Figure 5. Looking upstream at VRW2 (a low gradient rock weir) under low
(top photo), intermediate (middle photo), and high (bottom photo) water
level conditions. It is evident that orifice flow is the only active
flow regime under low water level conditions, while orifice, gap, and
over-weir flow are active simultaneously under intermediate and
high-water level conditions. VRW2 under low water level conditions
demonstrates the importance of embeddedness for enhancing fish passage
effectiveness, while VRW2 under high water level conditions demonstrates
the effect of ‘drowned conditions’.
Orifices
Orifices are challenging to survey in the field post-construction, and
were assumed to be minor with regards to facilitating fish passage in
the present study. In various river restoration projects, impermeable
geotextile layers are installed at the stream bed and upstream of the
weir crest to prevent orifice flow. However, this was not the case in
Weslie Creek, where geotextile layers were not used in the rock weir
design. Rather, smaller keystones and river stones were used to control
flow. Although the design concepts for Weslie Creek indicate that
keystones and footer stones are compacted with a mixture of 90 mm – 225
mm river stone, orifice flow is present based on field observations
(i.e., VRW2). It is possible that the river stone placed in the orifices
were transported downstream during large rain events, and consequently
provided opportunities for orifice flow. Based on observations in the
field, it is likely that orifices throughout the Weslie Creek rock weir
system are < 0.05 m in width and depth. Literature recognizes
rock weir designs that are purposely constructed with orifices as the
preferred pathways for fish passage (e.g., Ead et al., 2004). Further,
orifice design may enhance fish passage effectiveness via associated
turbulent structures (Silva et al., 2012), or by providing space for
energy dissipation through the structure.
Under low water level conditions, gap and over-weir flow may not be
activated over or through rock weirs. During these times, orifice flow
provides the only pathway for local fish species to travel upstream or
downstream, depending on their life stage and behavioural
characteristics. According to Kupferschmidt and Zhu (2017), velocity
through orifices must be less than the maximum burst speed of the local
fish species. This is also true for gap and over-weir flow. The
difficulty associated with evaluating the effectiveness of orifices for
fish passage, particularly during in-field analysis, is the
inaccessibility for sampling equipment. Although orifice flow is
considered non-negligible in Weslie Creek, sampling the required
geometries and flow beneath keystones was not possible. Silva et al.
(2012) analyzed fish passage effectiveness through orifices, however the
research was conducted in a flume setting with the appropriate equipment
for measuring velocity in such confined spaces.
Distance Between Rock
Weirs
The characteristics of pool features surrounding rock weirs upstream and
downstream are an important consideration for fish passability.
According to Martens and Connolly (2010), suitable pool features provide
refuge opportunities, habitat for rearing, and leaping pools for local
fish species. The distance between rock weirs (i.e., the length of the
pool feature) also influences flow through/over the downstream rock weir
by dissipating energy and maximizing flow resistance (Wang et al.,
2009). Pool features at Weslie Creek that were less than 4.0 m in length
were more likely to produce flows that exceed local fish species’ burst
swim speeds (m/s) and therefore reduce fish passability (Table 2). In
contrast, the pool features that were greater than 4.0 m in length
provide 100% fish passability under all water level conditions (Table
2). This is supported by literature that suggests pool length is the
primary geometric dimension that influences flow through both
conventional and nature-like fishways (Wang et al., 2009; Bermudez et
al., 2010).
It was determined that as pool length increases, the total number of
opportunities for fish habitat and/or refuge also increases, with few
outlying instances. In terms of fish habitat and/or refuge, it is
important to note that although recirculation zones were not identified
in all pools under all water level conditions (Figure 3), the measured
velocities were below fish species’ critical swim speeds. To recognize
all possible locations for fish habitat and/or refuge in the Weslie
Creek reach, further analysis is required to identify the sustained swim
speeds for local fish species. For example, low cross-sectional velocity
values (i.e., 0.02 m/s) were measured at pool features in Weslie Creek
and most likely facilitate fish habitat and refuge, however the
sustained swim speeds appropriate for local fish species are unknown. As
such, only locations with stagnant or recirculation zones were used as
indicators for fish habitat and/or refuge in Weslie Creek. Since
sustained swim speeds are less than burst swim speeds, it is likely that
such low cross-sectional velocity values do support local fish habitat
and/or refuge conditions (Beamish, 1978).
In terms of pool length, there are conflicting goals between channel
stability and fish passage (Thomas et al., 2000). With a greater pool
length, the distance between rock weirs increases, and creates a greater
drop height between rock weirs. Fewer rock weirs throughout the reach is
problematic for channel stability due to a larger gradient.
Additionally, fewer rock weirs throughout the reach is problematic for
fish passage due to the greater drop heights local fish species are
required to maneuver. It is recommended that pool lengths (the distance
between rock weirs) be large enough to provide suitable conditions for
passage, habitat, and refuge, but not undermine channel stability. The
conclusion from this analysis should be applied in future natural
channel design projects to ensure pool features are measured to an
appropriate length to provide maximum opportunities for 100% fish
passability and channel stability.
Evaluating Effective Rock Weir Design and
Construction
According to Lucas and Baras (2008), river restoration efforts (such as
fishways) should provide 90% overall passage efficiency for diadromous
and potamodromous fish species to be considered functional. Fish
passability through a reach is a function of three components:
appropriate water depth, velocity, and gradient for leaping (Williams et
al., 2012). It is important that such components are suitable for the
target fish species within the system, both at the rock weir structures
and pool features (Williams et al., 2012). The results of this research
suggest that fish passability through the reach is most effective and
longitudinal connectivity is most complete under low flow conditions
(Figure 3). This is likely attributed to the number of active gaps
available for fish passage. Although not all gaps facilitated the
appropriate velocity for local fish passage, 9/10 rock weirs have at
least one suitable pathway. With 90% overall passage efficiency based
on local velocity measurements, the rock weirs at Weslie Creek are
considered functional under low water level conditions. Excessive
velocities through flow pathways that inhibit fish passage upstream is
recognized as the most likely cause of passage failure and
non-functionality through rock weir systems (Knaepkens et al., 2006).
The mark-recapture of non-salmonid species through a pool-weir system in
Belgium yielded 0%, 8%, and 29% fish passage effectiveness for
bullhead (Cottus gobio ), perch (Perca fluviatilis ), and
common roach (Rutilus rutilus ), respectively. These fish species
are larger and have stronger swimming capabilities than species local to
Weslie Creek. This demonstrates that despite the size of the fish
species and their swimming and/or leaping capabilities, where velocities
exceed burst swim speeds, the rock weir system is not functional.
The general consensus concerning rock weirs and fish passage is that
there is a lack of standardized monitoring protocols for evaluating
effectiveness or fish passability (Silva et al., 2018). Further, fish
passage analyses are common in the literature, however their measure of
effectiveness differs. The structural differences between rock weirs and
other nature-like fishways also contributes to challenges for comparing
fish passage results. For example, PIT-tagging is common for fish
passage monitoring in larger species (e.g., Tummers at al 2016; Martens
and Connolly, 2010). However, in small-bodied fish, such as those in
Weslie Creek, a different approach is needed. Rather than observing
where the fish go, the hydrodynamics and geometries of the system were
used to evaluate fish passage feasibility given physiological abilities
of the expected local fish populations. In addition to the different
methods and the different measures of fish passage effectiveness, it is
likely that rock weir structure and design differences contribute to
differences in fish passability results. Target fish species with
different burst swim speeds will require different conditions (i.e.,
rock weir design and water level) to facilitate fish passage. As such, a
relationship may exist between the size/swimming characteristics of fish
species and appropriate conditions for fish passage effectiveness.
Large-bodied fish species require greater water depth and can employ
stronger burst swim speeds to maneuver rock weirs, while small-bodied
fish species require smaller water depth and require lower velocities
through rock weirs for passage. Such relationships must be considered in
rock weir designs for effective fish passage for the target fish
community.