Dam break in rectangular channels with different upstream-downstream widths Publication date: October 2019 Source: Advances in Water Resources, Volume 132 Author(s): A. Valiani, V. Caleffi Abstract The classic Stoker dam-break problem (Stoker, 1957) is revisited in cases of different channel widths upstream and downstream of the dam. The channel is supposed to have a rectangular cross section and a horizontal and frictionless bottom. The system of the shallow water equations is enriched, using the width as a space-dependent variable, together with the depth and the unit discharge, which conversely depend on both space and time. Such a formulation allows a quasi-analytical treatment of the system, whose solution is similar to that of the classic Stoker solution when the downstream/upstream depth ratio is sufficiently large, except that a further stationary contact wave exists at the dam position. When the downstream/upstream depth ratio is small, the solution is richer than the Stoker solution because the critical state occurs at the dam position and the solution itself becomes resonant at the same position, where two eigenvalues are null and the strict hyperbolicity of the system is lost. The limits that identify the flow regime for channel contraction and channel expansion are discussed after showing that the nondimensional parameters governing the problem are the downstream/upstream width ratio and the downstream/upstream initial depth ratio. After the introduction of the previous analytical framework, a numerical analysis is also performed to evaluate a numerical method that is conceived to suitably capture rarefactions, shock waves and contact waves. A second-order method is adopted, employing a Dumbser-Osher-Toro Riemann solver equipped with a nonlinear path. Such an original nonlinear path is shown to perform better than the classic linear path when contact waves of large amplitude must be captured, being able to obtain specific energy conservation and mass conservation at the singularity. The codes, written in MATLAB (MathWorks Inc.) language, are made available in Mendeley Data repository.

Crab burrows as preferential flow conduits for groundwater flow and transport in salt marshes: A modeling study Publication date: October 2019 Source: Advances in Water Resources, Volume 132 Author(s): Kai Xiao, Alicia M. Wilson, Hailong Li, Carolyn Ryan Abstract Crab burrows can act as preferential flow conduits for pore water-surface water interactions in salt marshes, but the effect of preferential flow on subsurface transport in these tidally-influenced systems is not fully understood. We used numerical models based on salt marshes of North Inlet, South Carolina, to investigate the impacts of crab burrows on porewater salinity. This modeling effort was inspired by field results from North Inlet, where prior field studies that used a combination of tension samplers and passive diffusion samplers measure salinity in crab burrows and in the adjacent sediment matrix found that the minimum salinity (28 PSU) reported by the tension samplers was larger than the maximum salinity (26 PSU) reported by passive diffusion samplers. Two kinds of numerical models were developed to investigate the effect of crab burrows on tidally-driven groundwater flow and salt transport. In the equivalent-continuum model (ECM), crab burrows were included via a shallow surface layer with hydraulic properties representing a bulk average of sediment matrix and crab burrow properties. In the preferential flow model (PFM), an independent high-permeability material was embedded in the surface muddy layer to explicitly simulate preferential flow conduits. The simulated results showed that both models can depict the effect of crab burrow on soil saturation and salt transport in salt marshes. The presence of crab burrows can greatly increase tidally-driven water exchange, improve the intensity and duration of soil aeration and enhance salt transport in salt marshes. The effect of crab burrows on groundwater flow and salt transport varied spatially from the creek bank to marsh interior. PFM models demonstrated that salinity is likely to differ between crab burrows and the sediment matrix, which supports observed differences in results between tension samplers and passive diffusion samplers. These findings may have important implications for practical pore water sampling and hydrochemical investigation in coastal wetlands.

A full-scale fluvial flood modelling framework based on a high-performance integrated hydrodynamic modelling system (HiPIMS) Publication date: October 2019 Source: Advances in Water Resources, Volume 132 Author(s): Xilin Xia, Qiuhua Liang, Xiaodong Ming Abstract Full-scale fluvial flood modelling over large catchments has traditionally been carried out using coupled hydrological and hydraulic/hydrodynamic models. Such a traditional modelling approach is not well suited for the simulation of extreme floods induced by intense rainfall, which is usually featured with highly transient and dynamic rainfall-runoff and flooding process. This work aims to develop and demonstrate a modelling framework to predict the full-scale process of fluvial flooding from the source (rainfall) to impact (inundation) over a large catchment using a single high-performance hydrodynamic model driven by rainfall inputs. The modelling framework is applied to reproduce the flood event caused by the 2015 Storm Desmond in the 2500 km2 Eden Catchment at 5 m resolution. Without intensive model calibration, the predicted results compare well with field observations in terms of inundation extent and gauged water levels across the catchment. Sensitivity tests reveal that high-resolution grid is essential for accurate simulation of fluvial flood events using a 2D hydrodynamic model. Accelerated by multiple modern GPUs, the simulation is more than 2.5 times faster than real time although it involves 100 million computational cells inside the computational domain. This work provides a novel and promising approach to assess and forecast at real time the risk of extreme fluvial floods from intense rainfall.

A model of local thermal non-equilibrium during infiltration Publication date: October 2019 Source: Advances in Water Resources, Volume 132 Author(s): Thomas Heinze, Johanna R. Blöcher Abstract A realistic temperature estimation is crucial for many earth-science applications, ranging from hydro-thermal systems to plant physiology. The most common approach to calculate the temperature in multi-phase systems assumes immediate local thermal equilibrium (LTE) between the phases. However, local thermal equilibrium between the phases is not applicable in various scenarios like during the infiltration of rain or melt water in frozen soil, limiting the applicability of the approach and inhibiting the implementation of separate initial and boundary conditions for non-equilibrium situations. In local thermal non-equilibrium (LTNE) models, phase temperatures are described separately to the cost of additional differential equations and an explicitly formulated heat transfer between the phases. Especially a cumbersome parameterization of the explicit heat transfer restricts the use of the LTNE models in multi-phase conditions so far. In this work, we derive a general local thermal non-equilibrium model for dynamic, partly saturated porous media. Heat transfer between the phases is described explicitly using well-known semi-empirical parameterization accounting for velocity changes of the mobile phases. The change in volume fraction introduces an additional term in the heat equation, causing a coupling with the hydraulic model. We validate our model with a numerical simulation of historic experimental data from soil infiltration experiments of warm and cold water into drained soil, posing a perfect example of local thermal non-equilibrium conditions between the phases. Experimentally obtained mixture temperatures are reproduced within experimental accuracy. We further show the benefits of our model by applying it to rainwater infiltration into cold soil. Besides a consistent formulation of initial and boundary conditions, the derived model allows physically based conclusions about the thermal state of the separate phases.

Seawater intrusion and retreat in tidally-affected unconfined aquifers: Laboratory experiments and numerical simulations Publication date: October 2019 Source: Advances in Water Resources, Volume 132 Author(s): Xiayang Yu, Pei Xin, Chunhui Lu Abstract Based on combined laboratory experiments and numerical simulations, this paper examined seawater intrusion (SWI) and seawater retreat (SWR) processes caused by abrupt inland watertable changes in a laboratory-scale unconfined aquifer (length of 7.7 m and thickness of 1.0 m) subjected to a synthetic sinusoidal tide. The results showed that the salinity distribution was relatively stable and that SWI and SWR processes were almost temporally symmetrical given relatively large horizontal hydraulic gradients (0.0269, 0.0209 and 0.0149) between the inland watertable and the mean sea level. However, the salt distribution changed significantly in response to the inland watertable variations when the horizontal hydraulic gradient was relatively small (0.0030). The speed of the SWI and SWR response to the inland watertable variations was temporally asymmetric, e.g., SWR was quicker than SWI by a factor of 9 with respect to the observed saltwater wedge toe locations. As a relatively thick mixing zone (transition between freshwater and saltwater zones) was induced by the tide, simulated saltwater wedge toe locations, as indicated by the 5%, 50% and 95% isohalines, changed inconsistently. Different hysteresis behaviors were found in the relationship between the SW toe locations and total salt mass stored in the aquifer. Sensitivity analyses demonstrated that the response of both SWI and SWR to the inland watertable variations could be prolonged by a decreased tidal amplitude or decreased tidal period.

Effect of aperture field anisotropy on two-phase flow in rough fractures Publication date: October 2019 Source: Advances in Water Resources, Volume 132 Author(s): Zhibing Yang, Dongqi Li, Song Xue, Ran Hu, Yi-Feng Chen Abstract The void space geometry of rough fractures is one of most important factors controlling two-phase flow in fractured media. This paper presents a numerical study on the effect of aperture field anisotropy on two-phase flow properties in rough fractures. By using a power spectrum based method, we generate multiple realizations of synthetic rough fractures with aperture fields of different anisotropy factors. Fluid–fluid displacement in these fractures is simulated by a modified invasion percolation model. It is found that the spatiotemporal distribution of fluid phases is strongly influenced by the aperture field anisotropy. On average, both the nonwetting phase saturation and the entrapped wetting phase saturations at breakthrough decrease with increasing anisotropy factor; but the specific interfacial area is larger for a higher anisotropy ratio. Relative permeabilities to both phases in the direction parallel to the displacement increase with the anisotropy factor, indicating a reduced phase interference due to aperture field anisotropy. Empirical equations are proposed to link the relative permeabilities to the anisotropy factor. These results improve our understanding of immiscible displacement in rough fractures and can be useful for predicting two-phase flow in fractured media at the large scale where geomechanical and chemical processes produce anisotropic roughness.

A novel method for well placement design in groundwater management: Extremal optimization Publication date: October 2019 Source: Advances in Water Resources, Volume 132 Author(s): Fleford Redoloza, Liangping Li Abstract Well placement design refers to finding the optimal well locations to install with a set of constraints. This is important for both petroleum engineering and water resource management. This study presents a novel optimization method for well placement design in groundwater management. The proposed method, EO-WPP, is based on the Extremal Optimization (EO) algorithm. EO works by modifying the components of a solution that contribute the least to its overall performance. EO-WPP extends the EO algorithm to the fields of groundwater management and well field optimization for the first time. Groundwater Management program (GWM) is coupled with EO-WPP and used to rank wells in terms of pumping rate, given well locations. In the first testing phases of this work, EO-WPP was applied to a problem of simple geometry and a simple synthetic model in order to study its performance and its emergent spatial behaviors. Results show that the proposed method was faster than Particle Swarm Optimization (PSO) and the Broyden–Fletcher–Goldfarb–Shanno (BFGS) algorithms. EO-WPP then was applied to a field problem involving the Aberdeen groundwater model in South Dakota. The results show that EO-WPP was able to generate a series of possible of well fields that can be used to pump effectively groundwater from the Elm aquifer.

Vertically-averaged and moment equations for flow and sediment transport Publication date: October 2019 Source: Advances in Water Resources, Volume 132 Author(s): Francisco N. Cantero-Chinchilla, Oscar Castro-Orgaz, Abdul A. Khan Abstract Simulation of river flow processes including sediment transport is usually conducted using the shallow water flow equations, which are based on a hydrostatic pressure distribution. To increase the accuracy of predictions in a variety of scenarios involving horizontal length scales of the order of vertical length scales, an improved representation of the vertical flow structure is necessary. The mathematical approximation to field variables like the velocity and fluid pressure must be enhanced during the depth-integrating process. Therefore, this paper presents a 1D non-hydrostatic flow and sediment transport model developed by using the method of the weighted residuals into the RANS equations. Using continuity, momentum, and moment equations, the fluid pressure distribution is modelled using a quadratic predictor with perturbation parameters to deviate the vertical momentum balance from the hydrostatic law. The flow equations are a generalized non-hydrostatic flow solver, where the fluid density variation due to suspension of sediments and the bed deformation due to erosion-sedimentation processes are accounted for. A hybrid semi-implicit finite volume-finite difference numerical scheme is developed to solve the system of conservation laws. Two different approaches are used to model the sediment transport processes: (i) Unified computation of the total-load transport and (ii) separate computation of suspended and bed loads. The accuracy of the non-hydrostatic model is demonstrated by comparison with experimental data, highlighting better results accounting for separate determinations of the suspended and bed loads in highly erosive flows. Graphical abstract

Aging and mixing as pseudo-chemical-reactions between, and on, particles: Perspectives on particle interaction and multi-modal ages in hillslopes and streams Publication date: October 2019 Source: Advances in Water Resources, Volume 132 Author(s): David A. Benson, Michael J. Schmidt, Diogo Bolster, Ciaran Harmon, Nicholas B. Engdahl Abstract The particle-tracking method was recently extended to allow inter-particle mass transfer and arbitrarily complex reactions by allowing each particle to represent any number of distinct chemical compounds. This methodology allows the tracking (and broadening due to mixing) of the age probability density function (PDF) on each particle. Aquifer heterogeneity leads to channeling and multi-modal age PDFs in stream samples. This observation supports the concept of age classes but clearly shows the more complicated interplay of dispersion, mixing, and travel times on the age distributions.

A dynamic data-driven method for dealing with model structural error in soil moisture data assimilation Publication date: October 2019 Source: Advances in Water Resources, Volume 132 Author(s): Qiuru Zhang, Liangsheng Shi, Mauro Holzman, Ming Ye, Yakun Wang, Facundo Carmona, Yuanyuan Zha Abstract Attributing to the flexibility in considering various types of observation error and model error, data assimilation has been increasingly applied to dynamically improve soil moisture modeling in many hydrological practices. However, accurate characterization of model error, especially the part caused by defective model structure, presents a significant challenge to the successful implementation of data assimilation. Model structural error has received limited attention relative to parameter and input errors, mainly due to our poor understanding of structural inadequacy and the difficulties in parameterizing structural error. In this paper, we present a dynamic data-driven approach to estimate the model structural error in soil moisture data assimilation without the need for identifying error generation mechanism or specifying particular form for the error model. The error model is based on the Gaussian process regression and then integrated into the ensemble Kalman filter (EnKF) to form a hybrid method for dealing with multi-source model errors. Two variants of the hybrid method in terms of two different error correction manners are proposed. The effectiveness of the proposed method is tested through a suit of synthetic cases and a real-world case. Results demonstrate the potential of the proposed hybrid method for estimating model structural error and providing improved model predictions. Compared to the traditional EnKF without explicitly considering the model structural error, parameter compensation issue is obviously reduced and soil moisture retrieval is substantially improved.