Darcy's law and seepage
groundwater flowpermeabilityhydraulic conductivityseepage velocity
Groundwater through soil: hydraulic gradient, Darcy discharge, the faster real velocity in the pores, and layered permeability.
Hydraulic Gradient
Hydraulic gradient as the loss of total head divided by the length of the flow path, the dimensionless driving force behind all seepage.
Darcy's Law for Groundwater Flow
Darcy's law for laminar flow through soil: discharge equals hydraulic conductivity times hydraulic gradient times gross cross-sectional area.
Seepage Velocity from Discharge Velocity
Actual seepage velocity through the pores, obtained by dividing Darcy's fictitious discharge velocity by the porosity of the soil.
Equivalent Horizontal Permeability of Layered Soil
Thickness-weighted equivalent permeability for flow parallel to the bedding of two soil layers, the parallel-resistance case of stratified seepage.
How they fit together
Darcy found in 1856, while designing Dijon's water supply, that flow through sand is proportional to the hydraulic gradient — head lost per unit distance travelled. The constant of proportionality, hydraulic conductivity, spans about ten orders of magnitude from clean gravel to intact clay, a wider range than almost any other engineering property.
The distinction that matters on site is discharge velocity versus seepage velocity. Darcy's v assumes flow across the whole cross-section, but water only moves through the pores, so the actual particle speed is v divided by porosity — typically two to three times faster. Use discharge velocity to size a dewatering pump and seepage velocity to predict when a contaminant arrives; swapping them makes a plume look slower than it is.