4. FISH PASS INSTALLATIONS CURRENTLY IN USE

All fish pass intallations in Latin American rivers, except one, are the pool and weir-type (Table 1).

4.1 Argentina

Fish pass facilities have been installed in only 3 dams. These are in the Carcarañá river, a tributary of the Paraná river, in Santa Fe Province. They were built in the thirties and are approximately 2.5 m high. Originally, they consisted of a sloping floor and a system of lateral wings which served as deflectors. They were totally ineffective and an attempt was made in 1960 to convert them into pool and weir-type structures, which worked well with similar fish fauna in Brazil (Bonetto, 1980). The modifications enabled Prochilodus platensis and Salminus maxillosus to pass upstream. Its efficiency was nevertheless assessed as being low (Bonetto et al., 1971).

4.2 Brazil

Brazil's first fish pass was built on the Pardo river in the Plata basin in 1911 (Table 1). It is still in use and allows the passage of some “piracema” fish (Godoy, 1985).

Another ladder was built in the Cochoeira de Emas dam, Pirassununga, Sao Paolo State, on the Mogi Guassu river in 1920–22. It was to pass fish over a height of 3 m, but was small and not very efficient. It was demolished in 1942 and replaced by a pass consisting of 5 pools and weirs, which operated until 1966. It allowed fish to surmount a height of 5 m and was very efficient (see Section 6.1). The dam was remodelled in 1983–84 and the pass now surmounts a height of 3 m (Godoy, 1985). Each pool and weir is 15 m wide, 5 m deep and 0.75 m high.

A fish pass designed to surmount a height of 16 m according to Godoy (1985) and 13.5 m according to Castello (1982) was installed in the Pirajú dam, on the Paranapanema river in Sao Paolo State. Castello (1982) reports that it consists of 25 steps, each 0.50 m high. One measures 6 × 5 × 4 m, two others 6 × 5 × 3 m and the 22 remaining 3 × 5 × 2 m. It is 85 m long. It describes a wide curve and its entrance is located below the turbine discharge hydraulic jump. Plant employees say that it is efficient, but Castello (1982) is of the opinion that two or three more steps would be needed for it to operate at low water.

The Salto Mauá pass on the Tibagi river, Monte Alegre, Paraná State is 6 m high, according to Godoy (1985) for a dam 20 m high, according to Castello (1982). It is the pool and weir-type and is used by Prochilodus sp., Salminus sp., Pseudoplatystoma sp., Astyanax spp., Acestrorhynchus sp., Pimelodus sp and Plecostomus sp.

Three pool and weir-type passes have so far been built on the Jacquí river in Rio Grande do Sul State (Table 1). All three surmount a height of 5 m (Godoy, 1985). The Fandango pass, Cachoeira do Sul, is used by Salminus maxillosus in its reproductive migration (Castello, 1982). According to Godoy (cited by Castello, 1982), it consists of a 1 m wide pool and weir structure and is also used by Prochilodus sp. and Plecostomus sp.

A pool and weir type pass was installed on the Taquarí river, a tributary of the Jacuí, in Bom Retiro do Sul, to help fish surmount a height of 9 m. The fish species ascending the river via this pass are the same as those using the Fandango pass (Castello, 1982).

Table 1

Operational and planned fish pass facilities in Latin America

Type: 1. pool and weir;
2. elevator and bottom sluice;
3. Borland-type elevator;
4. combined pool and weir/elevator

(a) Were built originally with sloping floors and were equipped with flow deflectors, but were not efficient

(b) Do not work well due to lack of maintenance

(c) Final decision not yet taken

(d) See 6.2

(e) May be modified, but final decision not yet taken

Godoy (1985) reports that fish passes were installed in 23 dams (açudes) in Northeastern Brazil between 1957 and 1980 and all are operating satisfactorily. The first of these structures, built in the Piloes dam, Paraiba State, enabled 1 380 fishes/hour to pass in the winter of 1960 (Fontenele, 1961, cited by Gurgel, 1981).

A 16-pool fishway of 6.5 m total height, was built in the Mendubim dam, Açu, Rio Grande do Norte State. The migratory movement of the fish was studied over a period of four years and the passage of 98 312 specimens (hourly average 332) of Prochilodus cearensis was recorded (Gurgel, Silva and Duarte, 1977).

A pool and weir-type fish pass, 163 m long and 15 m wide, was built in the Poçodo Barro dam, Morado Nova, Ceará State, in 1980. A fish count has been in operation since 1980 (Fontenele, 1981, 1984, cited by Godoy, 1985).

4.3 Venezuela

A fish pass was built in an irrigation dam on the Guanare river, which is part of the Orinoco river basin, in Portuguese State. It is approximately 3 m high and is used as a migratory passage for Prochilodus platensis. There are certain flaws in the entrance design. The entrances are situated upstream of the sluices, which means that some fish are attracted by the sluice outflow. Insufficient maintenance and poaching in the pools have been reported (Lilyestrom and Taphorn, 1978).

4.4 Argentina - Uruguay

The Salto Grande dam in the middle reach of the Uruguay river (Figure 2), some 30 m in height, is an obstacle to migrating fish. There is a significant Prochilodus platensis fishery in the lower reach of the Uruguay river (Vidal, 1967; Bonetto et al., 1971); catches are insignificant in the middle reach and improve again at the point where the river forms the border between Argentina and Brazil (Figure 2). The fishery is exploited almost exclusively from the Brazilian side (Boiry and Quirós, 1985). In the early seventies, the bi-national body responsible for the construction and maintenance of the Salto Grande dam consulted Argentine and Uruguayan fishery biologists regarding the possibility of installing a fish pass structure in the dam. The reply was negative, based on the fact that (a) virtually no data was available on the migratory dynamics of the fish in the Uruguay river and, (b) the fish had the possibility of ascending the Paraná river instead. It was already common knowledge that fish migrated upstream mainly in the autumn (late March, April and May). Despite the recommendation to the contrary, the Salto Grande Joint Technical Commission decided to proceed with their plan to install a fish pass in the dam. The construction engineers included two Borland-type locks in their project. This decision appears to have been based exclusively on the fact that the height to be surmounted exceeded 20 m, which meant that it was not wise to use pool and weir-type passes. Another reason might have been that Borland-type locks are relatively inexpensive (see Section 9).

At that time, very little was known about the fish migrations in the Uruguay river and the size of stocks travelling in the area where the dam was to be located. One thing was clear, however, and that was that some fish tagged in the lower Uruguay and the La Plata rivers migrated up the Uruguay river (Bonetto and Pignalberi, 1964; Bonetto et al., 1971).

The studies on fish movements and migrations mentioned earlier and the investigations conducted by Godoy (1957, 1959, 1967) in Brazil, only provided data on the mean cruising speeds of some two or three fish species, particularly Salminus maxillosus and Prochilodus platensis and these data did not refer to the Uruguay river.

The decision to install Borland-type locks was taken in an attempt to compensate for, or perhaps in spite of, the total lack of data on the Uruguay river's fish stocks. Locks of this kind had already been found to be efficient for passing small fish or fish of limited swimming ability, cheap to install in dams 18 to 60 m high and inexpensive to operate (Clay, 1961). The limitations of such locks were also known, particularly with respect to fish-handling capacity.

The Borland-type locks in the Salto Grande dam are similar in design to those described in the literature (Clay, 1961), but on a physical scale in keeping with a river whose mean annual flow reaches 4 500 m³/sec. Basically they consist of two chambers: the lower or collection chamber and the upper chamber or fish exit, the two linked by an inclined connecting shaft (Figure 6). The fish entrances are located in the collection pool, downstream of the lower chamber. The maximum volume of the lower or collection chamber is 370 m³, but level in this chamber depends on the water level downstream. Under ideal conditions, such a lock can handle 37 000 kg of fish per operation cycle. This limit would appear difficult to attain, given that the weakness of Borland locks is their inability to clear all fish from the upper chamber (Clay, 1961; Larinier, 1976). The exit from the uper chamber is adjacent to the weir.

The fish collection pools, which communicate with each lock, are located between the weir and each of the powerhouses (Figure 7). The fish entrances face the weir area and are below the weir's hydraulic jump. The system would not operate with the weir open, even though this is not stated in the construction company's operating manuals because if it did, it would be physically impossible for the fish to reach the fish entrances (Figure 8).

Water flow and fish entry and exit are obtained by means of a system of synchronized sluice-gate operation. The upper sluice gate (C1) separates the upper chamber from the water of the reservoir, and the lower sluice-gate (C2) separates the lower chamber from the fish collection pool (Figure 9). The lower sluice-gate was originally designed to consist of two sections, an upper and a lower section. The lower was linked to the upper by a system regulating part of the water outflow so that water level is the same as in the river downstream and lock operating conditions are thus maintained (CTM Salto Grande, 1978). Only the upper section is currently in use.

Attraction water is provided by the lock system. A further supply of attraction water is obtained as a result of the pressure difference in the turbine discharge area and the collection pool (Figure 6). This auxiliary supply varies depending on the number of turbines in operation.

Each lock operates in such a way as to allow water to flow from the upper level to the lower level, thus attracting the fish into the system and encouraging them to move in the opposite direction to the flow and enter the reservoir upstream. The system may be operated automatically or manually (CTM Salto Grande, 1978). The entire process consists of four stages:

Stage 1: Fish enter the lock

The fish which have collected in the collection pool are attracted into the lower chamber. The upper sluice-gate remains partly open to admit a flow of about 0.5 m³/sec. The lower sluice-gate remains open in order to maintain a flow rate of 0.25 m/sec in that section.

Stage 2: Water level rises in the lock

The upper sluice-gate remains in the position it was in during the first stage. The lower sluice-gate closes and water flows into the lock until the reservoir water level is attained.

Stage 3: The fish leave the lock

The upper sluice-gate opens fully, while the lower remains partly open to allow a flow of 0.6 m³/sec. The rate of flow in the exit section of the lock is 0.25 m/sec. The fish must now swim out of the upper chamber, encouraged by the current moving in the opposite direction.

Stage 4: The level in the lock drops

The upper sluice-gate returns to its Stage 1 position and the lower opens sufficiently to allow the lock to empty without water velocity exceeding 0.25 m/sec.

Figure 6  Borland-type fish locks in the Salto Grande dam
A: Sectional elevation
B: Plan
1. Fish entrance; 2. Collection pool; 3. Lower chamber; 4. Connecting shaft; 5. Upper chamber; 6. Turbine discharge zone; 7. Weir zone; 8. Auxiliary water entrances (Delfino, Baigún and Quirós, 1986).

When the lock is empty the fish entrance sluice in the bottom chamber opens and the cycle is repeated.

Prior to its construction, the design was modified, in order to make the lock more versatile (National Inland Fisheries Department, 1978; Delfino, Baigún and Quirós, 1986). Manual operation became possible and fish entrance and exit times could be set within a range of 20 to 60 minutes. Flows in the lock could be varied between 0.5 and 1.0 m³/sec and higher flow velocities could be attained in the section separating the lower chamber from the fish collection pool, in other words, in the lower sluice-gate section. A control device for each lock was also included. It comprised an analogical signalling panel which enables the position of the sluice-gates and the water levels in the system to be determined. It was also possible for the operator to set fish entrance (Stage 1) and exit (Stage 2) times (Figure 9). These devices were not placed in the hydroelectric plant's turbine and sluice-gate control room, despite the recommendation that they should be (Delfino, Baigún and Quirós, 1986). It was also recommended that a single-channel conductivity bridge type fish counting device be installed. Given the size of the top chamber, the device did not detect fish less than 50 cm in length. Finally, a multi-channelled device was installed, but later had to be removed since the large shoals of fish, particularly Prochilodus platensis, using the system tended to collide with it (Delfino, Baigún and Quirós, 1986).

Before final installation of the sluice-gates, the contractor suggested a series of modifications, including: a device to prevent pieces of wood or refuse entering from upstream and damaging the system; a mechanical system for encouraging the fish to leave the upper chamber and; a by-pass valve in the lower sluice-gate to control the emptying of the lock and thus prevent turbulence which could confuse or disorient the fish entering. Only the last of these recommendations was carried out. Even then, it was expected that large numbers of fish would move toward the turbine discharge when the weir was closed (see Section 6.2). Another recommendation, which involved providing lighting in the three chambers so as to simulate river conditions, was not implemented (Delfino, Baigún and Quirós, 1986).

A by-pass valve was installed both for the reason given above and in order to solve the mechanical problem of vibration of the lower sluice-gate (C2) during emptying. At the present time, the bottom chamber is emptied by means of this valve and not by the sluice-gate method described in the original design (Figure 9).