Tutorial 2
Building a multi-layer heterogeneous, anisotropic model with various boundary conditions
Introduction
This second tutorial builds on the model created in the first tutorial. The first tutorial model was two-dimensional and steady, but this second tutorial will guide you through creating a model that has a multi-layer three-dimensional (3D) region in an area near the middle reach of the river. The well will be changed from fully-penetrating to partially-penetrating. This model may be built with the Demo version of Anaqsim, and will contain two levels in the multi-level region. More levels are possible with the Deep Designer licensed version of Anaqsim and there is some instruction about that at the end of the tutorial.
Opening the First Tutorial Model
Either begin with the model you had after competing the first tutorial, or download and then open the following file tutor1.anaq from Anaqsim. To open the file in Anaqsim, select File / Open from the main menu, then locate and open the tutor1.anaq file. Next select File / Save As and save this under another name such as 'tutor2.anaq'.
Zoom in on the area where you will make the model multi-level (2D) so that the resulting view looks something like the following image. In this case, the basemap (available in the C:\Program Files\Yellow Sub Hydro\Anaqsim\Documentation folder) is displayed in gray. The model elements are displayed in green.
Adding New Domains for the Multi-Layer Area
In the area near the well and the middle river reach (h=144 to h=108), we will create a region with two levels instead of one. The first step is to create new domains for this region. Select Model Input / Domains / Confined and/or Unconfined to bring up the domains data table, which contains two domains 'all' for most of the model, and 'recharge basin' for the recharge basin area. These two domains have the same properties, but differing recharge rates.
We will add 2 new domains for the multi-level region. The quickest way to create the new rows is to duplicate the two existing rows. Select both existing rows as is shown below.
Then right-click and choose Copy Selected Rows and then right-click and select Paste New Rows. Edit the first 5 columns of the new rows so that they look like the following table. Columns 6+ will all be unchanged, since these domains all have the same properties, just different elevations.
Note that the bottom elevation of the new two-level area is 80, a bit lower than the bottom elevation of the 'all' domain at 82. This is just to show that elevations may change laterally in a model if you want them to. The '3D upper' domain is in level 1 and is unconfined with a base at elevation 95. The '3D lower' domain is in the 2nd level (level numbers increase with depth) and is confined with a constant saturated thickness of 15, going from elevation 80 to 95.
At this point, save your model with File / Save from the main menu or Ctrl-S.
Interdomain Line Boundary to Define the 3D Region
We have defined the two new domains that will apply in the 3D region, but still need to define the area where those domains exist. Anaqsim does this with external line boundaries that refer to the domains. In this case, we will create one new interdomain line boundary that will represent the lateral transition from the single-level 'all' domain to the multi-level 3D area. At interdomain boundaries, the conditions imposed are continuity of flow and continuity of head across the boundary.
First digitize the interdomain boundary. Select the Plot tab, and make sure that under the plot menu Snap Settings / Snap to Elements is checked. This allows the digitized interdomain line boundary to 'snap to' the same vertex coordinates as the river line boundaries. When the cursor moves over an existing vertex, an orange box will appear; if you click with the orange box showing, it will snap to the same coordinates. Digitize a closed polygon boundary like the yellow one shown below, making sure to snap to the river vertexes labeled 'h=114' and 'h=108'. You can go around the boundary in either clockwise or counter-clockwise order, but it is important to remember which way you went.
Next, select Model Input / Line Boundaries / Inter-Domain and add a new row so it looks like this:
The 'ID' boundary defined the limits of the recharge basin, and the new '3D limits' one will define the boundary of the 3D area. Click on the Edit button in the '3D limits' row and paste in the coordinates you just digitized, and then press OK.
Then, click on the Select button under Domains_left. If you digitized in clockwise order, select the 'all' domain and if you digitized in counter-clockwise order, select both '3D Upper' and '3D Lower'. Imagine walking on the ground surface along the line you just digitized; Domains_left would be to your left as you walk along the boundary and Domains_right would be to your right. This interdomain boundary makes a transition from two levels (3D upper, 3D lower) on one side to a single level (all) on the other. By doing this, interdomain boundaries can help you efficiently concentrate computing power and multiple levels in a limited area. Interdomain boundaries can also join a single domain on one side with a single domain on the other side.
Now click on the Select button under Domains_right. If you digitized in clockwise order, select both '3D Upper' and '3D Lower' and if you digitized in counter-clockwise order, select the 'all' domain.
To check your input, select Make Plot / Model Elements Only, then select level 1 and press OK, and you should see something like this in the area near the well.
The new interdomain line boundary is now green, since it is composed of elements in the model. Move the cursor over the vertexes of the interdomain line boundary to check that the left/right sense of the domains is correct. Move the mouse across the boundary, and observe the domain listed at the left. When you are outside the 3D area, you should see this:
When you are inside the 3D limits you should see this:
Now make a similar plot, but chose level 2 instead of level 1 and you will see something like this:
The river elements do not show up, since they are in level 1. Move the mouse across the boundary, and observed the domain listed at the left. When you are outside the 3D area, there is no level 2 and you should see a blank under Domain Name:
When you are inside the 3D limits you should see this:
Putting the Well and River Reach into the New Domains
The two domains that occupy the 3D area are '3D upper' and '3D lower', but our input still lists the middle river reach and the well as being in domain 'all', which is no longer correct. We need to change the input for these elements and put them in the correct domains.
First, bring up the well data by moving the cursor near the well, right-clicking, and then selectin Edit Nearest Well. Then change the domain of the well from 'all' to '3D lower':
Now the well is screened only in the '3D lower' domain from elevation 80 to 95, not in the '3D upper' domain above. In this manner, Anaqsim simulates partially-penetrating wells.
Second, edit the middle river reach by moving the cursor near this line boundary, right-clicking, and then selecting Edit Nearest Line Boundary. This will open the river line boundary table and highlight the middle reach row. Change the domain of the well from 'all' to '3D lower':
The river now will be in the upper layer and there will be vertical resistance and 3D flow with the underlaying 2nd level.
Save your model now, selecting File / Save from the main menu or Ctrl-S.
Spatially-Variable Area Sinks in 3D Region
In the 3D area there will be vertical leakage between the two levels. For each domain, this can be thought of as a distributed source/sink term like recharge, but it will vary with location. For example, near the well that is in '3D lower' there will be vertical leakage from '3D upper' to '3D lower' with leakage increasing closer to the well. To model this spatially-variable leakage, we need to use spatially-variable area sinks (SVAS). SVAS are required to simulate vertical leakage in multi-level areas of models, and also to simulate storage fluxes in transient models. In this steady model, SVAS are needed just in the 3D area where there is vertical leakage. Outside the 3D area, the model is single layer, steady, and the uniform recharge can still be modeled efficiently with a uniform area sink so there is no need to modify that input.
SVAS by Domain
SVAS can be input over the area spanned by a domain or the area spanned by polygon. For this model, we will use both. First, we will enter an SVAS that apples over the 3D area. Select Model Input / Area Source/Sink / Spatially-Variable, Domain, and complete a row like this:
This SVAS applies o the area spanned by the '3D upper' domain. SVAS are applied for all levels of a model, including those that underlie and/or overly the specified domain. Since '3D upper' and '3D lower' span the same area, we only need to create an SVAS for one of these domains. The result would be the same if we chose either '3D upper' or '3D lower' as the Domain. Condition_Top is set to Flux, meaning that the sink/source rate into the top of the the model is specified and equal to 0.0006, the same recharge rate that applies in the 'all' domain outside the 3D area. Condition_Bottom is set to Flux = 0, so the bottom of the model (bottom of '3D lower') is impermeable. The alternative top or bottom condition is head_dependent_flux, which is used for vertical leakage to a specified head at a surface water, for example.
Node_spacing is the approximate spacing between basis points in the SVAS. The leakage/storage fluxes are solved for perfectly at the basis points, and approximated with a smooth interpolated surface between basis points. Anaqsim will automatically populate the area of domain '3D upper' with basis points on a regular hexagonal grid of points, spaced about 80 units apart. To read more about SVAS, see papers by Fitts (2010) and Strack and Jankovic (1999).
To see how these basis points are distributed, select Make Plot / Model Elements Only, then select level 1 and press OK. You should see green '+' basis point symbols in the 3D area something like this:
A similar plot of level 2 would show basis points at these same locations. Move the cursor over some basis points to see the domain and top / bottom conditions at this point.
SVAS by Polygon
Where the rates of vertical leakage change over short horizontal distances, we need denser basis point spacing. We can use polygon SVAS and special well basis point spacing for this purpose. First, we will create a zone of denser basis points in the well / river vicinity using a polygon SVAS. Start by digitizing a polygon (either clockwise or counter-clockwise) that looks something like the yellow line in the following image. If you don't close the polygon, Anaqsim will close it for you from the first point to the last.
Then select Model Input / Area Source/Sink / Spatially-Variable, Polygon and edit the row so it looks like this:
Then click on the Edit button and paste in the coordinates of the polygon you just digitized. This will spread out basis points with a small spacing (25) inside the polygon. The domain basis points that were within this polygon will be automatically overwritten. The Nesting_level is used when you have multiple polygon SVAS that are nested within one another. Polygon SVAS with higher nesting levels overwrite polygon SVAS with lower nesting levels. We will input just one polygon SVAS, so nesting won't be an issue.
Save your input, and then select Plot Input / What to Plot and then check the SVAS_polygons box so that the SVAS polygons will be displayed in subsequent plots:
Select Make Plot / Model Elements Only and choose level 1 to see the distribution of basis points and the SVAS polygon (light blue):
