FEFLOW simulation of dynamic groundwater flow and solute transport

Transcription

1 FEFLOW simulation of dynamic groundwater flow and solute transport Post Doctoral Associate Aarhus University Research Foundation TATION presen

2 Outline Scientific motivation The FEFLOW model The FEFLOW Application Programming Interface (API) Threecodeexamples Pros and cons of FEFLOW Researcher Practitioners Summary 2

3 Scientific motivation Asses the impact of episodic ocean events on the exchange of fluid and salt between a coastal barrier aquifer and the ocean relative to the continuing effect of tides. 3

4 The conceptual model 2D, variably saturated groundwater flow and solute transport model Dynamic boundary conditions Inundation Seepage Ocean Recharge Seepage Recharge h t [ S 0 s( Ψ) + ε M ( Ψ) ] + q = Qh q = K r ρ ρ 0 ( s) Kf μ h + ρ0 [( s( Ψ) Dd I + D) C] + QhC = s( Ψ Qc q C ε ) 4 Poulsen et al (in press)

5 The FEFLOW model FEFLOW was selected among other codes for its ability to accommodate: Geometric complexity Variation in the parameter fields (hydraulic conductivity and anisotropy hereof) Dynamic boundary conditions by means of the FEFLOW Application Programming Interface 5

6 The FEFLOW API API (Application Programming Interface) is a particular set of rules and specifications that software can follow to communicate with each other. A scripting or programming language is required for wielding the API (e.g. LUA and C++). In layman's terms: The GUI is the users interface with the software. The API is the programmers interface with the software. APIs are implemented in many types of software from advanced computer games to operating systems (e.g. the Windows API). The FEFLOW API offers almost complete control over the modeling process via dynamically linked libraries and API functions/procedures. The FEFLOW API is accessed by C++ modules. 6

7 Code examples The following examples serve to illustrate the increased flexibility provided by the FEFLOW API 1. Read sea level time series into FEFLOW and store them for later use 2. Before each time step set boundary conditions according to the current sealevel (inundation, seepage and recharge) 3. Calculate seepage and submarine groundwater discharge and append them to a custom budget file 7

8 Example 1: Input 8

9 Example 2: Dynamic boundary conditions 9

10 Example 3: Customizing budgets 10

11 Pros of FEFLOW as a researcher 1. FEFLOW is well-documented. 2. The FEFLOW API is its ultimate source of strength. Its design and implementation is very solid and it is easy to use. 3. Finite elements are useful for complex geometries and non-continues variations in parameter fields. 4. The parallelized algebraic multigrid solver. 5. Several formulations of the Richards equation with conservative properties and varying robustness to the nonlinearity of the retention curve. 6. Mesh smoothing and quantification of obtuse triangles. The position of vertices when using triangular elements can be set to predefined points. 11

12 Cons of FEFLOW as a researcher 1. FEFLOW does not allow more than one instance at a time. 2. Consequently, inverse modeling/model calibration is limited considering recent advances in cloud and parallel computing (e.g. parallel- and BEO PEST). This problem can not be addressed via the API. 3. Closed source code. 4. FEFLOW costs money. 5. Fixed grid formulation of the solute transport equation. This restricts the Péclet number when compared to particle tracking based Eulerian-Lagrangian methods available to e.g. MT3DMS (USGS). 12

13 Pros of FEFLOW as a practitioner 1. The FEFLOW GUI is well designed and mostly free of bugs. It is my experience that untrained practitioners can more easily set up complex models in FEFLOW compared to e.g. Groundwater Vistas (MODFLOW GUI). 2. The workflow in FEFLOW is streamlined and intuitive compared to MODFLOW GUIs. The latter seem to have almost endless menu systems to accommodate the myriad of modules available to MODFLOW. 3. Additional functionality can be accommodated by the API. This, however, requires basic coding skills. 4. A FEFLOW web forum is available. The forum is moderated by FEFLOW developers who will address questions about FEFLOW posed by forum users. 13

14 Cons of FEFLOW as a practitioner 1. MODFLOW and SUTRA are free of charge (however it is tedious to set up models without non-free GUIs like Groundwater Vistas). 2. The FEFLOW GUI supports simple model calibration via an API PEST module. MODFLOW or SUTRA in conjunction with PEST are much more powerful alternatives at the moment. 3. FEFLOW can not be started from a Windows remote desktop session even though the USB dongle is inserted in the remote computer. This is just plain annoying. 4. Some obvious functionality is still missing like for example head difference calculations in 3 dimensions (can be accommodated by the FEFLOW API). 14

15 Summary FEFLOW is a powerful and sophisticated forward model for both researchers and practitioners. The FEFLOW GUI is well designed and effective in use especially for untrained practitioners. The API is FEFLOWs ultimate source of strength. However, it requires basic coding skills to wield it. Only one instance/executable of FEFLOW can be initiated with the current licensing policy for single seat users. This severely limits the use of FEFLOW in inverse modeling considering recent advances in parallel and cloud computing. 15

16 Multiple instances example: MATLAB Estimate geothermal parameters by Markov Chain Monte Carlo inversion of borehole temperature records 417 inversions of model runs = forward model runs! Parallelized by running a MATLAB instance per core (Xeon 6-core server, 12 hyper threaded cores). 16