4 | DISCUSSION
The rapid assessment procedure used in this study provided valuable
information on structural integrity and function with relatively little
effort in the field. All 137 structures within the 8-km study reach were
surveyed within a two-day period each year, typically by a team of 3-4
individuals. Although the rankings were qualitative, the ordinal nature
of data supported an analytical approach for evaluating structure
performance, including investigating differences between years and
structure types. Overall, instream structures met the performance
objective of 90% stable and functional at the three year mark following
completion of construction. However, structural integrity and function
diminished over time and the likelihood of poorer rankings increasing
following a 38-year flood that occurred in 2019. This suggests that the
lifespan of some structures may be dependent on the magnitude and timing
of flows that occur following completion of instream construction
(Roper, Konnoff, Heller, & Wieman, 1998). Engineering design, including
both the prescription and specifications for treatments, also played a
role in structural performance, as some structure types performed better
than others. Results indicate that some structure types should not be
used in similar geomorphic settings and others would benefit from a more
robust design approach.
The observed failure rate for instream structures (26%) was comparable
or lower when compared to similar studies, which reported a mean failure
rate of 40% across studies with a range of 15-75% (Babcock, 1982;
Frissell & Nawa, 1992; Schmetterling & Pierce, 1999; Miller & Kochel,
2013). Boulder-toe structures were the most likely to exhibit issues
with integrity and erosion. The study site is located in a wide alluvial
valley where the Arkansas River has historically migrated across the
floodplain. Prior to disturbance, the stream banks were dominated by
woody riparian vegetation with willows stretching across much of the
valley floor. Boulder toe was prescribed in some locations with the
intention of preventing erosion into fluvial tailings deposits. However,
this treatment did not perform well due to the dynamic nature of the
system, and typically failed when erosion occurred behind the boulders
that were intended to armor the stream bank. In similar systems with
meandering riffle-pool morphology and stream banks dominated by woody
riparian vegetation, we recommend that bioengineering techniques
(Giordanengo et al., 2016) be utilized to stabilize stream banks rather
than more hardened approaches, such as boulder toe. Log toe structures,
considered a bioengineering approach, performed better than boulder toe
with respect to integrity and erosion, but still exhibited failure rates
over 20% during the study period. However, logs will degrade with time
and, unlike boulders, will not be a permanent feature in the river as it
evolves. Furthermore, the addition of large wood has been shown to
improve stream habitat (White et al., 2011; Jones et al., 2014), while
increasing hyporheic exchange and the retention of sediment, nutrients,
and organic matter (Wohl et al., 2016).
High failure rates were also observed for log-vane structures. The
failure of log vanes was likely associated with inadequate engineering
design. The post hoc analysis of log-vane failures indicated that
the likelihood of failure increased with flatter log-vane slopes.
Flatter slopes were associated with a greater angle of departure from
the stream bank, which may have exposed more surface area of the log to
erosive forces in the channel. Boulders were used to anchor the end of
the log vanes in the channel and typically protruded above the elevation
of the streambed. These boulders were prone to movement and the number
and size of anchor boulders was not clearly specified in the plans.
Another common failure mechanism for log vanes was the undermining of
footer logs, which may have been prevented by installing erosion control
fabric on the upstream face of the header and footer logs. In addition,
the length of logs utilized for the log vanes was relatively short,
typically less than 5 m. Longer logs would have allowed the end of the
log vanes to be buried into the stream bed and then anchored with
appropriately sized boulders. The shorter logs entailed pinching the end
of the vanes between two or more boulders, jeopardizing the integrity of
the structures if boulders moved slightly. For future projects that
utilize logs vanes, we recommend that specifications err on the side of
longer logs and that the structures are engineered to withstand a
specific design flow (Rafferty, 2013) that corresponds with project
objectives and constraints.
Cobble toe, grade control in the main channel, and boulder clusters
exhibited the best performance with respect to integrity and erosion.
Cobble toe was typically utilized in locations where the river was
relatively straight and erosional forces were lower, particularly when
compared to the outside of meanders bends where scour and bank erosion
may be more likely. Grade control structures in the main channel were
typically placed in straight reaches as well, either to backwater an
upstream pool or provide additional stability in locations where the
channel had been realigned. Although some boulder clusters were placed
in pool habitats, the majority (>80%) were placed in
straight, riffle sections to increase channel complexity. The placement
of these structure types in straight, less erosive, reaches may have
contributed to the lower integrity and erosion rankings. At high flows,
pools may scour with the coarser material mobilized from the pools being
deposited over the riffles that form in the straight sections between
meander bends (Lisle, 1979). The sediment transport processes in
meandering rivers with riffle-pool morphology make pools more prone to
erosion and riffles more prone to deposition, which may explain some of
the observations from this study. For instance, log vanes that were
placed in riffles were more likely to receive higher rankings for
deposition than log vanes that were placed in pools. Deposition was also
more likely in years with lower magnitude floods (i.e., 2017, 2018, and
2020), suggesting that depositional processes may have been more
prevalent in these years, while scour and erosion would be more
prevalent in years with larger magnitude floods (Backcock, 1982;
Frissell & Nawa, 1992). Issues with erosion resulted in more problems
with structural integrity, whereas deposition may have affected the
function of structures but did not typically lead to failures. Sediment
deposition may also have adverse impacts on fish and benthic
macroinvertebrates (Henley, Patterson, Neves, & Lemly, 2000).
The change in RPD also varied between locations that were classified as
riffle or pools prior to instream construction. Pools that were
developed in riffle locations resulted in a greater initial change in
RPD when compared to preexisting pools because the preexisting RPD was 0
m for all riffles. In addition, riffle locations are typically shallower
than pools (Lisle, 1979), so it is not surprising that pool development
in riffle locations resulted in larger RPD changes than the enhancement
of existing pools. Furthermore, it is not surprising that the pools
developed in riffle locations filled in to some degree following the
first runoff cycle, whereas existing pools exhibiting less change in RPD
between the as-built condition and first runoff cycle. The main effect
of structure type did not have a significant effect on the change in RPD
for either riffle or pools habitats, suggesting there were no
differences among structure types regardless of the preexisting
morphology. Years post construction was a significant effect for both
the riffle and pool models, but this effect differed between
morphologies. For riffles, the change in RPD associated with the
as-built condition was greater than all other years, indicating that
construction activities increased RPD and initially improved overwinter
habitat for trout. However, pools that were developed in preexisting
riffle locations exhibited a significant decline in RPD following the
first runoff cycle. Despite this change after the first runoff cycle,
the cumulative change in RPD within preexisting riffles was positive
over the study period, increasing by 0.26 m. These results suggest that
over excavating pools can still result in improved RPD even if those
pools fill in to some degree.
The change in RPD at preexisting pools differed from the observations at
riffle locations, with a smaller net increase of 0.13 m over the study
period. Although years post construction was a significant main effect,
the interpretation of that effect was confounded by the significant
interaction between years post construction and structure type. Three
structure types or combinations exhibited significant difference for the
as-built condition at preexisting pool locations. The changes in RPD at
log toe structures and log vane/log toe combination were greater than
the changes at log vanes. Log vanes were either placed at the head of a
pool, within riffle habitats to create pocket water and increase habitat
complexity, or adjacent to fluvial tailings deposits to decrease near
bank shear stress and reduce erosion risk. When log vanes were
constructed in riffle locations, a relatively small pool was excavated
downstream of the vane. These pools were typically shallower than the
pools at meander bends where log toe and log toe/log vane combination
treatments were often utilized, which may explain the difference in
response between these structure types for the as-built condition. The
boulder vane/boulder toe combination also differed from other structure
types in year 1, exhibiting greater pool scour when compared six other
structure types or combinations. The combination of a boulder vane and
boulder toe treatment likely concentrated energy over the end of the
vane while also armoring the stream bank, directing energy to the stream
bed and resulting in greater scour. The hardened approach associated
with the boulder vane/boulder toe combination resulted in a different
RPD response, where scour appeared to be greater than other structure
types in years 1 and 2, while deposition was greater in year 3.
Although the approach utilized in this study provided useful information
regarding project objectives associated with the integrity and function
of instream structures and their effect on pool development, the study
was not without limitations. The rapid assessment procedure could be
developed further by incorporated a qualitative habitat evaluation,
which would provide valuable information for project managers when
considering the need for maintenance at specific structures. As the
project design was developed independently from the monitoring design,
there was imbalance in sample sizes for some structure types, which may
have affected the results from statistical analysis. The logistics of
building such a large stream restoration project also entailed
implementing the project over two construction seasons, which precluded
the inclusion of discharge as a covariate and led us to utilize years
post construction as a factor instead. The magnitude and duration of
annual flood events surely affected the amount of sediment transport
that occurred, which in turn affected the severity of erosion,
deposition, and pool scour that occurred. Furthermore, the qualitative
nature of the rapid assessment rankings introduced some subjectivity to
the evaluation, but that was mitigated to some degree by having the same
observer present for all surveys. Despite these limitations, the study
indicated that structures met the performance objective for integrity
and function and there was a positive increase in RPD and associated
overwinter habitat. Our results suggest that the lifespan of some
instream structure may be limited to a decade or less, as structural
integrity and function declined over time. However, the longevity of
structures may not be the most important consideration if they improved
channel resilience by supporting the establishment and growth of woody
riparian vegetation, which occurred within a few years of project
completion (Cubley et al., 2022). Although the inference from this case
study is somewhat limited, we hope that our results can inform any
future maintenance needs for this project as well as structure selection
and design for future stream restoration projects located in similar
geomorphic settings.