PORTUGUESE FISH LADDERS OPERATING CONDITIONS: AN OVERVIEW

António N. Pinheiro

Member I-5671, Ph.D., Assistant Professor, Department of Civil Engineering,

Instituto Superior Técnico (IST)

Teresa Ferreira

Ph.D., Assistant Professor, Department of Forestry, Instituto Superior de Agronomia (ISA)

Address: IST, DECivil, Secção de Hidráulica

Avenida Rovisco Pais, 1049~001 Lisboa, Portugal

Tel: 351 1 8418144; Fax: 351 1 8418144; E-mail: apinheir@civil.ist.utl.pt

Abstract: The construction and operation of mini-hydro schemes affects the fish communities inhabiting the streams. One of the main effects is the interruption of these small barriers is the prevention of the free circulation of the fish populations during the different stages of their lives.

The ecological effects of the construction of mini-hydro schemes is an issue not yet sufficiently studied, as well as the characterisation of ecological changes displayed by the fish populations and the alterations they suffered due to those works.

This paper is based on an extensive work in co-operation with by the National Forestry Services and was performed by the Instituto Superior de Agronomia with the collaboration of the Instituto Superior Técnico, both of the Lisbon Technical University.

An eco-hydraulic overview of Portuguese mini-hydro fish-ladders operation is presented. The different hydraulic parameters used and their qualitative classification are discussed.

Finally, some remarks about the overall performance of the fish ladders are presented and the necessity of extensive research on this issue is also discussed.

Keywords: eco-hydraulics, mini-hydro, fish-ladder

1    INTRODUCTION

Due to the construction of mini-hydro schemes, most of them with small concrete weirs, and to the necessity of guaranteeing that the fish populations are still able to migrate along the streams, a significant number of fish ladders have been built in Portugal in the two last decades. Figure 1 shows a typical fish ladder in a Portuguese mini-hydro scheme.

The criteria used to design the fish ladders have been mainly based on LARINIER (1992), and particularly address salmonids (trout and salmons). The most frequent species in Portuguese rivers with mini-hydro schemes, however, are not salmonids, but cyprinid assemblages, although trouts can also be present and sometimes be dominant.

Fig.1    Fish ladder in a Portuguese mini-hydro scheme

Although the Portuguese legislation compels the Owners to build fish passes whenever requested by the Forestry Authorities, no efficiency evaluation or operation monitoring is required afterwards. So, there has been no significant feedback about the adequacy of the used design criteria or about their effective utilisation by the fish populations.

The Division of Inland Fisheries of the National Forestry Services, which is the Authority for fish passes, promoted a first study to characterise the present situation of mini-hydro fish ladders. This study included hydraulic and ecological aspects, namely the field sampling of fish populations, upstream and downstream of the weir, so as to evaluate its ecological effects and obtain information about the effectiveness of the fish ladders operation.

This paper presents the results of the hydraulic performance, the implantation and attractiveness adequacy, as well as maintenance issues and the accessibility to the weir (road conditions) and to the fish ladder, considering maintenance requirements.

2    FISH LADDERS DESIGN CRITERIA

The design of fish passes and fish ladders in Portugal followed, in most cases, the design criteria proposed by Larinier (1992).  These criteria, which are summarised in Table 1, were developed especially for salmonid species, which present significant swimming and jumping capabilities.

Table 1    Fish ladders. Design criteria proposed by Larinier (1992)

(1) salmons and sea trouts ; (2) trouts ; (3) allis shad ; (4) salmonids

Although the fish populations of Portuguese rivers can be dominated by trout, most fish assemblages found were in fact composed of cyprinid communities, dominated by Iberian chub, Iberian nase and Iberian barbel, which do not present such outstanding physical capabilities.

Nevertheless, some of them, especially potamodromous species such as barbel and nase, are able to jump small slopes, and it is recognised that they are able to use fish ladder designed with the criteria referred to.  However, it is also recognised that more suited design criteria should be redefined in order to design new fish ladders and adapt some of the existent ones.

One of the most important parameters to be considered is the power dissipation per unit of water volume in each of the basins of the fish ladders. The turbulence generated in this process in the main cause for the fishes to get tired when passing the ladders. The other is, of course, the water level difference between successive basins.

So, it is considered important to establish a research programme aiming to adapt Table 1 design criteria to the Portuguese fish populations.

Table 2    Appraisal of Portuguese mini-hydro fish ladders

3    PORTUGUESE MINI-HYDRO FISH LADDERS EVALUATION

As already mentioned in the Introduction, it was considered of utmost importance to evaluate the actual eco-hydraulic operation conditions of the Portuguese mini-hydro fish ladders, which include most of the existent fish ladders in Portugal. Bochechas (1995) had evaluated the performance of Portuguese fish locks (Borland type).

To do that, a field survey was carried out to assess the local operating conditions and to compare them with the design specifications. A score classification was established to a set of parameters. These parameters intended to classify the most relevant issues, concerning the operationality of each fish ladder and allowed proposing a final overall classification of the fish ladders operating conditions. The parameters and the scores, as well as the obtained results, are presented in Table 2.

The criterion to attribute the Overall appraisal classification was based on the fact that to be considered Good or Acceptable, a certain fish ladder must allow the fishes to try to pass the ladder, to enable the progression of the fish along the ladder and finally to reach the upstream reservoir.  The parameters related  to human  accessibility were not considered to be determinant for the operationality of the fish ladder but important to determine how easy the maintenance can be performed.

In Fig.s 2 to 4, some of the detected problems are illustrated. In Fig. 5 a fish ladder considered with good operating conditions is shown.

Fig.2   Significant sediment deposition in a fish ladder basin

Fig.3  Attractiveness problem of downstream entrance  due to strong competition of another water outlet

Fig.4    Excessive first step height and obstruction of the downstream entrance. Inaccessible entrance

Fig. 5    Fish ladder with no downstream obstruction and good attractiveness

4    CONCLUSIONS

The analysis of Table 2 shows that more than 50 % of the fish ladders were considered Non operational or with Unacceptable operating conditions, which turn out to be disturbing figures. However, a significant number of problems, like obstructions or a first step too high, are easy to correct. These problems only subsist because there is no Portuguese legislation stating the need of monitoring and efficiency proofing after the construction of those ladders.

Acknowledgements

The authors, which were responsible for the execution of the study underlying this paper, would like to point out the significant work developed by Filipa Reis and Susana Santos in their graduation final report and to Jorge Bochechas, of the National Forestry Services, by the support given during the field survey.

References

Bochechas, J. (1995) - Operating conditions and efficiency of the fish passes: The cases of Crestuma-Lever e Belver Dams. MSc. Thesis, Instituto Superior Técnico, Lisbon, Portugal.

Larinier, M. (1992) - Passes à Bassins Successifs, Prébarrages et Rivières Artificielles. Bulletin Français de la Pêche et de la Pisciculture, nº 326-327, Paris, France.

Reis, F.; Santos, S. (1999 ) - Fish Passes in Mini-Hydro Schemes. Characterisation and eco-hydraulic appraisal (in Portuguese). Environmental Engineering graduation Final Report. Lisbon (Portugal), Instituto Superior Técnico.