The hydraulic gradient is the head loss per unit length in the direction of flow, and is also equal to the slope of the hydraulic grade line. It is useful in connection with extraction of groundwater from an aquifer through wells for water supply, irrigation, … In the table at the left the units are gpd/ft2. Putting these two proportionalities together gives the following equation: Q = flow rate of liquid through the porous medium, typically in ft3/sec. movement is neither directly downward nor directly toward the channel Then i = hL/L and Darcy’s Law can be given as Q = KAi. A very porous medium with little resistance to flow will have a high value for K, while a tightly packed medium with high resistance to flow will have a low value for K. The approximate range of K values for several types of soil are given in the table at the left. The maximum groundwater gradient and flow direction are based on the plane formed by the squared heads. Darcy’s Law gives the relationship among the flow rate of the groundwater, the cross-sectional area of the aquifer perpendicular to the flow, the hydraulic gradient, and the hydraulic conductivity of the aquifer. actual flow paths in fractured rocks will follow the gradient as the hydraulic (where V is the velocity of the groundwater flow, K is the hydraulic conductivity, and i is the hydraulic gradient). head dictates, but only as the physical presence of the fractures allows. m = viscosity of flowing liquid, lb-sec/ft2. diff./[length in dir of heat flow/(thermal conductivity)(area normal to heat flow)]}, Darcy’s Law: Q = hL/(L/kA) {liquid flow rate = head loss/[length in dir of flow/(hydraulic conductivity)(area normal to flow)]}. The curving path depicts the theoretical path of groundwater flow. When following groundwater flow paths from a hill to an adjacent stream, water discharges into the stream from all possible directions, including straight … The hydraulic gradient is a vector gradient between two or more hydraulic head measurements over the length of the flow path. The values of K in the table are for the flow of water through the indicated porous media. Groundwater gradients are pretty low. To first approximation, groundwater flows down-gradient (from high to low hydraulic head). Groundwater can move upward against gravity because move upward or downward. Hydraulic head is the level to which groundwater will rise in a well. Groundwater flow direction is reported as degrees clockwise from the positive y-axis defined by your x,y locations. Since dh/ds varies with position in an unconfined aquifer, we report … Reynold’s number (Re) for flow through a porous medium is defined as: Re = ρVL/μ, where ρ and μ are the density and viscosity of the liquid, V is the flow velocity (Q/A), and L is a characteristic length, typically taken as the mean grain diameter of the medium. water toward an area of lower pressure, the stream channel. Groundwater flows from high hydraulic head to low hydraulic head. γ = specific weight of flowing liquid, lb/ft3. Hydraulic Gradient. The hydraulic conductivity, K, is a constant for a given porous medium. All Rights Reserved. (The actual equation is Ω = [X Π] / Φ.) The velocity of groundwater flow is proportional to the magnitude of the hydraulic gradient and the hydraulic conductivity of the aquifer (see Chapter 12). tendency toward lateral flow is actually the result of the movement of For groundwater, it is also called the 'Darcy slope', since it determines the quantity of a Darcy flux or discharge. to flow laterally in the direction of the slope of the water table. Groundwater can actually For an unconfined aquifer, the calculation squares the input heads, then fits a plane through these squared heads. The gradient (the hydraulic conductivity (Π), and the effective porosity (Φ) are all quantities that need to be measured or estimated in order to calculate the speed (Ω) of groundwater flow. Darcy’s Law is an empirical relationship for liquid flow through a porous medium. The resulting The effect of the medium and the properties of the liquid flowing through it can be separated by the use of the specific pemeability, k, as shown in the following equation: k = specific permeability, ft2 (a property of the porous medium only).