HydroCAD^® Stormwater Modeling - Since 1986

Tailwater, Backwater, Coupled Ponds, & Pressure Flow

What is tailwater?

The default routing method (storage-indication) requires a static stage-discharge relationship.  This means the stage-discharge relationship is fixed for the duration of the routing, and cannot respond to independent factors, such as tailwater.  To avoid incorrect results, HydroCAD automatically checks the tailwater at each time step and issues a warning message if a potential tailwater condition is detected.

How do I handle tailwater?

There are several options for modeling tailwater conditions.

1) If the tailwater is fairly constant, you can set a fixed tailwater elevation for each pond.  In some cases, you may also be able to use an average or worst case elevation, depending on your specific design objectives.

2) If the tailwater "tracks" the headwater with a fixed difference, you can specify a differential tailwater.  This feature allows the tailwater to rise and fall with the headwater, always maintaining a specified distance below the headwater.  This allows accurate modeling of a much broader range of applications, including some coupled ponds.

3) Other tailwater conditions are best modeled with the new Dynamic Storage-Indication method. This procedure routes all nodes at the same time, allowing them to respond to downstream as well as upstream factors.  (The DSI procedure is generally preferred over the earlier Sim-Route procedure.)

What about interconnected ponds?

Just route one pond directly into the next.  If there is a tailwater dependency, you will get a message recommending the use of the DSI procedure, as described above.

Do not use a reach to connect the ponds.  Since a reach always operates under normal flow conditions, it cannot respond to tailwater, and will "break" the tailwater capability between the two ponds.  Instead, route one pond directly to the next.  If there is a connecting pipe, model this as a culvert outlet device on the upper pond.

If the connection between the two ponds is sufficient to keep the ponds equalized, another option is to model the two storage volumes as a single "pond", thereby avoiding the tailwater calculation entirely.  Just use a separate line on the pond storage tab for each volume.  If you wish, you can also include the storage contribution of the interconnecting pipe or channel.  When describing the outlets, you should list only the final controls that effect both ponds, excluding the interconnection.

How do I set a manual tailwater with the DSI procedure?

With the DSI procedure the tailwater for each node is automatically determined from the next node downstream.  If you want to set the tailwater by hand, use a link as the downstream node and enter the tailwater on the Elevation tab.  You can enter a constant tailwater elevation, a table of elevation vs. time values, or even a tidal variation.  Click Help on the Elevation tab for complete details.

Important: Using a fixed "flood elevation" as the tailwater will minimize the discharge from the HydroCAD model and maximize retention.  But in most cases the tailwater will actually vary over time, often peaking later than the smaller watershed being modeled with HydroCAD.  For an accurate simulation of these situations you should enter a complete elevation vs. time table, or simulate the receiving water body within the HydroCAD model.

What about the Hydraulic Grade Line?

HGL calculations are typical of steady-state pipe calculations, but are not readily calculated with hydrograph routing models (such as HydroCAD) where the flow is constantly changing.  However, HydroCAD does calculate the peak elevation at each node, which is the equivalent of the maximum HGL value at that point.

How do I model a catch-basin?

The best way to model a catch-basin is as a pond with a culvert outlet. This allows a more accurate culvert analysis to be performed on the pipe, including the effects of headwater and inlet losses.  Details here.

I've set a manual tailwater on my culvert, but it doesn't have any effect!

First, make sure you've entered an elevation and not a depth. The tailwater elevation must use the same elevation reference used to describe the inlet invert.

You should also review the stage-discharge curve (not the hydrograph) to be sure it displays the expected behavior.  A culvert should exhibit zero discharge until the headwater exceeds the specified tailwater.

If the pond is being properly modeled, and you still see little tailwater effect, you may have a situation where the tailwater has a minimal effect on the actual hydrograph routing.  This is why its important to check the stage-discharge curve, rather than assuming that all factors will have a significant impact on the final hydrograph routing.

I'm not getting any discharge through my culvert!

If you've specified a tailwater, its possible that the peak elevation in the pond never exceeds the tailwater. There will be discharge from a culvert only when the headwater exceeds the tailwater.

When I enter a tailwater elevation, I get a message that says "Backflow will be prevented."

This occurs when you specify a tailwater elevation that is higher than the inlet of a culvert. The message is telling you that your fixed tailwater causes a potential reverse flow within the culvert, but that HydroCAD will not produce any such flow. At any point where the tailwater exceeds the headwater, HydroCAD will produce a zero discharge.

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