modeling multi-layer flow and unsaturated flow: aem
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Modeling multi-layer flow and unsaturated flow: AEM solutions to themodified Helmholtz equation
Mark BakkerCivil Engineering, TU DelftBio and Ag Eng., Univ. of [email protected]
Sabbatical funding:TU Delft Grants Program
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Flow is at steady-stateResistance to vertical flow is neglected within an aquifer
Head is function of x and y, but flow is 3DFlow in leaky layers is verticalAquifer properties are piece- wise constant
Objective: Model multi-aquifer and multi-layer flow with an analytic, mesh free approach
Funding: US EPA, WHPA, WBL, Brabant Water, Artesia, Amsterdam Water SupplyCollaborators: Otto Strack, Vic Kelson, Ken Luther
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Vertical flow between aquifers or layers is computed with Darcy's law
leaky layeraquifer 1
aquifer 2
qz = k zh2−h1
H 1H 2/2=
h2−h1
c
layer 1
layer 2
qz = k zh2−h1
d=
h2−h1
c
: vertical hydraulic conductivity of leaky layerd : thickness of leaky layerc = d / : resistance
k z
k z
: vertical hydraulic conductivity of layers : thicknesses of layers
k z
c=H 1H 2/2kzH 1 , H 2
multi-aquifer flow multi-layer flow
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Flow in 3-layer system is governed by 3 linked differential equations
∇2 h2 =h2−h1
c1 T 2
h2−h3
c2 T 2
∇ 2 h1 =h1−h2
c1T 1
∇ 2 h3 =h3−h2
c2 T 3
∇ 2 h=E /T
h: hydraulic head (m)
E: sink term (m/d)
T=kH: transmissivity (m2/day)
c: resistance between layers (days)
General form of deq:
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System of differential equations may be written in matrix form of modified Helmholtz equation multi-layer theory of Hemker (1984)
∇ 2h1
h2
h3=
1c1 T 1
−1c1 T 1
0
−1c1T 2
1c1T 2
1c2 T 2
1c2 T 2
0 −1c2 T 3
1c2 T 3
h1
h2
h3
∇ 2h=Ah A is the system matrix and is function of aquifer properties
Hermann von Helmholtz
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General form of solution for confined system with N layers: Laplace part and Helmholtz part
h=hLe∑n=1
N−1
hn un
∇ 2 hL=0
∇ 2 hn=wn hn
General form of solution:
hL fulfills Laplace's deq:
hn fulfills modified Helmholtz deq:
unit vectoreigenvector n of system matrixeigenvalue n of system matrix
eun
wn
h = Q2T tot
ln re∑n=1
N−1 An
2K0r wn unExample:
(implemented formulation is in discharge potentials)
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Path lines started in top aquifer
Path lines started in bottom aquifer
A Line-sink in top aquiferDrawdown in top aquifer Drawdown bottom aquifer
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An area-sink on top of the aquifer sytem, here used for recharge
Path lines started at top of aquifer
Recharge inside polygon Water mount in bottom aquifer
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Multi-aquifer inhomogeneity in uniform flow
Red: k = 100 m/dBlue: k = 2 m/dGreen: leaky layer
Outside:k = 10 m/d
k = 20 m/d
Uniform flow
Heads in top aquifer
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Blue: Aquifer 1Red: Aquifer 2Green: Aquifer 3
well inaq 3
well inaq 2+3
well in aq 2
recharge to top aquiferriver segments(line-sinks)
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Path lines tracedback from the well
Determine sourceof well in aquifer 3
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Collector well in Sonoma County, CA; model with 12 layers
Example application: modeling radial collector wells with a multi-layer approach
WHPA, Bloomington, IN
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Heterogeneity in the vadose zonecauses advective spreading
Effective longitudinal spreading in saturated heterogeneous media quantified with analytic element solutions (Dagan, Fiori, Jankovic)
Analytic element solutions for unsaturated flow through heterogeneous vadose zones were developed recently (Bakker & Nieber 2004, VZJ, WRR)
(an open door)
Collaborator:John Nieber, Univ. of Minnesota
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Mathematical formulation
Darcy's law for specific discharge:
qx=−k ∂∂ x
qz=−k ∂∂ z k
ψ : pressure head (negative for unsaturated flow)k(ψ) : hydraulic conductivity function
Continuity of steady flow:
x
z
∂∂ x k
∂∂ x ∂
∂ z k∂∂ z −∂k
∂ z=0
Highly n
on-line
ar deq
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Kirchhoff potential and Gardner model
H =∫−∞
k sds
k =k s exp [−e]
Kirchhoff potential:
Hydraulic conductivity:(Gardner model)
ks : hydraulic conductivity at saturationα : parameter dependent on pore size distributionψe : air entry pressure head
substitution gives ....
Gustave Kirchhoff
H =k
Physically:
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Kirchhoff potential is governed by linear differential equation
=H exp[−z−zc/2]
∇2=2
4
Modified Helmholtz equation
∇2 H− ∂H∂ z =0
Define:
which gives
Approach: Superimpose analytic element solutions for H(x,z), by using solutions to the Mod. Helmholtz Eq.
Hermann von Helmholtz
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Analytic element solutions for circular and elliptical inhomogeneities
Hydraulic conductivity functions are different inside and outside (ks, α, and ψe may differ)
Separation of variables in radial or elliptical coordinates
Separate infinite series for inside and outside
Boundary conditions of continuity of pressure head and normal flow are met up to machine accuracy
k=k s e−e
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Commonly, k of finer soil is larger than kof coarser soil under unsaturated conditions
ψe = -0.15 m
ψe = -0.25 m
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Consider a finer-grained medium containing coarser grained inclusions
finer materialks = 0.36 m/dψe = -0.25 m/dα = 12.9 m-1 λ = 3.9 m-1
coarser materialks = 1 m/dψe = -0.15 m/dα = 21.2 m-1
λ = 6.2 m-1
coarser
finer
finer
uniform flow qz0
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0.9 m
In saturated flow, the coarser ellipses will attract the flow
Flow patternindependentof uniform flow amount
Effect of ellipses withkcoarser ≈ 3kfineris relativelysmall
start trace experiment
compute arrival times
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µ = 16.9 dσ = 0.57 dσ/µ = 0.03γ = 1.4
µ = 169 dσ = 5.7 dσ/µ = 0.03γ = 1.4
µ = 1690 dσ = 57 dσ/µ = 0.03γ = 1.4
qz0 = 1.8 cm/d
vz0 = 5.1 cm/d
qz0 = 0.18 cm/d
vz0 = 0.51 cm/d
qz0 = 0.018 cm/d
vz0 = 0.051 cm/d
Saturated flow: Breakthrough curve independent of uniform flow
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qz0 = 0.05ks,fine = 1.8 cm/d
vz0 = 12.8 cm/d0.9 m
Unsaturated flow: coarse-grained ellipses divert flow
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qz0 = 0.005ks,fine = 0.18 cm/d
vz0 = 2.56 cm/d
A smaller uniform flow creates a greater diversion by the ellipses
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qz0 = 0.0005ks,fine = 0.018 cm/d
vz0 = 0.51 cm/d
Coarse-grained ellipses in very small uniform flow behave almost as impermeable ellipses
start trace experiment
compute arrival times
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µ = 7.2 dσ = 0.99 dσ/µ = 0.14γ = 0.18
µ = 36 dσ = 8.9 dσ/µ = 0.25γ = 1.1
µ = 181 dσ = 59 dσ/µ = 0.33γ = 2.4
vz0 = 12.8 cm/d
vz0 = 2.56 cm/d
vz0 = 0.51 cm/d
Breakthrough curves for unsaturated flow
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Example of flow new with many ellipses(coming soon)
Accurate and efficient models can be made of flowthrough many (thousands?) of inhomogeneities
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Multi-layer solution of 3D flowto a radial collector well
Inhomogeneities in vadose zonehave much greater effect than insaturated zone and benefit fromanalytic modeling
Analytic modeling of head and flow in heterogeneous multi-layer systems and vadose zones
Mark [email protected]