Three-dimensional Layer-integrated Modelling of Estuarine Flows with Flooding and Drying

Details are given of the refinement and application of a thee-dimensional (3-D) layer-integrated numerical model of tidal circulation, with the aim of simulating severe tidal conditions for practical engineering applications. The mode splitting strategy has been used in the model. A set of depth-integrated 2-D equations are first solved to give the pressure gradient, and the layer-integrated 3-D equations are then solved to obtain the vertical distributions of the flow velocities. Attention has been given to maintaining consistency of the physical quantities derived from the 2-D and 3-D equations. A TWO=layer mixing length turbulence model for the vertical shear stress distribution has been included in the model. Emphasis has been focused on applying the model to a real estuary, which is geometrically complicated and has large tidal ranges giving rise to extensive flooding and drying. The model has been applied to three examples, including: wind-driven flow in a rectangular lake, tidal circulation in a model rectangular harbour, and tidal circulation in a large estuary. Favourable results have been obtained for both the simple and complex flow beds.     

Variation of roughness length of a mobile sand bed in a tidal flow

Velocity profiles, suspended sand concentration, and ripple shapes were measured over the tidal cycle on a mobile bed. Variations in ripple shape or dimensions, and the effects of mobile sediment could not account for the variations in bed roughness length, which increased systematically during the tide from 0.5 to 1.3 cm. The effects of acceleration could be the cause as they would lead to the same pattern of variation, but existing theory does not give the correct magnitude.     

Reconstruction of ancient sea conditions with an example from the Swiss Molasse

Ancient sea conditions can be estimated from the grain size, spacing and steepness of preserved ripple-marks. The element of greatest uncertainty in such reconstructions is the relationship between near-bed orbital diameter of water particles and the ripple spacing. This relationship is simple for vortex ripples of high steepness but is problematical for the low-steepness forms known as post-vortex, rolling-grain or anorbital ripples.

The existence field for wave ripples is between the threshold velocity for sediment movement and the onset of sheet flow, most low-steepness forms occurring close to the bed planation threshold. A range of maximum period of formative waves can be obtained using combinations of orbital diameter and orbital velocity, assuming linear wave theory to be a reasonable approximation.

Probable wave heights, wave lengths and water depths can be investigated using the transformation of wave parameters in shallowing waters and the constraints on wave dimensions provided by the wave-breaking condition. Given reasonable estimates of wave height, crude estimates of wave power allow a comparison of ancient wave-influenced sequences with modern counterparts.

Wave ripple-marks preserved in the Upper Marine Molasse of western Switzerland have been investigated. Results, which are in agreement with regional geology, suggest deposition in a seaway of approximately 100 km width, where moderate period waves (T = 3–6 s) were generated. The depositional facies belts were adjusted to the prevailing waves, tides and fluvial outflows.     

Simulation of wave power devices

Classical frequency and time domain models of a single degree of freedom wave power device are presented. In the time domain, a convolution integral is conventionally used to represent the fluid dynamic radiation force, characterised by added mass and damping in the frequency domain. This integral is replaced by an approximate ordinary differential equation (ODE) model which is faster and more convenient in simulations. A time domain model of the fluid dynamics of an oscillating water column (OWC) device is derived to illustrate the technique. Digital simulations of the OWC are used to compare the accuracy of the classical and ODE models. The simulation of the ODE model runs about six times as fast as the classical model based on convolution, yet characterises the fluid dynamics accurately.     

Multipole expansions for wave diffraction and radiation in deep water

A multipole expansion of the velocity potential is described for two- and three-dimensional wave diffraction and radiation problems. The velocity potential is expressed in terms of a series of multipole potentials. The wave terms and the local disturbance terms are represented by separated multipole potentials. Floating bodies and submerged bodies are treated in the same way. This approach differs from that of some other authors, who considered floating bodies and submerged bodies separately and derived entirely different multipoles. Semi-analytical solutions for a circular cylinder in two-dimensional motions are given. It is found that the local disturbance decays rapidly and steadily. The general application of the multipole expansion to arbitrary geometries is also presented, based on a method coupling multipoles to a boundary integral expression. Numerical results for several floating and submerged cylinders are presented.     

Trace element distributions in coastal waters along the US-Mexican boundary: relative contributions of natural processes vs. anthropogenic inputs

Trace element concentrations (Pb, Cd, Mn, Fe, and Zn) were measured along four surface water transects across the continental shelf off Baja California, to evaluate the magnitude of heavy metal contamination in the coastal waters along the US-Mexican boundary. These initial measurements of trace elements in Mexican neritic waters revealed offshore concentration gradients, with the highest levels in coastal waters with high salinities and nutrient concentrations. There were also longshore gradients, with lower concentrations in the southern locations. Although the relative enrichment of metals detected at nearshore stations along the US-Mexican border appeared to correspond to wastewater discharges in that area, these trace metal enhancements were found to be primarily associated with physical oceanographic processes (upwelling and advection), rather than anthropogenic inputs. This was demonstrated both by metal-nutrient correlations and multivariate statistical analyses. Mass balance calculations also indicated that about 1% of Cd, 9% of Zn, and 29% of Pb were from urban discharges within the area.     

Estimates of the characteristics of the horizontal turbulent exchange in the northwestern part of the Pacific Ocean

Estimates of the characteristics of the horizontal turbulent exchange (coefficients of horizontal exchange, scales of deviations of the velocity of geostrophic currents from the background flows, and horizontal scales of perturbations) in the upper ocean layer were obtained on the basis of the data of expeditions held in 1978 and 1980 in the northwestern part of the Pacific Ocean. It is shown that the characteristics of the horizontal turbulent exchange strongly depend on the structure of the background flows, and one of the causes of their variability is related to tropical cyclones. A tendency to a decrease (to different degrees) of the turbulent coefficients in the wake of a tropical cyclone is noted.     

On the relevance of iron adsorption to container materials in small-volume experiments on iron marine chemistry: Fe-aided assessment of capacity, affinity and kinetics

Iron chemistry in seawater has been extensively studied in the laboratory, mostly in small-volume sample bottles. However, little has been reported about iron wall sorption in these bottles. In this paper, radio-iron ⁵⁵Fe was used to assess iron wall adsorption, both in terms of capacity, affinity and kinetics. Various bottle materials were tested. Iron sorption increased from polyethylene/polycarbonate to polymethylmetacrylate (PMMA)/high-density polyethylene/polytetrafluoroethylene to glass/quartz, reaching equilibrium in a 25–70 h period. PMMA was studied in more detail: ferric iron (Fe(III)) adsorbed on the walls of the bottles, whereas ferrous iron (Fe(II)) did not. Considering that in seawater the inorganic iron pool mostly consists of ferric iron, the wall will be a factor that needs to be considered in bottle experiments.The present data indicate that for PMMA with specific surface (S)-to-volume (V) ratio S/V, both iron capacity (42 ± 16 × 10^− 9 mol/m² or 1.7 × 10^− 9 mol/L recalculated for the S/V-specific PMMA bottles used) and affinity (log K_Fe'W = 11.0 ± 0.3 m²/mol or 12.4 ± 0.3 L/mol, recalculated for the S/V-specific PMMA bottles used) are of similar magnitude as the iron capacity and -affinity of the natural ligands in the presently used seawater and thus cannot be ignored.Calculation of rate constants for association and dissociation of both Fe'L (iron bound to natural occurring organic ligands) and Fe'W (iron adsorbed on the wall of vessels) suggests that the two iron complexes are also of rather similar kinetics, with rate constants for dissociation in the order of 10 ⁻⁴–10^− 5 L/s and rate constants for association in the order of 10⁸ L/(mol s). This makes that iron wall sorption should be seriously considered in small-volume experiments, both in assessments of shorter-term dynamics and in end-point observations in equilibrium conditions. Therefore, the present data strongly advocate making use of iron mass balances throughout in experiments in smaller volume set-ups on marine iron (bio) chemistry.     

On the use of poorly soluble hygroscopic substances for modification of warm clouds and fogs

From analysis of the relationships between the equilibrium state of condensation nuclei and the relative humidity, a conclusion is made concerning the preferred use of poorly soluble substances for the generation of artificial condensation nuclei (ACN) designed for modification of warm clouds and fogs with the purpose of their dissipation and precipitation formation. The advantage of poorly soluble substances over commonly used soluble salts is that the finely dispersed part of the spectrum of the poorly soluble ACN does not deliquesce and so is not involved in the formation of cloud droplets. For experimental testing of the conclusions, preference was given to cement, whose main soluble substance is calcium oxide with a solubility of about 1 g/l. The spectrum of dispersed cement particles was measured and compared with the spectra of pyrotechnic flares widely used for modification at present. The process of formation of the cloud droplet spectra was simulated in the aerosol chamber of the Institute of Experimental Meteorology by decreasing the preliminarily generated excess pressure. It was found in these experiments that, compared to the spectra of particles formed on background condensation nuclei, the introduction of dispersed cement leads to the broadening of spectra and to a decrease in the concentration of droplets. Even at the early stage of condensation, droplets with radii of ∼20 μm appear. In this case, no “overseeding” phenomenon is observed, which, for soluble substances, manifests itself in an increase in the concentration of cloud droplets with a large ACN concentration. These effects indicate that, according to the existing concepts about the mechanism of warm-cloud modification with hygroscopic substances, the introduction of poorly soluble ACN (in particular, dispersed cement) below the base of cumulus clouds should stimulate coagulation processes and accelerate rain-formation processes. Considerations are given that the introduction of poorly soluble ACN into the already existing cumulus or stratocumulus clouds or fogs should also result in the acceleration of precipitation-formation processes or cloud dispersal. Comparison of cement powder with the well-known means of warm-cloud and fog modification is carried out. Original Russian Text ? N.P. Romanov, A.S. Drofa, N.S. Kim, A.V. Savchenko, G.F. Yaskevich, 2006, published in Izvestiya AN. Fizika Atmosfery i Okeana, 2006, Vol. 42, No. 1, pp. 80–91.