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Fish passage systems may provide a means to mitigate the barrier effect of dams on migrating fish species. Even though design criteria were initially developed for the fish fauna of temperate regions, they are widely used in tropical rivers with relative success. A fish passage concept has been developed for a hydroelectric power project on the Mekong River in the absence of suitable fish passage design criteria. There are several hydropower dams proposed for development on the mainstem of the Mekong River in Cambodia, Laos, Thailand and China and the impact of hydropower on fisheries is fast becoming a big issue. Located in Laos territory, the present project is one of the promising run-of-river hydropower projects identified by the Mekong Secretariat in 1994. The feasibility study began in 2007 and the outline (basic) design is being completed in 2010. The scheme includes a comprehensive fish passage system, which provides facilities for upstream and downstream passage. The proposed fish passage concept is based on twelve principles and gives emphasis on an adaptive project management, which includes planning, implementation and operation of the fish passage system and other structures of the project. It will cater to the large number of species and high biomass and especially to the variable flow regime and the lack of biological knowledge on the behavior of migrating species. © 2011 Elsevier B.V.

Pinzer B.R.,WSL Institute for Snow and Avalanche Research SLF | Pinzer B.R.,Paul Scherrer Institute | Schneebeli M.,WSL Institute for Snow and Avalanche Research SLF | Kaempfer T.U.,WSL Institute for Snow and Avalanche Research SLF | Kaempfer T.U.,AF Consult Switzerland Ltd
Cryosphere | Year: 2012

Dry snow metamorphism under an external temperature gradient is the most common type of recrystallization of snow on the ground. The changes in snow microstructure modify the physical properties of snow, and therefore an understanding of this process is essential for many disciplines, from modeling the effects of snow on climate to assessing avalanche risk. We directly imaged the microstructural changes in snow during temperature gradient metamorphism (TGM) under a constant gradient of 50 K mĝ̂'1, using in situ time-lapse X-ray micro-tomography. This novel and non-destructive technique directly reveals the amount of ice that sublimates and is deposited during metamorphism, in addition to the exact locations of these phase changes. We calculated the average time that an ice volume stayed in place before it sublimated and found a characteristic residence time of 2-3 days. This means that most of the ice changes its phase from solid to vapor and back many times in a seasonal snowpack where similar temperature conditions can be found. Consistent with such a short timescale, we observed a mass turnover of up to 60% of the total ice mass per day. The concept of hand-to-hand transport for the water vapor flux describes the observed changes very well. However, we did not find evidence for a macroscopic vapor diffusion enhancement. The picture of {temperature gradient metamorphism} produced by directly observing the changing microstructure sheds light on the micro-physical processes and could help to improve models that predict the physical properties of snow. © Author(s) 2012.

Hayek M.,AF Consult Switzerland Ltd.
Journal of Hydrologic Engineering | Year: 2016

A general analytical model for one-dimensional (1D) contaminant transport in infinite domain with time-dependent transport parameters is presented in this paper. The model is based on the advection-dispersion equation with a time-dependent flow velocity, a time-dependent dispersion coefficient, and a time-dependent distribution coefficient due to sorption. It takes into account a first-order irreversible reaction (decay), an arbitrary initial distribution of the contaminant, and an arbitrary space- and time-dependent sink/source term. The model can handle any time-dependent transport parameter. Analytical solutions are provided for both the advection dominant and the advection-dispersion transport equations. The proposed analytical solutions are general and they can be used for problems where one or more parameters are time-dependent. It is shown that the presented solutions can be reduced to other existent solutions where one transport parameter is assumed to be time-dependent. The general analytical solutions are obtained by using the Fourier transform, and they are presented in integral forms. Several closed-form solutions can be derived from the general integral form. Such closed-form solutions are of great interest since they allow the dependence of the solutions on the underlying physical parameters to be studied in an analytical manner. In particular, the author presents some closed-form solutions for the case of an initial step function and discusses through the numerical examples some insights about the errors that can be made by assuming constant parameters instead of time-dependent ones. These assumptions may lead to overestimated or underestimated concentrations. The proposed analytical solutions are useful for benchmarking numerical solutions to problems in hydrogeology and chemical engineering. They are also of great importance to the investigation of quantitative accuracy assessment. One of the presented closed-form solutions is used to compare with numerical solutions. © 2016 American Society of Civil Engineers.

Hayek M.,AF Consult Switzerland Ltd
Journal of Hydrology | Year: 2015

An analytical solution for one-dimensional steady vertical flux through unsaturated homogeneous soils is presented. The model assumes power law hydraulic conductivity and diffusivity functions. The soil domain is a finite-depth flow medium overlying a water table. A steady constant flux is applied at the top boundary while a constant saturation value is specified at the bottom boundary. The general form of the analytical solution expresses implicitly the depth as function of the liquid water saturation. It can be used to model both infiltration through the soil surface and evaporation from the bottom, depending on the sign of the flux boundary value. The analytical solution takes into account the prediction of a drying front in the case of evaporation from deep water table. Algebraic expressions of practical and theoretical importance are derived in terms of soil water parameters. These expressions include the stored mass in the system at steady state as well as the drying front when it exists. The general form solution can be inverted back to obtain exact explicit solutions when the power law parameters are related. Numerical results show the effects of soil type, surface flux, capillarity, and gravity on the saturation distribution in the soil. The analytical solution is used for comparing between models, validating of numerical solutions, as well as for estimating the hydraulic parameters. © 2015 Elsevier B.V.

Hayek M.,Paul Scherrer Institute | Hayek M.,AF Consult Switzerland Ltd. | Kosakowski G.,Paul Scherrer Institute | Jakob A.,Paul Scherrer Institute | Churakov S.V.,Paul Scherrer Institute
Water Resources Research | Year: 2012

One of the challenging problems in mathematical geosciences is the determination of analytical solutions of nonlinear partial differential equations describing transport processes in porous media. We are interested in diffusive transport coupled with precipitation-dissolution reactions. Several numerical computer codes that simulate such systems have been developed. Analytical solutions, if they exist, represent an important tool for verification of numerical solutions. We present a methodology for deriving such analytical solutions that are exact and explicit in space and time variables. They describe transport of several aqueous species coupled to precipitation and dissolution of a single mineral in one, two, and three dimensions. As an application, we consider explicit analytical solutions for systems containing one or two solute species that describe the evolution of solutes and solid concentrations as well as porosity. We use one of the proposed analytical solutions to test numerical solutions obtained from two conceptually different reactive transport codes. Both numerical implementations could be verified with the help of the analytical solutions and show good agreement in terms of spatial and temporal evolution of concentrations and porosities. Copyright 2012 by the American Geophysical Union.

This work develops a simple exact and explicit solution of the one-dimensional transient and nonlinear Richards' equation for soils in a special case of exponential water retention curve and power law hydraulic conductivity. The exact solution is obtained as traveling wave based on the approach proposed by Philip (1957, 1967) and adopted by Zlotnik et al. (2007). The obtained solution is novel, and it expresses explicitly the water content as function of the depth and time. It can be useful to model infiltration into semi-infinite soils with time-dependent boundary conditions and infiltration with constant boundary condition but space-dependent initial condition. A complete analytical inverse procedure based on the proposed analytical solution is presented which allows the estimation of hydraulic parameters. The proposed exact solution is also important for the verification of numerical schemes as well as for checking the implementation of time-dependent boundary conditions. © 2016 Elsevier B.V.

A general analytical model for one-dimensional transient vertical infiltration is presented. The model is based on a combination of the Brooks and Corey soil water retention function and a generalized hydraulic conductivity function. This leads to power law diffusivity and convective term for which the exponents are functions of the inverse of the pore size distribution index. Accordingly, the proposed analytical solution covers many existing realistic models in the literature. The general form of the analytical solution is simple and it expresses implicitly the depth as function of water content and time. It can be used to model infiltration through semi-infinite dry soils with prescribed water content or flux boundary conditions. Some mathematical expressions of practical importance are also derived. The general form solution is useful for comparison between models, validation of numerical solutions and for better understanding the effect of some hydraulic parameters. Based on the analytical expression, a complete inverse procedure which allows the estimation of the hydraulic parameters from water content measurements is presented. © 2016. American Geophysical Union. All Rights Reserved.

The paper presents certain exact solutions describing the vertical movement of a water pulse through a semi-infinite unsaturated porous column. The saturation-based form of the Richards' equation is used with special power law relative-permeability functions. Both capillary and gravity effects are taken into account. Three exact solutions are derived corresponding to three relative-permeability functions, linear, quadratic and cubic. The Richards' equation is nonlinear for the three cases. The solutions are obtained by applying a general similarity transformation. They are explicit in space and time variables and do not contain any approximation. They describe the evolution of the water saturation in the vertical column and they can be used to predict the post-infiltration movement of a finite quantity of water. Exact expressions of the masses of water leaving a given depth are also derived for the three cases. We analyze the effect of relative-permeability and capillary pressure. The proposed solutions are also useful for checking numerical schemes. One of the exact solutions is used to validate numerical solution obtained from an arbitrary initial condition. Results show that the numerical solution converges to the exact solution for large times. © 2014 Elsevier B.V.

Hayek M.,AF Consult Switzerland Ltd
Transport in Porous Media | Year: 2015

Attempts have been made to find exact solutions for the one-dimensional transient gas flow equation in porous media. By introducing a traveling wave variable, a traveling wave solution of the gas flow equation has been found. The traveling wave solution is presented in an explicit form of the space and time variables, and it takes into account both gravity and Klinkenberg effects (pressure-dependent permeability). We investigated the properties of the traveling wave solution and the effect of some parameters such as the Klinkenberg coefficient. A numerical study has been carried out, which confirms the stability of the traveling wave solution. The traveling wave solution is then used to derive two benchmark solutions defined over the semi-infinite domain. The first one assumes uniform initial gas pressure and non-uniform boundary condition, and the second assumes uniform boundary condition and non-uniform initial distribution of the gas pressure. The benchmark solutions are easy to use and are useful for validating numerical solutions. Two illustrative examples are presented in order to compare the benchmark solutions with the numerical solutions. The results show good agreements between the solutions. © 2014, Springer Science+Business Media Dordrecht.

Hayek M.,AF Consult Switzerland Ltd.
Computers and Mathematics with Applications | Year: 2014

We present an exact solution for a nonlinear diffusion equation by considering the radially symmetric ν-dimensional case in inhomogeneous medium. This exact solution describes the evolution in space and time of an initial distribution of a diffusing substance. The diffusion coefficient is assumed to be dependent on both, positional coordinate (radial distance) and concentration, with power law nonlinearity. The exact solution is novel and it was obtained thanks to general similarity transformation. A constant time is involved in the similarity variable which allows the definition of the initial amount of the diffusant in the system. We provide an exact expression of the front propagation of the diffusing substance. The proposed exact solution is useful for verification of numerical simulation tools as well as for comparison with laboratory experiments. © 2014 Elsevier Ltd. All rights reserved.

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