Flow Derived Hydraulic Structures
This section will convey how to configure and use the equation-based structures and flow table based hydraulic structures listed in the Flow Control Type drop-down menu in Hydraulic Structure Boundary Conditions form.
Whole Channel Rating Curve
EEMS provides the option of setting a boundary that is defined by a single rating curve to represent the river. This feature is useful if the user wants to create stepped flows for an IFIM habitat analysis.
The option Whole Channel Rating Curve acts much like a flow table, or flow time series, which may be split with a flow multiplier across different cells. Using the scroll bar in the Current Boundary Cell frame allows the user to see how the multiplier changes.
The user also has the option of setting a downstream cell that the flow will return into in the Downstream Cell frame. If this is left blank, then EE ignores the downstream end.
Figure 1. Hydraulic structure: Whole channel rating curve option.
When the user selects the Set button next to the Head Multiplier, several options are provided for the split approach as shown in Figure 2. The first option is to set the water surface elevation to use when the flow splits are applied. By default, this is the maximum value in the rating curve.
The second option is to select one of three approaches for calculating the flow splits. The user may split-flow based on DX, on DY, or on DX*DY.
Upstream Depth and Elevation Difference Options
The hydraulic structure boundary condition option that uses flow derived from upstream depth, or elevation difference, uses a head lookup table to describe the relationship between head and flow for that cell. This is selected with Flow Control Type menu. Several options are available for the flow control type including:
Upstream Depth
Elevation Difference
Elevation Difference with Flow Accelerations
If the Elevation Difference with Flow Acceleration option is selected the flow acceleration parameter flows are now squared and multiplied by an acceleration factor when passing through an inlet.
Flow Derived from Upstream and Downstream Elevations
Another option is to use flow derived from upstream and downstream elevations. In the example shown in Figure 4, Pacheco Outlet uses such a control type. In this case, it is necessary to have a matrix to describe the relationship between head and flow as shown in Figure 5. 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 or make a new one, they should click on Columns tab of the Hydraulic Lookup Table form then edit US Head and DS Head (Figure 6). The Head values will be updated when the user clicks back Data Series tab.
This matrix may be plotted by selecting the View time series plot button in the Hydraulic Lookup Table form. This displays the graph as shown in Figure 7. As there are often too many series to the plot, the user can hide some line by RMC to the Legend and deselect Visible check for some lines. The user may also select a specific upstream head to plot.
Flow derived from Upstream Depth or 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 inflow. 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 set up is shown in Figure 8.
In the equations for the head look-up tables, HQCTLU is set on a cell by cell basis. HCTLUA comes from Head Offset option.
The table for CTRL_1 is defined by the user as shown in Figure 9.
Note that the bridge must be of a size larger than the grid size for this option to work effectively.
It is recommended that users set a common low chord elevation for adjoining cells and pay attention to the bathymetry for those cells. While it is possible to set a different low chord elevation for adjoining cells, if they have different values then flow will pass from cell to cell across the bridge and cause model instability. In the same way, sudden change of bathymetry between the cells may also create oscillations. To prevent this it is suggested to use masks.
Flow Derived from Equation - Culverts
To assist the user define the input parameters for the culvert, a definition editor is provided as shown in Figure 10. This form can be accessed by clicking on Edit button in Figure 8 Here the user may define the Equation Name, allowing various culverts' dimensions and type to be defined.
The user should select the Structure Type from the drop-down menu. In this case, the option chosen is "culvert". The user should then select the Cross-section type. Depending on the options chosen the image of the culvert will change to match the user's selection. The user can then specify the culvert dimensions in meters, including upstream and downstream elevations of the pipe, length of the pipe, Manning's roughness coefficient, and diameter of the pipe as shown in Figure 10. An animation of an example culvert may be seen here.
The methods for determining the flow through culverts are based on the culvert flow type classification and analysis from Chow (1959). This methodology describes six different types of culvert flow based on the location of the control section within the culvert and the relative elevations of the head-water, tail-water, and culvert invert, and crown elevations in meters. The discharge is primarily computed using Manning's equation which can be expressed as:
Q= KS1/2
Where conveyance, K= (A/n)R(2/3)and where A = cross-sectional flow area (m); n = Manning's roughness coefficient, and R = hydraulic radius (m).
Flow Derived From Equation - Sluice Gates
The sluice gate hydraulic structure boundary has been enhanced to simulate the opening and closing of gates. Currently, two types of sluice gate operation are supported: operational time series, and operational rules.
With the operational time series option, the gate is opened or closed during the simulation based on a defined time series of the operational state and settings such as rate of the gate opening, and maximum opening height. Examples of the types of gates may be shown in Figure 12.
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a) Structures with upward opening | |
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b) Structure with downward opening | c) Structures with sideward opening |
Figure 12. Operation for different types of gates.
When using the equation-based hydraulic structures, if the user selects Flow derived from Equation: Sluice Gate, the Time Control drop-down menu will be displayed as shown in Figure 13.