Boundary Conditions

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Boundary Conditions

• Boundary conditions always influence a steady
  state
• solution but may not influence a transient
  solution.

• A steady state solution with all specified flux
  boundary
• conditions (including no flow) without specified
  head
• or head-dependent internal boundaries may not
  converge
• or will give a non-unique solution.

• Initial conditions provide reference heads for
  transient
• solutions so that it may be appropriate to set
  all boundary
• conditions to flux conditions.

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Boundary Conditions

• A specified head boundary acts as an infinite
  source or sink.

• Specified flux conditions allow calibration to
• hydraulic conductivity while specified head and
• no flow boundaries do not.

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Types of Boundary Conditions

• Specified head (including constant head)
• h f (x,y,z,t)

• Specified flux (including no flow)
• ?h/?l -ql/Kl

• Head-dependent conditions
• q -Kb (hb h)/L

Implementation in MODFLOW
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Head Dependent Boundaries (e.g., MODFLOWs River
Package)
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Q C (hR hijk)
hijk
m
RBOT
w
Gaining River Cell
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Q C (hR hijk)
RBOT
Losing River Cell
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Q C (hR hijk)
RBOT
Losing River Cell
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Q C (hR RBOT)
RBOT
Percolating River Cell
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Definitions
Riverbed Conductance (Kv/m) wL Leakance Kv/m
Resistance m/Kv
Kv vertical hydraulic conductivity of riverbed
sediments m thickness of riverbed sediments w
width of the river L length of the river within
the cell
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Resistance
River with sandy bottom 1 day Small stream
with silty bottom 1- 10 days Stream/lake in
till 100 days