Treatment of rock armoring feedback:

1. Ideally surface-flow should be maintained with nature-like meanders.
2. There is some evidence that small fish can make their way up through the gaps between the rocks but it is not ideal.
3. Finer material may need to be added if it is unlikely to be transported and deposited by natural processes.
4. Appropriate geo-textiles may need to be used. These may need to be impermeable.
5. Adding of finer material will need to be managed to ensure large rocks still interlock.
6. Armoring may need to extend further as velocities may be higher when all water is at the surface and the suface is less rough.
7. Provision should be made to include vegetation within and overhanging.
8. Precast channels can be designed to meander.
9. If precast channels are used then the joints would need to be flexible and water-tight similar to the floating ramps.
10. Precast channels would need to be anchored to ensure they do not tip and spill water over the sides.

This useful link to a very comprehensive Urban Storm Drainage Criteria Manual was shared by Iain Smith
https://udfcd.org/wpcontent/uploads/uploads/vol1%20criteria%20manual/08_Open%20Channels.pdf

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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

1. Excavation as part of structure installation
2. Suitable geo-textile laid
3. Precast channel fitted to apron at outlet. Also baffles fitted on apron
4. Rock rip-rap placed either side of ramp
5. Channel covered with rock rip-rap

Points to consider:

– Channel design
– Installation
– Cost
– Durability
– Effectiveness (short and long-term)
– Planting

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