2. Introduction
Three culverts put in place by a landowner near Tangent, Oregon have been deemed
impassable to adult and juvenile Chinook Salmon, Rainbow Trout, and Pacific Lamprey by the
local watershed council. Fish passage criteria are established by Oregon Administrative Rules
(OAR). In addition to OAR regulations, the ultimate design of a crossing structure that reduces
the amount of maintenance required by the land owner is desired.
The OAR (OAR 635-412-0035) has several criteria that must be met by all culvert designs
in the state of Oregon. First, the maximum difference between upstream and downstream
water elevation should be 6 inches maximum. During the time period when only salmon or
steelhead adults require passage, the max drop can be up to 12 inches. Second, the water
depth through the culvert should be at least 12 inches when adults require passage. A
minimum depth of only 6 inches is required for juvenile passage. A jump pool must be provided
for fish, and needs to have a minimum depth of 2 feet. The velocity range allowable is between
1 and 2 feet per second and the slope of the culverts must equal the surrounding long-channel
streambed profile. Lastly, the culvert width needs to be equal to or greater than the active
channel width.
The conditions of fish passage, based on velocities are given in a swim speed table
provided by Corvallis Forestry Research Community (Table 1).
Table 1:
Species
Max Swim
Speed
Rainbow
Trout
822 cm/s
Chinook
Salmon
670.5 cm/s
Pacific
Lamprey
109.7 cm/s
Thirteen cross sections of the river were measured along 839 feet of the river. Surface
elevation values were also measured at each station. Culvert data that included the culvert
dimensions, elevations, and location were collected. Lastly, roadbed characteristics were
measured. This data was then input into a program called HEC-RAS. The Hydrologic
Engineering (HEC) in Davis, California developed the River Analysis System (RAS) in order to
analyze channel flow and floodplain determination. HEC-RAS is a computer program that
models natural rivers and other channels and the hydraulics of water flow through these
natural passages. The collected data was calibrated within HEC-RAS. This was done by
3. manipulating coefficients within the program. The model was then run for the 95th and 5th
percentile flows of 27cfs and 3950 cfs provided by measured flows.
Current Conditions
After the model was calibrated for the river section with the existing culverts in place,
the current conditions were compared to the OAR regulations for culvert design. The model
was run at the 95th and 5th percentile flows of 27cfs and 3950 cfs (Table 2).
The max drop was found by finding the difference in water elevation heights before and
after the culverts and was reported to be nearly 13 inches for the 95th percentile flow and 24
inches for the 5th percentile flow. This lies outside of the 6 inch drop for juvenile fish and 12
inch drop for adults. The velocity range also fell outside of OAR regulations in both flows with a
top velocity of 2.92 ft/s and 16.33 ft/s respectively. Velocities in the 95th percentile flow
exceeded the top velocity for survival of Pacific Lamprey at 3.6 ft/s. Culvert slope also proved to
be an issue with the culvert slope measured to be around 0.02 compared to the surrounding
stream-bed profile slope of 0.038. The width of the culverts also caused issues with regulation
as they were not equal to or greater than the pre-existing channel bed. Minimum depth
requirements were met for both low and high flow percentiles, however it is noteworthy to
mention that the high flow would overtop the current roadbed. Reference Figure 1 and Figure
2 for cross sections of the culverts at the two flows.
Table 2
Max
Drop
(in)
Min
Depth
(in)
Jump
Pool
Dept
h (ft)
Velocity
Range
(ft/s) Culvert Slope Culvert Width
OAR
Criteria
6 or
12
6 or
12 2 1 to 2
equal/greater than surrounding
stream-bed profile (0.038)
equal/greater
than active
channel width
95th
Percentile
Flow (27
cfs) 12.48
69.48,
22.44,
33.48 7.53 2.92 0.0172, 0.0023, -0.0052
Criteria Not
Met
5th
Percentile
Flow (3950
cfs) 24
10, 6,
7
195.9
6 16.33 0.0172, 0.0023, -0.0052
Criteria Not
Met
4. Proposed Conditions
The active channel width was measured at 18.3 meters across. According to the
attached budget price sheets, a culvert with this diameter does not exist. The culverts must be
removed to attempt to restore the river. After the culvert and roadbed were removed from the
model, the same two flows were run. The velocity for both low and high flows exceeded
velocities for Pacific Lamprey and OAR regulations. The max velocities for low and high flows
were 4.81 ft/s and 9.34 ft/s respectively. Max drop, minimum depth, and jump pool depth,
however, met OAR criteria. The culvert slope and width are no longer applicable in this
instance since they have been completely removed. Reference Figure 3 and 4 for cross sections
of the river where the culverts had been removed.
Conclusion
It is imperative that the culverts be removed from the river in order to improve fish
passage. The current conditions fail to meet OAR 635-412-0035 requirements in terms of max
drop, velocity range, culvert slope, and culvert width. Velocities after the culverts are removed
still do not comply with OAR regulations and are too fast for Pacific Lamprey to swim upstream.
Further restoration would have to be implemented to slow river velocities. Since a culvert the
same width as the active river channel does not exist or is not economically feasible, one must
replace the culverts with an arch or a bridge to provide the landowner with a crossing structure
that complies with OAR. There are some uncertainties within the results. Swim speed data for
fish has a high variability depending on age and other fish health factors. The 5th and 95th
percentile flows do not account for all flows that can occur within this stretch of the river.
Finally, the realistic capabilities of meeting OAR velocity regulations should be analyzed. The
required velocities are low compared to a lot of river conditions.
5. Figure 1 – Low Flow, Culverts in Place
Figure 2 – High Flow, Culverts in Place
