Tutorial 21: Unstructured Grid Model Setup

Step 1 — Draw the Model Domain

Zoom to the area of interest: Diamond Springs Golf Course near Hamilton, Michigan. Go to 'Conceptual Model Tools' → 'DrawDomain' → 'DomainRect' and draw a rectangular domain centered on the golf course. This is a real location — the Global Base Model provides terrain, geology, and hydrology data automatically.

Step 2 — Add a River with Grid Refinement

Go to 'Conceptual Model Tools' → 'Lines' → 'DrawLine'. Click along Miller Creek to trace the river, then 'Save Shape'. Assign as a 'Two-way' head-dependent flux boundary condition. Then check 'Refined Grid' and click 'Edit Grid':

Grid Size Factor: 2 (both x and y)
Buffer: 0
Thresholds: 3 (smooth transition from coarse to fine along the river)

The river gets finer cells because surface water–groundwater interaction changes rapidly near the stream — coarse cells would miss the details of gaining and losing reaches.

Step 3 — Add a Pumping Well with Refinement

Go to 'Conceptual Model Tools' → 'Wells' → 'DrawWell'. Place a well just south of Miller Creek with a pumping rate of −15,000 m³/day. Check 'Refined Grid' and click 'Edit Grid':

Grid Size Factor: 4 (very fine around the well)
Buffer: 1 (one extra ring of refined cells)
Thresholds: 0 (abrupt transition — acceptable for isolated point features)

Wells create steep hydraulic gradients — the cone of depression changes rapidly near the well. A factor of 4 gives 16× the resolution, capturing drawdown accurately without refining the entire domain.

Step 4 — Add a Second Pumping Well

Place another well southeast of the first with a pumping rate of −10,000 m³/day. Apply grid refinement:

Grid Size Factor: 4
Buffer: 1
Thresholds: 5 (very smooth transition — many intermediate levels)

Note the different threshold setting: 5 here versus 0 in Step 3. This creates a much smoother transition from the parent grid to the refined well grid. Compare the two approaches visually when the grid is displayed in Step 12.

Step 5 — Add a Refinement Zone Around the Wells

Go to 'Conceptual Model Tools' → 'Zones' → 'ZoneRect'. Draw a rectangular zone encompassing both pumping wells, then 'Save Shape'. Select 'Polygon Only' under Zone Types (this zone defines refinement only, not property changes). Apply refinement:

Grid Size Factor: 2  |  Buffer: 0  |  Thresholds: 0

This creates an intermediate refinement level between the coarse parent grid and the very fine well grids — a middle layer in the refinement hierarchy.

Step 6 — Add an Injection Well

Add a well near the southern portion of the model with a pumping rate of +10 m³/day (positive = injection). No grid refinement needed for this small-rate well — the default grid resolution is sufficient.

Step 7 — Add a Wetland Zone

Go to 'Zones' → 'ZonePoly' and draw an irregular polygon near the southwest corner. In the 'Sources and Sinks Head Dependent' tab:

Check 'Two-way Head Dependent'
Zone Name: Wetland
Stage elevation: 190 m
Riverbed (wetland bottom): Stage minus 5 m
Leakance: 5 day⁻¹

The wetland is modeled as a head-dependent boundary — water exchanges between the aquifer and wetland depending on the head difference. If groundwater is above the wetland stage, the aquifer feeds the wetland (gaining). If below, the wetland recharges the aquifer (losing).

Step 8 — Add a Low Conductivity Zone

Go to 'Zones' → 'ZoneRect' and draw a zone just west of the injection well. Check 'Conductivity' and assign K = 0.1 m/day. This represents a clay lens or low-permeability geological feature that locally impedes groundwater flow.

Step 9 — Refinement Zone Around Southern Features

Draw a rectangular 'Polygon Only' zone encompassing the wetland, low K zone, and injection well. Apply refinement: Grid Size Factor = 2, Buffer = 0, Thresholds = 0.

Step 10 — Second Level of Refinement

Draw another 'Polygon Only' zone, smaller, around just the low K zone and injection well. Apply finer refinement: Grid Size Factor = 4, Buffer = 0, Thresholds = 0. This creates two nested levels of refinement — factor 2 around the broader area, factor 4 around the most critical features.

Step 11 — Set Grid Size and Solver

Go to 'DomainAttr' → 'Simulation Settings'. Set Grid NX = 30 (the parent grid has 30 cells in the x-direction). Click 'Solver Options', select MODFLOW, then select MODFLOW-6 as the solver. MODFLOW-6 is required for unstructured grids — earlier MODFLOW versions support only regular grids.

Step 12 — Display the Unstructured Grid

In Display Settings, change 'Main Display' to 'Contour+Color Map and Vectors'. Click 'Show Refined Grid'. After a moment, the Grid Refinement interface appears showing the complete unstructured grid — all refinement zones visible simultaneously. Notice how each feature created a local refinement patch, and how they nest within each other.

Step 13 — Zoom In to Inspect Refinement

Use the zoom rectangle tool to zoom into the area around the two pumping wells. You can now see the multi-level refinement in detail — the coarse parent grid transitioning through intermediate levels to the finest cells around each well. Compare the abrupt transition (Step 3, Thresholds = 0) with the smooth transition (Step 4, Thresholds = 5).

Step 14 — Simulate and View Parent Model Results

Go to 'Simulation Tools' → 'Simulate' and follow the prompts. The results displayed are for the parent (coarse-grid) model. The process for viewing the refined subgrid results is presented in the next tutorial (Tutorial 22).