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).