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HydroCAD Stormwater Modeling - Since 1986


Using the Simultaneous Routing procedure

What is the Sim-Route procedure?

Click for complete self-training materialsHydroCAD-6 introduced a new "simultaneous" routing procedure, abbreviated as Sim-Route.  By calculating all nodes at each time step (simultaneously) using the previous inflow, this procedure allows the analysis of flow networks that don't have a fixed flow order.  This allows the analysis of certain systems that have reversing flow, or even flow loops.

When should I use the Sim-Route procedure?

The Sim-Route procedure should be used only in situations with reversing flows or flow loops.  This includes cases where water flows first in one direction, and then changes direction.

The Sim-Route procedure is designed to handle ponds that provide a significant amount of storage in relation to the inflow hydrograph.  In general, if a pond is large enough to produce a significant attenuation of the peak inflow, it should work with the Sim-Route procedure.  A common application would be a culverted road crossing with tailwater effects, where each side of the road provides significant storage and is modeled as a separate pond.

When is the Sim-Route procedure not applicable?

Although the Sim-Route procedure can handle variable tailwater, the Dynamic Storage-Indication method is preferred for most tailwater situations.  Sim-Route should be used only for reversing flows or flow loops.  Customers who previously used the Sim-Route procedure with HydroCAD-6 will often obtain better results by switching to DSI routing in HydroCAD-7 or later.

bulletBefore using the Sim-Route procedure, review any other warning messages and be sure you have a true reverse-head situation.  Other modeling problems, such as overfilled storage, may create a false reverse-head situation which must be resolved first.
bulletPonds with very small amounts of storage are not good candidates for this procedure, and ponds with zero storage (catch basins) will not work at all.  In these cases, it may be necessary to use the DSI procedure.
bulletIf you have adjacent "ponds" that are equalized by an oversized connecting pipe, you should consider modeling them as part of a single storage volume, thereby eliminating the need to model tailwater or reversing flows at that point.
bulletKeep the model as simple as possible to meet your modeling objectives.  Consider how each element will behave and model it accordingly.  See the how-to tips.

How do I model a reversing flow?

To model a reversing flow, you need to define a reverse outlet between the nodes where the flow is reversing.  This is just like a standard outlet device, but routed in the reverse direction, that is, towards the "upper" pond.  The reverse outlet is actually the same device, but seen from the perspective of the "lower" pond.

For example, if you have pond P1 that normally flows into pond P2, you would define the reverse outlet in pond P2, and set the device routing to "Secondary".  (If the device is a culvert, this means it will have a negative slope, and the entrance loss (Ke) applies to any flow entering the "lower" end of the pipe.)  On the routing diagram, drag the secondary outflow handle back to P1, causing a double-ended arrow to appear.

What are "oscillations"?

Oscillations are rapid outflow fluctuations that can occur when the routing increment (dt) is too large for a particular reach or pond.  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.  Details here.

Why am I getting so many warning messages?

With a simultaneous 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.

How do I correct oscillations?

If you get an oscillation warning, click on the message for additional details.  In general, oscillations are resolved by setting a smaller dt for the project.  (You may need to use the smallest allowed dt of 0.01 hours.)  This causes the program to update the routing calculations more frequently, thus preventing the "overshoot" that typically triggers the oscillations.

Also be sure you don't have any unnecessary reverse outlets.  A reverse outlet is needed only in rare situations where the flow actually changes direction.  A reverse outlet is not required to handle normal tailwater, and may contribute to oscillations, as well as making them harder to isolate and resolve.

What if I still get oscillations?

First, try to use the DSI procedure, which generally provides superior results in a wider range of applications.  Stay with Sim-Route only if you must model a reversing flow, or analyze a system without a fixed flow order.

If oscillations still occur with the smallest dt, you may be attempting to model a pond or reach with too little available storage, such as a catch basin.  In general, the Sim-Route procedure is designed for modeling ponds of reasonable size, that is, ponds that take many dt increments to fill or empty.  If the available storage is small in relation to the inflow hydrograph, an accurate routing will require so small a routing increment as to make the procedure impractical.

To determine where the oscillations are originating, disconnect parts of the drainage system to see if you can isolate the problem.  Often you will find a particular node that is the source of the oscillation.  You may then be able to devise an alternate way to model that part of the watershed.

In some cases, you may be able to avoid reverse flows by combining adjacent storage volumes into a single pond.  Details here.  This will allow the use of a more stable routing procedure, such as DSI


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