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Modeling a Dam Breach

HydroCAD-10 provides a new "outlet device" for modeling a progressive dam breach.  The breach can begin at any desired time or water surface elevation, and develop over a user-specified interval.

A dam breach is modeled as flow through a trapezoidal weir in which the weir rise (height) increases linearly over time. The top of the breach is maintained at a constant elevation as the bottom of the breach decreases in elevation until the final breach height is achieved.

A dam breach may be initiated at a specific time, or when the water surface elevation in the pond reaches a specific elevation. After the breach begins to form, the height increases linearly over the specified time until the final height is attained.

Because the size of the breach varies over time, breach modeling requires the use of a dynamic pond routing procedure. This also allows the breach to respond to tailwater conditions created by a downstream reach or pond.

 For details see "dam breach" in HydroCAD help.

HydroCAD 9.1 and earlier

A limited simulation of a static dam breach can be performed with earlier versions of HydroCAD by using the following procedure.  However, HydroCAD-10 is required in order to model a progressive breach, or a breach that starts at any time other than the start of the calculation time span.  Update details here.

1) The pond must have an outlet device which approximates the behavior of the anticipated breach.  This could be a rectangular, trapezoidal, or custom weir, depending on the geometry of the breach.  (The pond should also include any other normal outlet structures that will continue to function during the breach, such as a culvert or riser structure.)

2) Set the pond starting elevation to the water surface elevation at which the breach will occur.  This will pre-fill the pond to this elevation, creating an immediate headwater above the breach.  If there are other ponds in the model, set their starting elevations to the appropriate value.

3) Optional:  If you want to model the effects of additional runoff occurring after the breach, include appropriate subcatchments and other elements as required to model the additional inflow.  If you only want to consider the initial pond volume, the pond does not need to have any inflows.

4) Set the beginning of the calculation time span to the time of the break:

A: If you have no runoff calculations (subcatchments) in the model, the starting time is arbitrary, and doesn't effect the outflow hydrograph.  A starting time of zero is recommended for this case.

B: If you're including runoff effects, set the starting time to the actual time at which you want the break to occur.  For example, if you're using a 24-hour rainfall, and you want to simulate a break that occurs at 14.2 hours, the time span must start at 14.2 hours.

5) Open a report window for the pond and view the results.

A:  Since the starting elevation exceeds the invert of the breach outlet, there will be an immediate spike in the outflow, which will decrease as the pond empties.

B:  To view the pond elevation vs. time, right-click the hydrograph plot and select the elevation curve.

C:  Adjust the end of the calculation time span as required to get an optimum view of the outflow.  A small break may require a longer time span, while a major break may empty the pond in an hour or less.

Determining the starting elevation

The starting elevation is often set to the maximum water level that will occur during a given event.  To determine this level with HydroCAD, use a separate copy of your HydroCAD model without the breach outlet, using a normal calculation time span (e.g. 5-20 hours).  This model can also be used to determine the starting elevation for any other ponds in the model.

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