Current structure in the Rhine region of freshwater influence

Observation with HF radar, ADCPs and conventional current meters in the Rhine ROFI system in the North Sea indicate that the dominant currents are due to tidal flow, with important contributions from winds and density-driven circulation. The tidal currents are dominated by the semidiurnal components, principally the M2, with tidal ellipses parallel to the coast and a semimajor axis of the order of 1 m s−1 at the surface. The wind-driven flow varies between 1 and 3% of the wind speed and the rotation varies with depth in qualitative agreement with Ekman theory. After removal of the tidal and wind-driven component the residual flow within this region is generally parallel to the coast (northeastward) with average surface speeds of about 10 cm s−1. A convergence zone in the surface flow was observed at the outer edge of the coastal zone. The vertical distribution of residual velocity closely follows the Heaps (1972) profile for density-driven flow, with slight deviations probably due to the tidal rectification (≈2 cm s−1)

An 11-year validation of wave-surge modelling in the Irish Sea, using a nested POLCOMS-WAM modelling system

In the future it is believed that extreme coastal flooding events will increase (in frequency and intensity) as a result of climate change. We are investigating the flood risks in the eastern Irish Sea posed by extreme storm events. Here, an 11-year simulation (01/01/1996–01/01/2007) including wave–current interaction has been validated. These data can then be used to investigate the potential for coastal flooding in the study area. To accurately model a storm event in the eastern Irish Sea both wave effects and the influence of the external surge need to be considered. To simulate the waves, we have set up a one-way nested approach from a 1° North Atlantic model, to a 1.85 km Irish Sea model, using the state-of-the-art 3rd-generation spectral WAve Model (WAM). This allows the influence of swell to be correctly represented. The Proudman Oceanographic Laboratory Coastal-Ocean Modelling System (POLCOMS) has been used to model the tide–surge interaction. To include the external surge we have set up a one-way nested approach from the 1/9° by 1/6° operational Continental Shelf surge model, to a 1.85 km Irish Sea model. For the high resolution Irish Sea model we use a POLCOMS–WAM coupled model, to allow for the effects of wave–current interaction on the prediction of surges at the coast. Using two classification schemes the coupled model is shown to be good and often very good at predicting the surge, total water elevation and wave conditions. We also find the number of low level surge events has increased in the study area over the past decade. However, this time period is too short to determine any long-term trends in the wave and surge levels

A numerical study of the ex-ROFI of the Colorado River

The freshwater discharge of the Colorado River into the Gulf of California has been reduced to negligible quantities since the construction of the Hoover Dam in 1935. These radical anthropogenic changes in the hydrography of the Colorado River Delta had striking repercussions on both physical and biological processes. Using historical river discharge data, the changes in the flow dynamics and hydrographic patterns before and after the drastic freshwater reduction are studied numerically, using a three-dimensional nonlinear shelf model. The results are applied to assess the environmental impact of the reduction of river discharge on the area. Satellite imagery is also used to compare our results with observed fronts

Asteroseismology of ZZ Ceti stars with fully evolutionary white dwarf models, I: The impact of tthe uncertainties from prior evolution on the period spectrum

Context. ZZ Ceti stars are pulsating white dwarfs with a carbon-oxygen core build up during the core helium burning and thermally pulsing Asymptotic Giant Branch phases. Through the interpretation of their pulsation periods by means of asteroseismology, details about their origin and evolution can be inferred. The whole pulsation spectrum exhibited by ZZ Ceti stars strongly depends on the inner chemical structure. At present, there are several processes affecting the chemical profiles that are still not accurately determined.Aims. We present a study of the impact of the current uncertainties of the white dwarf formation and evolution on the expected pulsation properties of ZZ Ceti stars.Methods. Our analysis is based on a set of carbon-oxygen core white dwarf models with masses 0.548 and 0.837 M⊙ that are derived from full evolutionary computations from the ZAMS to the ZZ Ceti domain. We considered models in which we varied the number of thermal pulses, the amount of overshooting, and the 12C(α,γ)16O reaction rate within their uncertainties.Results. We explore the impact of these major uncertainties in prior evolution on the chemical structure and expected pulsation spectrum. We find that these uncertainties yield significant changes in the g-mode pulsation periods.Conclusions. We conclude that the uncertainties in the white dwarf progenitor evolution should be taken into account in detailed asteroseismological analyses of these pulsating stars.Fil: de Gerónimo, Francisco César. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Althaus, Leandro Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Corsico, Alejandro Hugo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica La Plata; Argentina. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Romero, Alejandra Daniela. Universidade Federal do Rio Grande do Sul; BrasilFil: Souza Oliveira, Kepler. Universidade Federal do Rio Grande do Sul; Brasi

Toward representing wave-induced sediment suspension over sand ripples in RANS models

Parameterizations of near-bed sediment processes are commonly associated with the poor predictive skill of coastal sediment transport models. We implement a two-dimensional Reynolds-averaged Navier-Stokes model to directly assess these parameterizations by reproducing measurements obtained in large-scale wave flume experiments. A sediment transport model has been coupled to wave hydrodynamics and turbulence, and numerical experiments provide temporal and spatial variations of free surface, flow velocity, sediment concentration, and turbulence quantities. Model-data comparisons enable the direct assessment of how key suspension processes are represented and of the inherent variability of the sediment transport model. We focus on the different processes occurring above rippled beds versus dynamically flat beds. Numerical results show that increasing roughness alone is not sufficient to have good predictive capability above steep ripples. Some parameterization of the vortex entrainment process is necessary and a simple modification, which leads to constant sediment diffusivity above steep-rippled beds, is sufficient to obtain good predictions of wave-averaged suspended concentrations. Model-data comparisons for the turbulent kinetic energy are also presented and highlight the need to account for the effect of vortex entrainment on near-bed turbulence and transfer of momentum

Fate and pathways of dredged estuarine sediment spoil in response to variable sediment size and baroclinic coastal circulation

Most of the world’s megacities are located in estuarine regions supporting commercial ports. Such locations are subject to sedimentation and require dredging to maintain activities. Liverpool Bay, northwest UK, is a region of freshwater influence and hypertidal conditions used to demonstrate the impact of baroclinicity when considering sediment disposal. Although tidal currents dominate the time-varying current and onshore sediment movement, baroclinic processes cause a 2-layer residual circulation that influences the longer-term sediment transport. A nested modelling system is applied to accurately simulate the circulation during a three month period. The hydrodynamic model is validated using coastal observations, and a Lagrangian particle tracking model is used to determine the pathways of 2 sediment mixtures representative of locally dredged material: a mix of 70% silt and 30% medium sand and a mix of 50% fine sand and 50% medium sand. Sediments are introduced at 3 active disposal sites within the Mersey Estuary in 2 different quantities (500 and 1500 Tonnes). Following release the majority (83% or more) of the particles remain within the estuary due to baroclinic influence. However, particles able to leave follow 2 distinct pathways, which primarily depend on the sediment grain size. Typically the finer sediment moves north and the coarser sediment west. Under solely barotropic conditions larger sediment volumes (up to 5 times more) can leave the estuary in a diffuse plume moving north. This demonstrates the necessity of considering baroclinic influence even within a hypertidal region with low freshwater inflow for accurate particle tracking

Internal tides and tidal cycles of vertical mixing in western Long Island Sound

In estuaries, tidal period variations in the rate of vertical mixing have been observed to result from various causes: in Liverpool Bay and the York River, they have been attributed to tidal straining of the along-channel density gradient modulating stratification; in the Hudson River they arise from tidal modulation of the height of the tidal current bottom boundary layer (BBL). Along continental shelves, tidal period fluctuations in mixing have been observed to result from the dissipation of internal waves (IWs). Western Long Island Sound (WLIS) moored instrument records indicate that large near-bottom increases in dissolved oxygen (DO) and heat and a decrease in salt occur during the middle of the flood tide: an analysis of water mass signatures indicates that the transport involved is vertical and not horizontal. Temperature data from a vertical thermistor array deployed in the WLIS for 16 days in August 2009 clearly show a tidal cycle of IW activity creating a mean thermocline depression at midflood of approximately 25% of the water depth with individual IW thermocline depressions of as much as 50% of the water depth. Contemporaneous ADCP measurements show increases in shear due to IWs during the flood. Near-bottom internal wave activity is maximal at and after midflood and is correlated with near-bottom temperature and DO tendencies at both tidal and subtidal scales. We conclude that internal tides are an important vertical mixing mechanism in the WLIS through both increased shear from IWs and displacement of the pycnocline into the region of high shear in the BBL