Further to our recent mailout where we discussed mussel rope at perched/overhanging structures, it’s now time to look at mussel-rope when placed within culverts.
Mussel-rope is one of the tools that can be used to help improve fish passage where fast, laminar flow is present.
If it is not viable to remove an existing culvert or is not practical to fit baffles due to the small diameter (e.g. under 750mm or 2 &1/2 feet), then mussel-rope can be considered as a remediation tool.
Obviously, the rope does not retain bed material, create rest-pools or the complexity that baffles do. Therefore rope should only be used as a last resort, and it should not be used at new culvert installs.
Below are some CFD images that show the effects the mussel-rope has on the depth and velocity in culverts.
Many culverts are barriers to fish due to being perched/overhanging or having fast laminar flow.
In the video below, Tim demonstrates how mussel-rope can improve fish passage at perched/overhanging structures.
The rope simulates natural tree roots and fibres that some species have evolved to utilise when migrating upstream.
The type of rope being used here is “looped mussel-rope”.
Looped rope is proving to be an effective remediation tool at perched/overhung structures. It also has the benefit of not shedding fibres as readily as other types of ropes.
The Fish Passage Action Team have been working on a simple one-page guide to help with the remediation of common structures causing a barrier to fish.
The guide includes a link to an interactive decision tool and some helpful before & after examples.
Happy holidays!
Fish Passage Remediation (existing structures)
Overview
Fish passage remediation is less about target species, specific swimming speeds, average velocities, Mannings numbers etc., and more about understanding the medium in which fish have evolved to navigate, i.e. water, with the dynamics of complex flows and boundary layers all at a fine resolution.
Basic principles – “Match or better” the streamflow characteristics.
– Provide complex flows (non-laminar) – multi-directional flows with rest pools. – Length of fast flow zones between rest pools must be shorter than fish burst-swim capabilities. – Provide sufficient depth. – Ensure continuity – no overhangs. – Give fish a range of navigation path options. – Ideally, retain bed material.
General rules.
– Rope should not be used in culverts if baffles can be fitted, even if climbing species are targeted. – Ramps should include splash zones and substrate, e.g. rope. – Ramps should extend into the plunge pool as far as practicable. – Baffles should be spaced so that the rest pool from one extends easily up to the next. – Baffles may be considered artificial bed material in some situations, i.e. create similar hydrological conditions – If rope is fitted through a culvert that is also perched, a second set of rope should be fitted at the outlet. – Rope should only be attached at the upstream end. – Interventions should not significantly reduce culvert capacity or increase the risk of debris jams. – Tide-gates should be designed/modified to delay the closing as long as possible. Which tools to use where?
The above principles help to guide us towards selecting appropriate remediation tools. For more detailed guidance on the use of remediation tools click here:
Measures of success: Success should be outcome-focused and easy to measure or observe. – Complex flows similar to stream (depth, width, velocities) – Bed material retained. – Continuity – no perch or overhang. – Extended opening period for tide gates.
Please note that the linked resources above are kindly provided by ATS Environmental.
Many of you will be familiar with the use of mussel-rope to help improve fish passage over or through structures including culverts.
The rope helps to reduce the water velocity adjacent to the rope and also gives a tactile surface for small fish to wriggle through or over.
Rope is a good tool when there is no other option, however it does not create resting pools, add depth, or retain bed-material.
While we have installed many 1000’s of meters of mussel-rope, it is important to understand that there are limitations as to where mussel-rope can be used and where it is not suitable.
Mussel ropes can be used to enhance fish passage on existing structures where: –
Culverts are perched and fitting ramps is not practical.
It is not practical to install baffles e.g. culvert diameters <800mm
Other notes:
Based on our extensive experience, it is best not to attach the downstream end of the rope as this has been known to cause blockages when logs, debris etc gets caught under the rope.
Fixings should be roust e.g. stainless steel D-ring and clasp – NOT a waratah.
“Swimming lanes” are rarely achievable when installing ropes through smaller diameter pipes because these pipes typically have very low flow. It is even more challenging if the pipe is also long.
There is no evidence that “swimming lanes” are more effective than a number of ropes laying close beside each other.
When aiming to get fish up a perched or overhanging structure, it is best to first attach a strip of rubber in order to create a wetted margin. See Tim’s video link below.
Looped-rope is less likely to shed fibers than Super-Christmas-tree rope and there is no evidence of it being more likely to cause a blockage.
The cut ends of the ropes should be melted to prevent fraying.
If rope is used through a culver that is also overhanging, there should be twos sets – one set attached at the upstream end and finishing at the outlet, with a second set attached at the outlet hanging down into the plunge-pool.
In summary:
Mussel-ropes should only be used as a last resort when remediating existingstructures where ramps, baffles etc are not practical.
We are always willing to listen, help. share and learn, so please feel free to contact us however and whenever..
Tim Olley captured this rare footage of a juvenile redfin bully (Gobiomorphus huttoni) climbing up a retro-fitted mussel-rope with backing, into an overhanging culvert.
It reminds us that, given half a chance, many fish will navigate challenging obstacles.
What is not well understood, is what precise factors cause a fish to transition from a swimming to a climbing mode of locomotion.
It is now common practice to use large, similar sized, rip-rap (boulders) to manage downstream erosion at structures.
Above is typical example of a recent culvert upgrade where extensive rock armouring has caused water to be sub-surface and spread over a wide area during most flows, making upstream migration almost impossible for most fish.
The rubber strip and rope were added afterwards to try to help with fish passage.
We need your help to share ideas on how to better provide fish passage while still meeting erosion control requirements.
– Discussion Paper-
The challenge – maintaining fish passage through rock armouring, especially during low lows. Below is a concept that may provide passage for small fish, while still achieving erosion control. A series of precast channels maintain a flow path that may be completely covered with large rocks, providing gaps remain between individual rocks.
Installation process
Excavation as part of structure installation
Suitable geo-textile laid
Precast channel fitted to apron at outlet. Also baffles fitted on apron
