Rules for controlling hydraulic structures can be entered after an inline structure, lateral structure, or storage area connection has been added to the project. From the Unsteady Flow Data editor, add or select the given structure and then click on the Rules button (figure 14-40). This will bring up the Rule Operations editor as shown in figure 14-41. In the Gate Parameters table near the top of the editor, some initial information can be entered for any gate groups that are in the hydraulic structure.

Figure 14 41. Gate Rule Operations Editor

The Open and Close Rate controls how fast the gates can move. So if, for example, a rule operation required the gate to open one additional foot and the gate opening rate was 0.1 ft/min and the user had selected a one minute time step, it would take ten time steps for the gate to reach the new opening height. The Open and/or Close Rate can be left blank, which means the gate can move to any new setting in a single time step.
The Max and Min Opening will constrain the maximum and minimum gate opening settings. Building on the above example of opening the gate one additional foot, if the gate was at 3.5 feet and the maximum was set to 4 feet (even though the gate was 6 feet tall), over a five minute period, the gate would open to 4 feet and then stop. If the Max is left blank, then the gate maximum opening is limited only by the height of the gate. If the Min is left blank, then the minimum opening is fully closed (i.e. 0.0).

The Initial Opening provides the first setting for the gate. This opening height will be used during the initial backwater computation. The gate will be left at this setting until it is changed by a rule operation. The Initial Opening is required for all gate groups, if any, in the hydraulic structure and may not be left blank.

At the top of the editor, the user has the option of entering a description of the rule set. This can be a useful tool for documentation especially if the user has multiple plans with different rule operations.

Rule Sets

A group of rules for one hydraulic structure is referred to as a rule set. At the start of each time step, each rule set is evaluated to check for changes to the operation of the given hydraulic structure. Rule operations are performed from the first (top) rule to the last (bottom) rule. By default, each rule operation is evaluated once. However, branching operations (If/Then/Else, etc) can cause some rule operations to be skipped. No looping or jumping to prior rule operations is allowed. That is (during a given time step), a rule operation may not be performed more than once.

Note: A rule set is only called once during a time step, even if the program iterates during that given time step. (Whatever rules are "in force" at the start of the time step will apply during all of the iterations). At some point, a user option may be added to RAS to let the rule set be called for every iteration. This would allow the rule set to use a more "current" water surface and/or respond to stability problems.

When the Rule Operations editor is opened, the rules for that hydraulic structure are displayed beneath the Rule Operations heading. The rule set shown in figure 14-41 has 7 types of operations.

In this example, operation #1 gets the current gate opening. Operation #2 gets the current flow going through the gate. Operation #3 checks if the flow is less than 500 cfs. If it is, then operation #4 sets the gate opening to the current opening + 0.1 feet. After operation #4, control would jump to after the End If (operation #7). However, since there are no more operations after the End If, the rule set would be done for this time step.

If the flow is greater than or equal to 500 cfs, then operation #3 is false. In this case, control would jump to operation #5. Operation #5 checks if the flow is greater than 750 cfs. If it is, then operation #6 will close the gate 0.1 feet. In either case, the rule set would again be finished for this time step.