Center Station
The lateral, cross section station of the breach centerline. The breach will start at this lateral station (e.g. 50 m or the midpoint of the dam in Figure 1).

Max Possible Bottom Width.
The maximum bottom width of the final breach. 

Note:

The top width will generally be larger, based on the breach side slopes.

Min Possible Bottom Elev.
The model will erode through the dam until it reaches this elevation. This is the vertical limit of the breach. HEC-RAS will not erode to a negative elevation (e.g. for an arbitrary laboratory datum) with this method. Adjust the model datum so the Min Possible Bottom Elev: is positive if this value would otherwise be negative.

Note:

The current version of HEC-RAS will not erode below elevation Zero. 

Left Side Slope
Define the left side slope of the final breach opening. Side slopes in the breaching editor are a horizontal to vertical (H:V) ratio or the base divided by the height. (Remember, displays in HEC-RAS are vertically distorted, so these slopes may appear steeper in HEC-RAS editors.) For example, a side slope of 2 represents 2 horizonal units for every vertical unit or a side slope that is twice as wide as it is high. Note: the H/V convention in HEC-RAS is opposite the convention in the stand alone version of DLBreach.

Right Side Slope
Define the right side slope of the final breach opening (Horizontal : Vertical – see note in previous entry).

Note: Unlike the other breach algorithms in HEC-RAS DLBreach assumes the right and left side slopes are the same. Therefore, if a DLBreach model defines different breach side slopes, HEC-RAS will average the side slopes and use the average for both the right and left side slopes in DLBreach.

Figure 2: Diagram of the breach geometry variables in the left pane. In this example, the breach reached the minimum possible elevation but the widening stopped before it reached the maximum width.

Breach Weir Coeff
Enter the weir coefficient that HEC-RAS will use to compute flow through the breach. HEC-RAS uses the broad crested weir equation to compute flow through the breach for overtopping breaches (or after the piping breach collapses). This is the broad crested weir coefficient for that breach.

Failure Mode
Select the failure mode in this drop-down menu. DLBreach includes the same two failure modes as the other breaching methods in HEC-RAS, Piping or Overtopping. The interface will update based on this choice, activating different data fields appropriate for each method.

Piping Parameters and Data:

A piping failure starts with an enclosed flow channel that forms through the embankment. HEC-RAS simulates this flow channel with uniform diameter pipe and uses the orifice equation to compute flow in the pipe. DLBreach uses cohesive or non-cohesive transport equations to compute pipe sidewall erosion and increases the diameter of the pipe as the flow erodes material. When the pipe grows large enough that it can no longer hold a competent "ceiling" in the breach material, it collapses, and transitions into an overtopping breach phase.
Erosion rate tends to be directly and non-linearly related to flow, making the pipe expand faster at higher pipe flow rates. Therefore, the total breach time can be very sensitive to the initial breach diameter and the pipe flow parameters.

Figure 3: Piping geometry parameters and coefficient.

Piping Coefficient
HEC-RAS computes flow through the pipe – during the piping phase of a breach – with the orifice equation. The orifice equation has a coefficient that regulates the flow efficiency through the pipe. A Piping Coefficient of 1 would be completely efficient. The HEC-RAS default for a piping breach is 0.5. Good data on the conduit efficiency of piping failures are not available. But it is likely that these natural conduits are less efficient than the straight, smooth pipes used to simulate them. So reducing the flow efficiency with this coefficient is appropriate. This coefficient will regulate flow through the piping failure and will affect the breach time. It is also one of the most significant difference between DLBreach in HEC-RAS and the stand alone DLBreach equation.

Initial Piping Elev
Define the center elevation of the piping conduit. HEC-RAS and DLBreach assume that the piping conduit is horizontal (only has one elevation) and rectangular. As the conduit expands, it grows symmetrically from this initial elevation. The Initial Piping Elevation will be the center of the piping breach until the pipe ceiling collapses and it transitions to an overtopping breach.

Initial Piping Diameter
Define the initial "diameter" for the piping conduit. Piping conduits in HEC-RAS are square. Therefore, the "diameter" is actually the width and the height of the piping conduit. Piping failures progress slowly at small diameters (with smaller flows and lower erosion rates), making the breach time sensitive to this parameter.

Overtopping Parameters and Data:

Pilot Breach (aka Mass Wasting Feature) (Overtopping Failure Only). The other breaching functions in HEC-RAS have a "Mass Wasting" feature, which simulates a pressure build up behind the embankment that "blows out" a chunk of the embankment quickly. DLBreach does not use the mass wasting model. However, overtopping breaches in DLBreach require a "Pilot Breach," a starting notch that guides the overtopping erosion. HEC-RAS uses the Mass Wasting interface to define the pilot breach for DLBreach. 

The Pilot Breach option requires a Width and Bottom Elevation.

Note:

The bottom elevation convention is different than the Pilot Channel Depth convention in DLBreach.

Duration
The Duration is optional. If the Duration is zero or blank, the full Pilot Breach will form immediately.
Trigger Failure At
HEC-RAS has three overtopping breach-initiation options. Because DLBreach starts simulations once the breach begins, HEC-RAS users can select any of these three options to initiate a physical breach simulation just like they would for the other breach methods. The three methods allow users to initiate the breach:

  • at a specific time (Set Time),
  • when the water reaches a specific a water surface elevation (WS Elev), or
  • after the water surface has exceeded a specified water surface elevation for a specified period of time (WS Elev + Duration) – with the option to add a second elevation that triggers immediate failure (regardless of duration).

Starting WS
If the water surface trigger elevation (WS Elev) is selected, enter the trigger a water surface elevation into this field. HEC-RAS monitors the water surface elevation at the breach Center Station the user enters. Dam breaches have horizontal water surface elevations, so the water surface elevation is the same across the embankment. But levees have sloped water surfaces, making the water surface site-specific.

WS Elev+Duration
The WS Elev+Duration trigger mechanism monitors the time that the water on the embankment exceeds a threshold and triggers the failure when the water exceeds that elevation for a specified time. This option has four additional fields or options:

Threshold WS
HEC-RAS will monitor the time that the water surface exceeds this water surface elevation at the breach Center Station. If that time exceeds the Duration Above Threshold (next field), HEC-RAS will trigger the breach and start the DLBreach simulation. 

Duration Above Threshold:
This is a time in hours. If the water surface elevation at the breach Center Station exceeds the Threshold WS for the Duration Above Threshold, DLBreach will initiate the breach.

Immediate Initiation WS:
The WS Elev+Duration option supports a failure mechanism where a moderate water surface saturates and loads an embankment long enough to cause a failure. However, the same embankment might fail immediately if the water surface gets high enough. The Immediate Initiation WS initiates a breach, without waiting for the Duration Above Threshold, if the water surface at the breach Center Station, reaches this (higher) water surface.
three additional fields of data to enter. The first variables Threshold WS. This variable is the water.

Set Time
If the user selects the Set Time option, then a starting date and time to initiate the breach must be entered.