EIFAC 2006: DAMS, WEIRS AND FISH

Fish passage experience at small-scale hydro-electric powerplants in France

Michel Larinier

� Springer Science+Business Media B.V. and FAO 2008

Abstract There are more than 1,700 small-scale

hydro-electric power stations in France today, which

can be found on the majority of rivers with migratory

species in them. This article gives an overview of the

different types of fish facilities in use at these small-

scale hydro plants. The relative advantages and

drawbacks of each type of fish pass are discussed,

with reference to the requirements of migratory

species and the site-specific constraints. Emphasis is

placed on the problems of attraction and mainte-

nance. The article also mentions the various

techniques used to evaluate existing or recently

constructed fish passes. Experience in using bypass

facilities combined with trashracks for downstream

juvenile salmonids and eels are related. The author

points out the severe cumulative impact which may

occur due to the existence of several small-scale

hydro projects on the same river. He presents his

view on the priorities for research on fish passage

facilities, especially on ‘‘fish friendly’’ small-scale

hydropower intakes.

Keywords Fishways � Downstream bypass �Small-scale hydropower plant � Upstream and

downstream migration

Small-scale hydropower plants in france

Hydropower is the second source of electric power

generation in France. It accounts for about 14% of the

country’s total electricity production, as compared to

76% for nuclear power and 10% for classical thermal

power. The total production of hydraulic power in a

normal year is 70 TWh.

In general, the technical term ‘small-scale hydro

electricity’ encompasses all hydropower plants with a

production capacity of less than 10 MW. This is the

limit fixed by the International Union of Producers and

Distributors of Electrical Energy (UNIPEDE), which

distinguishes among small hydroelectric plants (2–

10 MW), mini-plants (500 kW–2 MW), micro-plants

(20–500 kW) and pico-plants (\20 kW). This 10 MW

boundary between small and large-scale hydroelectric

plants is however somewhat arbitrary when considering

fish passage issues and mitigation measures.

There are approximately 1,700 small-scale hydro-

power plants with a production capacity lower than

10 MW in France today. They have a combined total

capacity of 1,800 MW and jointly produce an average

of approximately 7.5 TWh in a normal year, which

represents around 10% of the total hydroelectric

Guest editors: R. L. Welcomme & G. Marmulla

Hydropower, Flood Control and Water Abstraction:

Implications for Fish and Fisheries

M. Larinier (&)

CSP-CEMAGREF, Institut de Mecanique des Fluides de

Toulouse, Allee du Professeur Camille Soula, 31400

Toulouse, France

e-mail: larinier@imft.fr

123

Hydrobiologia (2008) 609:97–108

DOI 10.1007/s10750-008-9398-9

power production and only 1.5% of the country’s total

electricity production. Among these small-scale hy-

droplants, 75% have a production capacity of less than

1 MW each and produce less than 1/5 of the total for

small-scale hydropower production. 1,400 of these

installations are owned by independent producers, 200

by the French Electricity Board (EDF) and 100 by

large private companies.

Experts have estimated that a new capacity of

1,000 MW should be generated by small hydroelec-

tricity by the year 2010 to meet the objective of the

European Renewable Energy Directive. According to

a recent study by ADEME, small-scale hydropower

has an overall estimated growth potential of 620 MW

corresponding to a production of around 4 TWh. This

includes the building of new plants (1/3) and the

upgrading of existing plants (2/3).

The introduction of this directive increased interest

in the development of new projects and in the

modernisation of existing installations. However,

existing environmental policy still creates serious

obstacles to the construction of new installations.

Small-scale hydropower stations can be found on

most French rivers throughout the country. The main

production regions are however located in mountain-

ous and foothill areas, mainly in south-east and south-

west France (the Midi-Pyrenees, Rhone-Alpes and

Provence-Cote d’Azur regions) which account for

more than 60% of the total small hydroelectric power

production.

Most of the schemes are run-of-river schemes asso-

ciated with relatively low dams: the amount of water

running through the power plant is determined by the

water flowing in the river. They range from high head

schemes (from more than 100 m high) in upland areas

that have a flow-through capacity of a few hundred litres

per second to low head schemes located in lowland areas

with a head of a few metres and flow-through capacities,

which may be greater than 100 m3 s-1. There may thus

be small-scale power installations on rivers which

support populations of diadromous fish species as well

as potamodromous and riverine species.

Fish passage legislation

The French Environment Code (Article L 432-6)

requires that any hydro scheme in watercourses or

parts thereof, in the list specified by decree, must

include facilities to guarantee the passage of migra-

tory fish. Existing obstructions must comply with the

provisions of this Article within 5 years following

publication of the list of migratory species by river

basin or sub-basin, as specified by the responsible

Minister, without compensation. The decrees estab-

lishing the list of species for each river were

published between January 1986 and December 1999.

The diadromous migratory species taken into

account by the law are salmon Salmo salar (L.),

sea-run brown trout Salmo trutta (L.), sea lamprey

Petromyzon marinus (L.), allis shad (Alosa alosa

(L.)), sturgeon (Acipencer sturio L.) and European

eel Anguilla Anguilla (L.). The only riverine species

considered to be ‘migratory species’ are brown trout

Salmo trutta (L.), Northern pike Esox lucius (L.) and

European grayling Thymallus thymallus (L.).

For new hydroplants, or during relicensing of

existing hydropower facilities, fish passes may be—

and are generally—required by the authorities, even

on rivers which have not been classified as ‘migra-

tory’ rivers by law, so that fish passes can be built for

riverine species on all new or relicensed obstructions.

Since the promulgation of the European Framework

Water Directive, a more determined effort is being

made to take into account all species in order to

restore the longitudinal connectivity in rivers.

The law stipulates that the owner of the obstruction

caused by any dam is obliged to ensure the operation

and maintenance of these facilities, i.e. is responsible

for providing effective facilities for fish passage.

The same law (Environment Code, Article L. 432-

5) requires that any hydro scheme to be constructed

on a watercourse must include facilities to guarantee

a minimum ecological flow in the river to ensure the

continued existence, passage and reproduction of the

species that populate the waters at the time when the

hydro plant is constructed. This minimum flow may

not be less than one tenth of the mean annual daily

discharge of the watercourse at the hydro scheme

(evaluated on the basis of a minimum of five

consecutive years of data), nor less than the total

flow immediately upstream of the installation, if the

latter is lower.

If necessary, facilities may also be required to

prevent fish from entering the intake and discharge

channels.

The law relating to the use of hydraulic energy

(October 1919), which was subject to several

98 Hydrobiologia (2008) 609:97–108

123

subsequent amendments (particularly in 1980), stip-

ulated that nobody may dispose of hydraulic energy

without an authorisation (for a capacity lower than

4.5 MW) or a concession (for a capacity higher than

4.5 MW). The technical specifications for the con-

cession or authorisation lay down several mitigation

measures for ensuring an environmental flow and the

free passage of fish. This law also introduced the

notion of ‘reserved’ rivers where neither new permits

nor concessions can be granted. This classification

was at first limited to classified ‘migratory’ rivers, but

was later extended to other rivers, with the aim of

protecting rivers which have not yet been affected by

hydro electric installations.

Upstream fish passage facilities

This part briefly describes the various types of

fishway usually found at small-scale hydroplants.

Denil fish passes

Two designs of baffle (or ‘Denil’) fish passes are

commonly used in France:

• The ‘plane baffle’ or ‘standard’ Denil fish pass,

developed by White & Nemenyi (1942). The

width of the baffles usually varies from 0.60 m

for trout up to 1.00 m for salmon and sea trout.

These fish passes generally have slopes of

between 15 and 20%.

• The ‘super-active’ bottom-baffle fish pass (Lari-

nier et al., 2002), in which herringbone-patterned

steel baffles are placed on the bottom while the

two sides of the channel are kept smooth. This

fish pass generally has slopes of between 10 and

16% and the height of the baffles varies from

0.08 m to a maximum of 0.20 m for large fish.

The width of such a design is not limited; several

unit-patterns can be juxtaposed according to the

size of the river and the design discharge.

Denil fish passes are relatively selective and are

only used for species which have sufficient swim-

ming speed and endurance. They are used for large

rheophilic species and in particular salmon, sea trout

and marine lamprey, for which they appear to be

particularly effective. They tolerate only slight vari-

ations in the upstream water level.

Most Denil fish passes have been installed in small

coastal rivers in Normandy and Britanny and were

designed to accommodate sea-run brown trout and

salmon passage at low weirs while a few have also

been installed at small-scale hydropower facilities in

foothill areas to pass brown trout by means of baffles

with reduced dimensions (Larinier et al., 2002).

Pool fish passes

The pool-type fish pass is the most frequently used

fish pass at small-scale hydroplants. The discharge in

these passes, depending on the size of the river, can

vary from less than 0.1 m3 s-1 in small brooks or

mountain streams to more than 2 m3 s-1 in large

rivers (Larinier et al., 2002).

The drop per pool generally adopted is around

0.30 m for Atlantic salmon, sea-run brown trout and

brown trout, 0.20–0.30 m for shad, and between 0.15

and 0.30 m for other target-species, depending on the

species and their length. Pool volume is determined by

the criteria of dissipated power per unit pool volume.

The maximum values are 200–250 watts m-3 for

salmonids, 150 watts m-3 for other species, namely

shad, cyprinids and in all cases for small pools. Pool

shape and dimensions are determined by a number of

factors including the pool volume, the flow pattern in

the pool, the species concerned and their size. The

length of pools can vary from 1.2 m for a small trout

fish pass designed for less than 0.1 m3 s-1, to more than

4.5 m for a fish pass designed for 1 m3 s-1, with a mean

length of 2.5–3.0 m for fish passes designed for 0.3–

0.5 m3 s-1. Consequently, the slope of a pool fish pass

can vary greatly, from less than 7% to more than 25%,

with the most frequent values ranging from 10 to 12%.

The most commonly used pool-type fish pass at

small-scale hydroplants in France is the alternate

deep notch and submerged orifice fish pass (Larinier

et al., 2002). Such fish passes can accommodate

moderate upstream water level variations without the

need for installing any upstream flow-regulation

section.

Vertical slot fish passes have the great advantage

of operating correctly without any regulating

device—by tolerating significant variations in

upstream and downstream water levels—and by

allowing fish to pass from the bottom up to the

surface of the pool. However, a fairly high flow

discharge is required in fish passes for large

Hydrobiologia (2008) 609:97–108 99

123

migratory fish such as Atlantic salmon, while taking

the minimum size of the slot and the minimum depth

in the pool into account.

Experience has shown that when a pool pass has

been correctly designed (in terms of drops, level of

turbulence and flow pattern in the pools), it is not very

selective and can be crossed by most species present in

the river. The main problem for some small species can

be the significant time spent in very large pools: small

species tend to ‘get lost’ and to remain trapped in

recirculation eddies, which occur in large pools. It is

already clear that the introduction of rough obstacles on

the bottom can help small benthic species to pass

through. For other small species, a solution may be to

reduce the size of pools where possible, or to find ways

of reducing the size of the recirculation eddies by

installing several obstacles in the pools. A study of

vertical slot fish passes is underway to characterise flow

in terms of speed, turbulence, flow patterns and to see

which devices may be used in pools to help guide small

species (Tarrade et al., 2006).

Pre-barrages

Pre-barrages are often an efficient and inexpensive

solution to enable fish to clear fairly low obstacles.

They are made of several walls or weirs downstream

of the obstacle, creating large pools which break up

the drop to be cleared (drops of 0.40–0.60 m). The

configuration of the weirs and drops among pools

depends on the target-species: for salmonids, a

plunging flow is acceptable and the walls among

pools can be vertical whereas for most other species it

is better to progressively dissipate energy and reduce

velocities on rough ramps to enable the fish to move

through by swimming. Pre-barrages are a very

common solution adopted for trout on low head

diversion dams in upland areas (Larinier et al., 2002).

Natural bypass channels

There are currently many fewer natural bypass

channels in France than pool or Denil fish passes at

small-scale hydroplants. This type of facility is

particularly suitable if the fish pass has to be installed

near a dam whose structure cannot be changed and

when there is sufficient space on the bank. The slope

for this type of structure generally varies from 2 to

5%, depending on the target species. The major

disadvantage of this type of device is its overall

dimension and the difficulty of taking significant

variations in the upstream level into account. The

energy is generally dissipated by rows of blocks or

weirs creating a series of drops of variable height

(from 15 to 30 cm). The design criteria are very

similar to those for pool fish passes. If the level of

upstream water varies significantly, a control section

has to be installed upstream to limit the flow in the

facility and the most efficient system generally

recommended is a section of a vertical-slot fish pass

(Larinier et al., 2002).

Fish locks and fish elevators

Fish locks and fish lifts are only rarely used at small-

scale hydroplants: the head at small-scale hydro dams

is generally limited, the operation of such facilities is

too sophisticated and their maintenance much more

cumbersome than that of a more conventional fish

pass.

Over the last 15 years a few fish locks were built

in France at small hydroplants with only 3–6 m head

differences where the space available was too limited

for installing a conventional fish pass. The whole

system, including the downstream holding pool, was

generally left open and is very similar to a navigation

lock. The principal limits to this kind of facility are

the discontinuous nature of its operation and the

difficulty of optimising its operating cycle when

several species with very different behaviour are to

be passed (Larinier et al., 2002).

Attraction flow and maintenance

The attraction of a fish pass, i.e. the fact that fish find the

entrance more or less rapidly depends on its location in

relation to the obstruction, particularly the location of its

entrance and the hydraulic conditions (flow discharges,

velocities and flow patterns) near these entrances. The

discharge through the fish passage facility must be

sufficient to compete with the flow in the river during

the migration period. It is difficult to give precise

criteria, but generally the flow passing through the fish

pass must be of the order of 2–5% of the competing

flow. The competing flow can be either the turbine

discharge, either the ecological flow or the spilling

discharge at the dam (Larinier et al., 2002).

100 Hydrobiologia (2008) 609:97–108

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In the case of a small-scale hydroelectric plant,

where all the flow discharge passes through the

turbines, migrating fish will be attracted to the turbine

draft tubes. The entrance to the fishway must

therefore be adjacent to the powerhouse, preferably

in the riverbank.

Where the plant is located on a diversion canal, it

is often difficult to decide whether it is preferable to

install the fishway at the dam or at the powerhouse. A

careful study must be made of water flow regimes at

each location and of plant operation during the

migrating period. Fish can be attracted to the

continuous flow from power generation or else to

the dam in the case of frequent spills during the

migration period. At times of low flow, most of the

fish will be attracted to the turbines, while during

periods of higher flow, fish can congregate below the

dam. Should the diversion canal be very long, the fish

will be much more likely to become trapped in one of

the arms (tailrace or river) and have a limited chance

of rapidly finding a single fishway. Where possible, it

is thus better to provide two separate fishways, one at

the powerhouse, the second at the diversion dam.

Several small-scale hydro plants with long discharge

canals are now equipped with two fish passes in

South-West rivers supporting diadromous species.

It is not always possible to install a fishway at the

powerhouse, particularly on high head schemes

where the river flow is conducted to the turbine in a

very long pressurised conduit to the power house

sometimes several kilometres downstream. The

effective performance of a fish passage facility

situated at the diversion dam depends entirely, in

such cases, on a continuously significant and suffi-

cient ecological flow being released in the bypassed

section of the river.

The use of electric repellent devices has been

envisaged as an alternative to the construction of a

second fishway at the power station for preventing

fish from entering the tailrace. A few electric screens

have been installed, but their efficiency was not

straightforward, and their installation has even been

considered more detrimental than useful on short

tailraces: some fish can pass through and remain

trapped between the screen and the turbines (Chan-

seau & Larinier, 2001). The repelling electric-field

screen must be associated with high local velocities

in the tailrace, which are not possible to maintain

during high and even medium flow at low-head,

small-scale powerplants.

Fish behaviour is not the only factor to take into

account when choosing or designing fish pass facil-

ities. Exposure to floods, protection against debris,

maintenance and control problems are also important.

Adequate maintenance is vital to the successful

operation of a fish pass. Lack of maintenance of

facilities is a recurring problem in France, which is

why the Adour Garonne Basin Agency recently

introduced a system of bonuses to incite owners to

correctly maintain structures.

Sedimentation is a factor to be taken into account

when designing a pool fish pass in upland areas, where

the sediment transport in the river can be significant

during high flows. While the power dissipation in the

fish pass is generally sufficient to avoid the deposition

of sand, the filling of the fish pass with gravel can be

expected on rivers which transport gravel during floods.

The location of the fish pass must be carefully studied to

avoid high deposition areas. It is possible to install a

gravel trap upstream, and in all cases, the maintenance

access to the pass should be made easy. The depth of

pools has to be reduced to a minimum and their width

proportionally increased. Tests were performed on

hydraulic models for designing self-maintaining pool

fish passes for gravel rivers, but their use still remains

very limited.

Efficiency of fish passes

The legal obligation on owners to achieve a result for

the free circulation of fish has led to the need to

specify the concept of efficiency for fish passes,

which is not easy to define—it has, in fact, never been

properly and clearly defined in the regulations—and

even more so to demonstrate. Is a pass, which is

effective for a given species, one that allows the

passage of at least one individual, or of the total

population present below the obstruction, or of a

specified fraction of this population, or only of all

individuals that enter the facility? Should the time

taken to pass upstream from the obstruction also be

taken into account? The definition of efficiency may

be quite different for the same fish facility, depending

on the species considered. We could distinguish

effectiveness and efficiency:

Hydrobiologia (2008) 609:97–108 101

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Effectiveness of a fish pass is a qualitative

concept, which consists in checking that the

pass is capable of letting all target species pass

through within the range of environmental

conditions observed during the migration

period.

The efficiency of a fish pass is a more

quantitative description of its performance. It

may be defined as the proportion of stock

present at the dam which then enters and

successfully moves through the fish pass in

what is considered to be an acceptable length of

time.

For salmon and other catadromous species, the

whole of the migrating population should be able to

pass through any obstruction located downstream

from spawning areas. If, moreover, this watercourse

is equipped with many obstacles it will be necessary

to minimise delays to migration caused by these

obstacles so that the migrating fish arrive at the

spawning areas in time. On the other hand, if the pass

is located further upstream of the river within the

spawning grounds, the requirements for percentage

and time taken may be less stringent given that the

fish may reproduce downstream and that the motiva-

tion to migrate may be variable. Whatever the case,

the fish pass must be sufficiently efficient so as not to

constitute a limiting factor for the long-term main-

tenance of migrating stock

The quantitative evaluation of efficiency of

upstream fish passage requires knowledge of the

number of fish available for passage as well as the

number that actually pass the dam. Mark-recapture

and radiotracking are valuable techniques for overall

evaluation of the efficiency of fish passes in terms of

percentage of passage and delays to migration and the

cumulative impact of different obstacles on a migra-

tion route (Chanseau et al., 1999). When there are

several obstructions close to each other on the same

river, a quantitative estimate of upstream passage

efficiency can be obtained more easily from fish

passage counts at the dam of interest and the next

lower dam. The efficiency is expressed as the

proportion of the passing fishing counted to the

number of fish available for passage (i.e. those that

were passed above the lower dam).

An example is the efficiency obtained through

radio-tracking on four fish passes located at small-

scale hydroplants on the Gave de Pau (mean annual

discharge 90 m3 s-1) in the South-West of France

(Fig. 1). The Artix (turbine discharge 80 m3 s-1) and

Biron (turbine discharge 110 m3 s-1) power plants

are recent installations equipped, respectively, with a

vertical slot fish pass (design flow 0.7 m3 s-1,

auxiliary flow 2 m3 s-1) and a natural bypass channel

Migration delay

0

20

40

60

80

100

1 day 3 days 1 week 2 weeks 3 weeks > 1 month

% s

alm

ons

BAIGTS (efficiency 35,3%)

Migration delay

0

20

40

60

80

100

1 day 3 days 1 week 2 weeks 3 weeks > 1 month

% s

alm

ons

SAPSO (efficiency 74%)

Migration delay

0

20

40

60

80

100

1 day 3 days 1 week 2 weeks 3 weeks > 1 month

% s

alm

ons

BIRON (efficiency 100%)

Migration delay

0

20

40

60

80

100

1 day 3 days 1 week 2 weeks 3 weeks > 1 month

% s

alm

ons

ARTIX (efficiency 93,8%)

Fig. 1 Fish passes efficiency and migration delays at four dam on the Gave de Pau river

102 Hydrobiologia (2008) 609:97–108

123

(design flow 4 m3 s-1). The Sapso (turbine discharge

m3 s-1) and Baigts (turbine discharge 90 m3 s-1)

plants are much older installations equipped, respec-

tively, with a Denil fish pass and a mixed pool-Denil

fish pass which location is far from optimum. The

poor results obtained by radiotracking at the two last

small-scale hydroplants convinced the authorities to

require the owners of these two last structures to

build new and more efficient fish passes.

Experience gained with radiotracking of salmon

showed that an efficiency of 95–100% can be

obtained on recent well-designed fish passes with

delays of a few hours to a few days (Chanseau et al.,

1999).

The radio-tracking study undertaken on the Gave

de Pau from 1995 to 1998 to evaluate the passability

of about 30 obstructions (among which 20 small-

scale hydroplants) for upstream migration showed

that:

– 16 structures allowed all of the migrating fish to

pass through without significant delays,

– 10 structures were more serious obstacles to

migration in terms of delays or blocking part of

the population, but were still acceptable,

– 5 structures, of which several were located on the

downstream part of the migration route and of

older design, were major obstacles.

It was estimated that only 13% of salmon reach

the first spawning zones. The current objective is to

increase this percentage to 80% by improving

existing facilities and building new ones on the

three most critical obstacles. A new fish lift was

recently built at the Baigts powerplant (Larinier

et al., 2005).

For riverine species the main biological objectives

of the fish pass are to ensure the longitudinal

connectivity of the river and avoid fragmentation of

the population. Since it is not generally possible to

ascertain the size of the downstream population and

the proportion of it willing to clear the obstruction,

the facility may be considered to be effective if it is

used by a certain number of individuals, in relation to

the population in place. Simply counting the fish

upstream of the fish pass by visual inspection,

trapping or video checks may provide an adequate

indication of the efficiency of the pass.

In practice, rigorous evaluation of fish passes

efficiency is only rarely performed, except for

diadromous species such as salmon and shad. Fish

passes are generally determined to be satisfactory by

regulatory agencies based on substantially less infor-

mation, such as direct observation of fish passage,

and in most cases conformance with design criteria

and satisfactory maintenance.

Downstream migration

Downstream migration involves diadromous species:

juveniles of anadromous species, adults of catadro-

mous species and certain anadromous species (repeat

spawners). In France, considering the high number of

installations on most rivers, hydroelectric power-

plants are the principal issue to be dealt with to

ensure safe downstream migration of diadromous

species.

The downstream fish passage at hydroelectric

power dams for potamodromous and resident species

is generally considered less stringent: if they can

move downstream during their life cycle, this species

migrates over limited distances and are in most cases

concerned by a few installations. The need to provide

passage for mitigation must be considered species-

and site-specific.

Downstream passage over spillways or weirs is

rarely a problem in France for fish at small-scale

hydroplants where dams are generally of moderate

height. Provided that fish are able to fall safely on the

downstream side, with sufficient depth at the base of

the dam and no over-aggressive baffles, then spill-

ways and weirs are usually considered to be the safest

way for fish to pass a dam.

Fish passing through hydraulic turbines are subject

to various forms of stress likely to cause high

mortality: probability of shocks from moving or

stationary parts of the turbine (guide vanes, vanes or

blades on the wheel), sudden acceleration or decel-

eration, very sudden variations in pressure and

cavitation.

Numerous experiments have been conducted in

various countries (USA, Canada, Sweden, Nether-

lands, Germany and France), mainly on juvenile

salmonids and less frequently on clupeids, eels and

other species, to determine the mortality rate due to

their passage through the main types of turbine (Bell,

1981; Monten, 1985; EPRI, 1987, 1992; Larinier &

Dartiguelongue, 1989; Holzner, 2000).

Hydrobiologia (2008) 609:97–108 103

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The mortality rate in impulse type turbines (i.e.

Pelton turbines) used on high head schemes is very

high, if not total.

For juvenile salmonids, the mortality rate in

Francis and Kaplan turbines varies greatly, depending

on the properties of the wheel (diameter, speed of

rotation, etc.), their conditions of operation, the head,

and the species and size of the fish concerned. The

mortality rate varies from under 5% to over 90% in

Francis turbines. On average, it is lower in Kaplan

turbines, between 5% and approximately 20%. The

difference between the two types of turbine is due to

the fact that Francis turbines are generally installed

under higher heads. Damage through turbines

installed at small hydroplants is generally higher

than that at larger plants, because of the smaller size

and the higher rotation speed of the turbines involved.

Mortality rate in adult eels is generally higher than

that observed for juvenile salmonids, because of their

greater length. The mortality rate may be 3–5 times

higher than in juvenile salmonids (depending on the

length of the eel in question), reaching a minimum of

10–20% in large low-head turbines, and more than

50% in the smaller turbines used in most small-scale

hydroelectric power plants (Monten, 1985; Larinier

& Dartiguelongue, 1989; Desrochers, 1995; Hadder-

ingh & Bakker, 1998; Holzner, 2000).

Studies were carried out between 1999 and 2005 to

assess the cumulative impact of small-scale hydro-

power plants on the main salmon rivers with the

objective of identifying the most critical rivers on

which efficient downstream fish facilities should be

installed as a priority (Bosc & Larinier, 2000;

Anonymous, 2002, 2004; Pallo & Larinier 2002).

This was done using a simple model characterising

the downstream migration phenomenon at each site and

evaluating the respective probabilities of fish passing

through the spillway and the turbine, taking into

account the hydraulic regime of the river during the

period of downstream migration, the turbine discharge

flow, the site characteristics and the probabilities of

damage caused by transit through the turbines. This

probability of damage was predicted using relationships

among mortality rate, length of fish and turbine

characteristics (Larinier & Dartiguelongue, 1989; Lar-

inier et al., 2002). Two examples of the results are

shown in Figs. 2 and 3.

Low mortality rates on the small-scale hydroplants

installed on the Gave d’Oloron are related to the large

turbines whose total capacity is much lower than the

river flow during migration. Higher mortalities rates

at some hydroplants on the Saison are related to

smaller turbines with a high rotation speed and/or a

high entrainment through the turbine.

The main problem is generally the cumulative

impact of a series of hydro-electric power plants

located on the same river. Considering passage at 10–

20 plants, even with a high mean survival rate of 95%,

or even 98% at each plant, the cumulative mortality

rate will be, respectively, between 40 and 19% and 74

and 33%. These indiscriminate impacts of small-scale

hydroplants can threaten entire fish populations.

The cumulative mortality rate for salmon smolts

during their downstream migration was estimated in

mortality rate (% at each plant)(Gave d’Oloron)

Légu

gnon

Gue

rlain

Sau

cède

Dog

nen

Mas

seys

Aut

erriv

e

Sor

de

0

1

2

3

4

5

6

mortality rate (% at each plant)(Saison)

Mou

lin D

atto

Tro

is V

ille

Got

ein

Mau

léon

Liba

rren

x

Gor

re

Ché

raut

e

Cha

ritte

de

bas

0

4

8

12

16

20

Fig. 2 Raw mortality rate figures (expressed in % of the fish

transiting through each installation) for juvenile salmon passing

through various installations on the Gave d’Oloron and Saison

104 Hydrobiologia (2008) 609:97–108

123

several South-western rivers prior to the installation

of any specific downstream facilities. The mortality

rate, depending on the year, may vary significantly

according to the flow conditions during the migration

period (Anonymous, 2002, 2004; Bosc & Larinier,

2000; Pallo & Larinier 2002) (Table 1).

The impact must be still higher on for silver eel,

due to the higher probability of them being wounded

when passing through turbines. Similar studies such

as those carried out for salmon should be launched

soon to evaluate the cumulative impact of small-scale

hydroplants on the downstream migration of silver

eel, given that technical solutions in this case are

much less straightforward than for salmon.

Downstream fish passage facilities

Downstream fish passage technology is much less

advanced than that for upstream fish passage facili-

ties. This is due to the fact that efforts to re-establish

connectivity for migrating fish began with the

construction of upstream fish passage facilities and

that downstream migration issues were acknowl-

edged and addressed much later. It is also because it

is much more difficult and complex to develop

effective facilities for downstream migration.

One solution to prevent fish from passing through

the turbines involves stopping them physically at

water intakes by means of bars or screens which must

have a sufficiently small spacing or mesh dimension

to physically prevent fish from passing. These

barriers have to guide fish towards a bypass, which

is done most effectively by placing them diagonally

to the flow, with the bypass in the downstream part of

the screen. Sufficient screening area must be provided

to create low flow velocities to avoid fish impinge-

ment and reduce hydraulic head losses. Uniform

velocities and eddy-free currents upstream of screens

must be provided to effectively guide fish towards the

bypass (Larinier & Travade, 1999).

In France most small-scale hydroplants were

constructed before the importance of downstream

migration facilities had been understood. Retrofitting

existing installations with fine screens acting as

physical barriers would require increasing the screen

mortality rate (% of total population)(Gave d’Oloron)

Légu

gnon

Gue

rlain

Sau

cède

Dog

nen

Mas

seys

Aut

erriv

e

Sor

de

0

1

2

3

4

5

6

mortality rate (% of total population)(Saison)

Mou

lin D

atto

Tro

is V

ille

Got

ein

Mau

léon

Liba

rren

x

Gor

re

Ché

raut

e

Cha

ritte

de

bas

0

4

8

12

16

20

Fig. 3 Raw mortality rate figures (expressed in % of the

population migrating downstream) for juvenile salmon passing

through various installations on the Gave d’Oloron and Saison

Table 1 Estimation of cumulative mortality rates for juvenile salmon passing small-scale hydroplants on South-western rivers

River Gave de Pau Gave d’Oloron Saison Salat Neste Correze Vezere

Basin Adour Adour Adour Garonne Garonne Dordogne Dordogne

Number of small-scale hydroplants 20 7 8 23 10 5 4

Min/Mean/Max cumulative

mortality rate (%)

9/19/30 1/3/8 10/18/25 31/50/64 16/28/38 9/15/22 2/6/9

Hydrobiologia (2008) 609:97–108 105

123

areas to reduce the approach velocities to such an

extent that it was considered unrealistic to do so.

Surface bypasses combined with existing conven-

tional trashracks or angled bar racks with relatively

close spacing have become one of the most fre-

quently prescribed fish protection systems for

juvenile salmonids at small hydroelectric power

plants in France. These structural guidance devices

act as physical barriers for larger fish (downstream

migrating adults) and behavioural barriers for juve-

niles. A research programme was undertaken to

assess the efficiency of such bypasses for juvenile

salmonids and to define the design criteria and

determine the limits to their use. The efficiency is

closely related to the fish length to spacing ratio and

to fish response to hydraulic conditions around the

front of the structure and at the bypass entrance. Tests

showed that under optimal conditions, efficiency

could reach 60–85% (Larinier & Travade, 1999).

Flow discharge in the bypass has also proven critical.

The design criteria currently in France call for a mean

discharge of 5% of the turbine discharge and for

juvenile salmon a spacing between bars of 2.5 cm,

even if efficient results have been obtained with bar

spacing of up to 4 cm, but this must be combined

with optimum flow patterns at intake (Croze et al.,

1999).

External vapour mercury lights have been tested

on several sites to improve bypass efficiency. If weak

intermittent or permanent lights seem to exert

significant attraction on juveniles, the potential of

lights to improve bypass efficiency was clearly

demonstrated on only a few sites (e.g. Guilhot power

plant on the Ariege river and Poutes dam on the

Allier river). All results indicate that the spacing of

the trashracks and hydrodynamic factors are the

predominant factors to be taken into account when

designing a downstream bypass (Larinier & Travade,

1999).

Several experiments were recently carried out in

France with acoustic and electrical barriers to divert

salmon smolts from a canal intake to a surface

bypass. The results were disappointing for both

devices, while the efficiency of the acoustic barrier

was practically nil (Gosset & Travade, 1999). The use

of behavioural barriers must be considered with

caution.

The problem of the downstream migration of eels

at hydroelectric power stations is critical in the light

of their size and the high damage rate that can results

from turbine passage. No specific solution has been

implemented in France due to the relatively recent

awareness of eel migration. The results of recent

experiments showed that the ‘behavioural’ repellent

effect of the trashrack is far less obvious than for

salmon, and that bypasses must be combined with

physical barriers with a maximum bar spacing close

to 2 cm (Gosset et al., 2005; Subra et al., 2005).

Experiments still have to be carried out to optimise

intakes and bypass design.

Stopping turbines during eel downstream migra-

tion peaks is a solution which has already been

considered, as is the capture of individuals upstream

of the obstacles. However, these solutions assume

that the period of downstream migration is suffi-

ciently short and can be predicted with sufficient

accuracy, which does not appear to be the case for the

European eel.

Conclusion

This article gives an overview of experience gained

in fish passage at small-scale hydropower plants in

France.

Mitigation measures, such as environmental flows

and installation of upstream and downstream fish

facilities, made it possible to limit to some extent the

negative impacts of these small-scale hydropower

plants, or to make the impacts at least more tolerable.

The residual impacts of these obstacles are, however,

cumulative. Experience shows that every obstruction,

even if fitted with ‘effective’ fish passage facilities,

creates at least some delay in migration. Clogging of

the fish pass with floating debris and insufficient

maintenance of the facilities, as well as the frequently

observed deficient ecological flow are additional

causes of delay in upstream migration or obstruction

to fish passage. The technology allowing for reason-

ably satisfactory downstream passage is now quite

well-developed for juvenile salmonids and can be

applied with some success; however, as regards other

species, e.g. eel, this technology is still inadequate. In

order to find efficient solutions for existing installa-

tions, it is obvious that a lot more intensive research

and development will be needed.

The main conclusion to be drawn from experience

gained in France, is that it is not good management

106 Hydrobiologia (2008) 609:97–108

123

practice to plan the construction of, or even envisage

keeping, more than a very limited number of small-

scale hydropower stations on rivers, for which the

policy is to protect or restore the population of

migratory species.

Research and development is also urgently needed

with respect to the design of ‘fish-friendly’ water

intakes at small-scale hydroplants, which have to take

both downstream migration (in particular for silver

eel) and maintenance problems into account. Efficient

solutions with no mortalities must be found for all

new small-scale hydroplants involving downstream

migration issues.

Several research and development programmes for

‘fish-friendly’ turbines are in progress, particularly in

France, for equipping plants with very small heads.

We hope that the development of such turbines will

not lead to a drastic multiplication of micro- and

pico-plants, which could reverse the actual tendency

to decommission small dams and weirs. One should

never lose sight of the limits to the effectiveness of

fish passage facilities. In addition to problems

relating to fish passage, the effect of small dams on

habitat must not be underestimated. The protection of

migratory species for a given dam must be studied in

a much wider context than the strict respect of fish

passage alone.

References

Anonymous, 2002. Simulation des mortalites induites par les

amenagements hydroelectriques lors de la migration de

devalaison des smolts de saumon atlantique. Propositions

d’amenagements. Le Gave d’Oloron et ses principaux

affluents. SIEE–GHAAPPE Report TE 01.08.03/BV/a:

33 p.

Anonymous, 2004. Simulation des mortalites induites par les

amenagements hydroelectriques lors de la migration de

devalaison des smolts de saumon atlantique. Propositions

d’amenagements. Le Gave de Pau et le Neez. SIEE–

GHAAPPE Report TE 03.04.03/BV/a: 52 p.

Bell, M. C., 1981. Updated compendium of the success of

passages of small fish through turbines. Fisheries Engi-

neering Research Program, U.S. Army Corps of Engineers,

North Pacific Division, Portland, Oregon: 294 p.

Bosc, S. & M. Larinier, 2000. Definition d’une strategie de

reouverture de la Garonne et de l’Ariege a la devalaison

des salmonides grands migrateurs. Simulation des mor-

talites induites par les amenagements hydroelectriques

lors de la migration de devalaison. GHAAPPE Report

RA.00.01: 75 p.

Chanseau, M., O. Croze & M. Larinier, 1999. Impact des

amenagements sur la migration anadrome du saumon

atlantique (Salmo salar L.) sur le Gave de Pau (France).

Bulletin Francais de la Peche et de la Pisciculture 353/

354: 211–237.

Chanseau, M. & M. Larinier, 2001. Etude de l’efficacite de

l’ecran electrique de l’amenagement hydroelectrique de

Peyrouse sur la Gave de Pau. GHAAPPE Report

RA.01.02: 13 p.

Croze, O., M. Chanseau, & M. Larinier, 1999. Efficacite d’un

exutoire de devalaison pour smolts de saumon atlantique

et comportement des poissons au niveau de l’amenage-

ment hydroelectrique de Camon sur la Garonne. Bulletin

Francais de la Peche et de la Pisciculture 353/354:

121–140.

Desrochers, D., 1995. Suivi de la migration de l’anguille

d’Amerique (Anguilla rostrata) au complexe Beauhamois,

1994. MILIEU & Associes Inc. pour le service Milieu

naturel, vice-presidence Environnement, Hydro-Quebec:

107 p.

EPRI, 1987. Turbine-related fish mortality: review and evalu-

ation of studies. Research Project 2694-4, Final Report:

102 p.

EPRI, 1992. Fish Entrainment and Turbine Mortality Review

and Guidelines. Stone and Webster Eng. Corp., Boston,

Massachusetts: 225 p.

Gosset, C. & F. Travade, 1999. Etude de dispositifs d’aide a la

migration de devalaison des salmonidae: barrieres com-

portementales. Cybium 23 (Suppl 1): 45–66.

Gosset, C., F. Travade, C. Durif, J. Rives & P. Elie, 2005. Test

of two types of bypass for downstream migration of eels at

a small hydroelectric power plant. River Research and

Applications 21, 1095–1105.

Hadderingh, R. H. & H. D. Bakker, 1998, Fish mortality due to

passage through hydroelectric power stations on the Me-

use and Vecht rivers. In Jungwirth, M., S. Schmutz & S.

Weiss (eds), Fish Migration and Fish Bypasses. Fishing

News Books, Oxford: 315–328.

Holzner, M., 2000. Untersuchungen uber die Schadigung von

Fischen bei der Passage des Kraftwerks Dettelbach.

Thesis Technische Univeritat Munchen: 335 p.

Larinier, M., M. Chanseau, F. Bau & O. Croze, 2005. The use

of radio telemetry for optimising fish pass design. In

Spedicato, M. T., G. Lembo & G. Marmulla (eds),

Aquatic Telemetry: Advances and Applications. Pro-

ceedings of the Fifth Conference on Fish Telemetry held

in Europe, Ustica, Italy, 9–13 June 2003. Rome, FAO/

COISPA: 53–60.

Larinier, M. & J. Dartiguelongue, 1989. La circulation de

poissons migrateurs: le transit a travers les turbines des

installations hydroelectriques. Bulletin Francais de la

Peche et de la Pisciculture Special Issue 312/313: 94.

Larinier, M. & F. Travade, 1999. The development and eval-

uation of downstream bypasses for juvenile salmonids at

small hydroelectric plants in France. In M. Odeh (ed.),

Fish Passage Technology. American Fisheries Society,

Bethesda, Maryland, USA: 25–42.

Larinier, M., F. Travade & J. P. Porcher, 2002. Fishways:

biological basis, design criteria and monitoring. Bulletin

Francais de la Peche et de la Pisciculture 364: 208.

Hydrobiologia (2008) 609:97–108 107

123

Monten, E., 1985. Fish and Turbines. Fish Injuries during

Passage through Power Station Turbines. Vattenfall,

Stockholm: 111 p.

Pallo, S. & M. Larinier, 2002. Definition d’une strategie de

reouverture de la Dordogne et de ses affluents a la

devalaison des salmonides grands migrateurs. GHAAPPE

Report RA.02.01: 60 p.

Subra, S., P. Gomes, S. Vighetti, P. Thellier, M. Larinier & F.

Travade, 2005. Etude de dispositifs de devalaison pour

l’anguille argentee. Comportement de l’anguille et test

d’un dispositif de devalaison a l’usine hydroelectrique de

Baigts de Bearn (Gave de Pau – 64). EDF R&D GHA-

APPE Report HP-76/05/025/A-RA.05.03: 89 p.

Tarrade, L., A. Texier, L. David, G. Pineau & M. Larinier,

2006. An experimental study of turbulent flow in vertical

slot fishways. Symposium on Hydropower, Flood Control

and Water Abstraction: Implications for Fish and Fisher-

ies, Mondsee, Austria, June 2006: 14–21.

White, C. M. & P. Nemenyi, 1942. Report on hydraulic

research on fish passes. In Institution of Civil Engineers

(eds), Report of the Committee on Fish Passes. Clowes

and Sons, London, Great Britain: 32–61.

108 Hydrobiologia (2008) 609:97–108

123