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Defining Spillways for HEC-HMS Storage Areas

The Spillways data panel of the Storage Area Data dialog box will be enabled when Outflow Structures is selected as the routing method. Otherwise, this data panel entry is disabled (i.e., grayed out). Spillways typically represent structures at the top of the dam that allow water to go over the dam in a controlled manner.
Spillways panel in the Storage Area Data dialog box

The following sections describe how to interact with the Spillways data panel of the Storage Area Data dialog box.

Spillway Specifications

A maximum of 10 spillways can be defined under the Spillway Specifications data table. The Spillway Type dropdown entry defines how the dam crest geometry is to be defined. This dropdown combo box provides the following entries:

  • None (default)
  • Broad Crested
  • Ogee
  • User-Defined

The Flow Direction dropdown combo box entry defines where the spillway flow is being routed. This dropdown combo box provides the following entries:

  • Main (default)
  • Auxiliary

Broad Crested Spillway Specifications

This section will only be enabled when the Spillway Type entry of the Spillway Specifications data table is selected as Broad Crested.
Broad Crested Spillway Specifications

This section contains the following options:

  • Crest elevation
    This entry defines the crest elevation of the spillway.
  • Spillway width
    This entry defines the width of the spillway. Clicking the [Pick] button will cause the dialog box to temporarily disappear and allow the user to measure the width of the spillway from the Map View.
  • Discharge coefficient
    This entry defines the broad crested discharge coefficient used in spillway computations. Clicking the […] button displays the Weir Coefficients lookup dialog box that contains broad crest weir coefficients.
    Weir Coefficients

Ogee Spillway Specifications

This section will only be enabled when the Spillway Type entry of the Spillway Specifications data table is selected as Ogee. The ogee spillway allows for controlled flow over the top of the storage area according to the weir flow assumptions. However, the discharge coefficient in the weir flow equation is automatically adjusted when the upstream energy head is above or below the design head.
Ogee Spillway Specifications

This section contains the following options:

  • Crest elevation
    This entry defines the crest elevation of the spillway.
  • Spillway width
    This entry defines the width of the spillway. Clicking the [Pick] button will cause the dialog box to temporarily disappear and allow the user to measure the width of the spillway from the Map View.
  • Number of Abutments
    This integer spin control defines the number of spillway abutments that exist. This spin-control offers a range from 0, 1, and 2.
  • Abutment material
    This dropdown combo box entry defines the material that the spillway abutments are constructed of. This dropdown combo box provides the following entries:
    1. Concrete (default)
    2. Earthen
  • Approach depth
    This entry defines the approach depth of the spillway. If there is no approach channel, the depth should be the difference between the spillway crest and the bottom of the storage area.
  • Approach loss
    This entry defines the approach loss of the spillway. This represents the energy loss that occurs between the main storage area and the spillway. If there is no approach channel, the approach loss should be zero.
  • Apron elevation
    This entry defines the elevation of the spillway apron.
  • Apron length
    This entry defines the length of the spillway apron. Clicking the [Pick] button will cause the dialog box to temporarily disappear and allow the user to measure the length of the spillway apron from the Map View.
  • Design head
    This entry defines the design head of the Ogee spillway.

User-Defined Rating Curve Specifications

This section will only be enabled when the Spillway Type entry of the Spillway Specifications data table is selected as User-Defined.
User-Defined Rating Curve Specifications

The user-defined spillway rating curve option can be used to represent spillways with flow characteristics that cannot be represented by the broad crested or ogee weir assumptions. This allows the user to define an elevation versus discharge curve that represents the spillway discharge as a function of storage area pool elevation. Currently, there is no ability to include submergence effects on the spillway discharge. Therefore, this option should only be used for storage areas where the downstream tailwater stage cannot affect the discharge over the spillway.

The Spillway rating curve dropdown combo box allows the user to select an already defined spillway rating curve. Clicking the […] button displays the Spillway Rating Curve Data paired data dialog box that allows the user to define a new elevation discharge rating curve.
Spillway Rating Curve Data

Gate Specifications

In the Gate Specifications data table, up to ten separate gates can be used per spillway.
Gate Specifications

The data table under this section contains the following parameters:

  • Gate Type
    This dropdown combo box entry defines how the gate type is being defined. This dropdown combo box provides the following entries:
    1. None (default)
    2. Sluice
    3. Radial
  • Gate Width
    This entry defines the width of the gate.
  • Number of Gates
    This integer spin control sets the number of identical gates that are defined. This spin-control ranges from 1 to 10.

Sluice Gate Specifications

This section will only be enabled when the Gate Type entry of the Gate Specifications data table is selected as Sluice.
Sluice Gate Specifications

This section contains the following options:

  • Discharge coefficient
    This entry defines the sluice gate discharge coefficient used in spillway computations. Clicking the […] button displays the Gate Coefficients dialog box that displays assistance for the gate coefficients.
    Discharge coefficient
  • Submerged orifice coefficient
    This entry defines the submerged orifice discharge coefficient used in the spillway computations. Clicking the […] button displays the Orifice Discharge Coefficients lookup dialog box containing orifice discharge coefficients.
    Orifice discharge coefficients
  • Gate opening height
    This entry defines the fixed height of the gate opening.

Radial Gate Specifications

This section will only be enabled when the Gate Type entry of the Gate Specifications data table is selected as Radial.
Radial Gate Specifications

This section contains the following options:

  • Radial discharge coefficient
    This entry defines the radial gate discharge coefficient used in spillway computations. Clicking the […] button displays the Gate Coefficients dialog box that displays assistance for the gate coefficients.
    Radial discharge coefficient
  • Trunnion exponent
    This entry defines the radial gate trunnion exponent used in spillway computations.
  • Opening exponent
    This entry defines the radial gate opening exponent used in spillway computations.
  • Head exponent
    This entry defines the radial gate head exponent used in spillway computations.
  • Trunnion height
    This entry defines the trunnion height of the gate opening.
  • Submerged orifice coefficient
    This entry defines the submerged orifice discharge coefficient used in spillway computations. Clicking the […] button displays the Orifice Discharge Coefficients lookup dialog box containing orifice discharge coefficients.
    Orifice discharge coefficients
  • Gate opening height
    This entry defines the fixed height of the gate opening.

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