Showing papers on "Waves and shallow water published in 2013"

Simulating high‐frequency atmosphere‐ocean mass variability for dealiasing of satellite gravity observations: AOD1B RL05

Abstract: [1] An improved version of the OMCT ocean model with 1° spatial resolution provides bottom pressure anomalies for the new release 05 of the GRACE Atmosphere and Ocean Dealiasing Level 1B (AOD1B) product. For high-frequency signals with periods below 30 days, this model explains up to 10 cm2 of the residual sea level variance seen by ENVISAT in large parts of the Southern Ocean, corresponding to about 40% of the observed sea level residuals in many open ocean regions away from the tropics. Comparable amounts of variance are also explained by AOD1B RL05 for colocated in situ ocean bottom pressure recorders. Although secular trends contained in AOD1B RL05 cause GRACE KBRR residuals to increase in shallow water regions, we find a reduction of those residuals over all open ocean areas, indicating that AOD1B RL05 is much better suited to remove nontidal high-frequency mass variability from satellite gravity observations than previous versions of AOD1B.

Response of land surface fluxes and precipitation to different soil bottom hydrological conditions in a general circulation model

Abstract: [1] Very different approaches exist in land surface models (LSMs) to describe the water fluxes at the soil bottom, from free drainage to zero flux, and even upward fluxes if the soil is coupled to a water table. To explore the influence of these conditions on the water cycle in a unified framework, we introduce new boundary conditions in the ORCHIDEE LSM, which is coupled to the atmospheric general circulation model LMDZ. We use a zoomed and nudged configuration centered over France to reproduce the observed regional weather. Soil moisture and evapotranspiration increase ranging from free drainage to impermeable bottom, then by prescribing saturation closer and closer to the surface. The corresponding response patterns can be related to both climate regimes and soil texture. When confronted to observations from the SIRTA observatory 25 km south of Paris, which exhibits a shallow water table, the best simulations are the ones with prescribed saturation. The local precipitation, however, is only increased if the new bottom boundary conditions are applied globally. The magnitude of this increase depends on the evaporation and on the relative weight of local versus remote sources of moisture for precipitation between Western and Eastern Europe. This suggests that the summer warm/dry bias of many climate models in this region might be alleviated by including a sufficiently realistic ground water description.

Analyzing near water surface penetration in laser bathymetry – A case study at the River Pielach

Abstract: Recent developments in sensor technology yielded a major progress in airborne laser bathymetry for capturing shallow water bodies. Modern topo-bathymetric small foot print laser scanners do no longer use the primary near infrared (NIR) signal (=1064 nm) but only emit and receive the frequency doubled green signal (λ = 532 nm). For calculating correct water depths accurate knowledge of the water surface (air-water-interface) is mandatory for obtaining accurate spot positions and water depths. Due to the ability of the green signal to penetrate water the first reflections do not exactly represent the water surface but, depending on environmental parameters like turbidity, a certain penetration into the water column can be observed. This raises the question if it is even feasible to determine correct water level heights from the green laser echoes only. In this article, therefore, the near water surface penetration properties of the green laser signal are analyzed based on a test flight of the River Pielach (Austria) carried out with Riegl's VQ-820-G (532 nm) and VQ-580 (1064 nm) scanners mounted on the same airborne platform. It is shown that within the study area the mean penetration into the water column is in the range of 10–25 cm compared to the NIR signal as reference. However, as the upper hull of the green water surface echoes coincides with the NIR signal with cm-precision, it is still possible to derive water surface models from the green laser echoes only via statistical analysis of aggregated neighboring echoes and robustly keep the underestimation of the water level below 6 cm. This especially holds for still and stationary flowing water bodies.

Shallow water waves generated by subaerial solid landslides

Abstract: SUMMARY Subaerial landslides are common events, which may generate very large water waves. The numerical modelling and simulation of these events are thus of primary interest for forecasting and mitigation of tsunami disasters. In this paper, we aim at describing these extreme events using a simplified shallow water model to derive relevant scaling laws. To cope with the problem,twodifferentnumericalcodesareemployed:one,SPHysics,isbasedonaLagrangian meshless approach to accurately describe the impact stage whereas the other, Gerris, based on a two-phase finite-volume method is used to study the propagation of the wave. To validate Gerris for this very particular problem, two numerical cases of the literature are reproduced: a vertical sinking box and a 2-D wedge sliding down a slope. Then, to get insights into the problem of subaerial landslide-generated tsunamis and to further validate the codes for this case of landslides, a series of experiments is conducted in a water wave tank and successfully compared with the results of both codes. Based on a simplified approach, we derive different scaling laws in excellent agreement with the experiments and numerical simulations.

Freak waves in weakly nonlinear unidirectional wave trains over a sloping bottom in shallow water

Abstract: Using a Boussinesq model with improved linear dispersion, we show numerical evidence that bottom non-uniformity can provoke significantly increased probability of freak waves as a wave field propagates into shallower water, in agreement with recent experimental results [K. Trulsen, H. Zeng, and O. Gramstad, “Laboratory evidence of freak waves provoked by non-uniform bathymetry,” Phys. Fluids 24, 097101 (2012)]. Increased values of skewness, kurtosis, and probability of freak waves can be found on the shallower side of a bottom slope, with a maximum close to the end of the slope. The increased probability of freak waves is typically seen to endure some distance into the shallower domain, before it decreases and reaches a stable value depending on the depth. The maxima of the statistical parameters are observed both in the case where there is a region of constant depth after the slope, and in the case where the uphill slope is immediately followed by a downhill slope. In the case that waves propagate over a...

Observations of shallow water marine ambient sound: The low frequency underwater soundscape of the central Oregon coast

Abstract: A year-long experiment (March 2010 to April 2011) measuring ambient sound at a shallow water site (50 m) on the central OR coast near the Port of Newport provides important baseline information for comparisons with future measurements associated with resource development along the inner continental shelf of the Pacific Northwest. Ambient levels in frequencies affected by surf-generated noise (f < 100 Hz) characterize the site as a high-energy end member within the spectrum of shallow water coastal areas influenced by breaking waves. Dominant sound sources include locally generated ship noise (66% of total hours contain local ship noise), breaking surf, wind induced wave breaking and baleen whale vocalizations. Additionally, an increase in spectral levels for frequencies ranging from 35 to 100 Hz is attributed to noise radiated from distant commercial ship commerce. One-second root mean square (rms) sound pressure level (SPLrms) estimates calculated across the 10-840 Hz frequency band for the entire year long deployment show minimum, mean, and maximum values of 84 dB, 101 dB, and 152 dB re 1 μPa.

Two‐dimensional two‐layer shallow water model for dam break flows with significant bed load transport

Abstract: River flow dynamics and sediment transport are intimately interdependent. Their interaction governs a great diversity of flows with significant morphologic consequences. To predict the bed load transport induced by dam break waves, the proposed two-dimensional (2D) two-layer shallow water description considers an upper layer made of clear water and a lower layer made of a dense mixture of water and moving grains. Continuity and 2D momentum conservation are written for each layer, which allows the depth-averaged velocities to be distinct in magnitude and direction in both layers. The model accounts for the grain entrainment across the bed interface and for the mass and momentum exchanges between the flowing layers. The system of governing equations, written so that no loss of hyperbolicity occurs in the conservative part, is solved by a Harten-Lax-Van Leer finite volume scheme on an unstructured triangular mesh. The numerical model is tested against four dam break flows over mobile beds: a theoretical radial problem and three laboratory experiments in 1D and 2D configurations. © 2013 John Wiley & Sons, Ltd.

Classical Aspects of Hawking Radiation Verified in Analogue Gravity Experiment

Abstract: There is an analogy between the propagation of fields on a curved spacetime and shallow water waves in an open channel flow. By placing a streamlined obstacle into an open channel flow we create a region of high velocity over the obstacle that can include wave horizons. Long (shallow water) waves propagating upstream towards this region are blocked and converted into short (deep water) waves. This is the analogue of the stimulated Hawking emission by a white hole (the time inverse of a black hole). The measurements of amplitudes of the converted waves demonstrate that they appear in pairs and are classically correlated; the spectra of the conversion process is described by a Boltzmann-distribution; and the Boltzmann-distribution is determined by the change in flow across the white hole horizon.

Modeling tidal sand wave formation in a numerical shallow water model: the role of turbulence formulation

Abstract: Tidal sand waves are prominent dynamic bottom features in shallow sandy seas. Up to now, the processes controlling the formation of these bedforms have only been studied in stability sand wave models, in which geometry, boundary conditions and turbulence models are schematized. In this paper we present simulations of sand wave formation and migration with a numerical shallow water model (Delft3D), in which we restrict us to bedload transport and analyse the initial formation stage only. First, it is shown that the reproduction of the basic sand wave formation mechanisms in a numerical shallow water model requires careful treatment of model geometry, initial profile, vertical resolution and lateral boundary conditions. Second, an intercomparison between the Delft3D model and a nonlinear stability sand wave model is performed. Next, we compare the results for two of the built-in turbulence models: constant vertical eddy viscosity model (commonly used in stability sand wave models) and a more advanced spatially and temporally variable vertical eddy viscosity model (k–e turbulence model). Finally, the model results are compared with field data on sand wave length. The k–e turbulence model shows good agreement with the field data, whereas the constant vertical eddy viscosity model overestimates the wave length of the sand waves considerably

Nonlinear hydrodynamics in a Mediterranean lagoon

Abstract: The paper addresses the application of the non- linear hydrodynamics model (RANS (Reynolds-averaged Navier-Stokes) equations) in a wide semi-enclosed Mediter- ranean lagoon (Berre lagoon), considering three natural forc- ing functions, i.e., sea tide propagating through a long narrow channel, wind and runoff. Main attention is focused to char- acteristic velocities (at free surface and bottom) and to free surface elevation associated to each of these three mecha- nisms, with special attention to the nearshore areas (i.e., in shallow water). The most interesting result concerns wind effects which, due to Berre lagoon bathymetry, give rise to downwind coastal jets, alongshore, in shallow water areas. Such coastal jets were never mentioned before in Berre la- goon literature.

The pattern of surface waves in a shallow free surface flow

Abstract: [1] This work presents new water surface elevation data including evidence of the spatial correlation of water surface waves generated in shallow water flows over a gravel bed without appreciable bed forms Careful laboratory experiments have shown that these water surface waves are not well-known gravity or capillary waves but are caused by a different physical phenomenon In the flow conditions studied, the shear present in shallow flows generates flow structures, which rise and impact on the water-air interface It is shown that the spatial correlation function observed for these water surface waves can be approximated by the following analytical expression W(ρ)=e−ρ2/2σw2cos(2πL0−1ρ) The proposed approximation depends on the spatial correlation radius, σw, characteristic spatial period, L0, and spatial lag, ρ This approximation holds for all the hydraulic conditions examined in this study It is shown that L0 relates to the depth-averaged flow velocity and carries information on the shape of the vertical velocity profile and bed roughness It is also shown that σw is related to the hydraulic roughness and the flow Reynolds number

Shallow water anchor

Abstract: A shallow water anchoring mechanism for a vessel drives an upper arm between a raised position, in which the lower end of a pole is above the bottom of the hull and a lowered position, in which the lower end of the pole extends downward below the vessel so as to engage the bottom of a shallow body of water. This provides a shallow-water anchoring arrangement for a boat that does a minimum amount of damage to sea life, including oysters, and grasses or other vegetation growing on the bottom of the water, and allows a boater to anchor in shallow water and to leave the anchorage without bringing mud, grasses, or other debris into the boat. A manual disconnect arrangement allows the boater to pull the pole out of the water in the event of actuator or power supply failure.

The Numerical Simulation of Shallow Water: Estimation of the Roughness Coefficient on the Flood Stage:

Abstract: The results of the numerical simulations of the dynamics of shallow waters for Volga-Akhtuba Floodplain are discussed. The mathematical model is based on the system of Saint-Venant equations. Numer...

The UBO-TSUFD tsunami inundation model: validation and application to a tsunami case study focused on the city of Catania, Italy

Abstract: . Nowadays numerical models are a powerful tool in tsunami research since they can be used (i) to reconstruct modern and historical events, (ii) to cast new light on tsunami sources by inverting tsunami data and observations, (iii) to build scenarios in the frame of tsunami mitigation plans, and (iv) to produce forecasts of tsunami impact and inundation in systems of early warning. In parallel with the general recognition of the importance of numerical tsunami simulations, the demand has grown for reliable tsunami codes, validated through tests agreed upon by the tsunami community. This paper presents the tsunami code UBO-TSUFD that has been developed at the University of Bologna, Italy, and that solves the non-linear shallow water (NSW) equations in a Cartesian frame, with inclusion of bottom friction and exclusion of the Coriolis force, by means of a leapfrog (LF) finite-difference scheme on a staggered grid and that accounts for moving boundaries to compute sea inundation and withdrawal at the coast. Results of UBO-TSUFD applied to four classical benchmark problems are shown: two benchmarks are based on analytical solutions, one on a plane wave propagating on a flat channel with a constant slope beach; and one on a laboratory experiment. The code is proven to perform very satisfactorily since it reproduces quite well the benchmark theoretical and experimental data. Further, the code is applied to a realistic tsunami case: a scenario of a tsunami threatening the coasts of eastern Sicily, Italy, is defined and discussed based on the historical tsunami of 11 January 1693, i.e. one of the most severe events in the Italian history.