Case study:Fish pass on the Najerilla River

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Location: 42° 27' 54.52" N, 2° 42' 21.08" W

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Project overview

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Status	Complete
Project web site
Themes	Habitat and biodiversity
Country	Spain
Main contact forename	Rincón Sanz
Main contact surname	Gonzalo
Main contact user ID
Contact organisation
Contact organisation web site
Partner organisations
Parent multi-site project
This is a parent project encompassing the following projects	No

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Project summary

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The concerning weir was small and it served to supply irrigation communities in the area. In origin, the weir was of an approximate height of 40 cm. However, over time, an erosion was generated downstream the obstacle so its height increased to 1.5 meters and the weir became impassable. This was a problem since the community of fish in the river is very varied and this obstacle became a serious problem for its movement through the river.

Among the variety of existing fishways, in this particular case the chosen option was the installation of a rock ramp. This type of device mimics the natural conditions of the river. Usually have an inclined plane with a slope always ≤ 10%, in which blocks of stone of considerable size are inserted.

The advantages of this kind of devices are:

- It offers better conditions of passage (both upstream and downstream).

- Its appearance is better integrated with the environment.

- It allows the evacuation of flows (including ecological flows).

- It does not alter the structure of the obstacle.

- Low maintenance cost.

On the other hand the disadvantages are:

- It requires more space to be built.

- It needs more flow to ensure their functionality.

- It is only applicable to obstacles with small-medium heights (less than 2.5 m).

In this case, the Najerilla River presents marked contrasts of flow between high and low waters, so that it was proposed to make two sections of the fishway: a deeper central ramp of about 10 m wide and two shallower lateral ones of 4 m wide each. Stone blocks of about 1 meter diameter were placed so that no channels were formed where the water reaches a high speed and impedes the ascent of the fish. Finally, gravels have been embedded in the surface of the ramp in order to increase the roughness of the ramp bottom. The slope of the ramp was 5%

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Lessons learnt

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The implementation of monitoring programs is essential to evaluate the correct functioning of the fishways. In this regard, there is a lack of information about this action.

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