The Advanced-1 Training Package, which was recently added to
the AnAqSim Instructional Series, introduces an important new area of AnAqSim’s
capabilities: development of multiple layer groundwater flow models.
Unlike simpler AEM models, which generally only allow for
one model layer, AnAqSim can incorporate up to 10 model layers. Each layer can
have its own aquifer properties, its own hydraulic features (wells, line
sources and sinks, recharge or leakage areas, etc.), and its own boundary
conditions. This allows users to create
vertically-zoned aquifers, or vertical aquifer-aquitard sequences that can more
accurately represent geology within the aquifer domain and can simulate
three-dimensional groundwater flow conditions, such as vertical discharge to
surface water bodies or flow to partially penetrating features.
Similar in some ways to the new MODFLOW-USG software,
AnAqSim allows for flexible layering in a model – allowing more layers to be
added in an area of interest, and fewer layers in the far field area, as shown
below.
With this additional power and flexibility come some
additional requirements for model construction and testing to ensure an
accurate solution. The Advanced-1 Tutorial Series was designed to provide users
with detailed instruction on how to properly construct complex multi-layer
models, as well as tools and methods that should be utilized to analyze model
performance and evaluate aquifer response.
Topics covered include:
· When a multi-layer model may be necessary or
advantageous, and how to specify multiple layers in AnAqSim;
·
How and when extraction element basis points are
used to represent vertical leakage between layers;
·
Various methods for checking the accuracy of a
multi-layer model solution;
·
Tools and techniques for evaluation of 3-D
vertical flow conditions;
·
Proper assignment of model elements and
sub-domains to accurately represent hydraulic features;
·
How to identify and avoid errors when joining
model domains and model elements;
·
Adjustment of model settings to help improve
model convergence;
·
How to incorporate and accurately simulate flow
conditions within long, narrow domains (e.g. faults); and
·
Advanced pathline tracing techniques.
The concepts for properly constructing and testing a model are demonstrated in the tutorial series by constructing and examining simple, multi-layer models. The series begins with an overview lesson that explains the basic concepts and methods associated with the construction of multi-layer models in AnAqSim. The first tutorial exercise constructs a basic single-layer box model, and then adds multiple aquifer layers and a pumping well to demonstrate how the flow field changes as the model is transitioned from a single layer model with a fully penetrating well, to a multi-layer model with a fully penetrating lens, to a multi-layer model with pumping from a partially penetrating well. Pathline tracing is applied to examine the three-dimensional capture zone in the vicinity of the well screens. The importance of proper basis point spacing, as well as tools that can be utilized to analyze and test model performance, are also demonstrated.
The next set of exercises construct models to examine three-dimensional groundwater flow in a multi-layer area near a shallow river. These types of groundwater-surface water interaction models are often applied in ecosystem hydrology studies or conjunctive use water management studies. Multiple methods for representation of a river in an AnAqSim multi-layer model are explored (including the use of a line element within an aquifer domain, and a separate narrow domain with a head-dependent flux upper boundary condition), including how the depiction of vertical flow and discharge in the aquifer zone beneath the river is affected when employing each of these methods.
A set of exercises is also included that demonstrates the proper methods for joining an AnAqSim stream element to a lake or pond boundary. 2-D and 3-D models are constructed to explore the proper placement of stream and lake boundary control points. For the multi-layer model, the use of digitized polygon features to efficiently create more refined basis point spacing near surface water boundaries for greater model accuracy is also explored.
Finally, a model of a bedrock aquifer is created that incorporates a long, linear geologic fault feature with higher hydraulic conductivity. This transmissive fault model demonstrates the outstanding performance of a grid-free polyline-element modeling tool like AnAqSim in simulating small-scale high-resolution flow features that are oriented in ways that would be very difficult to design in a grid-based modeling system. The exercise demonstrates the proper techniques for designing these types of features, and employs pathline tracing to examine the effect of the fault on the groundwater flow patterns near the fault and in the vicinity of a nearby pumping well.
These tutorials should provide a great start in getting you
familiar with the concepts behind multi-layer AnAqSim model construction and
operation, for a variety of common three-dimensional model applications. They
cover the basic concepts, provide practical hands-on model building experience,
and teach valuable techniques for refining model design in key areas to improve
model solution accuracy.
