Global river models are an essential tool for both earth system studies and water resources assessments. As advanced physical processes have been implemented in global river models, increasing ...computational cost has become problematic for executing ensemble or long‐term simulations. To improve computational efficiency, we here propose the use of a local inertial flow equation combined with a vector‐based river network map. A local inertial equation, a simplified formulation of the shallow water equations, was introduced to replace a diffusion wave equation. A vector‐based river network map which flexibly discretizes river segments was adopted in order to replace the traditional grid‐based map which is based on a Cartesian grid coordinate system. The computational efficiency of the proposed flow routing and river network map was tested by executing hydrodynamic simulations with the CaMa‐Flood global river model. The simulation results suggest that the computational efficiency can be improved by more than 300% by applying the local inertial equation. It can be improved by a further 60% by implementing the vector‐based river network map instead of a grid‐based map. It is found that the vector‐based map with evenly distributed flow distances between calculation units allows longer time steps compared to the grid‐based map because the latter has very short flow distances between calculation units at high latitudes which critically limit time step length. Considering the improvement in simulation speed, the local inertial equation, and a vector‐based river network map are preferable in global hydrodynamic simulations with high computational demands such as ensemble or long‐term experiments.
Key Points
The computational efficiency of global river model simulation was improved
The local inertial equation was applied instead of the diffusive wave equation
Vector‐based river network maps enables faster calculation than grid‐based maps
The accumulation of large wood debris around bridge piers obstructs the flow, producing increased upstream water levels, large horizontal structural loadings, and flow field modifications that can ...considerably exacerbate scour. These effects have frequently been held responsible for the failure of a large number of bridges around the world, as well as for increased risk of flooding of adjacent areas. Yet little is currently known about the time evolution and processes responsible for the formation and growth of these debris piles. This paper is aimed at deciphering the whole life of debris accumulations through an exhaustive set of 570 experiments in which debris elements were individually introduced into a flume and accumulated at a pier model downstream. Our findings show that in all experiments, the growth of accumulations is halted at a critical stage, after which the jam is removed from the pier by the flow. This condition typically coincides with the time when the dimensions of the accumulations are maxima. The values of the accumulation maximum size display a clear dependence on flow characteristics and debris length distribution. On the other hand, other variables have shown much weaker effects on the geometry of the accumulations. For a given debris length, accumulations are wide, shallow, and long at low flow velocities but become narrower, deeper, and shorter with increasing velocities. A comparison of results of accumulations formed with debris of uniform and nonuniform size distributions has revealed that the former can be up to 2.5 times wider than the latter.
Key Points
Experimental results show that the growth of debris jams at piers is halted at a critical stage
New relations are proposed between the size of debris jams and flow and debris variables
Accumulations formed with uniform are considerably larger than with nonuniform debris
Recent studies have demonstrated the improved computational performance of a computer algorithm based on a simplification of the shallow water equations—the so‐called local inertial ...approximation—which has been observed to provide results comparable to the full set of equations in a range of flood flow problems. This study presents an extended view on the local inertial system, shedding light on those key elements necessary to understand its applicability to flows of practical interest. First, the properties of the simplified system with potential impact on the accuracy of the solutions are described and compared to the corresponding full‐dynamic counterparts. In light of this discussion, the behavior of the solutions is then analyzed through a set of rigorously designed test cases in which analytical solutions to the shallow water system are available. Results show a general good agreement between the local inertial and full‐dynamic models, especially in the lower range of subcritical flows (Fr < 0.5). In terms of steady nonuniform flow water profiles, the error introduced by the local inertial approximation leads to milder water depth gradients, which results in attenuated spatial changes in depth. In unsteady problems, the local inertial approximation leads to slower flood propagation speeds than those predicted by the full‐dynamic equations. Even though our results suggest that the magnitude of these errors is small in a range of floodplain and lowland channels, it becomes increasingly relevant with increasing Fr and depth gradients.
Key Points
Model behaviour in steady non‐uniform flow conditions was analysed.
Model behaviour in unsteady flow conditions was analysed.
Error patterns and error relations have been presented.
The ability of two‐dimensional hydrodynamic models to accurately and efficiently predict the propagation of floods over large urban areas is of paramount importance for flood risk assessment and ...management. Paradoxically, it is in these highly relevant urban domains where flood modeling faces some of the most challenging obstacles. This is because of the very high‐resolution topography that is typically required to capture key hydraulic features, which significantly increases the computational time of the model. One particularly interesting solution to this difficulty was recently proposed in the form of a numerical scheme for the solution of a simplified version of the shallow water equations, which yields a system of two explicit equations that captures the most relevant hydraulic processes at very high computational efficiency. However, some stability problems were reported, especially when this formulation is applied to low friction areas. This is of particular importance in urban areas, where smooth surfaces are usually abundant. This paper proposes and tests two modifications of this previous numerical scheme that considerably improves the numerical stability of the model. Model improvements were assessed against a structured set of idealized test cases and finally in the simulation of flood propagation over complex topography in a highly urbanized area in London, United Kingdom. The enhanced stability achieved by the new formulation comes at no significant additional computational cost and, in fact, the model performance can benefit from the longer time steps that are allowed by the new scheme.
Key Points
Solution of numerical instability reported in previous paper
Proposal of a high‐performance and robust formulation for flood propagation
This paper presents a new sub-grid flood inundation model obtained by upscaling the shallow water equations (SWE) to enhance the model efficiency in large-scale problems. The model discretizes study ...domains using two nested meshes. The equations are solved at the coarse mesh by a second-order accurate in space (i.e. piecewise linear reconstruction of variables) Godunov-type finite volume (FV) method, while the fine mesh is used to incorporate high-resolution topography and roughness into the solution. The accuracy and performance of the model were compared against a first-order version of the model recently proposed by the authors and a second-order conventional FV model using artificial and real-world test problems. Results showed that improved accuracy is delivered by the proposed model, and that at low-resolution meshes, the spatial reconstruction of variables of the numerical scheme plays a major role in the solution's accuracy.
Abstract
Context
Coronavirus disease 2019 (COVID-19) is a proinflammatory and prothrombotic condition, but its impact on adrenal function has not been adequately evaluated.
Case report
A 46-year-old ...woman presented with abdominal pain, hypotension, and skin hyperpigmentation after COVID-19 infection. The patient had hyponatremia, serum cortisol <1.0 µg/dL, adrenocorticotropin (ACTH) of 807 pg/mL, and aldosterone <3 ng/dL. Computed tomography (CT) findings of adrenal enlargement with no parenchymal and minimal peripheral capsular enhancement after contrast were consistent with bilateral adrenal infarction. The patient had autoimmune hepatitis and positive antiphospholipid antibodies, but no previous thrombotic events. The patient was treated with intravenous hydrocortisone, followed by oral hydrocortisone and fludrocortisone.
Discussion
We identified 9 articles, including case reports, of new-onset adrenal insufficiency and/or adrenal hemorrhage/infarction on CT in COVID-19. Adrenal insufficiency was hormonally diagnosed in 5 cases, but ACTH levels were measured in only 3 cases (high in 1 case and normal/low in other 2 cases). Bilateral adrenal nonhemorrhagic or hemorrhagic infarction was identified in 5 reports (2 had adrenal insufficiency, 2 had normal cortisol levels, and 1 case had no data). Interestingly, the only case with well-characterized new-onset acute primary adrenal insufficiency after COVID-19 had a previous diagnosis of antiphospholipid syndrome. In our case, antiphospholipid syndrome diagnosis was established only after the adrenal infarction triggered by COVID-19.
Conclusion
Our findings support the association between bilateral adrenal infarction and antiphospholipid syndrome triggered by COVID-19. Therefore, patients with positive antiphospholipid antibodies should be closely monitored for symptoms or signs of acute adrenal insufficiency during COVID-19.
This paper presents a new sub-grid flood inundation model aimed at high computational performance. The model solves the two-dimensional shallow water equations (SWE) by a Godunov-type finite volume ...(FV) method that uses two nested meshes. Runtime computations are performed at a coarse computational mesh, while a fine mesh is used to incorporate fine resolution information into the solution at pre-processing level. New upscaling methods are separately derived for each of the terms in the SWE based on the integration of the governing equations over subdomains defined by the coarse resolution grid cells. The accuracy and performance of the model are tested through artificial and real-world test problems. Results showed that (i) for the same computational (coarse mesh) resolution, the inclusion of sub-grid information delivers more accurate results than a single-mesh FV model and (ii) for the same accuracy and at low resolution, the proposed methods improve computational performance.
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