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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
- 5. Conceptual Model Development 3D Lithology Database
- 6. Conceptual Model Development 3D Lithology Database
- 7. Conceptual Model Development 3D Lithology Database
- 8. Conceptual Model Development 3D Lithology Database
- 9. Conceptual Model Development 3D Lithology Database
- 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
- 14. Clay Layer 1 Compaction 3D Subsidence Models
- 15. 1D vs. 3D SUB-WT Subsidence Results 840 860 880 900 920 940 960 980 1000 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1940 1960 1980 2000 2020 2040 2060 2080 2100 2120 GroundWaterLevel(ftNAVD88) Subsidence(ft) Year 1D vs. 3D Subsidence Model Comparison 3D Modeled Subsidence 1D Modeled Subsidence Head
- 16. 1D vs. 3D SUB-WT Subsidence Results Modeled Subsidence Results (Feet) Scenario 1 Scenario 2 Scenario 3 Scenario 4 Scenario 5 Scenario 6 Scenario 7 Scenario 8 Year 3D 1D 3D 1D 3D 1D 3D 1D 3D 1D 3D 1D 3D 1D 3D 1D 2020 0.4 0.5 0.4 0.5 0.4 0.5 0.3 0.5 0.3 0.4 0.3 0.4 0.3 0.4 0.3 0.4 2080 0.7 0.9 1.2 1.5 1.5 1.9 1.1 1.4 0.8 1.1 1.1 1.4 1.7 2.0 1.3 1.5 2117 0.8 1.0 1.2 1.5 1.5 1.9 1.1 1.4 1.0 1.2 1.2 1.6 1.8 2.2 1.3 1.6 3D - Percent of 1D Result 78% 77% 78% 78% 78% 78% 84% 82%
- 17. Water movement horizontally and vertically within aquifers and aquitards Groundwater Clay Layer 1 Clay Layer 2 Upper Aquifer Middle Aquifer Lower Aquifer Ground Surface Movement of water restricted to the vertical plane 1 D 3 D 3D Subsidence Models
- 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
- 19. QUESTIONS? 19
- 20. THANK YOU! 20
Editor's Notes
- 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.