FLOOD07304

• County: Statewide
• Community: State of Colorado
• Basin: Statewide
• Title: Galesville Dam Breach Inundation Study
• Date: 9/21/1988
• Floodplain - Doc Type: Educational/Technical/Reference Information

Calibration and sensitivity computer runs were performed to
assess the models representation of the river system and the
effect of breach parameter assumptions. The flood of December,
1964. was a major regional flood approximating a 100-year
event. This flood was input to the model using the routing
feature as a check on calibration. The computed routed flood
had good temporal and spatial comparison with observed values.
At the gaging station on the South Umpqua River near Brockway,
the computed peak elevation was approximately 2.7 feet higher
than that observed, the computed maximum flow was within 3
percent of that observed, and the computed time of peak
elevation was equai to that observed (out of 44 hours from start
of routing to time of peak). Had the gage datum of the model
been equal to that of the actual gage, the difference in
computed and actual peak elevations would have been about 1.5
feet.

To assess the time of breach formation, breach times of 10 and
30 minutes, and 1 and 2 hours were input to the model. The
results showed that the breach times had little effect on peak
elevations, and that the difference in time to peak elevations
is about equal to the difference in breach formation times (Fig.
2 & 3).

)

To assess the shape and size of the breach, four assumed breach
geometries were input to the model. These geometries were a 300
foot wide rectangle 138.5 feet deep; 300 foot base width
trapezoid, 138.5 feet deep, 1H to 2V side slopes; 150 foot wide
rectangle 138.5 feet deep; 300 foot wide rectangle 69.5 feet
deep (Fig. 4 & 5). The results showed that for beyond about 5
miles below the dam, both of the 138.5 foot deep rectangles, and
the trapezoid shape, had essentially the same peak elevations.
The 69.5 foot deep rectangle had peak elevations about 10 feet
lower. The time of peak elevation was essentially the same for
the 300 by 138.5 foot rectangle and the trapezoid shape. The
peak elevation for the 150 by 138.5 foot rectangle ran about
15-20 minutes later. The peak elevation for the 300 by 69.5
foot rectangle was about a half hour later 10 miles below the
dam, and about one hour later further downstream.

In Figure 5 it can be seen that there is a large variation in
time to peak elevation about 80 miles downstream compared to the
trend upstream of this location. For the 300 by 69.5 foot
rectangle, the shorter time to peak below this location is
probably accounted for by most or all of the flow being
contained in the channel downstream of here, and consequently,
less overbank flow is occurring to retard/attenuate the flood
hydrograph. As for the 150 by 138.5 foot rectangle having a
longer time to peak below this location, this can be accounted
for by attenuation or a "flattening off" of the flood hydrograph
peak. For example, at the gaging station near Brockway (86.1
miles downstream), two hours before the peak the river stage was
only about 0.2 feet less. On Figure 5, the points circled as
"time to 751 of peak stage" demonstrate that the advancing flood
wave is tending to agree with the trend of upstream time to
peaks.

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