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Last modified
1/25/2010 7:10:20 PM
Creation date
10/5/2006 2:35:53 AM
Metadata
Fields
Template:
Floodplain Documents
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
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<br />MAXIMUM BREACH OUTFLOW DISCHARGE AND DEPTH CALCULATION <br /> <br />To determine the maximum breach outflow (Qbmax) from the dam the user <br />must first obtain the following data values: <br /> <br />As - Reservoir surface area (acres) at maximum pool level <br /> <br /> <br />H - Depth (ft) of maximum pool level above final breach elevation <br /> <br /> <br />Br Average final breach width (ft) <br /> <br />tf - Time of failure (minutes) <br /> <br /> <br />Qo - Additional (non-breach) outflow (cfs) at time tf (i.e., <br /> <br /> <br />spillway flow, turbine flow, and/or crest overflow) (optional <br /> <br /> <br />data value, may be set to 0.) <br /> <br />Defaults: The reservoir surface area may be estimated to be equal to twice <br /> <br /> <br />the reservoir volume divided by the dam height. The breach depth, <br /> <br /> <br />H, may be set equal to the height of the dam. The average final <br /> <br /> <br />breach width may be estimated to be one to three times the height <br /> <br /> <br />of the earth dam and the entire width of a concrete arch dam. The <br /> <br /> <br />time of failure may be estimated to be near zero for a concrete <br /> <br />arch dam (see following subsection on instantaneous failure) while <br /> <br /> <br />the breach erosion rate for earth dams is generally around 2 feet <br /> <br />per minute (i.e., tf minutes = H/3). <br /> <br />These values are substituted into the broad-crested weir flow equation to <br /> <br /> <br />yield the maximum breach outflow (Qbmax) in cfs, i.e. <br /> <br />Q = Q + 3.1 B <br />bmax 0 r <br /> <br />( )3 <br />C <br />t + C <br />~ IH <br /> <br />(1) <br /> <br />.. <br /> <br />where C = <br /> <br />23.4 A <br />S <br /> <br />B <br />r <br /> <br />(2) <br /> <br />1-3 <br />
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