Surface Elevation Profiles Research Articles

Quantifying the buttressing contribution of landfast sea ice and melange to Crane Glacier, Antarctic Peninsula

Abstract. The January 2022 disintegration of multi-year landfast sea ice in the Larsen B embayment, Antarctic Peninsula, was closely followed by a significant acceleration of ice flow and ice-front retreat of numerous outlet glaciers. Crane Glacier was a notable example of this, with 6 km of its floating ice shelf lost to calving in the first month following the disintegration and a 3.4 % increase in terminus flow speeds over the same time period. In this study we quantify for the first time the buttressing stresses that were transmitted to Crane by the ice melange at the glacier outlet using the ice-flow model Úa. We constrained our model with high-resolution surface elevation profiles of the glacier and ambient melange and reconstructed the observed flow velocities by optimising the rheology rate factor throughout our model domain. This allowed us to quantify the stress regime across both the glacier and ice melange. Results showed that resistive backstresses were imparted to Crane by the ice melange with a mean buttressing ratio of ΘN=0.68 calculated at the glacier terminus (ΘN=1 implies no buttressing). In addition, diagnostic modelling showed an expected 19.2 kPa mean increase in extensional stress at the ice front following the disintegration of the ice melange. This perturbation in stress likely triggered the observed rapid calving over the near-terminus region, leading to the periodic loss of sections of Crane's buttressing ice shelf and thus further acceleration of ice flow in the subsequent months.

SPH modeling of dam-break bores on smooth and macro-roughness slopes

Understanding the kinematics and hydrodynamics of long waves in nearshore zones is currently one of the more pressing aims motivating research in the coastal engineering community. Owing to the simplicity of its setup, the dam-break mechanism is frequently adopted in laboratories to generate bores and model long-wave dynamics, kinematics, and runup processes in coastal regions. Numerical simulations have been used to replicate the results of laboratory experiments using a dam-break system. In this study, we aimed to evaluate the effect of different macro-roughness slopes owing to dam-break driven swash using an open-source particle-based program, DualSPHysics, based on smoothed particle hydrodynamics. To serve as a reference case, the swash flows of a dambreak-generated bore on a uniform slope were performed, and model–data comparisons were made for time-series analysis of the surface elevation, runup process and velocity profiles at selected points. Simulations of dam-break bores passing over slopes containing various configurations of macro-roughness elements were performed to replicate long waves impacting an array of structures in a waterfront area. Different configurations of slope roughness have varying effects on the flow hydrodynamics in the swash zone, which were examined in this study.

Investigation of the Spiral Wave Generation and Propagation on a Numerical Circular Wave Tank Model

A two-phase incompressible flow model in three-dimensional cylindrical coordinates is applied for oblique wave generation in a numerical circular wave tank. The governing equations are discretized by a finite volume method, and a mass source function is added to reproduce oblique waves through a spiral wave generator positioned at the center of the tank. The volume of fluid method is implemented to track the free surfaces between the air and fluid, and the zonal embedded grid system is adopted to obtain a grid-independent solution in the cylindrical coordinates. A permeable, circumferentially sloping topography (1:7), similar to a natural beach profile, is set up for investigating wave propagation and other characteristics. The simulation and physical experimental results are compared, which show a good agreement in randomly selected water surface elevation profiles and wave heights under the same wave and sloping conditions. The spiral waves are reproduced and propagated in a phenomenon similar to that observed in the physical experiment. The results also show that the wave-breaking positions differ in different wave conditions and suggest a relationship with cross-shore and longshore velocity distribution in terms of incident wave heights and wave-breaking positions in different wave periods on the same sloping topography. Furthermore, this model can be used to investigate the mechanism of longshore current generation and the influences of beach slope on the generated wave propagation in a sloping topography.

Numerical simulations of flow over a side weir for diversion structures and water intakes

Side weirs are hydraulic structures used to divert the excess flow from a main channel into a lateral one at diversion structures or intakes, in order to control the water surface elevation or reroute part of the discharge. Flow over a side weir is considered to be a typical case of spatially varied flow, with decreasing discharge along the main channel and varying water surface elevation along the lateral crest. The objective of the paper is to apply various numerical modelling approximations (1D, 2D and 3D) using HEC-RAS and ANSYS Fluent software to virtually reproduce the flow characteristics over the side weir along the diversion canal of the Valea Iasului hydropower plant (HPP), on Argeş River (Romania). Different geometries, mesh types and sizes and the appropriate initial and boundary conditions are used for two HPP operating scenarios (fully functional and completely shut down). The water surface elevation profile along the weir crest is computed, together with a qualitative comparison between the shape of the simulated flow nappe and in-situ visualizations. Quantitative results from the numerical simulation cases are given in terms of the computed upstream and downstream weir rating curves.

Simulating surface height and terminus position for marine outlet glaciers using a level set method with data assimilation

We implement a data assimilation framework for integrating ice surface and terminus position observations into a numerical ice-flow model. The model uses the well-known shallow shelf approximation (SSA) coupled to a level set method to capture ice motion and changes in the glacier geometry. The level set method explicitly tracks the evolving ice–atmosphere and ice–ocean boundaries for a marine outlet glacier. We use an Ensemble Transform Kalman Filter to assimilate observations of ice surface elevation and lateral ice extent by updating the level set function that describes the ice interface. Numerical experiments on an idealized marine-terminating glacier demonstrate the effectiveness of our data assimilation approach for tracking seasonal and multi-year glacier advance and retreat cycles. The model is also applied to simulate Helheim Glacier, a major tidewater-terminating glacier of the Greenland Ice Sheet that has experienced a recent history of rapid retreat. By assimilating observations from remotely-sensed surface elevation profiles we are able to more accurately track the migrating glacier terminus and glacier surface changes. These results support the use of data assimilation methodologies for obtaining more accurate predictions of short-term ice sheet dynamics.

Superharmonic instability of stokes waves

AbstractA stability of nearly limiting Stokes waves to superharmonic perturbations is considered numerically in approximation of an infinite depth. Investigation of the stability properties can give one an insight into the evolution of the Stokes wave. The new, previously inaccessible branches of superharmonic instability were investigated. Our numerical simulations suggest that eigenvalues of linearized dynamical equations, corresponding to the unstable modes, appear as a result of a collision of a pair of purely imaginary eigenvalues at the origin, and a subsequent appearance of a pair of purely real eigenvalues: a positive and a negative one that are symmetric with respect to zero. Complex conjugate pairs of purely imaginary eigenvalues correspond to stable modes, and as the steepness of the underlying Stokes wave grows, the pairs move toward the origin along the imaginary axis. Moreover, when studying the eigenvalues of linearized dynamical equations we find that as the steepness of the Stokes wave grows, the real eigenvalues follow a universal scaling law, that can be approximated by a power law. The asymptotic power law behavior of this dependence for instability of Stokes waves close to the limiting one is proposed. Surface elevation profiles for several unstable eigenmodes are made available through http://stokeswave.org website.

Flood inundation modeling using HEC-RAS: the case of downstream Gumara river, Lake Tana sub basin, Ethiopia

This study presents the preparation of floodwater depth, the velocity of the flood, and water surface elevation profile with the application of unsteady flow 2D Hydrodynamic models and GIS. Flood frequency analysis is used for the prediction of peak flow. The water depth up to 1 m inundated a large amount of area of 52.5134 km2, 52.4756 km2 and 52.4403 km2 for a 10-, 100-, and 1000 years return period respectively. Much amount of inundated area covered with a velocity value up to 0.15 m/s in all flood events, which is 53.207, 52.656, and 52.064 km2 for flood events of 10, 100, and 1000-year respectively. The highest water surface elevation in the 10, 100, and 1000-year flood events was observed between 1785 and 1787 m with an inundated area of 58.806, 58.843, and 58.827 km2 respectively. Finally, the result of this study is useful for early warnings, municipal planning purposes, emergency action plans, flood insurance rates, and ecological studies in the research area.

Hydraulic Model Calibration Using CryoSat‐2 Observations in the Zambezi Catchment

AbstractGeodetic altimeters provide unique observations of the river surface longitudinal profile due to their long repeat periods and densely spaced ground tracks. This information is valuable for calibrating hydraulic model parameters, and thus, for producing reliable simulations of water level for flood forecasting and river management, particularly in poorly instrumented catchments. In this study, we present an efficient calibration approach for hydraulic models based on a steady‐state hydraulic solver and CryoSat‐2 observations. In order to ensure that only coherent forcing/observation pairs are considered in the calibration, we first propose an outlier filtering approach for CryoSat‐2 observations in data‐scarce regions using a simulated runoff produced by a hydrologic model. In the hydraulic calibration, a steady‐state solver computes the water surface elevation (WSE) profile along the river for selected discharges corresponding to the days of CryoSat‐2 overpass. In synthetic calibration experiments, the global search algorithm generally recovers the true parameter values in portions of the river where observations are available, illustrating the benefit of dense spatial sampling from geodetic altimetry. The most sensitive parameters are the bed elevations. In calibration experiments with real CryoSat‐2 data, validation performance against both Sentinel‐3 WSE and in situ records is similar to previous studies, with Root Mean Square Deviation ranging from 0.43 to 1.14 m against Sentinel‐3 and from 0.60 to 0.73 against in situ WSE observations. Performance remains similar when transferring parameters to a one‐dimensional hydrodynamic model. Because the approach is computationally efficient, model parameters can be inverted at high spatial resolution to fully exploit the information contained in geodetic CryoSat‐2 altimetry.

Can Short‐Wave Nonlinearity Affect the Prediction of Wave Setup?

AbstractBased on high‐resolution laboratory data of instantaneous surface elevation and fluid velocity associated with the propagation of short waves over a gently sloping beach (GLOBEX project), the present work compares two methods for predicting the wave setup. A one‐dimensional (cross‐shore) model is considered to solve the balance equation that links the pressure gradient induced by the mean surface elevation profile with the wave radiation stress (including wave roller) and bottom stress profiles. When compared to measurements and using a wave‐by‐wave approach, the mean elevation appears to be significantly better predicted with a nonlinear approach based on stream function theory than with the commonly used linear approach based on Airy wave theory. At the shoreline, the linear method overestimates the wave setup by at least about while this error is globally reduced by a factor 2 to 4 in the nonlinear case. In the framework of this study, the combined contribution of bottom stress and wave roller to the wave setup appears secondary since it accounts for about of the predicted setup at the shoreline. Alternative computational methods are also considered to model the wave setup. In the linear case, using a spectral instead of the wave‐by‐wave approach greatly improves the wave setup predictions. This improvement is related to the disparity between the representative wave height (and thus the wave energy) obtained from spectral and wave‐by‐wave analysis when short‐wave nonlinearity becomes significant.

Wave- and current-dominated combined orthogonal flows over fixed rough beds

Orthogonal wave-current interaction often occurs in coastal regions, when waves approach the shoreline near-orthogonally and longshore currents are present. Notwithstanding its wide relevance, this phenomenon is far from being understood to a full extent, especially in the presence of bed roughness. Indeed the effects on the apparent roughness due to the combination of steady currents and surface waves propagating at right angles are still a matter of debate, both in the current and in the wave dominated cases.To this aim, the hydrodynamic effects of an orthogonal regular wave on a current propagating over a rough bed were investigated; two different rough beds, one sandy and one made of gravel, were considered. Both surface elevation and velocity profiles were acquired by means of an array of wave gauges and Micro Acoustic Doppler Velocimeters positioned within the tank. Here the range of the investigated parameters was significantly enlarged with respect to the existing literature studies.The present paper focuses on the analysis of the mean velocity profiles. More specifically, observation of velocity profiles shows that the current to wave velocity ratio and the bed roughness play an important role, by impacting the value of the apparent roughness. Indeed it was found that as far as the flow is current dominated the addition of waves on the current increases the flow resistance. The opposite happens when the flow is wave dominated, irrespective of the bed roughness type. In particular, the apparent roughness increases as far as the wave plus current to current only friction velocity ratio increases. The increase is more rapid if the wave plus current prevails on the current only friction velocity.

Assessment of ICESat‐2 Sea Ice Surface Classification with Sentinel‐2 Imagery: Implications for Freeboard and New Estimates of Lead and Floe Geometry

AbstractNASA's Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2) mission launched in September 2018 and is now providing high‐resolution surface elevation profiling across the entire globe, including the sea ice cover of the Arctic and Southern Oceans. For sea ice applications, successfully discriminating returns between sea ice and open water is key for accurately determining freeboard (the extension of sea ice above local sea level) and new information regarding the geometry of sea ice floes and leads. We take advantage of near‐coincident optical imagery obtained from the European Space Agency (ESA) Sentinel‐2 (S‐2) satellite over the Western Weddell Sea of the Southern Ocean in March 2019 and the Lincoln Sea of the Arctic Ocean in May 2019 to evaluate the surface classification scheme in the ICESat‐2 ATL07 and ATL10 sea ice products. We find a high level of agreement between the ATL07 (specular) lead classification and visible leads in the S‐2 imagery in these two coincident images across all six ICESat‐2 beams, increasing our confidence in the freeboard products and deriving new estimates of the sea ice state. The S‐2 overlays provide additional, albeit limited, evidence of the misclassification of dark leads due to clouds. Dark leads are no longer used to derive sea surface and thus freeboard as of the third release (r003) of the ICESat‐2 sea ice products. We show estimates of lead fraction and more preliminary estimates of chord length (a proxy for floe size) using two metrics for classifying sea surface (lead) segments across both the Arctic and Southern Ocean for the first winter season of data collection.

Wave-induced shallow-water monopolar vortex: large-scale experiments

Abstract

Flood Mapping Uncertainty from a Restoration Perspective: A Practical Case Study

Many hydrologic studies that are the basis for water resources planning and management rely on streamflow information. Calibration and use of hydrologic models to extend flow series based on rainfall data, perform flood frequency analysis, or develop flood maps for land use planning and design of engineering works, such as channels, dams, bridges, and water intake, are examples of such studies. In most real-world engineering applications, errors in flow data are neglected or not adequately addressed. However, because flows are estimated based on the water level measurements by fitted rating curves, they can be subjected to significant uncertainties. How large these uncertainties are and how they can impact the results of such studies is a topic of interest for researchers, practitioners, and decision-makers of water resources. The quantitative assessment of these uncertainties is important to obtain a more realistic description of many water resources related studies. River restoration in many areas is limited by data availability and funding. A means to assess the uncertainty of flow data to be used in the design and analysis of river restoration projects that is cost effective and has minimal data requirements would greatly improve the reliability of river restoration design. This paper proposes an assessment of how uncertainties related to rating curves and frequency analysis may affect the results of flood mapping in a real-world application to a small watershed with limited data. A Bayesian approach was performed to obtain the posterior distributions for the model parameters and the HEC-RAS (Hydrologic Engineering Center-River Analysis System) hydraulic model was used to propagate the uncertainties in the water surface elevation profiles. The analysis was conducted using freely available data and open source software, greatly reducing traditional analysis costs. The results demonstrate that for the study case the uncertainty related to the frequency analysis study impacted the water profiles more significantly than the uncertainty associated with the rating curve.

Reducing water entry impact loads on offshore marine structures by forced air entrapment

ABSTRACT In this study, the effect of air cushioning due to air entrapment on the water entry of cylinders is examined experimentally and numerically. Air entrapment is forced via corrugations on a cylinder. Emphasis is given to the early stages of penetration effecting the total impact forces. Jet flows, pileups and free surface elevation profiles are characterised and compared with the ones obtained with a smooth cylinder. At low Froude numbers (Fr < 3), while the jet flow does not separate and climbs up on the solid surface of the smooth cylinder, the flow separation occurs and a cavity is formed during the water entry of the corrugated cylinder. The maximum impact force is a function of the wetted length which depends on the flow separation point. The numerical results showed good visual agreement with the experiments. The impacts under air cushioning and early flow separation effects produced 30% smaller impact pressures.

Oblique focused wave group generation and interaction with a fixed FPSO-shaped body: 3D CFD simulations and comparison with experiments

This paper presents a numerical study of oblique focused wave group generation and interaction with a fixed FPSO-shaped body, with thorough validations against available experimental data. The 3D numerical model is based on the open-source toolbox OpenFOAM®, where the oblique waves are generated using multiple virtual segmented wave paddles. The surface elevation and velocity profiles on each paddle are derived based on the snake principle, which mimics the behaviour of wave paddles in the physical wave tank. Numerical tests are firstly conducted for focused wave groups propagating obliquely in an empty wave tank using the proposed scheme. By analysis of the surface elevation, it is found that reasonably good quality of oblique wave fields can be generated in the central area of the wave basin. Furthermore, investigations are carried out on the effects of wave angles on the harmonic structures of the wave groups using the phase-inversion method. It is shown that while the wave angle has minor effects on the linear and second order harmonics, the third order harmonic is altered by the wave angles, albeit its magnitude is very small. Finally, to show the effectiveness of the numerical oblique wave generation method and the importance of the angle effects in the wave-structure interaction process, simulations are carried out for the oblique focused wave group interacting with a fixed FPSO-shaped body. The effects of the wave incidence angle are clearly shown from the comparison of the integrated wave forces between the cases with different wave propagating angles.

Wavelet‐Based River Segmentation Using Hydraulic Control‐Preserving Water Surface Elevation Profile Properties

AbstractRiver flow models that consider satellite observations of the water surface elevation (WSE) require meaningful segmentation into reaches. Segmentation methods involve a trade‐off between the spatial sampling and error characteristics of satellite observations, both of which impact the flow model realism/accuracy. This paper investigates the spatial properties of WSE profiles (slope and concavity) and their link with hydraulic controls (HCs) and leverages this knowledge to advance river segmentation. Fine‐scale synthetic cases were analyzed to characterize HC effects on the remotely observable WSE properties. Following this, a wavelet‐based segmentation method was developed and subsequently tested on real rivers, including test cases with measurements characteristic of the Surface Water and Ocean Topography satellite mission. The results show that the local extrema of water surface (WS) concavity are good candidates for defining reach bounds while consistently preserving the HC signatures and local flow nonuniformities (deviations from equilibrium) from fine to large spatial scales.

On the Application of OpenFOAM® in Gravity Water Wave Generation and Wave-Structure Interaction

This article presents an investigation on the best practice for modeling of water wave generation and wave-structure interaction using a widely spread two-phase flow solver with a specific interface compression technique in OpenFOAM® (OpenFOAM Foundation Ltd., London, United Kingdom). A series of numerical experiments were conducted to examine the effects of the employed schemes, mesh resolution, time step resolution, and compression coefficient. Both surface elevation and velocity profile were considered as the criteria for assessment of the quality of the generated waves. The numerical experiments showed that by using a blending scheme between the Crank-Nicolson and Euler scheme, relatively high quality waves were produced, where the spurious current at the interface region was effectively reduced. Meanwhile, it was also recommended to apply a compression coefficient Cα = 1, the Courant number limit Co = .1, and a mesh resolution of 18 cells per wave height. This set of parameters was used to validate the numerical model for two sets of cases for wave forces on half-immersed horizontal cylinders. The results in general agreed well with the experimental data, although the inline forces were slightly but consistently overestimated. 1. Introduction In the past few decades, because of the increase in the computing ability and resources, computational fluid dynamic (CFD) approach has been gaining popularity in the naval hydrodynamic communities. Considering that a large portion of naval hydrodynamic problems are concerned with the free-surface flows, e.g., ocean waves and its interaction with structures, a free-surface model is needed to couple with the Navier-Stokes solver. Eventually, as a result of direct application of the first principles, the solver can model both inertia and viscous dominant flows. Even local wave breaking and violent flows can be properly resolved. Different methods have been proposed to track the interface of free-surface flows. This includes height function method and line segment method, which are the simplest techniques to represent and configure a free surface. However, they have deficiencies when two surfaces intersect or when a surface folds over on itself, which are overcome by the marker and cell method proposed in Harlow and Welch (1965). In their method, massless particles are used to track the free surface in a Lagrangian manner. Alternatively, the level-set (LS) method or volume of fluid (VOF) method can be used to implicitly capture the surface in an Eulerian way. The LS method was introduced in Osher (1988) where the interface is defined as the one on which a level-set function is equal to zero. This LS function is continuous across the interface. Therefore, it is not significantly influenced by the numerical diffusion. However, the originally proposed LS method suffers from mass conservation problems, which has been resolved in, e.g., Olsson and Kreiss (2005) and Olsson et al. (2007).

Benchmarking the Indian National CartoDEM against SRTM for 1D hydraulic modelling

ABSTRACTAccurate forecasts are a necessity in a world with frequent flood disasters and climatic anomalies. Studies have demonstrated that one of the primary sources of uncertainty in flood modelling is the topographic input data, the bathymetry in particular. As high-resolution elevation/bathymetry datasets are often unavailable in developing regions, the present study benchmarks the freely available Indian National Digital Elevation Model (DEM) CartoDEM, against the currently preferred global DEM, SRTM, for 1D-hydrodynamic modelling. Techniques for improving simulated Water Surface Elevation (WSE) profiles, while using cross-sections derived from these free DEMs are proposed. A distributed vertical error characterization using ground surveyed control points was performed and used to correct the DEM-extracted cross-sections. The approach was tested by simulating a 25-year return period flood in a reach of the Erai River, near Chandrapur city in Maharashtra, India. The results show an improvement of ∼84% in the root-mean-squared-error value for the WSE generated by the SRTM after correction, while the CartoDEM degraded by ∼41%. It was observed that while the uncorrected CartoDEM performs better than the uncorrected SRTM, the converse is true after the bias correction.

Anticipated Improvements to River Surface Elevation Profiles From the Surface Water and Ocean Topography Mission

Existing publicly available digital elevation models (DEMs) provide global-scale data but are often not precise enough for studying processes that depend on small-scale topographic features in rivers. For example, slope breaks and knickpoints in rivers can be important in understanding tectonic processes, and riffle-pool structures are important drivers of riverine ecology. More precise data (e.g. lidar) are available in some areas, but their spatial extent limits large-scale research. The upcoming Surface Water and Ocean Topography (SWOT) satellite mission is planned to launch in 2021 and will provide measurements of elevation and inundation extent of surface waters between 78° north and south latitude on average twice every 21 days. We present a novel noise reduction method for multitemporal river water surface elevation profiles from SWOT that combines a truncated singular value decomposition and a slope-constrained least-squares estimator. We use simulated SWOT data of 85-145 km sections of the Po, Sacramento, and Tanana Rivers to show that 3-12 months of simulated SWOT data can produce elevation profiles with mean absolute errors of 5.38-12.55 cm at 100-200 m along-stream resolution. Mean absolute errors can be reduced further to 4-11 cm by averaging all observations. The average profiles have errors much lower than existing DEMs, allowing new advances in riverine research globally. We consider two case studies in geomorphology and ecology that highlight the scientific value of the more accurate in-river DEMs expected from SWOT. Simulated SWOT elevation profiles for the Po reveal convexities in the river longitudinal profile that are spatially coincident with the upward projection of blind thrust faults that are buried beneath the Po Plain at the northern termination of the Apennine Mountains. Meanwhile, simulated SWOT data for the Sacramento River reveals locally steep sections of the river profile that represent important habitat for benthic invertebrates at a spatial scale previously unrecognizable in large-scale digital elevation models presently available for this river.

Anticipated improvements to river surface elevation profiles from the surface water and ocean topography mission

Anticipated improvements to river surface elevation profiles from the surface water and ocean topography mission