Organization amp Main Packages CE EN 547 BRIGHAM YOUNG UNIVERSITY Code Selection Model Design We are now at this spot in the model development process MODFLOW is generally a safe and dependable choice for a model code ID: 1027004
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1. MODFLOW – IntroductionOrganization & Main PackagesCE EN 547 – BRIGHAM YOUNG UNIVERSITY
2. Code Selection – Model DesignWe are now at this spot in the model development processMODFLOW is generally a safe and dependable choice for a model codeModel design = construction of numerical grid and model inputs
3. Developed by McDonald & Harbaugh of the USGS, 1983Public DomainMost widely used groundwater modelSteady-state or transient saturated flowVersions:MODFLOW 88MODFLOW 96MODFLOW 2000MODFLOW 2005MODFLOW-NWTMODFLOW-USG
4. Finite Difference Model3D Cartesian GridCell-Centered
5. Grid GeometryOrthogonal in xyThickness varies in z
6. Governing Equationwhere: Kxx, Kyy, Kzz = values of hyd. cond. along xyz axes h = total head W = Sources and sinks Ss = Specific storage t = time
7. Processes & PackagesMODFLOW is divided into a series of processes & packagesMajor tasks are organized as processesMore specific tasks are performed by packagesEach process may use one or more packages
8. Global ProcessSpatial discretization# rows, cols, layersTop/bottom elevationsTemporal discretizationStress periodsTime stepsUnitsPackage Selection
9. Ground Water Flow ProcessFormulation and solution of the ground water flow equation by the FD methodMain part of MODFLOW codeIncludesFlow package (BCF, LPF, or HUF)Source/sink packagesSolvers
10. Observation ProcessUsed for model calibrationCalculates model-simulated heads and discharges to sources/sinks
11. Required PackagesDiscretizationBasicOutput ControlFlow Package (BCF, LPF, HUF, UPW)Solver (SIP, SSOR, PCG, GMG, etc.)
12. Optional PackagesCHB – Time Variant Specified HeadDRN – Drain DRT – Drain Return FlowETS – Evapotranspiration SegmentsEVT – EvapotranspirationGAGE – Gage GHB – General HeadHFB – Horizontal Flow BarrierLAK – LakeMNW – Multinode WellMNW2 – Multinode Well 2RCH – Recharge RIV – River SFR – Stream Flow RoutingSTR – Stream-Aquifer InteractionSUB – Subsidence WEL – WellUZF – Unsaturation Zone Flow
13. Active/Inactive ZonesCells outside problem domain are marked as inactiveControlled by IBOUND array (1=active, 0=inactive, -1=specified head)
14. Virginia Coastal Plain Model
15. Flow PackagesDefines aquifer propertiesFour optionsBlock-Centered Flow (BCF)Layer Property Flow (LPF)* Hydrogeologic Unit Flow (HUF)Upstream Weighting (UPW), MODFLOW-NWT*We will use the LPF package in this course
16. LPF - Layer Property Flow PackageUser enters Kh, Kv and storage terms for all layersKv can be entered as directly or in terms of vertical anisotropyHorizontal anisotropy entered on a cell-by-cell basisTwo layer typesConfinedConvertible
17. Well PackageAssigned to individual cellsQ can be steady state or transientExtraction well (neg Q)Injection well (pos Q)
18. DRN - Drain PackageAssigned to individual cellsUsed to simulateAgricultural drainsSpringsCreek bedsRequired parametersElevationConductance
19. DrainsHijkCDDelevQ = CD (Hijk - Delev) orQ = CD (Delev - Hijk) for proper sign on QWhen the head is above the drain elevation:
20. DrainsHijkCDDelevQ = 0When the head is below the drain elevation:
21. ConductanceDarcy’s Law:whereq = flow ratek = hydraulic conductivityDh = head differenceL = flow lengthA = gross cross-sectional area
22. Conductance, Cont.Darcy’s law can be rewritten as:whereThe appropriate values for k, A, and L must be determined on a case-by-case basis
23. Sample Conductance CalculationLength of overlap of river on cell
24. Recharge PackageOne value assigned to each vertical columnRepresents recharge due to precipitationCan be steady state or transientInfiltration rate must be assigned in correct units [L/T]
25. Factors Affecting Recharge Rate
26. Factors Affecting Recharge RateRainfallRunoffSlopeSoil typeLand useEvapotranspirationSoil typeVegetation