10_Supp%2010%20Head

10_Supp%2010%20Head - HYDROGEOLOGY LECTURE NOTES Chapter 2...

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HYDROGEOLOGY LECTURE NOTES HYDROGEOLOGYLECTURENOTES(SPRING2004).DOC PRINTED ON 8/2/2005 7 Chapter 2 – Fluid Potential and Kinetic Energy Introduction One of the fundamental equations that govern groundwater flow is called Darcy’s Law (equation 2-1), Q = K A dh / dl (Equation 2-1) where: Q = discharge [L 3 /T] A = cross sectional area [L 2 ] K = hydraulic conductivity [L/t] dh / dl = hydraulic gradient or change in hydraulic head (h) per change in distance (l) [ · ] In plain English, this equation states: Discharge (i.e., volumetric flow) through a cross sectional area is proportional to the hydraulic gradient, and the constant of proportionality is a quantity called the hydraulic conductivity We will get into the details of the equation later in the course, but for now, let’s focus on the little “h” in the hydraulic gradient term, i.e., hydraulic head . Fluid Energy In simple, everyday terms, we think of hydraulic head as an elevation. More specifically, hydraulic head is the elevation of water in a manometer in a pressurized water pipe (Figure 2-1), or in a piezometer (Figure 2-2). - A manometer is a vertical tube in a pressurized water pipe used to measure pressure in the pipe. - A piezometer is a vertical tube with an open or slotted interval (usually called the screened interval or just the screen) inserted into the ground and used to measure hydraulic head in an aquifer; it is basically a well constructed for the sole purpose of measuring groundwater levels. Figure 2-1. Pipe of flowing water with manometers showing the loss of head along the flow path (© Uliana, 2001) Figure 2-2. Cross section of aquifer showing hydraulic heads in three wells (© Uliana, 2001) In actuality, the water levels are a manifestation of the fluid energy in a groundwater system, and measuring a water elevation in a well is really a measurement of the energy present in the fluid at a certain point in a groundwater system (Figure 2-2). An aquifer is a dynamic system of flowing water. The fluid energy varies throughout the system and is different from one point to the next. The result is that we find different water levels throughout the aquifer. These different water levels result in hydraulic gradients, and water movement is driven by those gradients
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HYDROGEOLOGY LECTURE NOTES HYDROGEOLOGYLECTURENOTES(SPRING2004).DOC PRINTED ON 8/2/2005 8 What do we mean by “energy”? Everything in the universe has some amount of energy associated with it, and that energy is present in various forms. Some sort of energy drives every natural process, and the key to understanding physical processes is in understanding the distribution of energy in a system. Potential Energy : energy stored in a piece of matter or at a point in a system; generally associated with position or with thermodynamics of the system (elevation, pressure, chemical, thermal) Kinetic : energy associated with motion (velocity) At every point in an aquifer, the fluid possesses some total amount of energy that is the sum
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10_Supp%2010%20Head - HYDROGEOLOGY LECTURE NOTES Chapter 2...

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