The form for setting the hydraulic control structure boundary condition is shown in Hydraulic Structures Boundary Condition BC – Derived. The hydraulic structure boundary condition uses a head lookup table to describe the relationship between head and flow for that cell and is set with the Flow Control Type menu. Several options are available for the flow control type including:
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Two examples of how different uses of flow control types are described below:
Flow derived from Elevation Difference with Low Chord
One use of hydraulic structures is to simulate a low chord, i.e. the bottom of a bridge. In this case, when flows are below the bridge deck they may be bi-directional, i.e. flows can be going upstream or downstream. However once the bridge is overtopped flow is only upstream to downstream.
EFDC uses the total flow rate for the flow calculations, therefore it requires the flow to be at the actual time when the cell reaches the low chord elevation. The elevation is then subtracted from the value obtained from the rating curve as this curve defines the relationship for the total flow around the bridge for the whole range of depths. It is necessary to subtract the actual flows from the curve to prevent a large jump in flow.To prevent instability at the transition the minimum number of time steps above the low chord may be provided by the user. An example of how this is setup is shown in Hydraulic Structures Boundary Condition BC – Derived.
In the equations for the head look up tables (Hydraulic Structures Boundary Condition BC – Derived), HQCTLU is set on a cell by cell basis. HCTLUA comes from cell options offset.
The table for CTRL_1 is defined by the user as shown in Hydraulic Structures Boundary Condition BC – Derived.
This boundary condition needs to be turned on for each cell. However, to enter one value for all the cells in a group the user may select all (CTRL A) and then apply to all the cells in that group. The 3D view of the backwater effect from the bridge is shown in Hydraulic Structures Boundary Condition BC – Derived. Note that the bridge must be of a size larger than the grid size for this option to work effectively.
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Figure 2 Hydraulic Structure Editing Form: Low Chord.
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Figure 3 Bridge (Low Chord) Hydraulic Structure in View3D.
Flow Derived from Upstream and Downstream Elevations
In another example, P Outlet uses a control type with flow derived from both upstream and downstream elevations as shown in Hydraulic Structures Boundary Condition BC – Derived. In this case it is necessary to have a matrix to describe the relationship between head and flow as shown in Hydraulic Structures Boundary Condition BC – Derived. Most data tables, such as those by the US Corp of Engineers, use the same downstream and upstream head. Therefore, if the user wants to change the table, they should select Edit Heads, and add a new table. When Accept Heads is selected a new row and column will be created.
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This matrix may be plotted by selecting the View Series | Current option. This displays the graph as shown in Hydraulic Structures Boundary Condition BC – Derived. As there are often too many series to plot, the user is prompted for a skip interval so that some lines are not displayed. The user may also select a specific upstream head to plot.
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