Discussion
Critical Design Components for Effective Fish Passage
Critical design components for effective fish passage include channel
gradient, keystone characteristics, and weir geometry. In nature-like
fishways features are used to ensure three-dimensional circulation
(downstream, across the stream, and within the water column). This is
achieved by their asymmetrical position using various keystone shapes
and sizes. The asymmetrical nature of the rock weirs assessed introduces
a secondary gradient and sinuosity through the cross-section, and
maintains channel stability under different water level conditions
(Figure 2). An asymmetrical rock weir design enhances fish passage
effectiveness by activating gap, and over-weir flow pathways – either
independently or simultaneously – depending on the water level
condition (Figure 3). One of the main objectives of rock weirs is to
reduce overall channel gradient in restored systems (Williams et al.,
2012). In creating smaller drops to reduce channel slope, a secondary
gradient is introduced at each rock weir structure, which is an
important design consideration for fish passage. Where gradient is
gentle, keystones appear to protrude from the channel bed, and where
gradient is steeper, keystones appear more embedded within the channel
bed (Figure 4). Furthermore, protruding keystones obstruct connectivity
between upstream and downstream flows. For example, at a low gradient
rock weir (Figure 4), the vertical distance between water level and weir
crest is large enough to reduce connectivity between upstream and
downstream flows.
The three rock weirs that allow 100% fish passability under all water
level conditions were associated with the highest rock weir gradient
levels through the reach (-0.086 – -0.144; Figure 3). A higher rock
weir gradient decreases the vertical distance between upstream water
level and weir crest, enhances keystone embeddedness, provides adequate
gap and over-weir flow pathways for movement, and ultimately maintains
longitudinal connectivity through the restored system. Although steep
slopes are considered a flaw in conventional fishway designs (Katopodis
and Williams, 2012; Williams et al., 2012), nature-like fishways provide
a naturally gradual slope through the reach (Roscoe and Hinch, 2010),
and as such, higher secondary gradients at the rock weirs are
acceptable. Additionally, the gradient of each rock weir structure
within a restored reach is easily identifiable and is appropriate for
use as a method to preliminarily monitor fish passability without
conducting extensive in-field data collection.
Keystone Characteristics
All rock weirs that provided 100% fish passability have a weir crest
width greater than 3.0 m (Table 3). Further, all rock weirs that allow
100% fish passability have a greater than average number of keystones,
and range of keystone sizes (Table 3). The 100% fish passage
effectiveness is likely attributed to the number of flow pathways that
are available given a greater number of keystones, as well as a range of
keystone sizes, to fill the cross-section. Rock weir throat width is a
parameter used to inform failure rates for in-stream structures, with
results suggesting that the larger the rock weir throat width, the less
common failure is (Varyu et al., 2009). However, studies evaluating the
relationship between rock weir throat width (or crest width for modified
rock weir designs) and fish passage are not available. Further research
is required to identify a robust relationship between rock weir width
and fish passage effectiveness. It is important to note that rock weir
keystones are selected based on hydraulic sizing to withstand high flow
events through a reach (Thomas et al., 2000). Additionally, a factor of
safety is applied to ensure conservative keystone sizing. Large
keystones are used for channel stability, which reduces the required
number of keystones in a rock weir structure, and further reduces
potential pathways for fish passage. Based on this research, and to
address the conflicting goals between channel stability and fish passage
in river restoration, it is recommended that keystones be sized to
enhance gap flow pathways for fish passage, but not undermine channel
stability, particularly during high flow events. This design
consideration can be incorporated to ensure resilience, fish passability
for the local fish community, and complete qualitative monitoring of
fish passage effectiveness following construction.
Embeddedness
As previously mentioned, the rock weirs in Weslie Creek are asymmetrical
(Figure 2) which provides a secondary gradient for fish passage to
enhance longitudinal connectivity depending on water level. However, the
rock weir that restricts gap and over-weir flow under low water level
conditions (VRW2) (Figure 3) has a low gradient, and is less embedded
near the weir crest than other rock weir structures (Table 1). As such,
the keystones protrude further out of the channel bed, reducing
longitudinal connectivity (Figure 4). This effectively decreases the
opportunity for upstream water to pass through gap or over-weir flow
pathways. Rather, the water moves as orifice flow and enters the
downstream pool through available gaps beneath the keystones. According
to Keller et al. (2012) many rock weir designs require ‘drowned
conditions’ to facilitate fish passage. This idea is consistent with
necessary conditions to enhance fish passage effectiveness at VRW2,
where opportunities for gap and over-weir flow pathways are only
available under intermediate and high-water level conditions (‘drowned
conditions’) (Figure 5). Depending on water level characteristics at the
rock weir system, design considerations should be applied to construct
rock weirs with a high degree of embeddedness, or opportunities for
‘drowned conditions’ to enhance fish passage. For example, Weslie Creek
experiences low water level conditions for the majority of the field
season and as such, rock weirs should be constructed with a high degree
of embeddedness for maintaining longitudinal connectivity. However, in
systems characterized by high water level conditions, constructing rock
weirs that can experience ‘drowned conditions’ is likely more important
than embeddedness. As such, if ‘drowned conditions’ are more common in
these systems, longitudinal connectivity and fish passage effectiveness
would increase through the reach. It terms of embeddedness, it is
important that the material size used for constructing rock weirs is
large enough to maintain structure stability and resilience to failure,
but small enough to remain embedded. These dimensions are based on
site-specific characteristics, as well as requirements for the target
fish community, to ensure passage is possible at critical times (i.e.,
spawning periods).