Simple Drain and Makeup Water Boundary Conditions

Note: The sample problems presented in this post can be downloaded here.

Figure 1: Base Configuration of Example Model

This post describes how to use two boundary conditions that maintain a target head in the event that head would exceed (simple drain) or undershoot (makeup water) a specified value. Conceptually we can think of these boundary conditions as one-way boundary conditions where water can only enter (makeup water) or exit (simple drain) the system and are active only when the target value is reached. We find these two boundary conditions very useful when trying to represent water management scenarios.

Figure 2: Location of Simple Drain or Makeup Water BCs

To illustrate how to use these boundary conditions we have included 3 simple models which all share the same base configuration (see figure 1).

The model is a simple box model with a depression (pond in the middle). The lateral boundary conditions at x = 0 m and x = 50 m are specified head boundary conditions that fluctuate between 7 and 10 m on a 30 day cycle as follows (see code below):

time value table

    0   9

    30 7

    60 10

    90 7

    120 10

    150 7

    180 10

    210 7

    240 10

end

This range of fluctuation is enough to move water in and out of the depression as shown in the animation to the right. This example is referred to as ‘Pond_noBC’ in the sample problem set.

Two additional models are included which show the impact of having either a makeup water or simple drain boundary condition specified in the pond with a target head of 9 m (1 m above the bottom of the pond), see Figure 2.

Simple Drain

For the simple drain case, water will be removed from the pond if it exceeds a pressure head of (1m), or a total head of 9 m. The boundary condition setup as follows:

!--------------------- Simple Drain Boundary Condition

use domain type

surface

clear chosen nodes

choose node

25 0 8

create node set

nsimplewdrain

boundary condition 

       type

       simple drain

       node set

       nsimpledrain

       time value table

       0, 1, 1e8              !Pressure head = 1 m, equivalent to total head of 9 m                                    at this location. 1e8 is the conductance term, and                                    for this case is set to some arbitrarily large value

       end

end

Makeup Water

Figure 3: Comparison of results between base case (no BC), simple drain and makeup water 

For the makeup water case, water will be removed from the system of pressure head drops below (1m) or a total head of 9 m. The boundary condition setup is shown below:

create node set

nmakeup

boundary condition 

       type

       makeup water

       node set

       nmakeup

       time value table

       0 1 1e8              !Pressure head = 1 m, equivalent to total head of 9 m at                                    this location. 1e8 is the conductance term, and for                                    this case is set to some arbitrarily large value

       end

end

Comparison

Figure 3 shows the observed head at an observation point in the lake a few meters away from the boundary condition location. You can see in Figure 2 that for the makeup water case (red dashes) when total head drops below 9 m water is added to maintain the target levels, and for the simple drain case (green dashes) when water exceeds 9 m water is removed from the system.

We typically use these boundary conditions when mimicking the influence of artificial structures in your model.