FLOOD05412

• County: Statewide
• Basin: Statewide
• Title: Geomorphically Effective Floods
• Date: 1/1/1995
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

COSTA AND O'CONNOR 51

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Fig_ 6. Photograph of the channel and floodplain about 100 m downstream from the Porter Hill dam_ Note the
complete lack of any identifying evidence that a large floM had ever passed this location. The tape marks the
maximum flood stage.

TABLE 1. Dam and flood characteristics of Reservoir No.3,
and Porter Hill Dam.

Feature

Reservoir No.3
Centralia, Wash_

Porter Hill near
Roseburg, Oreg_

Type. of dam
Date built
Date failed
Height (m)
Volume (m')
Flood depth (m)
Slope
Discharge (m'/s)
Peak w (w/m')

Concrete-lined
1914
1991
5_2
13,250
1.0
0_09
71
3,300

Earthen
early 1990s
1993
5_8
15.000
1.0
0_10
30
2,900

forms, but our analysis focuses on the erosional thresholds
of extreme floods.
For comparison to the Centralia and Porter Hill dam-
failure floods, we analyzed floods from three thoroughly
studied historic large-dam failures. one cloudburst-rainfall
flood along a sand-bed stream in Colorado, and the flood

of record along the Mississippi River (Table 2). We also
include two well-known paleofloods. or series of paleo-
floods, the Missoula and Bonneville floods, which resulted
from large natural-dam failures about 10,000 - 15,000
years ago along the Columbia and Snake Rivers.
These floods were selected because (a) we have done
field work at all sites except the Mississippi River, and are
familiar with the flood effects; (b) the floods were
unequivocally large and capable of generating substantial
hydrodynamic forces; and (c) we had the appropriate data
for our analysis for each flood_ All available hydraulic
data, except that from the Mississippi River, are from
indirect-discharge measurements. Gaging stations, where
present, were destroyed during the floods. Indirect-
discharge data increase the likely error, but our attempt is
to present a concept and approach that can be further
verified with better data. -
Field investigations and descriptive reports indicate
which floods were effective geomorphic floods, and which
were not. Little or no change occurred to channels or
floodplains associated with the floods from the two small
upland dams at Centralia and Porter Hill discussed

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