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 49

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Fig. 3_ Photograph of the floodplain below Reservoir No.3, Centralia. Wash" where there was only minimal
damage from the flood.

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/"" Reservoir No.3

Porter Hill Dam
......:::--.--.----...
----.---.-.--..--

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
TIME. IN MINUTES, FROM DAM FAilURE

Fig. 4_ Reconstructed triangular hydro graphs for the Centralia
and Porter Hill floods.

passed at about 3 mfs (Figure 6).
During large floods along high-energy fluvial systems,
floodplains can become unraveled, severely eroded, and
inundated by coarse gravel and debris [Nanson, 1986;
Jarrett and Costa, 1986]. A floodplain that is not ravaged
by an extreme flood is the exception, and requires
explanation. Some studies that documented the lack of
erosion and modifications to channels and floodplains

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accompanying large floods attributed the lack of land
surface disruption to insufficient stream power [Nanson and
Hean, 1985] or extraordinary stabilization of surfaces by
vegetation [Zimmerman and others, 1967]. These explana-
tions refer to thresholds of landscape resistance that must
be overcome by the flow for it to be effective. The
Centralia and Porter Hill floods generated peak stream
power values that were likely capable of surpassing
resistance thresholds offered by the alluvial valleys and
floodplains.
We infer that the lack of disruption of the valley floor
despite extraordinarily large peak stream power acting
against the floodplain was chiefly a consequence of the
very short duration of high stream power during the flood_
In both dam-failure floods, high flows were not sustained,
and the entire hydro graphs passed the study areas within
about 16 minutes (Figure 4). The maximum flood power,
while large- enough to greatly surpass landscape resistance
thresholds, lasted for only a small fraction of that time and
was not effective in breaking down floodplain vegetation
and eroding channels and floodplains. Thus high-energy
floods, of very short duration, may cause little geomorphic

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