One dimensional subsidence models are often used due to lack of available input data. While these are useful screening level models, integration of subsidence modeling into a three dimensional flow model should be considered, particularly if a flow model already exists. MODFLOW SUB-WT and SUB packages rely on the Terzaghi theory of one-dimensional consolidation to calculate subsidence. Although these calculations are based in the vertical dimension, use of these packages impacts the water balance and subsequent results of groundwater flow models. This presentation will provide details on the differences in model results between one and three dimensional subsidence modeling using the MODFLOW SUB-WT package.
FAIRSpectra - Enabling the FAIRification of Spectroscopy and Spectrometry
Comparison of One and Three Dimensional MODFLOW Subsidence Results
1. Comparison of One and Three
Dimensional MODFLOW
Subsidence Results
Dave Colvin - Leonard Rice Engineers
NGWA Groundwater Summit
Garden Grove, CA
May 7, 2012
1
3. Subsidence Overview
• Lowering of ground
surface
• Based on Terzaghi
equation
• Before pumping,
geostatic load (pressure
or stress) is balanced
• Pumping reduces pore
pressure; increases
stress on ‘skeleton’
• “Stress” = Change in
water level
Reference:
Poland (1984). Guidebook to Studies of Land
Subsidence due to Ground Water Withdrawal.
UNESCO. International Hydrological Programme
Working Group
4. Data Characteristics
• Long periods of record
• Accurate monitoring
results
• Varied collection
methods
• Varied data types
• Expensive
4
• Drilling and Well
Testing
• Remote Sensing
• Survey Based
• Geotechnical
Laboratory Analyses
• Geophysics
– Downhole
– Airborne/Surface
Data Collection Methods
Subsidence Modeling
Data Collection
10. Groundwater
Clay Layer 1
Clay Layer 2
Upper Aquifer
Middle Aquifer
Lower Aquifer
Ground Surface
Constant Head Cells
-defined by water level
elevations
or
-groundwater flow
model heads
1D Subsidence Concept
11. USGS MODFLOW SUB-WT
• Simulates vertical
compaction in
groundwater flow
models
• Simulates groundwater
storage changes in
aquitards (or interbeds)
• Calculates changes in
geostatic stress due to
lowering of the water
table
• Allows for time variable
hydraulic and
subsidence properties
Reference:
Galloway and Leake(2007) MODFLOW Ground Water Model - User Guide to the
Subsidence and Aquifer-System Compaction Package (SUB-WT) for Water Table
Aquifers. U.S. Geological Survey Techniques and Methods 6-A23.
12. Modeled Head Impacts
using SUB-WT
0.0
50.0
100.0
150.0
200.0
250.0
300.0
350.0
400.0
450.0
500.0
550.0
600.0
Drawdown(ft)
Year
3D SUB-WT Model
Drawdown Result Comparison
Base Model Well #1
SWT Model - Well #1
Base Model Well #2
SUB-WT Model Well #2 Maximum
difference of 27.8
feet
13. MODFLOW Subsidence Water Budget Impacts
Reference:
Faunt, C.C., ed., 2009, Groundwater Availability of the Central Valley Aquifer,
California: U.S. Geological Survey Professional Paper 1766
18. Conclusions
• Use of the MODFLOW
subsidence packages can
have significant impacts on
model predicted:
– Head
– Storage
• 1D subsidence modeling
may be more appropriate in
locations where:
– Available data are obtained
from one borehole or
location
– Geologic heterogeneity of
clay thickness or bedrock
structure is poorly
understood
– Regional groundwater flow
models are not available
• 1D models assume that
horizontal flow is
insignificant compared to
other storage and water
budget components
• 1D subsidence models over
predict subsidence by
approximately 20%
• 3D groundwater flow
modeling may be used to
obtain subsidence results in
one cell
• Availability and quality of
input data must be balanced
with the confidence needs in
the subsidence predictions
18
One dimensional subsidence models are often used due to lack of available input data. While these are useful screening level models, integration of subsidence modeling into a three dimensional flow model should be considered, particularly if a flow model already exists. MODFLOW SUB-WT and SUB packages rely on the Terzaghi theory of one-dimensional consolidation to calculate subsidence. Although these calculations are based in the vertical dimension, use of these packages impacts the water balance and subsequent results of groundwater flow models. This presentation will provide details on the differences in model results between one and three dimensional subsidence modeling using the MODFLOW SUB-WT package.
Investigation into evidence of laterally continuous clay layers possibly from lacustrine lake deposits.
Investigation into evidence of laterally continuous clay layers possibly from lacustrine lake deposits.
Investigation into evidence of laterally continuous clay layers possibly from lacustrine lake deposits.
Investigation into evidence of laterally continuous clay layers possibly from lacustrine lake deposits.
Investigation into evidence of laterally continuous clay layers possibly from lacustrine lake deposits.
