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HydroCAD Stormwater Modeling - Since 1986


Dealing with Oscillations

What are oscillations?

Oscillations are rapid changes (fluctuations) in the outflow hydrograph.  This occurs when the projected outflow overshoots the actual value, causing the flow to change rapidly back-and-forth (oscillate).  HydroCAD automatically checks each routing calculation to see if oscillations are present, and generates a warning message when required.  The presence of oscillations generally indicates that the routing results are not valid and should not be relied upon.

What causes oscillations?

Oscillations are generally caused by an abrupt change in the stage-storage or stage-discharge curve.  If these changes are too abrupt, the routing procedure can not track the curve properly, causing an overshoot in the calculated outflow, which can oscillate back-and-forth around the correct value.

Why am I getting so many warning messages?

With a dynamic routing, an oscillation in one node will propagate downstream and upstream throughout the entire drainage network, often triggering additional warnings in other nodes.  Therefore you should resolve all oscillation warnings before attending to other warnings or results.  Correcting the source of the oscillations will often resolve many other warnings.

Having a closer look

If you get an oscillation warning, have a close look at the hydrograph plot to determine the exact nature of the problem.  In this example, the oscillations appear as a thick line in the discarded outflow curve.
The oscillations are usually easier to see if you switch the hydrograph to a 2D display:

Note: To toggle between 2D and 3D displays, click the button to the right of the display.

If we zoom-in, we get an even better view:

Note: To zoom in on any graphical report, drag a selection rectangle over the area you want to view.  Start at the upper-left corner of the area, click and hold the mouse button, and drag to the lower-right corner.  Release the button to complete the zoom.   To undo the zoom and return to the original view, drag in the opposite direction (from lower-right to upper-left).

If we zoom-in again, the individual fluctuations are clearly visible.  Notice how they alternate back-and-forth around the green inflow line, which represents the correct flow in this example.
In this example, the oscillations are occurring in exfiltration from a flat-bottomed pond.  The flat bottom causes an abrupt increase in the surface area and exfiltration rate.  Setting the exfiltration phase-in depth to 0.01' allows the exfiltration to gradually increase from zero to full flow, resolving the oscillations.

How do I correct oscillations?

Click for complete self-training materialsOscillations can generally be corrected by one of the following solutions:

1) Reducing the time step (dt) is usually the first approach to addressing oscillations.  This will generally increase the accuracy of the calculations, without any adverse effects other than a (slightly) longer calculation time.  The minimum dt of 0.01 hours can be used for most situations.  The minimum value can be reduced further by customizing the TimeInc parameter on the Settings|Units screen.  However, smaller values rarely offer any benefit over the default minimum, and will significantly increase calculation time and memory requirements.

2) Oscillations can also be triggered by overfilled storage, or by other conditions that cause an abrupt change in any stage-storage or stage-discharge curve.  When modeling a very small "pond", such as a catch basin, better results may be obtained by using a "zero-storage pond" and neglecting the volume.

3) Oscillations can also occur when using an exfiltration velocity with a flat-bottomed pond, as shown on the example above. This situation can usually be corrected by setting the exfiltration Phase-in Depth to a small non-zero value, such as 0.01 feet.

4) As a last resort, you can set the Finer Routing parameter to 2 or 3. This causes the program to update the routing calculations several times at each time step, and can sometimes produce a more stable routing.  However, since the Finer Routing option can adversely affect certain results, it should be used with care, and only as a last resort. If a finer routing value of 2 or 3 doesn't resolve the situation, set the finer routing back to 1 and pursue other remedies. Do no leave the finer routing at 2 or 3 unless this actually solves the problem. Setting the finer routing above 4 is rarely effective, and can actually compromise the results.  Resolving the cause of the oscillations will produce better results than "masking" them with the Finer Routing parameter.

5) If oscillations still occur after making several adjustments, try to determine where the oscillations are originating.  This can be done by disconnecting parts of the drainage system to isolate the source of the problem.  Often you will find a particular node that is causing the oscillations.  You may then be able to devise an alternate way to model that part of the watershed, such as correcting an overfilled storage condition, or combing adjacent storage volumes into a single pond.

Remember, always keep the model as simple as possible in order to meet your immediate modeling objectives.  Test as you go and fix any problems ASAP.  More tips here.

What if the hydrograph looks OK?

If you don't see a clear oscillation pattern (as discussed above), the results may be acceptable, in spite of the warning message.  This can occur in systems with complex routing behavior, such as tailwater, where additional peaks can be introduced into the hydrograph during the routing process.  If you cannot see the oscillations visually, or there are only a couple of additional peaks, the results may be acceptable, and the oscillation warning can be ignored.


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