Sometimes it is of interest to model a scenario in which there is a dam failure. Two types of dam failure can be modeled in HEC-HMS, overtop and piping. For both types of breach methods, a trigger method, development time, and progression method are used to define when the failure initiates, how long it takes to attain maximum breach opening, and how the breach develops during the development time. Typically dam breaks would be modeled in HEC-HMS only for periodic assessments. At a certain size, RAS would be more meaningful, since it includes the ability to manage a nonlevel pool.

In order to model a dam break, the Outflow Structures routing method must be used. Only one dam break can be included in the reservoir.

Trigger Method

There are three methods for triggering the initiation of the failure: elevation, duration at elevation, and specific time. For the Elevation method, the breach will begin forming as soon as the reservoir reaches a specified elevation. For the Duration at Elevation method, the reservoir elevation must remain at or above a specified elevation for a specified length of time in order to initiate the breach. For the Specific Time method, the breach will begin opening at the specified time regardless of the reservoir pool elevation.

Once the breach has been triggered, the development of the breach is determined using a selected progression method over the development time. The development time defines the total time (in hours) for the breach to form, from initiation to reaching the maximum breach size.

Progression Method

The progression method determines how the breach grows from initiation to maximum size during the user-specified development time. Current progression method options are: linear, sine wave, and user curve.
The Linear method causes the breach to grow in equal increments of depth and width from initiation to the end of the development time.
The Sine Wave method causes the breach to grow quickly in the early part of breach development and more slowly as it reaches maximum size. The speed varies over the development time according to the first quarter cycle of a sine wave.
The User Curve method allows the modeler to specify a pattern for the breach growth by defining the relationship between percent of the development time versus percent of the maximum breach size.

Overtop Dam Break Method

The overtop dam break is designed to represent failures caused by overtopping of the dam. These failures are most common in earthen dams but may also occur in concrete arch, concrete gravity, or roller compacted dams as well. The failure begins when appreciable amounts of water begin flowing over or around the dam face. The flowing water will begin to erode the face of the dam.
The method begins the failure at a point on the top of the dam (or below the top) and expands it in a trapezoidal shape until it reaches the maximum size. The maximum breach size is defined using the top and bottom elevation, bottom width, and side slopes.
The bottom elevation defines the elevation of the bottom of the trapezoidal opening in the dam face when the breach is fully developed. The bottom width defines the width of the bottom of the trapezoidal opening in the dam face when the breach is fully developed.
Flow through the expanding breach is modeled using the weir flow equations:

Q=CLH3/2
(X)
where:
Q = Discharge over dam breach
C = Discharge coefficient = 1.7 or 1.35
L = bottom width of the dam breach = average side slope x head
H = Upstream energy head above dam breach
The size of the dam breach at each timestep is determined using the trigger, progression, and definition of the maximum dam breach size. The invert elevation is changing, along with the head and the length.

Piping Dam Break

The piping dam break is designed to represent failures caused by piping inside the dam. These failures typically occur only in earthen dams. The failure begins when water naturally seeping through the dam core increases in velocity and quantity enough to begin eroding fine sediments out of the soil matrix. If enough material erodes, a direct piping connection may be established from the reservoir water to the dam face. Once such a piping connection is formed it is almost impossible to stop the dam from failing.
The method begins the failure at a point in the dam face and expands it as a circular opening. When the opening reaches the top of the dam, it continues expanding as a trapezoidal shape. Flow through the circular opening is modeled as orifice flow while in the second stage it is modeled as weir flow.

The piping elevation indicates the point in the dam where the piping failure first begins to form.
The piping coefficient is used to model flow through the piping opening as orifice flow. As such, the coefficient represents energy losses as water moves through the opening.

Q=KA2gH
( 4 )
in which Q = flow rate; K = user-specified orifice coefficient; A = the cross-sectional area of the culvert, normal to the direction of flow; H = total energy head on outlet; and g is the gravitational constant.

Once the piping opening reaches the top of dam elevation, it transitions from pressurized piping flow to an open, overtopping breach. Then the outflow is modeled as weir flow, using the top elevation, bottom elevation, bottom width, left slope, and right slope to describe the trapezoidal breach opening that will be the maximum opening in the dam.