# Rocks and water Darcys Law Henri Darcy established

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Rocks and water ﻣﺤﺎﺿﺮﺓ

Darcy’s Law • Henri Darcy established empirically that the energy lost ∆h in water flowing through a permeable formation is proportional to the length of the sediment column ∆L. • The constant of proportionality K is called the hydraulic conductivity. The Darcy Velocity VD: VD = – K (∆h/∆L) and since Q = VD A ( where A = total area) Q = – KA (dh/d. L)

Transmissivity and Storage Properties • Two most important aquifer characteristics: 1. Ability to store groundwater 2. Ability to transmit groundwater • Transmissivity: Ease with which water moves through an aquifer (rate at which water is transmitted through a unit width of aquifer under a unit hydraulic gradient 3

Transmissivity T = Kb T: Transmissivity, units: [L 2/T] e. g. , m 2/d K: Hydraulic conductivity b: aquifer thickness Darcy’s Law with T instead of K 4

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example • What is the transmissivity of an aquifer that has a thickness of 20 m and a hydraulic conductivity of 15 m/d? • T = Kb = 20*15 = 300 m 2/d 6

Storativity (Coefficient of Storage) and Specific Storage 1. If water is removed from a confined aquifer: – – Hydraulic head decreases - water level in wells falls Fluid pressure decreases in the aquifer. Porosity decreases as the granular skeleton contracts (aquifer collapses slightly) The volume of water increases 2. In unconfined aquifer, main source of water is drainage of water from pores 7

Storativity (coefficient of storage) • Storativity (S): the volume of water that an aquifer releases from or takes into storage per unit surface area per unit change in head. • Storativity is a dimensionless property S = volume of water/(unit area) (unit head change) =L 3/(L 2 * L) = m 3/m 3 8

Storativity contd. In confined aq. S ranges from 10 -3 to 10 -5 Specific Storage is the volume of water that an aquifer releases from or takes into storage per unit surface area per unit aquifer thickness per unit change in head Ss = volume of water ____________ (unit area)(unit thickness)(unit head change) = 1/m S = Ss b 9

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