NEH 16 Chapter 1 - NRCS Irrigation ToolBox Home Page

More documents

Recommendations

Info

--Pressure Headat Point "P" =Elevation Headat Point "P" =7 r YDATUMFigure 1-1, Illustration of hydraulic headHydraulic Gradient = HI- HzLFigure 1-2, Illustration of hydraulic gradient
Piezometers convert pressure at a point to a physical pressure head the heightof the water column in the piezometer. This height is not hydraulic head, sinceit is only the term P/W in the equation. To find the hydraulic head at the point(lower end of the piezometer) the elevation (Z) of the point above the datum mustbe added to the pressure head. The elevation of the water surface in the piezometer,referred to the datum, is P/W + Z, and so is numerically equal to thehydraulic head at the lower end of the piezometer.Hydraulic gradientGround-water flow results from the force "available" to move water through thesoil due to differences in energy content; i.e., differences in hydraulic head.This is analagous to the flow of heat or electricity, where flow is due todifferences in temperature (heat potential) or differences in voltage (electricalpotential). Hydraulic gradient is the difference in hydraulic head at two points,divided by the distance between the points measured along the path -- of flow(fig. 1-2). In this figure, the plane of the paper is a vertical surface throughthe path of flow.Hydraulic Gradient =H1 - H2where L = distance measured along the path of flow, ft.Subscripts 1 and 2 refer to the points of the higher and lower hydraulichead, respectively; other units are defined in the preceding paragraphs.In a given flow system (fig. 1-3) each "particle" of water in the system has itscorresponding hydraulic head. All particles or points, of a given hydraulic head(Hi) lie in the corresponding equipotential surface (HI). All points of hydraulichead H2 lie in the equipotential surface Hz, and so on. The force tending to produceflow acts in the direction of greatest decrease in hydraulic head; i.e.,normal to the equipotential surface, as F at point P, or F' at point P'. Themagnitude of this force is proportional to the hydraulic gradient at the point.At the water table, the pressure component of energy (P/W) is zero relative toatmospheric pressure. Therefore, the hydraulic head H of a point at the watertable is Z, the elevation of the point above the datum.Water-table slope represents the hydraulic gradient of flow only under certainconditions. Hydraulic gradient may differ greatly from the water-table slopewhere there is significant upward or downward component of flow such as in thevicinity of pumping wells or subsurface drains, in flow from artesian aquifers,and in unsaturated seepage from canals. As shown in figure 1-4, slope of thewater table is H1 - H2 (or tangent of the angle) by definition. S is the hori-Szontal projection of the path of flow L. But the hydraulic gradient is H1 - H2L(or sine of the angle). On flat gradients and with parallel flow, the watertableslope is essentially the hydraulic gradient because S = L nearly (tangentis nearly the same as the sine for small angles). It should be noted that the
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 16 and 17: Figure 1-3, Equipotenrial surfaces-
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

NEH 16 Chapter 1 - NRCS Irrigation ToolBox Home Page

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?