Flow present in pond outlet structure even though the pond elevation is lower than the outlet struct


 Product(s):Bentley PondPack
 Version(s):08.11.01.56
 Environment:N\A
 Area:Calculations
 Subarea:N\A

Problem

Flow present in pond outlet structure even though the pond elevation is lower than the outlet structure elevation.

Solution

Assuming the flow you're seeing is not reverse flow due to a downstream tailwater higher than the upstream pond water surface, this is likely related to how PondPack routes the flow. PondPack does its routing by taking the elevation-area (or elevation-volume) curve for the pond and the outlet structure rating curve and creates an Elevation-Volume-Flow curve (a.k.a. EVQ). It then uses this curve to do the routing.  In order to do this it needs to do a bit of interpolation since the elevations in the HW range of the outlet structure may not exactly match up to those in the pond (only the minimum and maximum elevation will be the only “exacts”).

Take a look at the EVQ table in the Report Builder. Compare the number of elevations listed in the table, as well as the increment. Compare this to the number of elevations in the pond's elevation-area curve and to the outlet structure range.  These two sets of elevations have to be “merged” into the aforementioned EVQ table. It is likely that the pond has fewer elevations in the table then what is in the EVQ table in Report Builder. This impact the accuracy of the EVQ curve.

There are two things that can be done to improve the accuracy of this table.

1. Decrease the headwater elevation increment in the outlet structures.  This will create more HW elevations
2. Increase the number of elevations in the elevation-area curve of the pond.

The second option might be the best option, and if possible, including more data in the curve and using actual elevation-area values. If you do not have the data, it is possible to workaround this. With elevation-area tables you can interpolate the area if you put in a value of 0.0 as long as it is between two set elevations. You could put in elevations at 0.1 ft increments with an area of 0.0 and at 1/2 foot increments put in an actual area value. Though going to this length is not required.

In one example case, a small change to the table was make like so:

Pond Elevation (ft) Pond Area (acres)
50.30                    0.0010
50.50                    0.0000
50.75                    0.0000
50.90                    0.0000
51.00                    0.0500
52.00                    0.6000
53.00                    1.5000

If you notice, the area for elevations 50.5, 50.75 and 50.9 are 0.0 acres.  These areas will be interpolated by the engine and then used to compute volumes at those elevations. We then changed the headwater increment for the outlet structures to 0.1 feet instead of 0.5 feet. Please note that having actual non-zero values for the pond-elevation curve will be the most accurate option, but using interpolation with zero values can also work.

After making these changes and computing the model, the maximum pond surface elevation will likely change by a small amount. However, the EQV table should now be more accurate and the results from the outlet structure should now be accurate.



 Original Author:Scott Kampa