Flooding, Land Use, and Watershed Response
in the Blue Mountains of Oregon and Washington
Jim Fitzgerald and Caty Clifton
The northern Blue Mountains sustained heavy rain and rapid snowmelt
in November 1995 and rapid snowmelt over frozen soil in February
1996. The result was multiple record flood events, with the February
peak flows being more widespread and of higher magnitude (see figure
below).
The flood assessment focused on characterizing the events, inventorying
mass wasting features, mapping channel perturbations, estimating
flood magnitude and frequency, and evaluating the performance of
instream fish habitat structures and stream-road crossing culverts.
In addition to flooding, the storms triggered debris flows and slides
that commonly occurred in the rain-snow transition zone, in saturated
loam-clay-ash soil, and on steep slopes (30 to 80%). Debris flows
or torrents, the dominate feature, start as earthslides and sometimes
transported debris over a distance of one mile. Roading and logging
were associated with 37% of the observed mass wasting features.
High flows and mass wasting combined to produce a variety of channel
responses including: scouring of substrate and banks, sediment aggradation,
large woody debris accumulation, and lateral channel migration.
Fluvial responses differed with elevation and land use intensity.
Flood discharge of National Forest streams was estimated using the
indirect, slope-area method based on post-flood field evidence.
Flood frequencies were estimated using U.S. Geological Survey regional
flood equations. Flood magnitude and frequency varied by watershed
with some areas experiencing one or more “100-year” events (Umatilla
and Walla Walla) and others experiencing less than a “25-year” event
(Tucannon and Wenaha).
Management Interactions
Instream Structures - Results from field inventories indicate a
high rate (73%) of instream fish habitat structure survival. Survival
rate varied between individual watersheds with a high survival rate
(80%) in larger streams (5th to 6th order), while lower order streams
had a lower survival rate. Anchored rock weirs had the highest success
rate.
Findings and Recommendations:
- a high rate of structural survival
- expect some structures to move or shift during high flow events
- design structures that work with fluvial processes, not against
them
- limit use of “rigid” structures (e.g., cabled log-rock weirs)
Stream-Road Crossings - In roaded watersheds, a sample of
culverts at stream-road crossings indicated that approximately 50%
of the culverts failed. The failure rate varied by watershed. Culverts
in the Umatilla River watershed had a 5% failure rate compared to
those in the Tucannon River watershed, which experienced a 95% culvert
failure rate. Results indicated that debris and coarse sediment
caused the majority of culvert failures, rather than undersizing
for flow.
Findings and Recommendations:
- expect culverts to plug with debris during large floods and
design them to allow for overtopping
- design crossings to accommodate bedload carried by high flows
- assess upslope conditions to determine landslide and debris
flow potential, and unit discharge
- use catchment basins at inlets to capture sediment and debris
- evaluate the benefits of decommissioning damaged sites relative
to repair costs, assess needs, and downstream values
Conclusion
Forest investments, such as roads, hiking trails, and instream habitat
structures, were damaged as a result of mass wasting, erosion, and
channel migration. In addition, land use accelerated flood damage
in some areas by decreasing slope and channel stability and potentially
increasing flood magnitude. The post-flood assessments will be used
to improve understanding of watershed response to extreme hydrologic
events and to improve management practices to reduce damage from
future high flows.
Jim Fitzgerald, Hydrologist, Environmental Protection Agency,
Idaho Office, Boise, ID 83706.
Caty Clifton, Forest Hydrologist, Umatilla National Forest,
Pendelton, OR 97801.
A complete summary of this study, originally presented as a poster
presentation at the Inland Northwest Water Resources Conference,
Spokane, WA, April 28-29, 1997, is available from the authors. To
request a copy, phone or fax Caty Clifton at: Phone (541) 278-3822
or FAX: (541) 278-3730.
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