Abstract

1
Abstract Upland areas of the Willamette Basin
are associated with a long history of logging and
related road construction. Sediment derived from
forested areas can greatly impact the rates of
geomorphic processes and water quality in
downstream areas. Soil erosion on forest roads
can come from many different sources, with
controlling variables that include slope angle
and spatial position. A key event that has been
subject to much research is that of the 1996
storm and flood event in western Oregon. Intense
rainfall caused widespread erosion in forested
areas. Road erosion is dependent upon local
geology, forest condition, and construction
style. In addition, soil texture and degree of
vegetative cover also played a role in
controlling erosion rates. Forest roads in
clear-cut areas were associated with
significantly more sedimentation and higher
erosion rates.
Results and Discussion of Data The first study
found that the variability of sediment production
is high from road segment to road segment. In
this study there was not much sediment produced
but when it was found there was a lot of it
produced. They also found that soil that was
coarser produced less sediment than soil that was
fine so the soil texture had a large impact on
sediment production. The second study found that
fluvial features were used less than mass
movement of sediments and ¾ of movements were
mass while the remaining ¼ were fluvial. Most of
the sediment that ended up on the roads was from
debris flows while other avenues ended up off the
road (Figure 5). Fillslopes accounted for 2/3rd
of sediment being removed from roads and
hillslopes had most of the remainder. Older and
mid-slope road have the bulk production of
sedimentation and debris flow issues.
Figure 2. Example of a culvert (Akay and others).
Introduction The specific designs of forest
roads and placement of them can significantly
help lower the chances of sediment build up or
erosion that results from natural storms. Poor
planning of placement can endanger the natural
habitat as well as the workers who use the roads.
To be successful the planning must take into
account several different factors and must be
able to be altered when the actual construction
begins. Currently many forest roads are
threatened by flooding, landslides, and soil
erosion making them unsafe and unstable to use
(Figure 1). Many studies have been conducted to
figure out the best placement of forest roads and
how to protect pre-existing ones that have
already been built in precarious locations. The
information gained from these studies can help
determine the natural factors forest roads face.
Oregon forests have been good testing grounds
because they weather severe storms and flooding.
With each passing incident scientists can take
away what worked and what did not and incorporate
those factors into planning for new roads.
Figure 5. Example of a debris flow (O.D.F., 2000).
Figure 3. A plot layout sample (Black and Luce,
1999).
Conclusion Both studies resulted in outcomes
that helped users and builders of forest roads.
The first study found that since there were many
road segments studied and only a few were high
risk for sediment production that it would more
cost effective to focus engineering upgrades to
the few roads that were sediment prone. The
second study found that valley floor roads
provide more effective sediment passages then the
other two levels of roads. Now developers can
focus on how to maintain and perfect the passages
on the valley floors since they held up the best.
Since the studies there has been more research on
road alignment and its effect on erosion. These
studies have been focusing on how a road connects
with different sources such as streams and how
erosion and sediments are transported as well as
the costs of building new roads. Each of these
studies is taken into account when Oregon
Department of Forestry looks into planning for
more forest road developments. During the process
of new road construction the real life
application comes through and the process is
altered with the new information.
Methods Used Continued A second study looked
more exclusively at road placement and used roads
that were at different elevations and hillslope
positions as well as road age and density factors
(Figure 4). The elevations zone coordinated with
different weather positions with the low
elevation from 400-800m, middle elevation
800-1200m and higher elevation gt1200m. Then the
basin was sectioned off into upper slope,
mid-slope and valley floor zones. They then
observed the February 1996 storm and recorded the
results (Wemple, 2001).
Figure 1. Photo of flooded road (Akay and others,
2007).
Erosion
Methods Used With such wide variety of natural
factors, scientists have to obtain a large amount
of information based on everything from soil
conditions, weather, plant life, logging plans,
location, and many others. To study erosion
scientists use sediment traps to measure sediment
production. The sediment traps were 1.5 cubic
meter plastic bins that were first weighed with
water and sediment and then water only. The
flow came into the bins using culverts (Figure 2)
which could often get clogged and caused
additional flooding and sediment production, and
then end up in sediment traps (Figure 3). One
study tested two different types of soil textures
and tested eleven different features that effect
forest road sediment production inherent
erodibility and runoff producing capacity of soil
and running surface road segment length road
gradient amount of cutslope and running surface
treatments of cutslope, running surface, and
fillslope flowpath geometry as embodied in
insloping and outsloping and degree of rut
development slope position and aspect as they
affect soil moisture forest cover time elapsed
since construction road use and weather (Luce,
1999).
References Cited Akay, Abdullah., Aruga, K.,
Chung, W., Miyata, E., and Sessions, J., January
2007, Incorporating Soil Surface Erosion
Prediction into Forest Road Alignment
Optimization Internet Web Resource, URL
Http//www.lib.unb.ca/Texts/JFE/
January2007/jfe18_1art03.pdf ( last Updated
January 2007). Black, T., and Luce, C., August
1999, Sediment Production for Forest Roads in
Western Oregon Water Resources Research, v. 35,
No. 8, p.2561-2570. Jones, J., Swanson, F., and
Wemple, B., 2001, Forest Roads and Geomorphic
Process Interactions, Cascade Range, Oregon
Earth Surface Processes and Landforms, v. 26, p.
191-204. Oregon Department of Forestry, July
2000, Forest Road Construction Internet Web
Resource, URL http/www.oregon.gov/ODF/STATE_FORE
STS/docs/ management/roads_manual/RMSec4-Const.pdf
(last Updated July 2007).
Figure 4. Location map of testing sites (Jones,
Swanson and Wemple, 2001)