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CE361 - Lecture 02

CE361 - Lecture 02 - Water Resources Engineering Lecture 2...

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1/17/2009 1 Water Resources Engineering - Lecture 2 1 Notes by T. Wagener & P. Reed Penn State University Principles of Flow in Hydrosystems 2

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1/17/2009 2 Water Properties – Mass or Weight Mass density or density ρ (rho), is the mass per unit volume [kg/m 3 or Ns 2 /m 4 ]. (Mass density of water at 4 ° C is 1000kg/m 3 ). Density is often assumed constant, so that water is assumed i ibl i h d l d h d li I ibilit d t incompressible in hydrology and hydraulics. Incompressibility does not always mean constant density because salt in water changes the density of water without changing its volume. Specific weight γ (gamma) is the gravitational force (weight) per unit volume of water. (Specific weight of water at 4°C is 9810 N/m 3 or 62.4 lb/ft 3 ). The relationship between density and specific weight is: This is an important characteristic for example when we are dealing with water with different levels of salt content or different temperatures [e.g. Estuaries, density currents, …]. g γ ρ = Why is it Important? Example: Ocean Thermohaline Circulation Evaporation makes water more saline and denser Freezing makes water more saline and denser Cold water is denser than warm water
1/17/2009 3 Water Properties - Pressure Pressure , p , is the force F acting over an area A F dF Δ Pressure at a point is equal in all directions. The pressure at depth y (assuming no pressure on the water surface) is 0 lim A P dA dA Δ → = = P 5 y γ = Water Properties - Pressure

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