[Next Section] [Previous Section] [Table of Contents]

Full Equations Utilities (FEQUTL) Model for the Approximation of Hydraulic Characteristics of Open Channels and Control Structures During Unsteady Flow

U.S. GEOLOGICAL SURVEY WATER-RESOURCES INVESTIGATIONS REPORT 97-4037


5.12 FLOODWAY Command


Purpose: The FLOODWAY command can be used with FEQX, FEQXEXT, or FEQXLST to define a regulatory floodway for flood-insurance purposes. Further details are given in section 4.7.

Notes: A floodway-definition table, stored in a file, is applied with the FLOODWAY command to define the manner in which subsequent cross sections in the input file are modified. In this way, only one copy of the floodway table is needed for a stream system. This reduces errors and helps maintain consistency. Only two lines must be added to the input files to invoke the FLOODWAY command in FEQUTL. These two lines are the command FLOODWAY given above and the file name defined on Line 1.

LINE 1
Variable: FILNAM
Format: 5X, A48
Example: FILE = FLOODWAY.UMS
Explanation:

FILNAM is the file name containing the floodway table specifying which cross sections are used and the method applied to define the encroachment limits.

The floodway table consists of the following lines.

LINE 1
Variable: HEAD
Format: A80
Example: Floodway specification for West Branch Salt Creek
Explanation:

HEAD is a descriptive heading given by the user.

LINE 2
Variable: GLBCON
Format: 16X, F10.0
Example: Conveyance loss = 0.05
Explanation:

GLBCON is the global value for the fraction of total conveyance lost on one side of the stream. The total loss is twice the given value. This global value is used if the LOSS field in Line 5 for a cross section is left blank and the OPTION field in Line 5 contains CONV.

LINE 3
Variable: GLBELV
Format: 15X, F10.0
Example: Elevation loss = 0.7
Explanation:

GLBELV is the global value for the decrease in elevation from the standard flood level for defining the floodway using the elevation option. This global value is used if the LOSS field in Line 5 for a cross section is left blank and the OPTION field in Line 5 contains ELEV.

LINE 4
Variable: HEAD
Format: A80
Example: XSEC OPTN ELEV FEQBOT LEFT RIGHT LOSS
Explanation:

HEAD gives the headings for subsequent columns in the floodway table.

LINE 5 (Repeated as required to compute the floodway specification.)
Variables: TAB, OPT, ELEV, BOT, LEFT, RIGHT, LOSS
Format: I5, 1X, A4, 1X, F8.0, 4A8
Explanation:

TAB is the table number of the cross section to be modified. If a cross section with the given table number appears in subsequent input in FEQX, FEQXLST, or FEQXEXT, then that cross section is analyzed applying the information in the remainder of the line to define a floodway cross-section table. If a table number is given in TAB and no subsequent cross section with a matching number is present in the input, then the information is read but not used. If a cross section is input without an entry in the floodway table, then the cross section is processed as usual. The input is terminated by assigning a negative value for the table number.

OPT specifies the options for modifying the cross section. Valid options are ELEV, CONV, and USET.

The data required in the following fields depend on which option is selected.

If OPT = ELEV

ELEV is the water-surface elevation applied to compute the conveyance in the cross section. Normally, the water-surface elevation corresponding to the 100-year flood is utilized. The elevation in ELEV must be specified, and it is an error if the value given in ELEV is lower than the minimum point of the cross section or higher than the last point on either the left or the right end of the cross-section boundary description.

BOT is the elevation of the bottom of the cross section as specified in the profile in the FEQ schematization of the stream. This must be given only if it differs from the elevation of the minimum point in the cross-section boundary description. If left blank, the bottom profile given in the FEQ schematization for this cross section matches the minimum elevation in the cross-section boundary description. The symbol `same' is printed if the field is left blank.

LEFT is the limit of the encroachment from the left of the main channel. If an offset is to the left of LEFT (that is, farther from the main channel) is computed in the floodway-boundary determination, then that offset is retained. However, if the offset computed for the definition of the floodway is closer to the main channel than LEFT, the floodway boundary is limited to LEFT. In a case where the value in LEFT overrides the computed left boundary of the floodway, the decrease in the standard-flood elevation requested in LOSS is not attained, and the user must examine the FEQUTL output to determine what portion of the requested decrease was attained. If this information is left blank, the encroachment is unlimited and the symbol '+inf', denoting positive infinity, is printed as the limit.

RIGHT is the limit of the encroachment from the right of the main channel. RIGHT serves the same function as LEFT, but for the right side of the main channel. If this information is left blank, the encroachment is unlimited and the symbol '-inf', denoting negative infinity, is printed as the limit.

LOSS is a decrease from the standard-flood elevation given in ELEV. For example, if the floodway is to start at an elevation of 672 ft above sea level and the standard-flood elevation is 672.7 ft above sea level, then the value in the LOSS column should be 0.7 ft. If the LOSS column is blank, the global value of the decrease (Line 3) is applied.

If OPT = CONV

ELEV is the water-surface elevation applied to compute the conveyance in the cross section. Normally, the water-surface elevation corresponding to the 100-year flood is utilized. The elevation in ELEV must be specified, and it is an error if the value given in ELEV is lower than the minimum point of the cross section or higher than the last point on either the left or the right end of the cross-section boundary description.

BOT is the elevation of the bottom of the cross section as specified in the profile in the FEQ schematization of the stream. This must be given only if it differs from the elevation of the minimum point in the cross-section boundary description. If left blank, the bottom profile given in the FEQ schematization for this cross section matches the minimum elevation in the cross-section boundary description. The symbol 'same' is printed if the field is left blank.

LEFT is the limit of the encroachment from the left of the main channel. If an offset that is to the left of LEFT (that is, farther from the main channel) is computed in the floodway-boundary determination, then that offset is retained. However, if the offset computed for the definition of the floodway is closer to the main channel than LEFT, the floodway boundary is limited to LEFT. If the computed value for left or right floodway boundary corresponding to the requested reduction in conveyance is overridden by the input boundaries of the floodway (LEFT or RIGHT), the user must examine the FEQUTL output to determine what part of the requested reduction in conveyance was attained. Thus, if the specified value of LEFT results in a 3-percent reduction in conveyance on the left side of the channel, the requested reduction in conveyance is 10 percent, and the specified value of RIGHT is not limiting, then only 8 percent of the requested 10-percent reduction in conveyance is attained. If this information is left blank, the encroachment is unlimited and the symbol '+inf', denoting positive infinity, is printed as the limit.

RIGHT is the limit of the encroachment from the right of the main channel. RIGHT serves the same function as LEFT, but for the right side of the main channel. If this information is left blank, the encroachment is unlimited and the symbol '-inf', denoting negative infinity, is printed as the limit.

LOSS is the loss of conveyance utilized to determine the encroachment limits for the floodway. This loss value is evenly divided between the left and the right sides of the channel. If the LOSS column is blank, the global value of the reduction in cross-section conveyance (Line 2) is applied.

In both the ELEV and CONV options, the values of BOT, LEFT, and RIGHT are optional and may be left blank if the default values are suitable. LEFT and RIGHT can be applied to preserve a specified floodway width because these values always override any computed encroachment inside the specified limits.

In OPT = USET

ELEV is the water-surface elevation applied to compute the conveyance in the cross section. Normally, the water-surface elevation corresponding to the 100-year flood is utilized. The elevation in ELEV must be specified, and it is an error if the value given in ELEV is lower than the minimum point of the cross section or higher than the last point on either the left or the right end of the cross-section boundary description.

BOT is the elevation of the bottom of the cross section as specified in the profile in the FEQ schematization of the stream. This must be given only if it differs from the elevation of the minimum point in the cross-section boundary description. If left blank, the bottom profile given in the FEQ schematization for this cross section matches the minimum elevation in the cross-section boundary description. The symbol 'same' is printed if the field is left blank. In this case, specification of BOT is optional.

LEFT is a fixed left boundary for the floodway encroachment, which must be specified in this option.

RIGHT is a fixed right boundary for the floodway encroachment, which must be specified in this option.

LOSS is not required in this option.

A summary of the floodway table is printed at the completion of the output with the values of LEFT and RIGHT replaced by the limits computed by the selected option when appropriate.


[Next Section] [Previous Section] [Table of Contents]