Phreatic seepage flow through an earth dam with an impeding strip

New mathematical models are developed and corresponding boundary value problems are analytically and numerically solved for Darcian flows in earth (rock)–filled dams, which have a vertical impermeable barrier on the downstream slope. For saturated flow, a 2-D potential model considers a free boundary problem to Laplace’s equation with a traveling-wave phreatic line generated by a linear drawup of a water level in the dam reservoir. The barrier re-directs seepage from purely horizontal (a seepage face outlet) to purely vertical (a no-flow boundary). An alternative model is also used for a hydraulic approximation of a 3-D steady flow when the barrier is only a partial obstruction to seepage. The Poisson equation is solved with respect to Strack’s potential, which predicts the position of the phreatic surface and hydraulic gradient in the dam body. Simulations with HYDRUS, a FEM-code for solving Richards’ PDE, i.e., saturated-unsaturated flows without free boundaries, are carried out for both 2-D and 3-D regimes in rectangular and hexagonal domains. The Barenblatt and Kalashnikov closed-form analytical solutions in non-capillarity soils are compared with the HYDRUS results. Analytical and numerical solutions match well when soil capillarity is minor. The found distributions of the Darcian velocity, the pore pressure, and total hydraulic heads in the vicinity of the barrier corroborate serious concerns about a high risk to the structural stability of the dam due to seepage. The modeling results are related to a “forensic” review of the recent collapse of the spillway of the Oroville Dam, CA, USA

Analysis of vadose zone inhomogeneity toward distinguishing recharge rates: Solving the nonlinear interface problem with Newton method

Citation: Steward, D. R. (2016). Analysis of vadose zone inhomogeneity toward distinguishing recharge rates: Solving the nonlinear interface problem with Newton method. Water Resources Research, 52(11), 8756-8774. doi:10.1002/2016wr019222Recharge from surface to groundwater is an important component of the hydrological cycle, yet its rate is difficult to quantify. Percolation through two-dimensional circular inhomogeneities in the vadose zone is studied where one soil type is embedded within a uniform background, and nonlinear interface conditions in the quasilinear formulation are solved using Newton's method with the Analytic Element Method. This numerical laboratory identifies detectable variations in pathline and pressure head distributions that manifest due to a shift in recharge rate through in a heterogeneous media. Pathlines either diverge about or converge through coarser and finer grained materials with inverse patterns forming across lower and upper elevations; however, pathline geometry is not significantly altered by recharge. Analysis of pressure head in lower regions near groundwater identifies a new phenomenon: its distribution is not significantly impacted by an inhomogeneity soil type, nor by its placement nor by recharge rate. Another revelation is that pressure head for coarser grained inhomogeneities in upper regions is completely controlled by geometry and conductivity contrasts; a shift in recharge generates a difference Dp that becomes an additive constant with the same value throughout this region. In contrast, shifts in recharge for finer grained inhomogeneities reveal patterns with abrupt variations across their interfaces. Consequently, measurements aimed at detecting shifts in recharge in a heterogeneous vadose zone by deciphering the corresponding patterns of change in pressure head should focus on finer grained inclusions well above a groundwater table

Numerical simulation of seepage maps under dams with sheet piles on their ends

Seepage maps formed by both stream and equipotential lines, emerging under dams with sheet piles on their ends, can be determined by simulating the Laplace conjugate equations using a numerical technique such as the network method. Based on these maps, engineers can immediately deduce the amount of water circulating under the structure and design the sheet piles depth to safe values that allow to limit risks such as siphoning or erosion of the base of the dam. For a fix depth of the upstream sheet pile, seepage maps are shown for different configurations of the downstream sheet pile, in a 2D scenario with finite depth and with large extensions both upstream and downstream of the dam.We would like to thank the Séneca Foundation for the support given to this research, thanks to the scholarship awarded to María Encarnación Martínez Moreno to carry out her doctoral thesis

Network model for the numerical solution of groundwater flow. Application to partially penetrating retaining structures in geotechnical engineering

Based on the network simulation method, a precise numerical model is designed for the 2‐D groundwater flow in porous and isotropic aquifers. If a partially penetrating impervious barrier exists, groundwater will flow downstream circumventing the underground structure. The network model is solved free code. Thanks to the powerful mathematical calculation algorithms implemented in is this type of codes, the provided solutions are quite precise for a relatively small grid size, with practically negligible computing times. The proposed model is applied to illustrative problems, providing hydraulic isopotentials and stream lines, showing that as the dam penetration depth increases the hydraulic gradient downstream decreases, thus reducing the risk of hydraulic failure.We would like to thank the Séneca Foundation for the support given to this research and for the scholarships awarded to María Encarnación Martínez Moreno to carry out her doctoral thesis

Deterministic and Random Isogeometric Analysis of Fluid Flow in Unsaturated Soils

The main objective of this research is to use IGA as an efficient and robust alternative for numerical simulation of unsaturated seepage problems. Moreover, this research develops an IGA-based probabilistic framework that can properly account for the variability of soil hydraulic properties in the simulations. In the first part, IGA is used in a deterministic framework to solve a head-based form of Richards’ equation. It is shown that IGA is able to properly simulate changes in pore pressure at the soils interface. In the second part of this research, a new probabilistic framework, named random IGA (RIGA), is developed. A joint lognormal distribution function is used with IGA to perform Monte Carlo simulations. The results depict the statistical outputs relating to seepage quantities and pore water pressure. It is shown that pore water pressure, flow rate, etc. change considerably with respect to standard deviation and correlation of the model parameters

Nitrate movement under a ridge configuration: a field and model investigation

A substantial portion of the nitrogen (N) fertilizer applied under intensive Midwestern cropping is lost through nitrate-nitrogen (NO[subscript]3-N) leaching with percolating water. A tillage and fertilizer-placement system designed to isolate the fertilizer from downward water flow and to minimize NO[subscript]3-N leaching is desirable, both environmentally and economically. A ridge-tillage configuration, with placement of the potential NO[subscript]3-N source in the elevated portion of the ridge, appears to be one possible best management practice. Therefore, NO[subscript]3-N leaching under ridge tillage during the early growing season and immediately following fertilizer application is investigated;Past numerical modeling of water and solute transport for both saturated and unsaturated soil is reviewed. The finite element formulation for two-dimensional water and solute transport is presented. The FEMWATER-FEMWASTE computer code is used for simulation modeling and a comparison is made of the water and solute transport in ridge- and flat-tillage systems;Data from a field experiment indicate that placement of N fertilizer in the center of a ridge reduces NO[subscript]3-N leaching as contrasted to a similar placement for flat tillage, even though total water movement through both systems is comparable. Vertical NO[subscript]3-N movement is predominant (in contrast to horizontal movement) and increases as the amount of simulated rainfall increases;Results from model verification indicate that the two-dimensional model has potential application in predicting water and solute movement in the unsaturated soil profile. However, further modeling activities are needed (with additional subroutines to handle runoff-ponding conditions) to insure the validity of the model for microscale applications such as those in this particular study. With further refinements, the model should be a more useful tool to describe water and chemical movement through soil for various fertilizer placements and surface configurations

Ground-Water Modeling Issues in Ground-Water Development: Types of Models/Choosing the Right Model

26 pages. Contains 3 pages of references

Laboratory tests to study stability mechanism of rainfall infiltrated unsaturated fine-grained soil slopes developing into shallow landslides and their hydraulic properties

Thesis (Master)--Izmir Institute of Technology, Civil Engineering, Izmir, 2013Includes bibliographical references (leaves: 116-123)Text in English; Abstract: Turkish and Englishxiv, 123 leavesThis study consists of two parts. In the first part, saturated soils wetting band infiltration theories and the most widely used in the world by Lumb, 1975 and Pradel and Raad, 1993 compares theoretical predictions were compared with observed results which gave poor correlations. Results showed that both theories grossly underestimated wetting-band thicknesses. Because above mentioned two theories result in constant values, instead of giving values changing as functions of time. These theories need corrections, which indicate need for further studies. In the second part, hydraulic properties were determined (water-retention, hydraulic-conductivity) of locally obtained 3 undisturbed soils near saturation with a new Hyprop testing technique using the evaporation method. As the Unified Soil Classification System (USCS) does not distinguish inorganic clay colloids by size (size <0,001 mm or 1000 nanometers), Lazer Diffraction Method was used. Results have shown that under zero overall stress; Matric suction does not stay constant, but increases with time up to a maximum point and then decreases, whereas time to reach maximum matric suction increases with decreasing plasticity index (PI) and colloid content (c). While maximum matric suction increases with PI and c, hydraulic conductivity and volumetric water content decreases with increasing matric suction. Also, hydraulic conductivity at maximum matric suction decreases with increasing PI and c

Proceedings of the symposium on transient ground water hydraulics

CER63DEM-MWB70.December 1963.Issued with supplements.Errata included.Includes bibliographical references.The symposium was held at Colorado State University in Fort Collins, Colorado on July 25-27, 1963