Conclusion
As the use of rock weirs as a component of natural channel design in
small-scale watercourses (i.e., < 2 – 10 m channel width, low
gradient) becomes more common, it is necessary that appropriate
monitoring techniques are established to evaluate fish passage
effectiveness. Additionally, where rock weirs are constructed,
determining the critical design and structural components, including
placement, materials and geometry, that enhance fish passage
effectiveness is important for informing future designs. The modified
rock weir system at Weslie Creek was assessed for fish passage
effectiveness based on gap and over-weir flow under different water
level conditions. Fish passability was most effective (100% fish
passability through 9/10 rock weirs) under low water level conditions,
which was representative of the system during the majority of the field
season. Further, fish passability was least effective under intermediate
and high-water level conditions, likely due to the asymmetrical nature
of the rock weir structures and the low ratio of over-weir flow pathways
to gap flow pathways with more water in the system. These conditions
typically lasted for approximately three days, and observations from
adjacent pools suggest adequate refuge habitat while passage through the
weirs were limited.
The structural components of the rock weirs were compared to fish
passage effectiveness and critical design components were identified
based on gradient, keystone characteristics (i.e., number and size),
distance between rock weirs, embeddedness, and orifices. Critical design
components for effective fish passage include:
- Steep secondary gradients at rock weirs to enhance longitudinal
connectivity and active orifice, gap, and over-weir flow pathways
- A greater number of keystones and larger range of keystone diameters
at each rock weir to provide opportunities for energy dissipation and
various pathways (in terms of shape and size) for fish passage
- Greater keystone embeddedness to enhancing longitudinal connectivity
under all water level conditions
- A greater distance between rock weirs (greater pool length) to provide
opportunities for recirculation zones to form, and provide locations
for fish refuge
Design considerations for effective fish passage should not undermine
the conditions required to provide channel stability through the reach.
These findings should be applied in future river restoration works,
where natural channel design is used to address fish passage and
stability. Incorporating critical design components for rock weirs will
enhance fish passage efficiency, provide bed and bank stability, and
limit the requirements for post-construction monitoring.