Formation of the salinity minimum in the Mixed Water Region between the Oyashio and Kuroshio Fronts

The salinity minimum frequently occurring in the Mixed Water Region between the Oyashio and Kuroshio Fronts seems to originate from the salinity minimum at the density of 26.8σ_θ called the North Pacific Intermediate Water. We examined water exchange of this region with the Oyashio and the Kuroshio Extension using mixing ratio R_K defined as (θ - θ_OY)/(θ_K - θ_OY) × 100, where θ_OY, θ_K, and θ represent potential temperature of the Oyashio and Kuroshio Waters and their mixture on the isopycnal surfaces, respectively. CTD data were obtained by repeated observation from January 1990 to May 1991. R_K increases southward from the Oyashio Front to the Kuroshio Front with the range of −20 to 120%. The gradient of R_K on the isopycnal surfaces is large around the Oyashio Front above the 26.8σ_θ surface, while it is large around the Kuroshio Front below it. This agrees with the average R_K in the Mixed Water Region decreasing greatly with the increase of density at densities less dense than 26.8σ_θ. We calculated thickness and volume transport of the Oyashio between the isopycnal surfaces near the coast of Hokkaido. They increase largely with density at densities less dense than 26.8σ_θ. It is supposed that the salinity minimum in the Mixed Water Region is the upper limit of the water largely influenced by the Oyashio Water. Its density could depend only on the density structure of the Oyashio.     

Glacial hazards in the Rolwaling valley of Nepal and numerical approach to predict potential outburst flood from glacial lake

In recent years, climate change and retreating glaciers constitute a major hazard in the Himalaya of South Asia. Glacial lakes are rapidly developing or increasing due to climate change. The rapid development of the lake may cause outburst of the lake. The outburst discharge from the glacial lake can cause catastrophic flooding and disaster in downstream area. Therefore, it is necessary to investigate the impact of climate change on glacial lakes and to understand the characteristics of the glacial lake outburst. In this study, the field assessment of Tsho Rolpa Glacial Lake in the Himalaya of Nepal has been presented and the impact of climate change on this glacial lake has been discussed. The Tsho Rolpa Glacial Lake is the largest and most potentially dangerous glacial lake in Nepal. In addition, a numerical model has been also developed for computing the characteristics of glacial lake outburst due to moraine dam failure by seepage and water overtopping. The numerical model is tested for the flume experimental cases. The simulated results of the outburst discharge, the dam surface erosion, and the temporal variation of the moisture movement in the dam are compared with those obtained from the hydraulic model experiments. The moisture profile calculated by numerical model was agreeable with the experimental moisture profile. The simulated failure surface of the dam due to seepage by considering the suction in slope stability analysis gave more agreeable results than the Janbu's simplified method. The results of the outburst discharge and dam surface erosion also agreed with the experimental results.     

The distribution of total and methylmercury concentrations in soils near the Idrija mercury mine, Slovenia, and the dependence of the mercury concentrations on the chemical composition and organic carbon levels of the soil

Although the mining activity of the Idrija mine in Slovenia ceased in 1995, a large amount of mining dregs containing high concentrations of mercury remains in the area. The mining dregs were transported with river flow and deposition along the Idrija River. To estimate the dispersion and change in the chemical form of mercury, a total of 28 soil core samples were taken around the river. The individual core samples were separated into layers for the analysis of their chemical composition, carbon contents, total mercury (T-Hg) and methylmercury (MeHg) concentrations. The chemical composition measured by X-ray fluorescence spectrometry was useful to estimate the dispersion of tailings: the fluvial terrace soil had a chemical composition similar to that of the tailings and could be distinguished clearly from the forest soil. The highest T-Hg concentration, 1,100 mg kg⁻¹, was observed in the fluvial terrace soil near the mine. Although the concentration decreased gradually along with distance from the mine, concentrations higher than 200 mg kg⁻¹ of T-Hg were still observed in the fluvial terrace soil approximately 20 km downstream from the mine. In the vertical distribution of T-Hg in the hillslope soil, a higher value was observed in the upper layers, which suggests the recent atmospheric deposition of mercury. The concentration of MeHg was the lowest at the riverside and higher in the hillslope soil, which was the opposite of the T-Hg distribution. The total organic carbon content tracked similarly with the distribution of MeHg and a linear relation with a significantly high correlation coefficient was obtained. The distinction may be related to the different dispersion process of mercury, and the organic carbon contents may be an important factor for MeHg formation.     

Modelling artificial channel and land‐use changes and their impact on floods and sediment yield to the Ishikari basin

The potential for flooding and sediment transport is greatly affected by river channel form and changes in land use. Therefore the modelling of channel morphology prior to canalization and of land‐use change is important with respect to the prediction of floods and sediment yield and their consequences. A combination of land‐use transformation maps and soil properties shows certain decision rules for the conversion of forest into arable or vice versa. The model proposed, from this study, was used to simulate possible past and/or future channel and land‐use patterns. Subsequently, the outcome of this simulation was used to assess the risk of flooding, sediment transport and soil‐erosion under different conditions. In this study, channel morphology prior to canalization and land‐use change in the Ishikari basin, Hokkaido, Japan, were analysed by comparing three scenarios using a physical based channel and slope model. The results indicate that pre‐canalization channel morphology has a significant impact on flood peak, but no significant effect on sediment yield. In contrast, land‐use change has a significant effect on soil eroded from hillslopes, but no significant effect on flooding for Ishikari basin. This study also illustrates the challenges that a simple model, such as a physical based channel and slope model, can simulate large‐scale river basin processes using fewer hydrological data resources. Copyright © 2004 John Wiley & Sons, Ltd.     

Preservation of meandering river channels in uniformly aggrading channel belts

Channel belt deposits from meandering river systems commonly display an internal architecture of stacked depositional features with scoured basal contacts due to channel and bedform migration across a range of scales. Recognition and correct interpretation of these bounding surfaces is essential to reconstruction of palaeochannel dimensions and to flow modelling for hydrocarbon exploration. It is therefore crucial to understand the suite of processes that form and transfer these surfaces into the fluvial sedimentary record. Here, the numerical model ‘NAYS2D’ is used to simulate a highly sinuous meandering river with synthetic stratigraphic architectures that can be compared directly to the sedimentary record. Model results highlight the importance of spatial and temporal variations in channel depth and migration rate to the generation of channel and bar deposits. Addition of net uniform bed aggradation (due to excess sediment input) allows quantification of the preservation of meander morphology for a wide range of depositional conditions. The authors find that the effect of vertical variation in scouring due to channel migration is generally orders of magnitude larger than the effect of bed aggradation, which explains the limited impact bed aggradation has on preservation of meander morphology. Moreover, lateral differences in stratigraphy within the meander belt are much larger than the stratigraphic imprint of bed aggradation. Repeatedly produced alternations of point bar growth followed by cut‐off result in a vertical trend in channel and scour feature stacking. Importantly, this vertical stacking trend differs laterally within the meander belt. In the centre of the meander belt, the high reworking intensity results in many bounding surfaces and disturbed deposits. Closer to the margins, reworking is infrequent and thick deposits with a limited number of bounding surfaces are preserved. These marginal areas therefore have the highest preservation potential for complete channel deposits and are thus best suited for palaeochannel reconstruction.     

Experimental study on the rheological properties of polycrystalline solid nitrogen and methane: Implications for tectonic processes on Triton

To investigate the evolution of any processes on planetary surfaces in the outer Solar System, the rheological properties of non-water ices were studied by means of a sound velocity measurement system and a uniaxial deformation apparatus. A pulse transmission method was used to obtain longitudinal (V_p) and transverse (V_s) wave velocities through solid nitrogen and methane at temperatures ranging from 5 to 64 K and from 5 to 90 K, respectively. The measured velocities confirmed that the solid methane and solid nitrogen samples were non-porous polycrystalline samples without any cracks and bubbles inside. Compression tests at constant strain-rate were performed for solid nitrogen and methane at temperatures of 5-56 K and 5-77 K, respectively, at strain-rates of 10⁻⁴-10⁻² s⁻¹. Both brittle and ductile behavior was observed for solid nitrogen and methane under these conditions. The maximum strength of solid nitrogen was observed to be 9 MPa in the brittle failure mode, and that of solid methane was 10 MPa. These low strengths cannot support cantaloupe structures with the topographic undulation larger than several kilometers found on Triton’s surface, suggesting that other materials such as H₂O ice could underlay solid methane and nitrogen and support these structures.     

Past,present and future of a meandering river in the Bolivian Amazon basin

Field observations on small rivers of the Amazon basin are less common due to their remote location and difficult accessibility. Here we show, through remote sensing analysis and field works, the planform evolution and riverbed topography of a small river located in the upper foreland Amazon basin, the Ichilo River. By tracking planform changes over 30 years, we identified the factors that control meander migration rates in the Ichilo River: cutoffs, climate and human interventions. The data suggest that neck cutoffs are the main controls in the Ichilo River, with an annual density of 0.022 cutoffs/km. In addition, climate controls have been identified in the form of high-precipitation events that may have promoted cutoffs, an increase in meander migration rate and channel widening. The width distribution of the Ichilo River is well represented by general extreme value and inverse Gaussian distributions. The spatiotemporal variability of meandering migration rates in the Ichilo River is analysed in two locations where neck cutoffs are expected. Analysing the distance across the neck in these two points, we predict the occurrence of a new cutoff. The combined methodology of bathymetric surveys and structure from motion photogrammetry shows us the Ichilo riverbed topography and banks at high resolution, where two scour holes were identified. Finally, we discuss the impact of planform changes of the Ichilo River on communities that are established along its riverbanks.     

Morphodynamic effects of vegetation life stage on experimental meandering channels

Previous studies have demonstrated that riparian vegetation leads to channel transformation from a multi-bar to a single-thread channel planform. However, it still remains unclear how the presence of pioneer and mature vegetation affects the morphodynamics of single-thread meandering rivers. In this study, we therefore investigated the effects of vegetation strength on the morphodynamic evolution of an experimental meandering channel. Three physical laboratory experiments were conducted using alfalfa sprouts in different life stages – no vegetation, immature vegetation, and mature vegetation – to simulate different floodplain vegetation strengths. Our results demonstrate that vegetation plays a key role in mediating bank erosion and point-bar accretion, and that this is reflected in both the evolution of the channel bed as well as the sediment flux. The presence of mature vegetation maintained a deep, single-thread channel by reducing bank erosion, thereby limiting both channel widening and sediment storage capacity. Conversely, an unvegetated floodplain led to channel widening and high sediment storage capacity. Channel evolution in the unvegetated scenario showed that the active sediment supply from outer bank erosion led to slightly delayed point-bar accretion on the inner banks due to helical flow, deflecting the surface flow toward the outer banks and causing further erosion. In contrast, in the immature vegetation scenario, the outer banks were also initially eroded, but point-bar accretion did not clearly progress. This led to a greater width-to-depth ratio, resulting in a transition from a single- to a multi-thread channel with minor flow paths on the floodplain. The experimental results suggest that the eco-morphodynamic effects of young (low-strength) and mature (high-strength) vegetation are different. Notably, low-strength, early-stage vegetation increases channel complexity by accelerating both channel widening and branching, and therefore might promote the coexistence of multi-bars and pioneer vegetation.     

Numerical Investigations of Mean Winds Within Canopies of Regularly Arrayed Cubical Buildings Under Neutral Stability Conditions

Recently, several attempts have been made to model the wind velocity in an urban canopy in order to accurately predict the mixing and transport of momentum, heat, and pollutants within and above the canopy on an urban scale. For this purpose, unverified assumptions made by Macdonald (Boundary-Layer Meteorol 97:25–45, 2000) to develop a model for the profile of the mean wind velocity within an urban canopy have been used. In the present study, in order to provide foundations for improving the urban canopy models, the properties of the spatially-averaged mean quantities used to make these assumptions have been investigated by performing large-eddy simulations (LES) of the airflow around square and staggered arrays of cubical blocks with the following plan area densities: λ _p = 0.05, 0.11, 0.16, 0.20, 0.25, and 0.33. The LES results confirm that the discrepancy between the spatial average of wind velocity and Macdonald’s five-point average of wind velocity can be large in both types of arrays for large λ _p . It is also confirmed that Prandtl’s mixing length varies significantly with height within the canopy, contrary to Macdonald’s assumption for both types of arrays and for both small and large λ _p . On the other hand, in accordance with Macdonald’s assumption, the sectional drag coefficient is found to be almost constant with height except in the case of staggered arrays with high λ _p .     

An interpretation of the Nusselt-Rayleigh number relationship for convection in a spherical shell

Recent studies on the relationship between the Nusselt ( Nu ) and Rayleigh ( Ra ) numbers for base-heated convection in a spherical shell with a constant viscosity show that the power-law index is around 1/4, which is different from the value of 1/3 predicted by a simple boundary layer theory. We show that such a difference may be caused by the flow pattern due to the geometry. The flow pattern of the convection in a spherical shell at relatively low Ra , at least, less than 10⁶, is characterized by narrow upwelling and broad downwelling, which is similar to the opposite flow pattern of internally heated convection. Convection in the internally heated case predicts the power-law index of 1/4. We demonstrate this relationship based on the concept of 'local' Rayleigh ( Ra1 ) and Nusselt ( Nu1 ) numbers