wildlife passage for fish

Image Credit: "Lake Champlain Fish and Wildlife Resource Office-Great Brook" by US Fish and Wildlife Service - Recovery Act Team is licensed under CC BY 2.0.

Written by Armando Ponce de León

July 19, 2023

Rivers are incredibly complex systems in all their aspects. It is for this reason that in order to understand them in the best way, different sciences must intervene that, in principle, seem to be very distant from each other. However, it is the very complexity of fluvial systems that makes it necessary to look for that point of union between disciplines such as ecology, physics and geomorphology, in this way it is possible to imagine that aspects such as the analysis of the forces that intervene in a singular particle of sand, the evolution of a channel through time and the interactions that occur between the organisms that inhabit the ecosystem, will have to work together, complementing each other, if what we want is to have a comprehensive vision of what happens within these systems in order to really learn to manage them in an intelligent and respectful way.

Starting from this complexity, and seeing that the alterations that a fluvial system can suffer (such as the construction of drainages under roads) give rise to conditions that the river did not present before, we can infer that the impacts derived from these alterations will be diverse. To exemplify this, we can imagine a case in which a drainage of smaller dimension than the bed of a river is built, which is inhabited by salmon. In addition, the construction of a drain can mean a significant channelization of the channel, as well as the loss of the bottom substrate. Salmon depend on a certain configuration of variables to spawn, which have to do, among other factors, with the stability of the bottom of the channel and the diversity of forms such as the presence of meanders or rapids. But with the construction of the drainage, the conditions inside it will be different, firstly, we will be able to notice an increase in the speed of the water, due to the lower dimension of the drainage compared to that of the channel, which will make it difficult for the fish to migrate upstream. This will also cause the bottom of the channel to be unstable, reducing the survival of the eggs. In addition, channelization and the lack of bottom substrate represent a loss in the diversity of potential habitats for salmon to spawn.

With the study of these impacts, various investigations have been carried out with the aim of generating design principles for highway drainage that not only take into account the water flow, as is the case with conventional hydraulic design, but also three other fundamental aspects: passage of organisms, sediments and organic matter.

Among these principles are the following:

Design the slope of the drainage to match that of the river:

Well, abrupt changes in the slope can start processes of erosion, sediment deposition causing changes in the morphology of the river and affecting the species, or directly a steep slope can mean a barrier for the passage of organisms.

That the drainage is as aligned as possible with the orientation of the river.

For this it is necessary to consider how the flow of the river will move in the future

Design the drainage opening for a full channel ( bankfull ) or a little larger:

The full channel flow ( banfull flow ) is the rate at which water fills the channel before it begins to flow towards the floodplain in an equilibrium river.

This will allow the natural movement of sediments and remains of organic matter such as branches and leaves.

Provide a depth similar to that of the channel:

Very shallow depths can also represent a barrier to the passage of fish. This point is related to the width of the drainage because, if the structure is wider than what is needed, it is likely that aggradation (sediment deposit) will occur, creating insufficient heights in low flows. To achieve this, an alternative in multiple drains is to embed one of them below the bottom of the river, so as not to have problems with low water flows. In addition, it is important to mention that the minimum height will depend on the species, the life stage of the organisms and the season of the year.

Provide a continuous sediment bed with a roughness similar to that of the channel

A similar roughness will help to reduce the velocities generated in the drainage, which in turn helps smaller organisms to more easily navigate through the structure. In addition, this also promotes a continuity in the movement of sediments, helping to avoid excessive processes of erosion or aggradation.

An important aspect to take into account at this point is that the size distribution (granulometry) must be similar to that found at the bottom of the channel, so that the energy losses due to turbulence are similar both inside and outside the channel. sewer system.

Maintain continuity of sediment and organic matter transport (branches and trunks)

Favoring the continuity of sediment transport is something that is being built with all the previous points, avoiding extreme processes of erosion or aggradation and thus facilitating the connectivity of the processes that occur in the ecosystem, both physical and biological.

But in addition, understanding how and when sediment will move in a certain river will help to avoid creating disconnections in sediment transport that could create drainage barriers. Depending on the shear force exerted by the water on the bottom particles in a given avenue, these will move or remain at rest depending on the size of each one.

In the case of organic matter such as branches and trunks, due to the fact that this fleet, it will be necessary to implement a free edge above the channel, so that it can cross freely

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