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Entering Dam Break Data

Dams are an important part of infrastructure. They are used for a variety of purposes, including water supply, recreation, irrigation, and hydropower generation. If a dam fails, it can cause significant property damage and loss of life. Humans can never be able to prevent a dam break completely. However, they can plan for the possibility of a dam failure.

In order to plan for the possibility of a dam break, dam breach information is entered to simulate the dam break and analyze the sensitivities. The dam breach information is stored as a part of the current scenario. This is done to facilitate the evaluation of dam and levee breaching in a real-time river forecasting mode. The dam breach data is only used for unsteady flow models and is ignored in steady flow models.

Follow the steps below to enter the dam breach data:

  1. From the Input ribbon menu, select the Inline Structure Data command.
    Inline Structure Data input ribbon menu commandAlternatively, the user can choose the Inline Structure Data command from the Inline Structures dropdown menu of the Input ribbon menu.
    Inline Structure Data command in Inline Structures dropdown of input ribbon menu
  2. The Inline Structure Data dialog box will be displayed.
    Inline Structure Data dialog box
  3. From the Inline Structure Specifications dropdown combo box, select the Dam Breach option.
    Inline Structure Specifications dropdown combo box
  4. The following data panel will be displayed.
    Dam Breach data panel

The following sections describe how to enter dam breach data and interact with the above data panel.

Breach Control

The Breach Control section contains the Breach structure checkbox option. This checkbox is used to control whether the structure should be modeled with a breach failure. Unchecking this checkbox allows the modeler to turn off the structure breach without losing the defined data. By default, this checkbox is unchecked.

Breach Dimensions

This section requires the following data to be entered to define the final breach dimensions:

  • Breach centerline station
    This entry allows the user to enter the centerline station of the breach. The stationing is based on the inline structure. Alternatively, the user can click the [Pick] button to graphically select the breach centerline station from the Inline Structure Plot section.
  • Breach final bottom width
    This entry allows the user to enter the bottom width of the breach after it has fully developed. Alternatively, the user can click the [Pick] button to graphically select the breach bottom width from the Inline Structure Plot section.
  • Breach final bottom elevation
    This entry allows the user to enter the bottom elevation of the breach after it has fully developed. Alternatively, the user can click the [Pick] button to select the breach bottom elevation from the Inline Structure Plot section.
  • Breach left side slope (V:H)
    This entry allows the user to enter the left side slope of a trapezoidal breach. The values entered in the side slopes represent the horizontal to vertical ratio. By default, the software uses a value of 3.0 (i.e., 1:3).
  • Breach right side slope (V:H)
    This entry allows the user to enter the right side slope of a trapezoidal breach. The values entered in the side slopes represent the horizontal to vertical ratio. By default, the software uses a value of 3.0 (i.e., 1:3).
  • Breach weir coefficient
    This entry allows the user to enter a weir coefficient for the breach. For an overtopping failure or when the top of a piping failure collapses, this coefficient is used with the weir equation to compute the flow through the breach. By default, the software uses a value of 2.6. However, the user can adjust it depending on the type of dam and breaching process ranging from 2.0 to 3.2. Clicking on the […] button will display a Weir Coefficients lookup table dialog box.
    Weir Coefficients dialog box
  • Breach formation time
    This entry allows the user to enter the time required for the breach to fully form, from the start of some significant erosion to completion. The modeler should be very careful in selecting the breach formation time. If a linear breach progression rate is selected, then the breach time should be limited to when the breach begins to erode significantly and up to when the major portion of the breach is formed.

Breach Failure Details

This section requires the following data to be entered to define the breach failure:

  • Breach failure type
    This dropdown combo box provides two different breach failure types:

    1. Piping This failure type should be selected when the dam fails due to seepage through the dam, which causes erosion, which in turn causes more flow to go through the dam, which results in even more erosion. This failure will grow slowly at first, but will pick up speed as the area of the opening begins to enlarge. At some point during the breach, the embankment above the breach will begin to sluff, at which time a large mass wasting of the embankment will occur.
    2. Overtopping This failure type should be selected when the water surface overtops the entire dam and erodes its way back through the embankment, or when flow going over the emergency spillway causes erosion that also works its way back through the embankment.
  • Piping coefficient
    The Piping coefficient entry allows the user to enter a piping coefficient. This coefficient is used in the orifice equation while the flow is coming out of the dam in a piping model. This entry is only available when a piping breach failure type is selected. Otherwise, it is grayed out (i.e., unavailable). By default, the software uses a value of 0.5. Clicking on the […] button will display the Orifice Piping Coefficients lookup table dialog box.
    Orifice Piping Coefficients dialog box
  • Initial piping elevation
    The Initial piping elevation entry allows the user to enter an initial piping elevation. This elevation should be entered as the center of the piping flow when the breach begins. The Initial piping elevation entry is only available when a piping breach failure type is selected. Otherwise, it is grayed out (i.e., unavailable). The user can click the [Pick] button to graphically select the piping elevation from the Inline Structure Plot section.
  • Breach trigger
    This dropdown combo box entry allows the user to select one of three trigger methods for initiating the breach. The three trigger methods are: Exceeds WS Elevation, Exceeds WS Elev & Time, and Date & Time. Based on the option selected, the content of the section below this entry changes to define additional data.

    1. Exceeds WS Elevation
      On selecting this option, the following section will be displayed:
      Exceeds WS Elevation in Breach trigger dropdown combo box

      • The Breach trigger elevation entry allows the user to enter a water surface elevation. Exceeding the water surface above this value triggers the breaching of the structure. (check this sentence. Not sure if this is error or not.) Alternatively, the user can click the [Pick] button to graphically select the elevation from the Inline Structure Plot section.
    2. Exceeds WS Elev & Time
      On selecting this option, the following section will be displayed:
      Exceeds WS Elev & Time in Breach trigger dropdown combo box

      • The Breach threshold elevation entry allows the user to enter a water surface elevation. Exceeding (?? Ignore if “Exceed” is accurate) the water surface over the specified duration triggers the breaching of the structure. Alternatively, the user can click the [Pick] button to graphically select the elevation from the Inline Structure Plot section.
      • The Duration above threshold entry allows the user to specify the duration. Exceed(?) in the water surface above the threshold value triggers the breaching of the structure.
      • The Immediate initiation elevation entry allows the user to enter a water surface elevation. Exceed(?) in the water surface immediately triggers the breaching of the structure. Alternatively, the user can click the [Pick] button to graphically select the elevation from the Inline Structure Plot section.
    3. Date & Time
      On selecting this option, the following section will be displayed:
      Date & Time in Breach trigger dropdown combo box

      • The Start date entry allows the user to specify the date at which the breach failure will be initiated.
      • The Start time entry allows the user to specify the time at which the breach failure will be initiated.

Breach Progression

This section allows the user to define the progression of the dam breach. There are two different breach progression methods:

  • Linear
  • Sine Wave

Clicking on the [Set to Linear] button will cause the breach to grow in equal increments of depth and width from the beginning to the end of the development time. Clicking on the [Set to Sine Wave] button causes the breach to grow rapidly in the early stage of breach development and slow down as it reaches its maximum size.

This section has a table that allows the user to enter user-specified progression curve data for the formation of the breach. By default, the breach progression is assumed to be linear between the breach initiation and the full breach size. The user can enter user-specific breach progression curve data in the Breach Progression table.

The Breach Progression table contains two columns: Time Fraction and a Breach Fraction. The user can enter data into these columns ranging from 0 to 1. The user-entered data is plotted in the Breach Progression Plot. The breach progression curve data are then used during the breach formation time to adjust the growth rate of the breach.
Breach Progression section

Repair Breach

The Repair Breach section allows the user to have the breach fill back in during the unsteady flow simulation. This could represent attempts to fill a breach during a failure event or repair the breach after the event.
Repair Breach section

The Breach Repair section requires the following data to be entered for breach repair:

  • Start repair after full breach
    This entry defines the amount of time (in hours) it takes to start the repair process after the breach has occurred.
  • Total repair time
    This entry defines the total amount of time that it takes to complete the repair of the breach.
  • Final repaired filled-in elevation
    This entry defines the final elevation of the repaired breach. Alternatively, the user can click the [Pick] button to graphically select the repaired filled-in elevation from the Inline Structure Plot section.

About the Author Chris Maeder

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