The Downstream Control Function Rule

The Downstream Control Function rule describes a minimum or maximum flow or stage limit (e.g., flow requirement, channel capacity, flood stage) at a control point rather than an explicit limit on the reservoir release. However, like all ResSim rules, when the rule is evaluated for a given timestep, it must produce a desired release limit. The desired release limit that the downstream control rule calculates will be determined based on the downstream objective, the influence of routing to the control point, previous and future releases, and the cumulative local flows at the control point.

A Downstream Control rule can be used to implement or impose an implicit system operation. System operation occurs when two or more parallel reservoirs are operated together to meet a common objective, in this case—the downstream constraint. Once a Downstream Control rule is created at one reservoir, it will be included in the list of Existing Rules at each reservoir in the network that is upstream of the control point. All it takes to establish an implicit system operation is to use the same downstream control rule in each applicable reservoir's operation set. "Reservoir Systems" discusses ResSim's methodology for defining and managing system operation.

Defining a Downstream Control Function rule is very similar to defining a Release Function rule. In fact, the Downstream Control Function rule editor is an extension of the Release Function rule editor described above, so this section will focus on describing how to define the three attributes that are unique to the Downstream Control Function rule — Parameter, Flow Contingency, and Advanced Options.

Like all ResSim rules, a Downstream Control Function rule must be assigned to a release element when the rule is created. However, a Downstream Control Function rule can only be assigned to the reservoir itself because the rule evaluation logic must account for all releases from the reservoir, including releases through a diverted outlet, that could influence the flow or stage at the downstream control point.

To create a Downstream Control Function rule:

• Right-click on the zone in the Zone-Rules Tree to which you want to add the new rule.
• Select Add New Rule… from the zone's context menu. The New Operating Rule dialog will open.
• Select the reservoir from the Operates Release From: selector. It will be the first entry in the list.

• Select Downstream Control Function from the Rule Type selector. This selection will cause a new field — Downstream Location — to appear in the New Operating Rule dialog ("Figure: Reservoir Editor - New Operating Rule - Downstream Control Function").
  
  

  Figure: Reservoir Editor - New Operating Rule - Downstream Control Function

  
  

• Select a junction from the Downstream Location selector that represents the control point of interest. The list of available locations will include all the reaches and junctions in the current network that are downstream of the current reservoir. Reaches are included for completeness, but there are very few situations in which a reach is the appropriate selection for a downstream control point.

  

  Note — When you see a grey box drawn around a junction in the Map Display area of ResSim, this is a visual cue that the junction is a control point for a downstream control rule.

• Give the rule a name and click OK to complete the creation of the new rule and close the New Operating Rule dialog.

As illustrated in ("Figure: Reservoir Editor - Downstream Control Function Rule"), your new rule will appear at the bottom of the rule stack of the current zone in the Zone-Rules tree and the Downstream Control Function rule editor (in the edit panel) will show the name of your rule and the Downstream Location you selected when you created the rule.

Refer to the Release Function Rule section above for descriptions of the features and parameters that are common to both the Release Function rule and the Downstream Control Function rule. The features and parameters that are unique to the Downstream Control Function rule editor are described in the following sections.