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Groundwater Flow Principles: Understanding Point Water Head and Flow Dynamics

This module focuses on the essential principles of groundwater flow, including point water head, validity of Darcy’s Law, and the diffusion equation. Key aspects covered include flow in unconfined aquifers and refraction of flow lines using flowed nets. You'll learn to qualitatively and quantitatively estimate equipotential and flux lines, as well as discharge and recharge rates. The course encourages an intuitive understanding of groundwater motion through graphical approaches and practical boundary condition applications.

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Groundwater Flow Principles: Understanding Point Water Head and Flow Dynamics

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  1. ESS 454 Hydrogeology Module 3 Principles of Groundwater Flow • Point water Head, Validity of Darcy’s Law • Diffusion Equation • Flow in Unconfined Aquifers & Refraction of Flow lines • Flownets Instructor: Michael Brown brown@ess.washington.edu

  2. Outline and Learning Goals • Know the appropriate boundary conditions of head and flux for various types of boundaries • Be able to qualitatively and quantitatively estimate equipotential lines, flux lines, and discharge/recharge rates using flownets

  3. 2-D Reconstructions (Flownets) • Graphical solution to LaPlace’s equation • Semi quantitative • Important in building “intuitive” understanding of groundwater flow

  4. Major Assumptions • The situation is 2-D • Aquifer is • Homogeneous • Isotropic • Saturated • Steady-state, incompressible laminar flow • Known boundary conditions (rule of thumb L= 5xW)

  5. Boundary Types • No Flow • Flow lines are parallel to boundary • Equipotential lines are perpendicular • Constant Head • Flow lines are perpendicular • Adjacent equipotential lines are parallel • Water table (Known head) • No recharge: flow is parallel • With recharge flow is oblique down Standing water

  6. Overall Plan • Plot boundaries to scale • Sketch equipotential line (stubs) at boundaries • Near boundaries draw flow perpendicular to equipotential lines • Extend flow lines to connect recharge to discharge regions • Connect equipotential lines to insure that they are perpendicular to flow lines everywhere important!!!

  7. The process is iterative • Draw boundaries • Identify boundary conditions and sketch local flow • Pencil in trial equipotential and flow lines • Erase and adjust lines until a satisfactory net is achieved • Flow lines and equipotential lines should define nearly uniform equi-dimensional “squares” • Must be 90° angle between all flow lines and intersecting equipotential lines • Calculate flow as q’= K h p/f Where q’ is discharge per width p is number of flow tubes f is number of squares along flow tube h is total head difference

  8. Example 1 Sides are “Constant Head” Impermeable boundary Flow is perpendicular and equipotential lines are parallel h=40’ h=20’ Semi-quantitative analysis Top and bottom are “No flow” q’ is volume discharge per unit width K is hydraulic Conductivity p is number of flow tubes h is total head loss f is number of squares along flow tubes Impermeable boundary Flow is parallel and equipotential lines are perpendicular 4 20’ 9 Flow Tube q’=Kph/f

  9. Example 2 8’ 0 ft Constant head Constant head No flow Needs adjusting: not 90° No flow No flow No flow Any 2-D flow situation can be estimated by constructing a Flownet h=10 Try it yourself for another geometry h=1

  10. The End of Module 3 • Should have • a conceptual grasp of how water flows in aquifers • a. Flow perpendicular to equipotential lines • b. Boundary conditions • An understanding of the equations that control flow • Diffusion Equation • LaPlace’s Equation Coming up: Flow of water to wells

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