Yhd Soil and Groundwater Hydrology

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Yhd-12.3105 Soil and Groundwater Hydrology
Steady-state flow Teemu Kokkonen Tel Room: 272 (Tietotie 1 E) Water Engineering Department of Civil and Environmental Engineering Aalto University School of Engineering

Aquifer types Aquifer Confined aquifer
Latin: aqua (water) + ferre (bear, carry) An underground bed or layer of permeable rock, sediment, or soil that yields water Confined aquifer Between two impermeable layers Groundwater is under pressure and will rise in a borehole above the confining layer Unconfined aquifer (phreatic aquifer) Groundwater table forms the upper boundary

Aquifer Types

Some Terms Saturated zone (vadose zone)
Pore space fully saturated with water Groundwater level (water table) Is defined as the surface where the water pressure is equal to the atmospheric pressure In groundwater studies the atmospheric pressure is typically used as the reference point and assigned with the value of zero Capillary fringe Saturated (or almost saturated) layer just above the grounwater level Unsaturated zone Both water and air are present in the pore space

Hydraulisia johtavuuksia

Equation for Groundwater Flow Steady state 1D
Inflow per unit time Qi = 2 l s-1 Inflow per unit time and unit area (influx) qi = 2 l s-1 / 0.4 m2 = 0.5 cm s-1 When the water level in the container does not change it is in steady-state The influx and outflux must then be equal to each other Outflow per unit time Qo = 2 l/s Outflow per unit time and unit area (outflux) qo = 0.5 cm s-1

Equation for Groundwater Flow Steady state 1D
Darcy’s law Conservation of momentum Continuity equation Conservation of mass In steady state conditions the amount of stored water does not change The outgoing flux must equal the incoming flux

Equation for Groundwater Flow Steady state 1D
Conservation of mass Inserting Darcy’s law to describe the flux q yields: Under the assumption of homogeneity: Laplace equation in 1D

Equation for Groundwater Flow Steady state 3D
The groundwater equation just derived in one dimension is easy to generalise to three dimensions Analogous analysis to the previous slide yields for the homogeneous 3D case:

Equation for Groundwater Flow Steady state 2D
Typically thicknes of aquifers is relatively small compared to their areal extent, which justifies the assumption of essentially horizontal flow

Exchange of Water: Sink or Source
An aquifer can receive (source) or loose (sink) water in interaction with the world beyond its domain Source: recharge from precipitation, injection wells Sink: pumping wells In the groundwater equation added or removed water is described using a sink / source term

Equation for Groundwater Flow Steady state 2D, Sink / Source
qx(x) = 9 ? qx(x+Dx) = 6 ? qy(y) = 2 ? qy(y+Dy) = 5 ? Dx = 3 ? Dy = 2 ? b = 2 ? R = -1 ? Explain in your own words what water balance components the circled terms in the above equation represent. Use then the values given below to compute their values assuming that the derivatives are constant within the rectangular control volume. Give also units to the quantities listed below.

Equation for Groundwater Flow Steady state 2D, Sink / Source

Equation for Groundwater Flow Steady state 2D, Sink / Source
How does the equation change if the aquifer is homogeneous? How does the equation change if the aquifer is isotropic? Defining transmissivity T to be the product of hydraulic conductivity K and the thickness of the water conducting layer b yields:

Equation for Groundwater Flow Steady state 2D, Sink / Source
Does R vary in space? When? Does T vary in space? When?

Boundary Conditions Governing equation for groundwater flow
Describes how the water flux depends on the gradient of the hydraulic head (Darcy’s law) Requires the mass to be conserved To represent a particular aquifer boundary conditions need to be defined Boundary conditions describe how the studied aquifer interacts with the regions surrounding the aquifer

Boundary Conditions Two main categories
Constant head (fixed head, prescribed head) Dirichlet condition Water bodies (lakes, rivers) Constant flux Neumann condition Impermeable boundary is a common special case (clay, rock, artificial liners...)

Dx Homogeneous aquifer Let us derive a numerical approximation for the steady-state 1D groundwater flow equation. Step 1. How would you approximate the spatial derivative between nodes i and i+1? Step 2. How would you approximate the spatial derivative between nodes i-1 and i? Step 3. How would you approximate the 2nd spatial derivative around node i?

Geometric average Heterogeneous aquifer Dx Let us again derive a numerical approximation for the steady-state 1D groundwater flow equation. How to compute and ?

Numerical Solution 2D Steady-state Flow, Sink/source Term
j

Boundary Conditions Numerical Solution
Hydraulic head to be computed from the groundwater flow equation. Hydraulic head set to a fixed value representing the level of the lake. Hydraulic head value is ”mirrorred” across the no-flow boundary. Lake H = 10 m I II Clay (almost impermeable) HII = HI Hydraulic gradient across this line becomes zero => no flow