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Boundary Conditions

Boundary Conditions

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Boundary Conditions

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  1. 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.

  2. 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.

  3. 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

  4. Head Dependent Boundaries (e.g., MODFLOW’s River Package)

  5. Q = C (hR – hijk) hijk m RBOT w Gaining River Cell

  6. Q = C (hR – hijk) RBOT Losing River Cell

  7. Q = C (hR – hijk) RBOT Losing River Cell

  8. Q = C (hR – RBOT) RBOT Percolating River Cell

  9. 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

  10. Resistance River with sandy bottom = 1 day Small stream with silty bottom = 1- 10 days Stream/lake in till = 100 days