Figure 4 illustrates the differences between Darcy Velocity and Seepage Velocity. When calculating solute travel time however, the seepage velocity calculation ( V S, Equation 2) must be used and an estimate of effective porosity is required. For any calculation where the actual flow rate in units of volume per time (such as liters per day or gallons per minute) is involved, the original Darcy Equation should be used (calculate V D, Equation 1) without using effective porosity. In groundwater calculations, Darcy Velocity and Seepage Velocity are used for different purposes. Difference between Darcy Velocity (also called Specific Discharge) and Seepage Velocity (also called Interstitial Velocity). Typical values for total and effective porosity are shown in Table 1.įigure 4. The difference is that total porosity includes some dead-end pores that do not support groundwater. N e = Effective porosity - fraction of cross section available for groundwater flow (unitless)Įffective porosity is smaller than total porosity. V D = “Darcy Velocity” describes groundwater flow as the volume of flow per unit area per time (also units of length per time) V S = “interstitial velocity” or “seepage velocity” (units of length per time, such as m/sec) This quantity is called “interstitial velocity” or “seepage velocity” and is calculated by dividing the Darcy Velocity (flow per unit area) by the actual open pore area where groundwater is flowing, the “effective porosity” (Table 1): Since Darcy’s time, Darcy’s Law has been extended to develop a useful variation of Darcy's formula that calculates the actual velocity that the groundwater is moving in units such as meters traveled per year. Mitchell, Western Washington University). Conceptual explanation of Darcy’s Law based on Darcy’s experiment (Adapted from course notes developed by Dr. This law is a fundamental mathematical relationship in the groundwater field and can be expressed this way:įigure 3. In unconsolidated geologic settings (gravel, sand, silt, and clay) and highly fractured systems, the rate of groundwater movement can be expressed using Darcy’s Law. Representative values of total porosity ( n), effective porosity ( n e), and hydraulic conductivity ( K) for different aquifer materials Aquifer Material If groundwater is flowing and contains dissolved contaminants it can transport the contaminants by advection from areas with high hydraulic head toward lower hydraulic head zones, or “downgradient”.ĭarcy's Law Table 1.
The rate of change (slope) of the hydraulic head is known as the hydraulic gradient. Groundwater flows from areas of higher hydraulic head (a measure of pressure and gravitational energy) toward areas of lower hydraulic head (Figure 1). In unconfined aquifers, the hydraulic gradient can also be described as the slope of the water table (Adapted from course notes developed by Dr. Hydraulic gradient (typically described in units of m/m or ft/ft) is the difference in hydraulic head from Point A to Point B (ΔH) divided by the distance between them (ΔL).