KED) was used to interpolate soil properties to evaluate the ability of the adjusted distance metric to incorporate secondary data into interpolations. The new adjusted distance metric typically lowered the nugget associated with the semivariogram, thereby better representing small-scale variability in the measured data compared to semivariograms based on the traditional distance metric. The pattern of the resulting kriging interpolations using KED and OK based on the adjusted distance metric were similar because they represented secondary data and, thus, enhanced small-scale variability compared to traditional distance OK. This created interpolations that agreed better with what is expected for the real-world spatial variation of the measured properties. Based on cross-validation error, OK interpolations using the adjusted distance metric better fit observed data than either OK interpolations using traditional distance or KED.     

Atmospheric boundary layer structure and turbulent flux transfer over the Zhongshan Station area,Antarctica

ＡｔｍｏｓｐｈｅｒｉｃｂｏｕｎｄａｒｙｌａｙｅｒｓｔｒｕｃｔｕｒｅａｎｄｔｕｂｕｌｅｎｔｆｌｕｘｔｒａｎｓｆｅｒｏｖｅｒｔｈｅＺｈｏｎｇｓｈａｎＳｔａｔｉｏｎａｒｅａ，ＡｎｔａｒｃｔｉｃａTX＠曲绍厚＠高登义＠邹捍Ａｔｍｏｓｐｈｅｒｉｃｂｏｕｎｄａｒｙｌａｙ...     

Three-dimensional distribution analysis of phosphorus content of limestone through a combination of geostatistics and artificial neural network

One of the factors that determines the suitability of limestone for industrial use and its commercial value is phosphorus (P) content, i.e., the weight percentage of phosphorus contained in small quantities of limestone. Because P content changes locally, geostatistical techniques including semivariogram, ordinary kriging, and conditional indicator sequential simulation were used in this study to identify the spatial correlation of P content and to estimate its three-dimensional distribution in an open-pit mine. The P content data at 43,000 points of five different bench levels were analyzed. It was found that the horizontal semivariograms produced by using the data at the same bench level show anisotropic behavior and are represented by the sum of two spherical models with different ranges and sills. The twelve vertical semivariograms were also constructed from P content in boring cores. After these semivariograms were classified into four types, a multilayered neural network was applied to clarify the horizontal distribution of each one. One of the twelve semivariograms was assigned to an arbitrary grid point in the study area by the criterion that its type is the same as the one estimated at the point and the borehole site producing the semivariogram is the nearest to the point. With this technique, ordinary kriging combined with the semivariogram of borehole data provided a proper estimation of P content in the depth direction.     

Modeling Short-Term Spatial and Temporal Variability of Groundwater Level Using Geostatistics and GIS

Continuous depletion of groundwater levels from deliberate and uncontrolled exploitation of groundwater resources lead to the severe problems in arid and semi-arid hard-rock regions of the world. Geostatistics and geographic information system (GIS) have been proved as successful tools for efficient planning and management of the groundwater resources. The present study demonstrated applicability of geostatistics and GIS to understand spatial and temporal behavior of groundwater levels in a semi-arid hard-rock aquifer of Western India. Monthly groundwater levels of 50 sites in the study area for 36-month period (May 2006 to June 2009; excluding 3 months) were analyzed to find spatial autocorrelation and variances in the groundwater levels. Experimental variogram of the observed groundwater levels was computed at 750-m lag distance interval and the four most-widely used geostatistical models were fitted to the experimental variogram. The best-fit geostatistical model was selected by using two goodness-of-fit criteria, i.e., root mean square error (RMSE) and correlation coefficient (r). Then spatial maps of the groundwater levels were prepared through kriging technique by means of the best-fit geostatistical model. Results of two spatial statistics (Geary’s C and Moran’s I) indicated a strong positive autocorrelation in the groundwater levels within 3-km lag distance. It is emphasized that the spatial statistics are promising tools for geostatistical modeling, which help choose appropriate values of model parameters. Nugget-sill ratio (<0.25) revealed that the groundwater levels have strong spatial dependence in the area. The statistical indicators (RMSE and r) suggested that any of the three geostatistical models, i.e., spherical, circular, and exponential, can be selected as the best-fit model for reliable and accurate spatial interpolation. However, exponential model is used as the best-fit model in the present study. Selection of the exponential model as the best-fit was further supported by very high values of coefficient of determination (r ² ranging from 0.927 to 0.994). Spatial distribution maps of groundwater levels indicated that the groundwater levels are strongly affected by surface topography and the presence of surface water bodies in the study area. Temporal pattern of the groundwater levels is mainly controlled by the rainy-season recharge and amount of groundwater extraction. Furthermore, it was found that the kriging technique is helpful in identifying critical locations over the study area where water saving and groundwater augmentation techniques need to be implemented to protect depleting groundwater resources.     

Effects of Microtopographically Concentrated Recharge on Nitrate Variability in a Confined Aquifer: Model Simulations

Simulated recharge to a shallow confined aquifer indicates that locally concentrated recharge through microtopographically determined recharge areas (having elevation differences of less than 2 cm) can have a substantial effect on the spatial variability of nitrate-N concentrations [N-NO₃] in the upper meter of an aquifer. For an aquifer that is confined and near pressure equilibrium with the watertable, hydraulic mounds formed under recharge areas can cause temporary large local increases in hydraulic gradient and subsequent flux from the aquitard to the upper aquifer. These hydraulic surges carry nitrates from the aquitard to the upper aquifer, where they move as local plumes and disperse according to the properties of the prevailing aquifer flow system, until discharged, reduced to ammonia, or denitrified. Under conditions of maximum local recharge the ratio of aquifer nitrate concentration to till nitrate concentration ([N-NO₃]_a/[N-NO₃]_t) may approach an upper limit of 1. Understanding these processes is important for interpretation of nitrate and other solute measurements near the aquifer-aquitard boundary.     

The landscape Reynolds number and other dimensionless measures of Earth surface processes

An analogy between turbulent fluid systems and landscape drainage systems [Parker, G., Haff, P.K., Murray, A.B., 2001, EOS, Transactions of the American Geophysical Union, 82, pp. F564.] is suggested by the observation that transport in both systems can be approximated by diffusion with size-proportional effective diffusivities, with a cross-over at small scales to Fickian diffusion. The “landscape” Reynolds number of a typical fluvial landscape is estimated to be of order Re_L 10⁶ to 10⁹, these large values reflecting the relative efficiency of fluvial transport compared to creep. Re_L is the ratio of the large-scale effective diffusivity of rivers to the small-scale diffusivity of creep processes on hillslopes. The spatial dependence of the effective diffusivity produces rivers with logarithmic long-profiles, similar to the profiles of many rivers in nature, and analogous to the logarithmic dependence of mean fluid velocity on distance from a wall in turbulent flow. The landscape example suggests how other generalized “Reynolds numbers” can be constructed as ratios of large-scale to small-scale diffusivities to measure the efficiencies of complex processes that affect the surface. As an example, the global airline transportation network is estimated to have an efficacy relative to that of direct human mechanisms for transport of similar goods and materials of about 10⁸ as measured by a corresponding “technology” Reynolds number. The appearance of such large dimensionless numbers, pertaining to the consequences of human invention and design, reflects the emergence of the technosphere as an increasingly efficient overlay on the historical domain of biology and surficial geology.     

Anisotropic turbulence in weakly stratified rotating magnetoconvection

Numerical simulations of the 3-D MHD-equations that describe rotating magnetoconvection in a Cartesian box have been performed using the code NIRVANA. The characteristics of averaged quantities like the turbulence intensity and the turbulent heat flux that are caused by the combined action of the small-scale fluctuations are computed. The correlation length of the turbulence significantly depends on the strength and orientation of the magnetic field and the anisotropic behavior of the turbulence intensity induced by Coriolis and Lorentz force is considerably more pronounced for faster rotation. The development of isotropic behavior on the small scales—as it is observed in pure rotating convection—vanishes even for a weak magnetic field which results in a turbulent flow that is dominated by the vertical component. In the presence of a horizontal magnetic field the vertical turbulent heat flux slightly increases with increasing field strength, so that cooling of the rotating system is facilitated. Horizontal transport of heat is always directed westwards and towards the poles. The latter might be a source of a large-scale meridional flow whereas the first would be important in global simulations in case of non-axisymmetric boundary conditions for the heat flux.     

Computational fluid dynamics and the physical modelling of an upland urban river

This paper describes the application of a commercially available, three-dimensional computational fluid dynamic (CFD) model to simulate the flow structure in an upland river that is prone to flooding. Simulations use a rectangular channel geometry, smooth sidewalls and a bed topography obtained from the field site that contains a subdued pool–riffle sequence. The CFD model uses the RNG κ– turbulence closure scheme of Yakhot and Orszag (J. Sci. Comput. 1 (1986) 1), as implemented in FLUENT 4.4.4, with a free surface. Results are shown for numerical runs simulating a 1:100 year return interval flood. Output from the numerical model is compared to a physical model experiment that uses a 1:35 scale fibreglass mould of the field study reach and measures velocity using ultrasonic Doppler velocity profiling (UDVP). Results are presented from the numerical and flume models for the water surface and streamwise velocity pattern and for the secondary flows simulated in the numerical model. A good agreement is achieved between the CFD model output and the physical model results for the downstream velocities.Results suggest that the streamwise velocity is the main influence on the flow structure at the discharge and channel configuration studied. Secondary flows are, in general, very weak being below the resolution of measurement in the physical model and less than 10% of the streamwise velocity in the numerical model. Consequently, there is no evidence for a ‘velocity dip’. It is suggested that the subdued topography or inlet morphology may inhibit the development of secondary flows that have been recorded in previous flat-bed, rectangular open channel flows. A significant corollary of these results is that the morphological evolution of the pool–riffle sequence at high discharges may be controlled primarily by the downstream distribution of velocity and sediment transport with little role for lateral sorting and sediment routing by secondary flows. This paper also raises a number of issues that may be of use in future CFD modelling of three-dimensional flow in open channels within the geomorphological community.