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Figure 1-3, Equipotenrial surfaces--.. 2,--Hydraulic Gradient at Water Table =LHI - Hz1. Slope of Water Table .--1 SHI- H2Figure 1-4, Difference between hydraulic gradientand slope of the water table
water table is not invariably a path of flow; water may be flowing down from orup into the unsaturated zone, thus crossing the water table.Paths of flow (streamlines)The force due to hydraulic gradient tends to move water along the line of forcenormal to the equipotential surfaces. Whether the flow moves actually in thesame direction as the line of force depends on whether the soil has the samehydraulic conductivity in all directions. If the soil is "isotropic," i.e., ifits hydraulic conductivity is the same in all directions, the path of flow willbe along the lines of force and perpendicular to the equipotential surfaces.If the soil has a higher hydraulic conductivity in one direction than in anotherdirection, the path of flow will not be perpendicular to the equipotential surface.Such a soil is said to be "anisotropic." Water-laid soils often havebedding planes or particle orientation causing them to be anisotropic. Thepaths of flow in anisotropic soils will be perpendicular to the equipotentialsurfaces at points where the lines of force are exactly parallel to or normalto the bedding plane. A soil with microstratification (thin layers with widelydifferent hydraulic conductivities) will cause water to flow in a way similarto the flow in an anisotropic soil.Many flow systems common in soil drainage may be studied in two dimensions ratherthan three, because of uniformity in the third dimension. Equipotential linesthen represent the intersection of the plane of the paper with the equipotentialsurfaces. An example of this representation is the flow into a system of paralleldrains, where the flow is at right angles to the drains.A two-dimensional system on the X-Y plane is illustrated in figure 1-5. The soilis isotropic (hydraulic conductivity is uniform in all directions) in figure 1-5.Figure 1-6 shows another soil, with a line of force normal to the equipotentialline and at an angle b to the vertical axis Y. But in this soil, which isanisotropic,the horizontal hydraulic conductivity Kh exceeds the vertical hydraulicconductivity Kv. The direction of flow is not along the line of force, butalong a line closer to the horizontal axis. It may be shown that the angle thepath of flow makes with the horizontal isKa = tan-' Kh zanThus, the flow pattern may be computed and drawn for an anisotropic system ifthe equipotential lines are known, and if the relative hydraulic conductivitiesKv and Kh are known.In analyzing the direction of flow of ground water, the investigator should beaware of the effect of anisotropic soils on the flow pattern.Flow nets and boundary conditionsFlow in the saturated zone often is studied by means of graphic representationsof hydraulic head and paths of flow. Cross sections are taken through the problemarea, usually in vertical planes. Lines connecting points of equal hydraulic head
Page 5 and 6: Classification of drainage methodsT
Page 7 and 8: occur where the soil is either slow
Page 9 and 10: apidly permeable deposits may serve
Page 12 and 13: Bernoulli's theorem is used to comp
Page 14 and 15: --Pressure Headat Point "P" =Elevat
Page 18 and 19: Figure 1-5, Flow direction in isotr
Page 20 and 21: ISOTROPIC SOILI -eDrainIimpervious
Page 22 and 23: This equation may be used to estima
Page 24 and 25: conditions if the curvature of the
Page 26 and 27: ModelsSand or soil is sometimes pla
Page 28 and 29: Drainage coefficients for pumping p
Page 30 and 31: This equation neglects the curvilin
Page 32 and 33: References(1) DARCY, H.1856. Les Fo

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