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Biological Journal of the Linnean Society (1994), 51: 25-36. With 6 figures
Tidal power and the aquatic environment of
La Rance
C. RETIERE
Museum National D’Histoire Naturelle Laboratoire Maritime, 17 Avenue George V,
35801 Dinard, France
The Rance basin, on the northern coast of Brittany, is the only site where a full-scale evaluation of
the ecological impact of a tidal power scheme, after 20 years of operation, has been made. The
isolation of the estuary, during the construction phase, was particularly damaging to the
environment. Gradually, after the scheme was put into service, an increasingly diverse flora and
fauna became established. The patterns of distribution of this flora and fauna, their groupings into
ecological units and the nature of their interrelationships, indicate a variable degree of biological
adjustment to the new environmental conditions. Migratory organisms are able to pass via sluice
gates and turbines. However, the new ecological equilibrium, established in the space of 10 years,
remains fragile and, being linked to the degree of stability of abiotic conditions, depends to a large
extent on the operating conditions of the power station.
ADDITIONAL KEY WORDS:-environmental
communities - trophic relationships.
effects
-
environmental equilibrium
-
CONTENTS
Introduction . . . . . . . . . .
The tidal power installation . . . . . .
Effects on the environment
. . . . . .
Effects of construction
. . . . . .
Effects of operation . . . . . . .
Ecological consequences of operation . . . .
Distribution of benthic species and communities
Species biology . . . . . . . .
Primary and secondary production
. . .
Movements and migration
. . . . .
Trophic relationships . . . . . . .
Utilization of the basin by overwintering birds
Conclusions . . . . . . . . . .
References
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INTRODUCTION
A number of investigators have attempted to predict the effects of various tidal
power projects on the marine environment, in various parts of the world, (e.g.
Martin, 1970; Gordon & Longhurst, 1979; Shaw, 1980; Larsen & Topinka,
1984). The Rance Basin, on the Armorican seaboard of the Channel, is the only
site which offers the opportunity to make a full scale assessment of the ecological
impact of a tidal power scheme after 20 years of operation (Retiere, 1989).
0024-4066/94/001025
+ 12 $08.00/0
25
0 1994 The Linnean Society of London
26
C. RE’I‘IERE
Dinard
Tidal Power Station
I
19”
30
N
t
1
Norman-Breton
Gulf
.
Figure 1. The Norman Breton Gulf and the Rance Estuary.
Of the seas of north west Europe, the Western Channel has some of the largest
tides: the spring equinox tidal amplitude reaches 15.5 m in the Bay of Mont St
Michel and 13.5 m a t the mouth of the Rance river. The latter is a medium-sized
river, approximately 100 km in length, with a low discharge (average 7 m3
sec-’). The estuary forms a succession of narrow channels and wide basins,
extending approximately 20 kilometres. It is at the mouth of this river, between
the towns of Dinard and St Malo, that the tidal power scheme was built.
Construction began in 1963 and was completed in 1966, creating a reservoir
with a useful capacity of 184 million cubic metres occupying a surface area of
2200 hectares at the 13.50 m level (Fig. 1 ) .
+
THE TIDAL POWER INSTALLATION
The installation is 750 m in length, anchored to granite bedrock 13 m below
zero datum level as shown on the marine charts. The structure carries a major
road and, from west to east, comprises a lock, the generating station proper with
its 24 bulb units, a rockfill dike and a 115 m wide movable dam made up of 6
~~
Figure 2. The barrage and the tidal power station.
sluice gates capable of passing a total flow of 9600 m3 sec-I under a 5 metre head
(Fig. 2).
I n addition to a 10 MW alternator, each bulb unit incorporates a 4-blade
horizontal turbine running at 94 rpm and capable of handling a flow of
275 m3 sec-'. These bulb turbines can handle water flowing in either direction,
and are thus able to generate power both during filling and emptying of the
basin. This type of operation is referred to as a double action cycle (Fig. 3 ) .
1
With pumping
I
7
Single action a t drainage
I
With pumping
1
Double action
Figure 3. Tidal curves compared with the water level variations (single and double actions) in the
basin (from Electricit; de France).
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C. RETIERE
Transfer of water from the sea into the basin takes place through the turbine and
sluice gate combination and via the turbines alone in the opposite direction.
EFFECTS O N T H E ENVIRONMENT
Effects of construction
Construction was carried out dry inside three cofferdam enclosures. With the
exception of small amplitude discharges ( 1 m) a t 2-week intervals, for flushing
purposes, the estuary was isolated from the open sea for 3 years between 1963
and 1966 (Rouvillois, 1967). During this period, the conversion of the estuary
with a large tidal range into a constant-level basin virtually eradicated the
marine fauna and flora; however, some populations of highly euryoecious
invertebrate and fish species flourished.
Effects of operation
The operating constraints of the installation impose highly specific ‘tidal’
conditions on the waters in the basin. Tidal range varies between 4.0 and 5.5 m,
depending on whether the turbines are operating in one or both directions.
Mean water level is raised by approximately 2.5 m and there is a particularly
long high water stand.
Reduction in tidal range is correlated with a reduction in the surface area of
the intertidal zone. At spring tide, prior to the scheme, this area was estimated at
70% of the total surface of the basin, whilst today the exposed zone accounts for
only 50%.
Prior to the scheme, the Rance was a ria, with slight haline stratification,
along which authors identified a maritime downstream sector followed by a high
salinity section giving way to a low salinity sector (Fischer, 1929; Fischer-Piette,
1931, 1933; Marie, 1938; Priou, 1947). Today, only two elements can be
identified: the marine reservoir, in which the deepwater salinity remains higher
than 30%0, and the estuary proper, upstream (Retiere & Richard, 1980). The
junction between ‘brackish’ and ‘marine’ has moved upstream from St Suliac to
Port St Jean.
The violent sluice and turbine currents have modified the distribution of the
sediments, whose origin, nature, texture and distribution had been extensively
studied between 1954 and 1959 (Berthois & Berthois, 1954, 1955; Bourcart &
Roa Morales, 1957, Bourcart, 1959). Thus, in 1971, Retiere reported that parts
of the river bed in proximity to the barrage had become eroded, sandbanks had
shifted and the bed was more or less covered with gravel or pebbles (Retiere,
1979). At the same time, the long periods of slack water had promoted
deposition of fine particles in the coves and bays (Retiere & Richard, 1980).
However, the sedimentary balance appears to reflect a natural evolution of the
estuary, influenced by the absence of any large reservoir of suspended material in
the coastal waters near the mouth of the Rance (Han, 1982). These changes,
which in any event are rather limited in scale, (Laboratoire Central
D’hydraulique de France, 1982), were most pronounced during the first few
years following startup of the installation (Fig. 4).
LA RANCE ENVIRONMENT
Figure 4. Sedimentary modifications (from Retiere & Richard, 1980). A, Eroded bottoms, B,
Pebbles locally embedded by coarse sands; C, Moved banks; D, Fine particle sedimentation areas; E,
Clean heterogenous sands; F, Muddy deposits.
ECOLOGICAL CONSEQUENCES OF OPERATION
Distribution of benthic species and communities
After 20 years of operation, the basin is rich in species: records show no less
than 110 species of polychaetes (Retiere, 1979; Lechapt, 1983; Lang, 1986), 47
species of decapod crustaceans (Le Calvez, 1986) and 70 species of fish (Le Mao,
1985).
Within the intertidal zone, however, the vertical range of algae is limited to
5-6 m instead of 13 m (Lang, 1986; Herpe, 1986). The irregularity of
C. RETIERE
30
m A
13 *
12.
11.
10.
9.
4
3
2
1
January
80
February
80
March
80
April
80
1
I
m B
1312
1110.
98-
7.
6.
5-
4.
3.
2.
1~
I
I
1
Figure 5. Tidal curves ( A ) and variations of high and low water levels in the basin ( B ) (from Lang,
1986).
fluctuations in water level (Fig. 5) has resulted in the establishment of a unique
community in the littoral zone, within which the belts described for the true
intertidal range are not clearly manifested.
LA RANGE ENVIRONMENT
31
Melinna palmata
Figure 6 . Benthic communities (from Retiere, 1979). A, Spisula elliplica - S. ovalis - Saccocirrus
papillocercus coarse sand community; B, Abra alba - Melinna palmata muddy fine sand community. C,
Impoverished facies of the Abra alba - Melinna palmata community; D, Epifaunal hard bottom
community; E, Neph&s cirrosa clean sand community.
Subtidally, in 1971, Retiere identified four major benthic communities
(Fig. 6). A period of 5 years had been, therefore, sufficient for the establishment,
albeit in limited diversity and on certain substrata formerly occupied by
intertidal fauna, of the subtidal communities present at the periphery of the
Normand-Breton gulf (Retiere, 1976; 1979). Species maps prepared 5 years
apart (1971/1976) showed clearly that the stability of the abiotic factors was
reflected in the spatial distribution of the organisms (Retiere & Richard, 1980:
Lechapt, 1983).
Between 1972 and 1982, monitoring of the principal infaunal components of
the Melinna palmata facies of the Abra alba-Corbula gibba community indicated
that a period of increase in species richness and density (up to 1976) was
followed by a period of stabilization (Clavier et al., 1983). Throughout the
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C. RElIERE
observation period, six populations of polychaetes alone accounted for 80% of
the infaunal abundance and 90% of its biomass, thus largely controlling the
community dynamics. Study of these populations (Retiere, 1976, 1979; Clavier,
198I; Rivain, 1983; Lechapt, 1983) clearly indicated that recolonization
followed recruitment from populations closest to the estuary, irrespective of the
lifespan of the species or the length of the pelagic phase within their life-cycle.
Passive transport of adults also made a significant contribution to their
establishment in the Rance Basin. Despite rapid stabilization of environmental
conditions once the installation was commissioned, it took 10 years (1966-1976)
to establish a new community balance. This has subsequently evolved via the
natural mechanisms of intra and inter specific rela tionships, unrelated to the
initial disturbance, with the long-lifespan species imposing the general trend.
Species biology
Reproduction and growth of organisms do not appear to have been affected
by the new conditions. The Rance is a spawning ground for some fish such as
Clupea harengus and a nursery ground for others (Pollachius pollachius).
Catadromous eels (Anguilla anguilla, Conger conger) enter as larvae and leave to
breed. Not less than 30 fish species are confirmed breeders in the area. Studies on
the biology of many species of fish (Le Mao, 1985), suggest that the reproduction
and growth of organisms within the basin have not been affected by the barrage.
As another example, the ormer, Haliotis tuberculata is present at high densities in
northern Brittany (Gaillard, 1958) and compared with populations living on the
coast, those in the Rance had a more extended period of laying but not
significantly different growth rates (Clavier & Richard, 1985).
Primary and secondary production
Primary production is higher in the upstream regions (Lacaze et al., 1976)
particularly in spring. Even in the lower Rance, however, figures are higher than
in the adjoining sea (Grall, 1972). Secondary production and biomass fluctuate
from year to year, but mean annual biomass of the subtidal Abra alba-Corbula
gibba community in the Rance (15-18 gm-2 (Retiere, 1979) is double that of
intertidal flats (8.5 gm-'y) (Clavier, 1981) and of subtidal areas outside
(9.5 grnp2y) (Dauvin, 1984). Production (P) and production to biomass (P/B)
ratios of Nephgs hombergii (P = 6 grnp2y, P/B = 1.2) (Retiere, 1976) and
Ampharete acutifons ( P = 1.5-9.0 gm-*y; P/B = 2.5-6.0) (Clavier, 1981) in the
Rance are, however, similar to measured values for the same species in the
Lynher River, Britain (Price & Warwick, 1980).
Movements and migration
Being transition zones between coastal and fresh waterways, estuaries
normally provide essential lines of passage for numerous species. Movements
vary in scope and affect one or more phases of their life cycle. To what extent
does the presence of a barrage and the operation of turbines impede such
movements? This problem has been analysed by Le Mao (1985) who, in
addition to fish, focused attention on two invertebrates of economic interest, one
LA RANCE ENVIRONMENT
33
a creeping spider crab, Maia squinado, and the other a swimming cuttlefish, Sepia
oficinalis.
Immature M . squinado occupy the basin throughout the year, but adults arrive
in the spring in sufficient numbers to provide a fishery (De Kergariou, 1971).
For them, the barrage is no real obstacle. Sepia oficinalis also provides a seasonal
fishery. I n 1982, between April and June, Le Mao (1985) estimated production
at 25-35 tonnes, and in good years this can rise to 40 tonnes. Similarly, the
abundance of migratory fish suggests that passage through the turbines is
effective. Le Mao (1985) concluded that the passage of both fish and
cephalopods through the turbines takes place without major problems because of
their large diameter and low rotational speed (but see Dadswell & Rulifson,
1994). A degree of indirect mortality is, however, detectable: crossing the
structures throws shoals into disarray, providing the opportunity for predation
by fish-eating birds such as gulls, cormorants and auks.
Trophic relationships
Le Mao (1985) discussed the food webs within the Rance basin. Within the
pelagic communities, dominant plankton-feeders were the fish Sprattus sprattus,
Sardina pilchardus, Atherina presbyter and Clupea harengus. Feeding on these were
squid, Loligo vulgaris, L. forbesi, and larger fish Scomber scombrus, Trachurus
trachurus.
Benthic fish communities on hard substrate included herbivores (Spondyliosoma
centharus, juvenile Blennius gattorugine) , species which feed off the sessile
zoobenthos (Blenniuspholis) or vagile zoobenthos (Ciliata mustela, Trisopterus luscus,
Symphodus melops) and fish-eating species, which also feed on decapod
crustaceans, (Pollachius pollachius, Dicentrarchus labrax, Enophrys bubalis) .
O n soft sediments, some fish feed preferentially on the permanent (Gobius niger)
or temporary meiobenthos (juvenile Solea vulgaris and Pleuronectes platessa) , whilst
adult plaice and Callionymus bra prefer to feed off the macrobenthic infauna
Vagile decapod crustaceans form a significant proportion of the prey base and
diet of Dicentrarchus labrax, Anguilla anguilla and Platichthys Jesus. The cuttlefish,
Sepia oficinalis heads the food-chain on soft sediments. This chain has a more
hierarchical structure in the marine sector than in the estuarine sector where
crustaceans and benthic fish occupy the same trophic level.
It is important to consider whether the links established between the various
ecological compartments of the Rance basin are liable to be affected by the
operating practices of the power station. I n this respect, the consequences of the
abrupt level variations, recorded in June 1983 following the changeover from
single to double action operation and causing periodic exposure of otherwise
constantly submerged mudflats, are very revealing. The abundance of young
plaice, Pleuronectes platessa, declined by around 80% at a critical time-since
recruitment ceases in July. Many young plaice were found stranded on the mud
or having succumbed to the rise in temperature of the water in the puddles
where they had sought refuge, and invariably exposing themselves to predation
by birds. Colonization by gobies Pomatoschistus minutus followed the high
mortality. Le Mao (1986) also noted an increase in the vacuity coefficient of
surviving plaice which could not be attributed to prey depletion since, by the
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C. RETIERE
following month, the incidence of Ampharete acutzfrons was increased in the gut
contents of gobies.
Even a brief destabilization of environmental conditions, following a change in
the operating pattern of the station, is liable to have lasting repercussions on the
functioning of the ecosystem.
Utilization of the basin by overwintering birds
Every year, the Rance receives an influx of several thousand wintering birds.
Surveys by LF Mao el al. (1986) and Schricke & Bourgault (1982) have
indicated that the larger surface area of permanent water favours diving ducks
and grebes, especially Podiceps nigricollis. Since 1970, the numbers of
overwintering shelduck, Tadorna tadorna, and brent geese, Branta bernicula, have
increased and the concentrations of small waders have remained high.
Development of the estuary as a tidal power scheme has not had any adverse
effect on the avifauna.
CONCLUSIONS
It is possible to draw a number of general scientific conclusions from the
investigations conducted in the Rance basin (Retiere, 1984):
( 1) T h e tidal power installation is, to varying degrees, biologically permeable:
it permits recolonization of a modified environment and normal exchanges
between estuarine and coastal waters.
(2) A new balance is attained after about 10 years; the system then functions
on the basis of intra and inter-specific relationships, independently of the initial
disturbance.
(3) This new balance is entirely dependent upon the mode of operation of the
installation; a regular pattern stabilizes abiotic conditions.
(4) Sudden variations in environmental factors (associated with production
requirements or maintenance of the installations), albeit of short duration, may
have severe ecological effects at various levels in both space and time.
( 5 ) T h e success of a tidal power scheme depends upon striking the correct
balance, which it is possible to define, between operational constraints and
biological equilibrium.
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35
Clavier J, Richard 0. 1985. Etudes sur Ics ormeaux d a m la rkgion de Saint-Malo. Rapport &Etude de
I‘Association pour la Mise en Valeur du Littoral de la Co’te d’Emeraude: 285 pp.
Dadswell MJ, Rulifson RA. 1994. Macrotidal estuaries: a region of collision between migratory marine
animals and tidal power development. Biological Journal of the Linnean Sociely 51: 93-1 13
Dauvin JC. 1984. Dynamique d’ecosysthes macrobenthiques des fonds sedimentaires de la baie de Morlaix et
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