Presentation by Betsy Romero Verástegui (Deltares, Netherlands) at the Hydrology Suite User Days (Day 3) - Groundwater modelling, during the Delft Software Days - Edition 2023 (DSD-INT 2023). Thursday, 30 November 2023, Delft.

1. Example of unstructured
MODFLOW 6 modelling in
California
Betsy Romero Verástegui
Toine Vergroesen
30 November 2023

2. Alameda creek project
Current situation
• Long periods of water scarcity
• Decreases in the levels of some aquifers that do not
recover during rainy seasons.
• Water demand exceeds supply in dry periods.
• In the creek: Use for flood control but it is getting filled
with sediments -> need to dredge.
Study area:
Alameda creek – California (USA)
2
Using iMOD tools and MODFLOW 6 to simulate the
effects of deepening the Alameda creek.
Objective:
• Know whether the deepening of the Alameda creek will
affect the infiltration from the nearby infiltration lakes
under different dredging scenarios.
DSD-INT
2023
|
Hydrology
User
Days

3. First model
3
• MODFLOW 6
• Unstructured grid: rectangular, nested
• Lakes area refinement
• Steady state
• 2 aquifers and 1 aquitard in between.
• Get the highest resolution where the
Quarry Lakes border Alameda Creek.
80 ft
40 ft
20 ft
10 ft
DSD-INT
2023
|
Hydrology
User
Days

4. Further steps needed
4
• Enhance the resolution for the
entire Alameda Creek (i.e.
increasing the density of the
model's grid cells throughout):
refine calculations in the creek
area given its importance in the
evaluation
• Reassess the groundwater
system and potential impacts of
the creek deepening
• Additional geohydrological data
• Transient simulation
• Improve calibration
DSD-INT
2023
|
Hydrology
User
Days

5. Current model (improved)
5
• MODFLOW 6
• Non-uniform unstructured Voronoi grid
• Discretization along the creek area
• Unconfined aquifer
• Fully 3D
• 4 discretization zones: 300 ft, 150 ft, 16 ft, 8/16 ft
• 8ft: 283 300 model cells, minimum size: 9 ft2
• 16ft: 173 520 model cells (38.75% less model cells), minimum
size: 22 ft2
DSD-INT
2023
|
Hydrology
User
Days

6. Data implemented
6
• Detailed data input for
water system
elements: lakes, river,
drains -> extra grid
refinement in these
areas
• Other spatially-varying
conditions: sea,
hydraulic properties,
recharge, well
extractions

7. Tools used
7
• iMOD Python toolbox
• Conversion of input data in several formats to
unstructured grid
• Delft3D river data implementation
• Along with snakemake, creation of an automated
workflow (particularly useful for scenarios simulation)
• Results postprocessing: Maps, comparison with
observed heads, water balance calculation
• MODFLOW 6
• Unstructured grid capabilities
• Model simulation
DSD-INT
2023
|
Hydrology
User
Days

8. Results
8
Run times:
• Steady state 16 ft grid approx. 2 to 3 minutes.
• Steady state 8 ft grid approx. 50% longer.
• 16ft grid 2 year on daily basis approx. 7 hrs
• Heads calculation
• Steady state results comparison with
observed
• Detailed water balance specifically for
lakes and river area
DSD-INT
2023
|
Hydrology
User
Days

9. Results
9
• Sensitivity analysis: for flows and heads (90
scenarios in total)
• Transient calibration
• Analysis of system behaviour under different
dredging scenarios
DSD-INT
2023
|
Hydrology
User
Days

10. Experiences
10
• Conversion of input files to unstructured grid
• Use of MODFLOW 6
• Transient simulation and data creation
• Scripting very helpful in the process of analysis and
understanding the model: easily re-run and adapting the model
• In the future: include variable density modelling
DSD-INT
2023
|
Hydrology
User
Days