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Rating Curve Management and Display Tool

Tuesday, November 22, 2016 Jeff Schuyler

SiteHawk Rating Curve Tool

This blog is about a specific software tool that Eyasco has developed to aid some customers that have to measure river and stream flow.  This is particularly important for maintaining minimum flows where fish migrate up fresh water streams to spawn. 

What Is a Rating Curve?

A rating curve is a relationship between stage (water level) and flow or discharge at a cross section of a river. Stage is the height of a water surface above an established point and flow (or discharge) is the volume of water moving down a stream or river per unit of time. Flow values can be obtained by applying these rating curve formulas to stage measurements.

In the example above the discharge is 40 cubic feet per second (cfs) when the stage equals 3.3 feet. The dots on the curve represent measurements (concurrent) of stage and discharge used to develop the curve. To develop a rating curve flow measurements are taken manually at different stage levels.  This is because the slope or curve of a river bank can change enough with water depth so that the relationship between water height and flow is non-linear.  This process is called "rating" the stream.  Historically log relationships were used to create the rating curves because they resulted in a near straight line on a log plot.  However, in more recent years polynomial equations have gained favor due their ability to better handle low-flow conditions.

If the channel geometry changes enough then more than one rating curve at a specific location may be necessary. For example, the picture below illustrates how there can be different volumes of water depending on the shape of the river bank, and there may be one rating curve for water heights less than 2 feet, and another for water levels greater than 2 feet.

Change in channel geometry also creates different stage-flow relationships If the geometry of the river channel changes due to erosion or deposition of sediment at the stage measurement site then a new rating curve has to be developed.  Over time then a specific rated location will have many rating curves.  Deriving historic or real-time flow from these curves requires application of the correct rating based on the stage values and/or the date and time.

The Rating Curve Tool

The Rating Curve tool included with SiteHawk allows users to enter and manage rating curves and to derive plots of flow based on application of these curves to stage measurements. 

Each rating curve entry allows a user to define a date range for application.

And a minimum and maximum stage level

The application will apply rating curves based on their applicability to a specific level measurement, and generate continuous, historically accurate flow data.  The tool can chart both the stage and flow values. Changing the polynomials on the application will reflect instantaneously on the chart the next time it is generated. Graphs can be generated for viewing or export from the user interface.

Applying the Rating Curve In Real Time

Once a user has entered the rating curves for a specific rated stream location, they can generate plots of flow manually, or decide to use the curves to generate flow data in real time  - as if the flows were actually being calculated or measured in the field!  The benefit of using this approach is that flows can be displayed in real time on charts (Infopages) and maps (SiteHawk).  

The Rating Curve tool is a powerful application that allows users to manage and organize their rating curves, generate flow calculations, visualize their calculations with graphs, and automate flow calculations for real-time monitoring.


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