Reservoir Discharge Controlled by Rating Curve

This model demonstrates how to simulate reservoir discharge using a predefined Head-Discharge rating curve, offering an alternative to detailed hydraulic calculations. The outflow rate is determined based on the calculated head difference between the reservoir water level and the downstream tailwater elevation. This approach is particularly useful for representing outlets with complex hydraulics or when a specific operational discharge relationship is required based on available head.

 Figure 1: Screen capture of the GoldSim model

The model dynamically calculates the reservoir water surface elevation based on inflows and outflows and then determines the discharge rate using a lookup table representing the outlet's rating curve. The components include:

• Reservoir: Represents the reservoir itself using a Pool element, tracking the volume of water stored.
• Inflow: A placeholder (e.g., Stochastic element) representing water flowing into the reservoir.
• Elev_Vol_Table: Defines the relationship between the reservoir's water surface elevation and the volume of water stored. This is used to calculate the current water surface elevation based on the current volume in the Pool element. 
• Tailwater_Elev: Defines the downstream tailwater elevation, which may be constant or variable depending on downstream conditions.
• Depth: Calculates the head difference between the reservoir water surface elevation and the tailwater elevation, ensuring it's non-negative. It uses the current water surface elevation (from the Elev_Vol_Table) and the Tailwater_Elev.
• Rating_Table: This takes the calculated 'Depth' as an input and outputs the corresponding 'Discharge Rate' based on the user-defined curve (Head vs. Discharge).
• Outflows: The Pool element has withdrawals representing potential losses (Evaporation placeholder) and the output of the "Rating_Table".

Why Use a Rating Curve Approach?

Employing a Head-Discharge rating curve for reservoir simulation offers several advantages, particularly in certain modeling contexts:

• Conceptual Modeling: Rating curves can effectively represent the combined hydraulic behavior of complex outlet structures (e.g., non-standard weirs, gated spillways under specific operating rules, culverts) where deriving analytical equations is difficult or impractical.
• Use of Empirical Data: If observed or empirically derived discharge data (related to water level or head difference) is available, a rating curve provides a direct way to incorporate this real-world information into the model.

While it may abstract some of the underlying physics compared to the explicit hydraulic calculation method, the rating curve approach is a powerful, flexible, and widely used technique when appropriate for the modeling objectives and available data.

Example Simulation Results

Below is an example time history chart showing the simulated reservoir water level compared to the downstream tailwater elevation, along with the resulting inflow and the discharge rate determined by the rating curve. This illustrates the model's response based on the defined head-discharge relationship.

Figure 2: Example time history results showing water level, tailwater elevation, inflow, and rating curve discharge.