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Transient Modelling of Groundwater Flow, Application to Tunnel Dewatering

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Transient Modelling of Groundwater Flow, Application to Tunnel Dewatering

  1. 1. 1 Transient Modelling of Groundwater Flow Application to Tunnel Dewatering E.J. Wexler, P.Eng. Earthfx Incorporated February 13, 2013
  2. 2. 2 Outline  Introduce the SEC Case Study  Review general concepts of numerical modelling  Show how basic principles and models can be extended to more complex settings  Discuss results of SEC model and insights gained
  3. 3. 3 SE Collector Sewer Environmental Assessment  Done for Conestoga-Rovers & Associates, Earth Tech (Canada) Inc. and Regional Municipality of York  Study to look at impact of construction dewatering on groundwater and baseflow in nearby streams  Previous construction (16th Ave) needed large-scale dewatering after boring into Thorncliffe aquifer  SEC alignment designed to pass mostly through Newmarket Till  Design used sealed shafts and EPBM when in aquifers (now EPBM for entire run)  Study used MODFLOW to investigate baseline impacts and contingencies (e.g., TBM rescue and delays).
  4. 4. 4 Southeast Collector Trunk Sewer Links to York-Durham Sewer System to Duffins Creek WPCP Hydrogeologic investigation by CRA
  5. 5. 5 Alignment is 15 km long Four tunnel boring machines 3.6 m sealed concrete liner Shafts for access and turning TBM Shaft 13 for connection to existing sewer Construction currently underway
  6. 6. 6 Dewatering applications for groundwater models:  How long will we have to pump to reach target levels?  How much will we have to pump to maintain levels?  What are the impacts on:  nearby wells  nearby streams (baseflow) and wetlands  Can we optimize pumping rates and minimize impacts?
  7. 7. 7 What is a groundwater model?  A model is a simplified representation of a real physical system  We want to analyze response in simple system and extrapolate  Need to simplify because we often have limited knowledge of subsurface geology  Limited data on hydraulic properties  Inputs (e.g. recharge) are highly variable in time and space.
  8. 8. 8 Mathematical groundwater model:  Based on two simple principals  Darcy’s Law: Flow is proportional to change in head (gradient)  q = - K dh/dx (K is hydraulic conductivity)  Conservation of mass:  Flow out – Flow in = Decrease in storage  Flow in/out can be related to heads through Darcy’s Law  Change in storage can also be related to change in head through the storage coefficient  Two basic types of models: analytical and numerical
  9. 9. 9 Analytical solutions for predicting drawdown:  Integrate the GW flow equation directly. Get “closed form” solution  Steady-State:  Single and multiple wells (e.g., Theim equation)  Line sources (e.g., DF drain discharge)  Transient  Single well – constant discharge (Theis)  Single well – constant drawdown (Lohman)  Late time (straight-line) solutions (Jacob)  Multiple wells (super-position) and boundaries  Leaky aquifers and partial penetration (Hantush)  Multiple aquifers (Neuman-Witherspoon)  Recharge and regional flow  Change in streamflow  Hunt (1999) and others
  10. 10. 10 Analytical solutions example:  What pumping rate do I need to get a 2 m drawdown at 30 days at the edge of a 100 m wide site. T is 650 m2/d, S is 0.0015  If there was a stream 500 m from the well with a bed thickness of 1 m and a K’ of 0.086 m/d, how much flow would be coming from the stream?
  11. 11. 11 Limitations of analytical solutions:  Need to assume infinite extent or that h=Ho at some radius of influence  Simple geometry  Uniform properties  Theis eqn assumes single, infinite aquifer with no recharge and fully penetrating well  Simple stream geometry and properties.  Note: More complex solutions can address specific limitations.  Image wells and superposition can help deal with boundary issues
  12. 12. 12 Numerical Models:  Finite Difference Methods:  Break area into a rectangular grid  Approximate derivatives in GW flow equation with expressions relating to heads in neighbouring cells  Flows must satisfy mass balance criteria  Solve for heads at centre of cell  Finite Element Method  Break area into triangular or rectangular mesh  Approximate head in element as simple function of heads at nodes and take derivatives  Combine with weighted residual method to minimize error  Solve for heads at each node
  13. 13. 13 Groundwater Modelling Programs:  Many codes available  MODFLOW-2005 is a finite difference code  developed by U.S. Geological Survey  open source and free (www.usgs.gov/software)  Many user-interfaces (e.g. Visual MODFLOW or GW- Vistas) available for purchase  FEFLOW 6.0 is a Finite-Element Code  developed by DHI-WASY  closed source  built in GUI  Which method is better? FD Guy FE Guy
  14. 14. 14 Numerical models have many important features:  Multiple aquifers and aquitards  Irregular geometry and discontinities  Irregular boundaries  Spatial variability in hydraulic properties  Variation in recharge rates  Multiple pumping sources  Confined/unconfined transition  Interaction with streams Warning: All models are simplifications. Not all features can be represented and are often unknown. Simplifying assumptions, and extrapolations should be identified.
  15. 15. 15 Model information requirements (Conceptual Model):  Model geometry  Model extent should be determined by natural hydrologic boundaries  Layer thickness (B) and continuity  Aquifer and aquitard properties (K, T, S, Sy)  Boundary conditions (heads and inflows at physical limits of model)  Initial Conditions (heads at t=0) Simple conceptual model for a well in a confined aquifer Assumes infinite areal extent
  16. 16. 16 SEC Model extents: Included all of Duffins Creek and Rouge River watersheds All overburden layers and weathered bedrock Large model but better able to analyze affects on streams
  17. 17. 17 Question: Local versus sub-regional models:  Dewatering analysis may only need a local-scale model  Impact assessment needs to consider effects beyond site boundary  Detailed information may exist only on site  Process and extrapolate from other information:  Surficial geology and bedrock maps  Aquifer maps  MOE WWIS and UGAIS geotechnical data  Larger scale model should not sacrifice detail at local scale
  18. 18. 18 Model grid design:  Model grid should be refined (i.e., small cells or elements) around area of interest  Often use expanding grid to reach model boundaries  Uniform grids are better for regional models because all features (e.g., streams) are of interest
  19. 19. 19 Portion of SEC Model grid: Uniform 100-m cell size outside of SEC study area. Down to 2.5 x 2.5 m near Shaft 13
  20. 20. 20 SEC Model layer geometry defined by analyzing borehole data Many monitoring wells and geotechnical boreholes installed for SEC Other data obtained from YPDT database
  21. 21. 21 SEC Model Geology: Good geologic control along the alignment Less detail at depth (e.g., to locate bedrock valleys) Information about Newmarket Till extent used in design Tunnel passes through TAC and ORAC at some locations
  22. 22. 22 Geology section outside SEC area inferred primarily from MOE WWIS geologic logs Location errors, ft-m conversion errors, and other data quality issues add to difficulty in interpretation process Potentiometric surfaces from MOE WWIS static water levels
  23. 23. 23 Three main types of Boundary Conditions:  Known head at boundary  Constant or time-dependent  Lakes and large rivers  Known flow at boundary  No-flow at stream divides  Impermeable boundaries (aquifer base)  Head-dependent flow  Leakage across confining units  Leakage across stream beds H=H0 No Flow No Flow 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 No Flow Model for a well in a confined aquifer with simple boundaries
  24. 24. 24 SEC Model uses natural hydrologic boundaries No-flow boundaries at regional groundwater divide Lateral boundaries defined by Rouge/Duffins watersheds Constant head (72.5 masl) at Lake Ontario
  25. 25. 25 Boundary Conditions for Dewatering:  Specifying flow at a well or multiple well points:  Useful if you need to know the time to achieve target drawdown  Can provide detailed pumping schedule (e.g., if using multiple wells on different benches)  Specified Head  Once target is achieved, head can be maintained with decreased rate of pumping  Can set head and determine inflows from mass balance  MODFLOW CHD package allows you to turn on constant head boundaries. We modified to turn them off again.  Dewatering ahead of TBW was simulated with moving CH boundary Click for Animation
  26. 26. 26 Aquifer properties:  Aquifer tests conducted by CRA:  Provide local information on T and S  Regional aquifer and aquitard properties  K’s inferred from previous studies and lithologic log data  Aquitard properties inferred previous work (e.g., Gerber and Howard) and regional ORM model calibration  Local data incorporated and K’s refined
  27. 27. 27 Aquifer Inflows:  For simple models, recharge can be estimated and refined through calibration  For SEC, groundwater recharge determined through separate water budget analysis  Used USGS Precipitation-Runoff model (PRMS)  Daily water balance calculated for each model cell  Daily climate data inputs (P, Temp, Solar Radiation)  Soil Properties and land use (e.g., % impervious and vegetative cover density) from available mapping  Simulated 7 years and averaged results
  28. 28. 28 SEC Model recharge: High values on Oak Ridges Moraine and Iroquois Beach Lower recharge on Halton and Newmarket Till and urban areas
  29. 29. 29 Aquifer Outflows:  For dewatering with wells or wellpoints, discharge rates are specified.  For drains, a control elevation is specified.  For SEC Model, all permitted groundwater takings were represented  Streams represented by MODFLOW rivers or drains  Discharge calculated internally based on difference between stage and aquifer head  Stage is assumed constant (other MODFLOW packages adjust stage based on upstream inflows and leakage)
  30. 30. 30 Model calibration:  After we define geometry, boundary conditions, aquifer properties, and inflows, we still need to calibrate to observations.  For SEC, calibration targets were observed potentials and average baseflow in stream  K’s and recharge primary calibration factors  MOE WWIS water levels  Data quality problems, large number makes them useful  Baseflow estimated for HYDAT gauges  Automated base- flow separation methods not exact
  31. 31. 31 Match of simulated (blue) to observed (red) is reasonable White areas are “dry”, heads are below base of ORAC
  32. 32. 32 Match aquifer test results to calibrate storage properties Also tried to match simulated and observed 16th Ave dewatering
  33. 33. 33 Modelling dewatering and streamflow depletion:  Pumping near a stream can induce surface water infiltration.  More likely, pumping can reduce amount of water that would naturally discharge to the steam.  Impacts depend on pumping rate, proximity to stream, and aquifer properties (transmissivity and storage), and streambed properties
  34. 34. 34 Modelling dewatering and streamflow depletion:  Lag between start of pumping and change in flow  Due to high storage (Sy)  May not see in short-term test  Recovery is also lagged
  35. 35. 35 SEC Dewatering analysis – Baseline Scenario:  Four Tunnel Drives:  TBM run in regular mode through Newmarket Till  Run in EPBM mode through aquifers  Shafts:  Sealed shafts in aquifers; open shafts in till.  Shaft 13 needs dewatering for 5 months at end for connection to old sewer  Water takings:  300 L/m at Shaft 11 for construction  All other water from municipal supply  (now permitted for 220 L/min for construction and 300 L/min for seepage and dewatering).  Change in baseflow determined by subtracting simulated baseflow from simulated discharge under baseline conditions
  36. 36. 36 Schedule for Tunnel Drives and Shaft construction (not current schedule) Green indicates no expected impact (sealed shafts and EPBM mode)
  37. 37. 37 Click for Animation Simulated Drawdowns and Change in GW Discharge to Streams
  38. 38. 38 Simulated change in baseflow (GW discharge to stream) Largest changes occur as TBM approaches channel
  39. 39. 39 Click for Animation Contingency Simulations: Stuck TBM on Drive A
  40. 40. 40 Conclusions:  Numerical models can account for complex geology, multiple aquifers and aquitards, and regional flow conditions  Transient flow modelling is needed to represent behavior of groundwater system under short-term and longer-term dewatering  Models can account for decreasing inflows over time  Can account for change in aquifer storage and seasonal changes in recharge  Can be used to assess time-dependent changes to groundwater discharge to streams  Models results and understanding of geology helped in route selection and dewatering design to minimize impacts.

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