FLOOD06918

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: The NWS Simplified Dam Break-Flood Forecasting Model
• Date: 5/16/1986
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

The distinguishing characteristic of each curve family is the Froude
number developed as the floodwave moves downstream. The distinguishing
characteristic of each member of a family is the ratio of the volume in the
reservoir to the average flow volume in the downstream channel. Thus it may
be seen that to predict the peak flow of the floodwave at a downstream point
the user must first determine the desired distinguishing characteristic of
the curve family and member. This determination is based on the calculation
of the Froude number and the volume ratio parameter. To specify the
distance in dimensionless form, the distance parameter must also be
computed. The equations required for these computations, as well as the
equations for determining the peak stage at forecast points are presented in
Appendix I.

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The time of occurrence of the peak flow at a selected cross section is
determined by adding the time of failure to the peak travel time from the
dam to that cross section. The travel time is computed using the kinematic
wave velocity which is a known function of the average flow velocity
throughout the routing reach. The times of first flooding and "de-flooding"
of a particular elevation at the cross section may also be determined when
the user provides the selected elevation.

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III. MODEL CONFIGURATIONS

The SMPDBK model was developed for use in dam failure analyses when
time is short or where main frame (large) computer facilities are
unavailable to the user. For the first reason above, the model's data
requirements have been kept to a minimum. For the second reason, the
model's computational requirements have also been kept to a minimum such
that it can be employed with small (mini- or micra-) computer systems or
even with a simple, non-programmable, hand-held calculator. To allow for
these three applications, the model has three versions. The first version,
known -as the "manual" version, allows the user, employing only an inexpen-
sive calculator and the routing curves provided in Appendix la, to produce
forecasts of the dam-break floodwave peak discharges, stages, and travel
times at selected downstream points. To accomplish this, the user simply
follows the step-by-step guide provided in Appendix I for making the
required calculations, providing data input where necessary, and selects the
indicated values from the routing curves.

The second version, known as the "FORTRAN Mini/Micro-computer" version
has been, as its name implies, developed for use on mini-computers or micro-
computers with a FORTRAN compiler. The computational procedure followed by
this.verion is the same as that followed in the manual version; however, the
computations are made by the computer rather than the user, thus signifi-
cantly reducing the time-required to prepare a forecast. This version was
developed for interactive use and prompts the user for information (data) as
it is needed, automatically using default values (where applicable) when the
user is unable to provide them. Included in this version is a "short"
option that may be employed when the only channel cross-section data
available is that describing the dam face; the model assumes a constant
channel configuration and provides forecast information at seven user-
specified points downstream. A copy of the FORTRAN code for this version
and an example run are provided in Appendix II.

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