Darcys Law Flashcards
Darcys law
Q=q.A
Q= volumetric ground water flow rate (m3/s) q= the specific discharge/darcys flux (m/s) A= cross sectional area (m2)
Darcys law can also be written as
Q= -k . >h/>l . A
Q= volumetric flowrate (m3/s) K= hydraulic conductivity (m/s) >h/>l= hydraulic gradient (i) unitless A= the cross sectional (m2)
Hydraulic conductivity
symbol: K
Units: m/s
Hydraulic conductivity can be written mathematically as
K= k. (pw.g/uw)
k= permeability (m2) pw= density of water (1000kg/m3) g= gravity at 9.8 m/s2 uw= the viscosity if fresh water at 20c = 1E-3 kg/m.s
Hydraulic conductivity is dependent on
a) properties of the rocks/sediments
increase in porosity = increase in K
b) properties of the fluids in the aquifers
Hydraulic gradient
symbol: i or >h/>l
units: unitless
2 major driving forces to groundwater flow
a) gravitational force – water will flow from high elev to low elev
b) pressure force (see photo in notes)
The head at a particular point is defined mathematically as
h= z + P/pw.g or h= z + (psy)
h= head (m) z= elevation head (m) P= pressure (PA or Kg/m.s2) pw= density of water (1000 kg/m3) g= gravity at 9.8 m/s2
Specific discharge
units: q
formula: q= v . n
n= porosity
Flow velocity
units: v