Basic Subsurface Flow

Subsurface flow refers to any flow below the surface of the ground which includes low flow (base flow) and quickflow (subsurface stormflow). Part of the infiltrated rainfall circulates more or less horizontally in the superior soil layer and appears at the surface through drain channels. Subsurface flows in water-bearing formations have drainage slower than superficial flows, but faster than groundwater flows. With this, subsurface flow is of becoming a great interest and importance beyond the conventional hydrological literature, for it is responsible for the transport of water beneath the earth's surface.

law of darcy, confined and confined aquifers

darcy's law

Darcy's law is a phenomenologically derived equation that describes the flow of liquid in a porous medium. It was formulated by Henry Darcy which expresses the conservation of momentum and is analogous to Fourier's law. Darcy's law is used to describe any liquid flows, which gives the relationship between flowrate of groundwater, cross-sectional area of the aquifer, the hydraulic gradient, and hydraulic conductivity of the aquifer
Darcy's law and characteristics of aquifer are provided as shown in the left figure.

sample problems

confined and unconfined aquifers

The geological formations of groundwater is classified into four, which are:

Aquifer
Aquitard
Aquiclude
Aquifuge

From the point of view of geological structure and expansion, aquifer is divided into two categories:

Confined Aquifers - Are found at such places where pressure of groundwater is comparatively more than atmospheric pressure due to non-permeable layers.
Unconfined Aquifers - These are aquifers in which the groundwater level works like the upper layer of the saturation zone. Wavy form and slope of groundwater constantly changes depending on groundwater replenishment and permeability.

groundwater flow in confined aquifer

The groundwater flow in confined aquifer can be calculated by first deriving the equation as shown in the figure.

SAMPLE PROBLEMS:

groundwater flow in unconfined aquifer

Using the concepts of differential and integral calculus, groundwater flow in an unconfined aquifer can be obtained using the given relation as shown.
Seepage, on the other hand, which is based on the assumption that the hydraulic gradient and water table slope are the same is presented via Dupuit's equation.

sample problems

RADIAL GROUNDWATER FLOW IN CONFINED AQUIFER

Steady radial groundwater flow in confined aquifers can be calculated using the Thiem Equation
However, when the groundwater flow becomes unsteady, computations would be complicated to solve using ordinary differentiation and integration for non-steady groundwater flow. Instead, partial differentiation is used via Theis Equation

sample problems:

radial groundwater flow in unconfined aquifer

Steady Radial groundwater flow in unconfined aquifer can be calculated using Dupuit's formula. This equation is used for obtaining the hydraulic conductivity of the aquifer as well as the drawdown from the well into the phreatic surface.

sample problems:

TIME TRAVEL OF GROUNDWATER IN CONFINED AQUIFER

The travel time of groundwater in confined aquifer can be calculated using the relations of groundwater velocity, Darcy's law, hydraulic flux, and other groundwater parameters including hydraulic conductivity and transmissivity. This is provided by the equations illustrated on the left figure.

assessment tasks:

PROBLEM SET #4

Basic Surface Flow_BARTOLOME (1).pdf

QUIZ #4

BASIC SUBSURFACE FLOW.pdf

VIDEO PRESENTATION LINK: https://youtu.be/q0pLAShRaWE

REFLECTION:

Subsurface flow is important in studying watershed behaviors. Understanding the behavior of water underground allows us to perceive and find out the relationship between the surface water in a catchment and groundwater resources. It is highly significant to study the aspects of subsurface flow for civil engineers to know the required capacity of the water reservoir in order to provide adequate water from underground and on the surface, which can be used for irrigation purposes and in supplying potable water systems in various municipalities, especially during droughts condition. These include studying the concept of darcy's law and the assumptions, as well as groundwater flow and groundwater flow in confined and unconfined aquifers.

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