Overlying Water Layer Research Articles

SV-wave scattering by an underwater circular cylindrical canyon in a saturated poroelastic half-space

To study the effects of underwater terrain on plane seismic waves (P-SV wave) propagation, the theoretical solution of ground displacement, strain, rocking, and energy for the underwater terrain is derived based on Biot’s theory. Numerical results in terms of incident angle and Poisson’s ratio are illustrated for various porosities and degrees of solid frame stiffness. The results show that the response of a saturated-fluid porous soil under overlying water is controlled by the solid frame stiffness and Poisson’s ratio. Considering the effect of complex site amplification, the scattering theoretical solution of plane SV wave by a circular-arc canyon with overlying water is first derived based on the free-wave field above obtained. The effects of the incident frequencies, the incident angles, and the circular arc canyon on the ground motion of the soil-water interface are investigated parametrically. In particular, the effect of overlying water on ground motion is studied, and the results indicate that the existence of an overlying water layer absorbs the energy of a seismic wave to a certain extent. Meanwhile, the accuracy of the results is also verified by reducing the complex site to that without an overlying water layer and no canyon. Finally, based on the theoretical solution derived above, the coherence functions and the underground variable seismic motion of the site also are simulated, respectively. Compared with ground motions, the numerical results show that underground motions are smaller, which indicates that the canyon has a certain amplification effect on the seismic motions.

Efficient simulation of stochastic seismic response of long-span bridges in river valleys using hybrid BEM-FEM

Determining ground motions is essential for seismic analysis of long-span bridges located at complex sites. However, there is a lack of earthquake records for local site conditions. Thus, this study focused on the effect of local site conditions on the stochastic response of long-span high-pier rigid frame bridges crossing a valley with an overlying water layer. For this purpose, a hybrid boundary element and finite element method (BEM-FEM) is proposed to investigate the stochastic seismic response of long-span bridges at such a complex site. First, a simulation method for 3-D spatially varying ground motions ascribed to wave scattering from irregular surface topography, sedimentary soil, and water layers was established based on the BEM combined with the spectral representation method. Furthermore, the statistical properties of the bridge response were explored based on the FEM and Monte Carlo simulations. The advantage of this work is that it reveals how the seismic wave propagation in this particular valley topography and water layer affects the stochastic response of the bridge considered. Moreover, the results indicate that seismic wave scattering leads to a significant energy redistribution of the input waves in the tri-directions related to the surface location, and a decrease in the coherency loss functions at all frequencies. Owing to this change, the peak ground accelerations (PGAs), bridge response means, and response variability are doubled. The beneficial effect of the water layer on the PGAs is weakened, while the adverse effect of hydrodynamic pressure dominates. The parameter analysis results demonstrate that a 10-m-deep sedimentary soil layer produces a 47% increase in the internal force of the pier bottom. The solid-liquid dynamic coupling effect should be accurately quantified for bridges located at saturated sites. Considering the local site conditions, the coefficient of variation (COV) of the response of the pier top exceeded 0.3, indicating strong randomness.

Study of Scholte wave dispersion curves and modal energy distribution using a wavefield numerical simulation method

Surface waves travelling along the subsurface interface between water and sediment are called Scholte (or generalised Rayleigh) waves. In this paper, we examine the characteristics of Scholte wave dispersion curves and their modal energy distribution through synthetic simulation tests. Analytical solutions of the dispersion equation for a homogeneous isotropic layered medium describe the propagation characteristics of the fundamental and higher modes, but do not provide information about the relative energy distributions of those wave modes. In real situations, the dispersion curves with different modes have different energies corresponding to different frequency bands. It is instructive to study and analyse the behaviours of Scholte wave dispersion curves by using joint dispersion equation solutions and numerical simulations results. Dispersion equation solutions are used to validate the numerical results, which in turn are used to assess energy distribution for both the fundamental and the higher modes. We observe that the thickness of the overlying water layer and the elastic properties of the ocean bottom play important roles in the dispersion characteristics of Scholte waves. The dispersion of the soft ocean-bottom model (whose S-wave velocity of the first solid layer is lower than the P-wave velocity of water) is dominated by the fundamental mode and appears to be less dependent on the thickness of the overlying water layer. In contrast, the dispersion of the hard ocean-bottom model contains significant energy in the higher modes. Higher modes will shift to lower frequencies and will be spatially closer (in the frequency direction) when the water layer thickness increases. The dispersion characteristics of the real data are in a good agreement with the conclusion of a soft ocean-bottom model. Our modelling approach of combining analytical solutions of the dispersion equation with numerical modelling can facilitate Scholte wave exploration in water-covered areas.

Numerical model on the flow dynamics around the sediment-water interface in the tidal coastal area

Investigating flow dynamics around the sediment-water interface is one of difficult issues in marine environment. It has important significance to study the physical, chemical and biological activities between the overlying water and sedimentary layer with tidal forcing. In this study, we established a numerical model to investigate the flow dynamics around the sediment-water interface in the tidal coastal area. The model reflected the hydrodynamics of periodic reciprocating unsteady flow in the overlying water layer and the fully coupled simulation of flow movement between the overlying water layer and sedimentary layer. A sedimentary layer, an overlying water layer, and an air layer were all treated as the fluid zone. The unsteady Reynolds-averaged Navier-Stokes equations, and the Reynolds stress model with porosity, were solved by the finite volume method. Moreover, the drag source term in the momentum equation was modified as the Darcy-Forchheimer extended form. The model showed a strong performance in simulating the hydrodynamics of coupled flow around the sediment-water interface with tidal forcing in a coastal area. With considering the sedimentary layer, the benthic boundary layer velocity distributions were more accurate than those obtained by a wall function model. Around the sediment-water interface, there was inertial loss in the flow. Furthermore, velocity increased with increasing porosity, and velocity gradient became larger. Compared with models with Darcy's Law, the numerical model in this study had better performance in the turbulent characteristics of sediment-water interface layer. The model can lead to better understanding of the exchange mechanisms of oxygen, nitrogen and nutrient between overlying water and sediment.

Shaking-Table Tests for Immersed Tunnels at Different Sites

Immersed tunnels are typically built in areas subjected to ground motion. Therefore, an evaluation of the seismic performance of the soil-tunnel system is essential. A series of shaking-table tests was conducted to study the influences of the site soil and overlying water layer on the seismic responses of soil deposits and an immersed tunnel. Detailed information on the experiment setup is provided with special focus on the similitude relationship, fabrication of the model system, measurement setup, and loading procedures for a simulation of the seismic waves. Three groups of tests at different sites in dry sand, saturated sand, and saturated sand with an overlying water layer were carried out using the same seismic excitations. The seismic responses of the soil deposits and the dynamic responses of the tunnel model were obtained. The experiment results indicate that, when considering only horizontal earthquake excitations, soil liquefaction significantly influences the propagation of seismic waves and the dynamic responses of the tunnel, whereas the water layer has no obvious effects on the dynamic performance of the ground or tunnel. Furthermore, the acceleration responses of the tunnel elements were analyzed qualitatively, and the joints are deemed important elements in an antiseismic immersed tunnel design.

In-situ measurement of free trace metal concentrations in a flooded paddy soil using the Donnan Membrane Technique

The field Donnan Membrane Technique (DMT) has been used successfully to measure in-situ free trace metal concentrations in surface waters. However, it has not been applied previously in submerged soil systems including flooded paddy rice fields. We tested this technique in a column experiment with a flooded paddy soil contaminated with trace metals and compared the DMT measurements with predictions from a geochemical speciation model. Flooding led to a strong gradient in the redox potential (Eh) along the soil column, and the pH and concentrations of Mn, Fe, and dissolved organic carbon increased with decreasing Eh. Total dissolved trace metal concentrations decreased from the overlying water layer to the soil layers, which might be ascribed to the elevated pH outweighing the effect of the increased DOC concentrations under anaerobic conditions. Also, free trace metal concentrations were lower in the soil solution of the upper and bottom soil layers than in the overlying water layer, largely due to the increased pH under anaerobic conditions. The DMT measurements and model predictions were similar in the aerobic water layer, except for Pb. In both anaerobic soil layers, however, Cu and Pb were poorly predicted. The applied geochemical modeling approach, which is frequently being used and performs well under aerobic conditions, does not fully capture all the chemical processes occurring under anaerobic conditions. Overall, the field DMT proved to be a useful tool for the in-situ measurement of free trace metal concentrations in flooded paddy soils.

Nitrate-Nitrogen Dynamics and Nitrogen Budgets in Rice-Wheat Rotations in Taihu Lake Region, China

Nitrate-nitrogen (NO−3-N) dynamics and nitrogen (N) budgets in rice (Oryza sativa L.)-wheat (Triticum aestivum L.) rotations in the Taihu Lake region of China were studied to compare the effects of N fertilizer management over a two-year period. The experiment included four N rates for rice and wheat, respectively: N1 (125 and 94 kg N ha−1), N2 (225 and 169 kg N ha−1), N3 (325 and 244 kg N ha−1), and N0 (0 kg N ha−1). The results showed that an overlying water layer during the rice growing seasons contributed to moderate concentrations of NO−3-N in sampled waters and the concentrations of NO−3-N only showed a rising trend during the field drying stage. The NO−3 -N concentrations in leachates during the wheat seasons were much higher than those during the rice seasons, particularly in the wheat seedling stage. In the wheat seedling stage, the NO−3-N concentrations of leachates were significantly higher in N treatments than in N0 treatment and increased with increasing N rates. As the NO−3-N content (below 2 mg N L−1) at a depth of 80 cm during the rice-wheat rotations did not respond to the applied N rates, the high levels of NO−3-N in the groundwater of paddy fields might not be directly related to NO−3-N leaching. Crop growth trends were closely related to variations of NO−3-N in leachates. A reduction in N application rate, especially in the earlier stages of crop growth, and synchronization of the peak of N uptake by the crop with N fertilizer application are key measures to reduce N loss. Above-ground biomass for rice and wheat increased significantly with increasing N rate, but there was no significant difference between N2 and N3. Increasing N rates to the levels greater than N2 not only decreased N use efficiency, but also significantly increased N loss. After two cycles of rice-wheat rotations, the apparent N losses of N1, N2 and N3 amounted to 234, 366 and 579 kg N ha−1, respectively. With an increase of N rate from N0 to N3, the percentage of N uptake in total N inputs decreased from 63.9% to 46.9%. The apparent N losses during the rice seasons were higher than those during the wheat seasons and were related to precipitation; therefore, the application of fertilizer should take into account climate conditions and avoid application before heavy rainfall.

Laboratory impacts into dry and wet sandstone with and without an overlying water layer: Implications for scaling laws and projectile survivability

Abstract— Scaling laws describing crater dimensions are defined in terms of projectile velocity and mass, densities of the materials involved, strength of the target, and the local gravity. Here, the additional importance of target porosity and saturation, and an overlying water layer, are considered through 15 laboratory impacts of 1 mm diameter stainless steel projectiles at 5 km s−1 into a) an initially uncharacterized sandstone (porosity ˜17%) and b) Coconino Sandstone (porosity ˜23%). The higher‐porosity dry sandstone allows a crater to form with a larger diameter but smaller depth than in the lower‐porosity dry sandstone. Furthermore, for both porosities, a greater volume of material is excavated from a wet target than a dry target (by 27–30%). Comparison of our results with Pi‐scaling (dimensionless ratios of key parameters characterizing cratering data over a range of scales) suggests that porosity is important for scaling laws given that the new data lie significantly beneath the current fit for ice and rock targets on a πv versus π3 plot (πv gives cratering efficiency and π3 the influence of target strength). An overlying water layer results in a reduction of crater dimensions, with larger craters produced in the saturated targets compared to unsaturated targets. A water depth of approximately 12 times the projectile diameter is required before craters are no longer observed in the targets. Previous experimental studies have shown that this ratio varies between 10 and 20 (Gault and Sonett 1982). In our experiments ˜25% of the original projectile mass survives the impact.

A Laboratory Study of Sediment and Contaminant Release during Gas Ebullition

Significant quantities of gas are generated from labile organic matter in contaminated sediments. The implications for the gas generation and subsequent release of contaminants from sediments are unknown but may include enhanced direct transport such as pore water advection and diffusion. The behavior of gas in sediments and the resulting migration of a polyaromatic hydrocarbon, viz phenanthrene, were investigated in an experimental system with methane injection at the base of a sediment column. Hexane above the overlying water layer was used to trap any phenanthrene migrating out of the sediment layer. The rate of suspension of solid particulate matter from the sediment bed into the overlying water layer was also monitored. The experiments indicated that signifi-cant amounts of both solid particulate matter and contaminant can be released from a sediment bed by gas movement with the amount of release related to the volume of gas released. The effective mass transfer coefficient of gas bubble-facilitated contaminant release was estimated under field conditions, being around three orders of magnitude smaller than that of bioturbation. A thin sand-capping layer (2 cm) was found to dramatically reduce the amount of contaminant or particles released with the gas because it could prevent or at least reduce sediment suspension. Based on the experimental observations, gas bubble-facilitated contaminant transport pathways for both uncapped and capped systems were proposed. Sediment cores were sliced to obtain phenanthrene concentration. X-ray computed tomography (CT) was used to investigate the void space distribution in the sediment penetrated by gas bubbles. The results showed that gas bubble migration could redistribute the sediment void spaces and may facilitate pore water circulation in the sediment.

Experimental evaluation of the water ingression mechanism for corium cooling

Experiments were performed to assess the significance of water ingression cooling in the quenching of molten corium. Water ingression is a mechanism by which water penetrates into cracks and pores of solidified corium to enhance cooling that would otherwise be severely limited by the low thermal conductivity of the material. Quench tests were conducted with 2100 °C melts weighing ∼75 kg composed of UO 2, ZrO 2 and chemical constituents of concrete. The amount of concrete in the melts was varied between 4% and 23%. The melts were quenched with an overlying water layer; three tests were conducted at a system pressure of 1 bar and four tests at 4 bar. The measured cooling rates were found to decrease with increasing concrete content and, contrary to expectations, are essentially independent of system pressure. For the lower concrete content melts, cooling rates exceeded the conduction-limited rate with the difference being attributed to the water ingression mechanism. Measurements of the permeability of the corium “ingots” produced by the quench tests were used to obtain a second, independent set of dryout heat flux data, which exhibits the same trend as the quench test data. The data was used to validate an existing dryout heat flux model based on corium permeability associated with thermally induced cracking. The model uses the thermal and mechanical properties of the corium and coolant, and it reproduces the very particular data trend found for the dryout heat flux as a function of concrete content. The model predicts that water ingression cooling would be most effective for concrete-free corium mixtures such as in-vessel type melts. For such a melt the model predicts a dryout heat flux of ∼400 kW/m 2 at a pressure of 1 bar. The results of this study provide an experimental basis for a water ingression model that can be incorporated into computer codes used to assess accident management strategies.

Relationship of Sediment Redox Conditions to Methyl Mercury in Surface Sediment of Louisiana Lakes

Surface sediment from three Louisiana Lakes containing overlying water layer spiked with 2 µg/g (2 ppm) mercury were incubated under oxygenated (air) and nonoxygenated (N2) conditions for determining the impact of oxygen status of overlying water on methylation of Hg in surface sediment from these lakes. The added mercury resulted in a greater than ten fold increase in methyl mercury (MeHg) as compared to native concentration of MeHg. The increase in methyl Hg production was less in sediment in which overlying water was exposed to oxygen rather than nitrogen. Results suggest that methyl Hg production would be less in lakes containing an oxygenated water column. In parallel microcosm studies without added mercury, MeHg decreased in sediment when redox potential of sediment suspension was increased from −200 mV to + 50 mV. Results of these studies demonstrate the importance of oxygenation or redox condition of surface sediment on mercury methylation and demethylation. Sediment conditions, which either reduce methylation or enhance demethylation in surface sediment, will limit the bioavailability of MeHg to the aquatic environment.

EFFECT OF CONVECTION IN PONDED WATER ON THE THERMAL REGIME OF A PADDY FIELD

Flooded conditions in a lowland paddy field provide a unique condition critical to the growth and development of rice plants, particularly their early growth. In order to illustrate the phenomena of a surface water layer on the thermal regime, a vertical temperature profile was measured in the paddy field, and the results were confirmed with model simulations under infrared and solar radiation, which were regarded as non-convective and convective systems, respectively. The results show that solar radiation is absorbed at the soil surface and heat energy is transferred to the overlying water layer by convection and to the underlying soil by conduction. Convection continues even at night as a result of the temperature differences between the boundaries of the water layer. Convection in the water layer plays an important role in defining the thermal regime of a paddy field.

Sensitivity analysis of a simulation model of methane flux from the Florida Everglades

To determine parameters having the greatest influence on methane flux, I developed a 7-compartment, 4-layer model of methane production in the Florida Everglades. The four layers consist of a bottom sediment layer, a top, active sediment layer, an algal-mat layer, and an overlying water layer. The compartments are acetate (a major end product of anaerobic decomposition), sulfate-reducing and methane-producing bacteria (major competitors for acetate), sulfate, methane, methane-oxidizing bacteria, and oxygen. Acetate production, depth of the active sediment layer, diffusion rate of oxygen into the sediment, and methane oxidation rate had the greatest influence of methane flux. A two-way sensitivity analysis showed that a change in one of these parameters could enhance or diminish the influence of another. Acetate production was the most sensitive parameter; as it increased, the rate of methane flux increased. Increased oxygen diffusion or methane oxidation diminished the effect of acetate production. Increased depth of the active sediment layer enhanced the effect of acetate production. As depth increased, the amount of acetate produced per area increased, increasing methane production and methane flux. At depths greater than 0.22 m, methane flux asymptotically tapered off. The increased depth increased the distance over which methane must diffuse. This constrained the amount of methane flux from the system. At shallow depths, increased oxygen diffusion and methane oxidation produced significant changes in methane flux. With increasing depth, the influence of these parameters became insignificant. This was attributed to the smaller contribution of oxygen diffusion to the larger volume of sediment, which constrained oxygen concentration and thus methane oxidation.

Crustal structure and magmatism of North Atlantic continental margins

Enormous variations exist along rifted margins of the North Atlantic in the magmatic activity that accompanied continental breakup. The North Atlantic margins exhibit a full range of magmatic behaviour. One end of the spectrum (commonly termed ‘volcanic’ margins) is found in the northern North Atlantic off Greenland, Norway and northwest Britain, where huge volumes of igneous rock were added to the continental crust as it rifted, and the adjacent oceanic crust is considerably thicker than normal. The other end of the spectrum (so-called ‘non-volcanic’ margins) occurs further south off France and Spain, where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust. Results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced. The volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle, and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere. Large quantities of igneous rock were produced as the continents broke up to form the North Atlantic where the newly initiated Iceland plume brought abnormally hot mantle beneath the rift. Much smaller quantities of melt were produced further south, where mantle temperatures were normal during rifting. The distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust. Where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years, as is common during the initial stages of rifting, then the rising asthenospheric mantle has time to cool by conduction. Heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region. As a consequence of the reduced mantle temperatures, considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts. This explains the abnormally thin oceanic crust found adjacent to the non-volcanic rifted continental margins in the North Atlantic. As the new ocean widens, the spreading centre becomes isolated from the continental margins on either side by newly formed, warm oceanic lithosphere, and the rate of rifting generally increases. The oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces, and all the melt formed by decompression of the mantle solidifies close to the spreading axis. Basaltic melts can intrude continental crust far more easily than oceanic crust. Basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts. This redistributes melt laterally towards the continent in both volcanic and, to a lesser extent, in non-volcanic margins and may be a contributory factor in leaving unusually thin crust in the first-formed ocean immediately adjacent to some rifted margins. It also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the Rockall area northwest of Britain.