Hydraulic Gradient and Seepage Velocity

Reference

The hydraulic gradient is the driving force of groundwater flow, and combining it with conductivity and porosity tells you how fast water actually travels.

The equation

i = Δh / L, v = K i, vs = v / n
  • i is the gradient: head difference over flow distance
  • v is the Darcy velocity in meters per day
  • vs is the seepage velocity through the pores
  • n is effective porosity, dimensionless

Worked example

Two wells 80 meters apart differ in head by 2 meters, so i = 0.025. With K of 12 meters per day the Darcy velocity is 0.3 meters per day, and with an effective porosity of 0.3 the water itself moves about 1 meter per day, the number that matters for contaminant travel time.

When to use it

The seepage velocity, not the Darcy velocity, sets arrival times of plumes and tracers. Use effective porosity rather than total porosity, since much of the pore space in fine soils barely conducts.

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