1. Introduction
A fundamental service for any inhabited
centre, but which is essential for a predominantly tourist area
such as the Isle of Elba is that of proper supplies of drinking
water. In the case in question, serious problems are encountered
with the low supply from sources during the Summer, which leads
to frequent interruptions to the water supply and the presence
of pollutants in the water that compromise it for drinking use.
An original solution is proposed here which is perfectly suited
to such a particular area as that in question and it is felt that
it will solve the problem once and for all making the island autonomous
and self sufficient.
2. The current situation of water supply on the Isle of Elba
The Elba drinking water supply is currently provided by means
of local aqueducts fed by springs and wells and in order to meet
continuously growing demand it has been connected to mainland
supplies in the Val di Cornia through a long submarine tunnel
(see Fig. 1).
During particularly dry periods in the Summer serious crises frequently
occur which threaten to compromise the local economy. The causes
are to be sought in the precariousness of the supplies from Val
di Cornia which, because of the size of the mainland and island
areas it serves is no longer able to produce water of sufficient
quantity and quality to meet the demand. Pollution by boron in
quantities higher that the percentages permitted by law is particularly
serious and is currently only tolerated because of special exceptions
to the norms that have been granted.
Water supplies on a "shift" basis to which resort must
be made is also serious and not only does this produce understandable
hardship due to the continuous interruptions to the supply but
it also pollutes the mains networks because of the intermittent
absence of pressure in pipes and the consequent entry of contaminated
water or substances harmful to health.
3. The improvement works planned by the utility company
The water company starts, in its programme
of works intended to solve the serious problems described above,
with the assumption that it will continue to rely on supplies
from Val di Cornia, over which, as already mentioned, there are
serious doubts regarding their effectiveness. It also intends
to increase local supplies above all by constructing new wells.
This measure is also wrong because in my opinion the wells are
destined to run into problems because of the poor quality of the
catchment area just at the time of greatest need, which is during
the particularly dry Summer period with considerable numbers of
tourists present.
4. Improvement projects
One of the improvement projects most
discussed in recent years, without, however leading to any implementation
due to the uncertainty over the results, is above all that based
on the construction of artificial reservoirs to be created by
constructing dams at Pomonte and Patresi (see Fig. 1). These works,
implemented with success in other localities afflicted by water
shortages are not feasible because, in the case of the Isle of
Elba, they present various problems such as the difficulty of
appropriating the land for large surface reservoirs, the serious
damage to the environment caused by the artificial lakes which
must be emptied in the Summer to use the content and finally because
of the huge water losses caused by evaporation. Another solution,
which perhaps deserves more attention than it has been given,
is described in the paper "Uso degli acquiferi locali per
la regolazione delle risorse idriche dell'Isola d'Elba" ("The
use of local aquifers for regulating water") by Prof. Pier
Gino Megale of the University of Pisa. It involves the construction
of an underground reservoir with a capacity of 2,000,000 cu. m.
by means of impermeable diaphragm walls which surround the plain
of Marina di Campo (see Fig. 1 ).
A further proposal concerns the use for
both drinking water and water for complementary purposes and for
irrigation of water recycled from public sewers after proper treatment.
This solution, often adopted in compliance with legal requirements
concerning drinking water, is not compatible, in the case of Elba
with the high degree of fragmentation of the sewer network which
consists of a large number of small sewage treatment works, difficult
and extremely costly to run to which, in the case of the non drinking
water, would be added the need to construct and manage a dual
distribution network.
The last project on the drawing board is for the installation
of desalination plants for salt or sea water which would not only
be very costly but would also be unsuited to the large variations
in the island's demand for water only being able to produce modest
quantities constantly over time.
5. The tunnel-reservoir
An examination of rain fall statistics
over recent decades confirms that annual volumes of precipitation
on the Isle of Elba are more than sufficient to meet the demand
for water but that there is a timing imbalance between the period
of intense rains which normally fall in the Autumn-Winter-Spring
period and that of the Summer when precipitation is very slight
(see Fig. 2).
What is missing to fully meet demand for drinking water and the
peak consumption in the Summer due to the presence of tourists,
is a large reservoir which, instead of allowing the large quantities
of water present out of season to run unused into the sea as happens
at present, collects and accumulates them to then use them when
they are needed, without having to resort to the Val di Cornia
or to the desalination of seawater with the problems and costs
that accompany them.
Since such an immense work would end up by doing irremediable
harm to the beautiful countryside on Elba if built on the surface,
it would be better to construct it underground where the conditions
for storing it are better.
The underground construction considered best for the purpose is
a tunnel excavated in rock and lined in concrete to which the
unusual, but very compatible, role of storing water is assigned
(see Fig. 3).
The work that results is singular but not new in the drinking
water supply sector. It adapts itself perfectly to the geography
of Elba as shown by the following examination of its particular
features and favourable local circumstances. The route, above
all. Since the western area of the island where the Monte Capanne
mountain is located has the most rainfall statistically and that
from an orographical viewpoint it consists of a series of small
valleys and gulleys which come down from the summit of the mountain
fanning out to feed a vast catchment basin, the route planned
winds 360° round the mountain itself, crossing or passing
under the rain channels making it possible to capture substantial
flows of water that collect there during the rainy seasons. The
tunnel, just like a road tunnel, constitutes the required reservoir
with a capacity of 2,000,000 cu. m. with an internal diameter
of 10 m. and a length of approximately 25 km as shown in figures
3 and 4. It should be noted here that of all the possible designs
for a large capacity reservoir, only that of a tunnel with a small
cross section and considerable length makes it possible to drain
extremely large areas and reach water sources, by branch tunnels
if necessary, where ever they may lie, even if very far one from
the other. Since, what is more, the tunnel is free to take any
direction because the only constraint is given by the depth of
the bottom and the crown which must remain constant for the whole
route, it has the additional advantage of being able to run through
ground that is most suitable for its construction and maintenance
both with regard to the quality of the material and the availability
of water in the ground through which it passes. The actual route,
to be decided at the final design stage on the basis of in-depth
analysis and studies, may therefore be very different from the
general indication given in the drawings without this making any
substantial changes to its functionality illustrated here. There
must obviously be accessory works in the vicinity of the tunnel-reservoir,
such as those for capturing and channelling gully water as well
as, perhaps underground works, for siphoning, filtering and sterilising
water, taking into account that the reservoir must accumulate
drinking water only, ready for delivery to consumers. There may
also be additional works for channelling and raising water to
be captured and drawn into the tunnel for water from outside the
underlying catchment basin or at levels lower than the tunnel.
Then there are also secondary advantages to the work, not to be
underestimated. The excavation of a tunnel, following a phenomenon
which usually causes considerable difficulties in carrying out
works, but which is fortunate in this case, will end up by drawing
water into it from the water tables present below ground on Monte
Capanne. This presence is confirmed by the numerous natural springs
there and they will provide an important supply of natural drinking
water which will be particularly abundant during the rainy seasons.
Another advantage to take into account is the quality of the rock
through which the tunnel will run, consisting entirely of marvellous
Elba granite. Not only will it offer all the necessary guarantees
for the stability of the construction during and after the works,
but it will also provide large quantities of a stone product which,
properly exploited, could help to cover large part of the construction
costs of the tunnel. In this respect it is worth noting that at
present the quarry workers co-operative which operates at S. Piero
in Campo is obliged to import granite from China because of the
difficulties imposed by the current legislation on open quarry
work which causes so much damage to the environment. To prevent
this and allow this work to continue, or perhaps to increase,
it is planned to organize the excavation of some parts of the
tunnel-reservoir in such a way as to obtain blocks of granite
suitable for the activity described. The remaining part of the
material excavated, appropriately crushed and sifted, may be used
in various ways, as aggregate for concretes, sand for the restoration
of beaches eroded by tides or to extend others, for the construction
of road embankments and for squares and car parks or, finally,
to restore the environment in areas damaged by quarrying in the
past.
It must not be forgotten that the theatre in which the operations
described here occurs is an island with all the difficulties which
that involves for all types of supply including that of construction
materials. Another point in favour of the tunnel-reservoir option
is the possibility of large part of the urban centers being supplied
directly by gravity thanks to the high altitude at 150 meters
above sea level at a first approximation limiting the costly use
of pumps to a minimum. In this respect, it should also be recalled
that the supply network that currently connects the various Elba
aqueducts running water from Val di Cornia down the length of
the island from East to West can also be fed in the opposite direction
which is to say from West to East and therefore distribute, as
it stands and without the need for aqueduct construction, all
the water collected and stored in the tunnel-reservoir. Finally
the accumulation of large volumes of rain water curbs the flow
of rivers in flood helping to reduce damage which is often caused
when banks overflow and there are floods after particularly intense
rain. The fact remains, however, that the most important result
achieved by the works in the project is that of allowing Elba
to supply itself autonomously and avoid resort to costly and precarious
supplies of water from Val di Cornia, supplies which could suddenly
disappear for a variety of reasons including the very poor chemical
characteristics of the water which might at any moment result
in their use for drinking water being forbidden, the impossibility,
intrinsic to the Val di Cornia waterworks, of satisfying the whole
of the increased demand for water consumption and finally, the
possibility, far from remote, that the under water pipeline to
the mainland might break with maintenance and repair operations
greatly affected by the depth at which it lies beneath the sea.
If it is considered that soon all the water supplies for a large
area, which includes Elba, defined as Ambito Territoriale Ottimale
(ATO- optimum geographical area), will be provided by one large
public body, which will implement the Integrated Water Service,
then it is understandable that the presence of a huge reservoir
at one end of the area, which is also located in a decentralized
position such as the Isle of Elba compared to the other areas
included in the ATO, constitutes a great security factor for the
water supplies of the entire region. The connection with the mainland
through the existing underwater pipeline, currently essential
for supplying Elba, would then change its function becoming a
reserve for use only exceptionally for possible exchange of water
in the two directions, Isle of Elba-mainland and mainland-Isle of Elba, if needed due to unforeseen
events.
Finally there is the extremely important question of the cost
of the works. The construction of a tunnel like that suggested
would cost a good 150 million euro. However, estimates made by
a company specializing in works of this type find that the presence
of an extraordinary material such as the granite of which the
Capanne mountain is made would cover large part of the sum if
the work was properly organized.
6. General characteristics of the tunnel-reservoir
The tunnel-reservoir consists mainly
of a circular construction with an internal diameter of 10 m.
with the geometry of the route in a ring at a height of 150 metres
above sea level surrounding the western area of the island and
its heights including the Capanne and Perone mountains with peaks
at heights of 1,018 and 630 metres above sea level respectively
(see Fig. 2). Along the outer perimeter, there are points on the
route where the main valleys lie at which either the main tunnel
or access tunnels come to the surface making it extremely easy
with modest surface works to collect water or run it into the
tunnel from ditches and brooks after possible siphoning, filtering
and disinfection to be performed in the tunnel or in separate
installations perhaps also underground as well as to run water
from other sources such as wells or local aqueducts into the tunnel.
Another extremely useful characteristic is the ability to capture
water from water tables existing in the ground. The route of the
tunnel which runs perpendicularly across all the gullies and small
valleys in the zone (see Fig. 3), guarantees that all the sources
of water that the tunnel passes beneath during excavation will
end up by being drawn into the tunnel as a result of faults and
fractures in the rock. It will therefore be extremely easy to
construct works to collect and regulate the water in the points
of intersection with the water table, without prejudicing the
possibility of intercepting and deviating them, if necessary,
through external drainage piping (see Fig. 5). Each entry of water
must in fact be kept under quantitative and qualitative control
by means of automatic measuring apparatus and continuous data
transmission. The tunnel-reservoir, excavated for its entire length
through rock strata, will be lined entirely with reinforced concrete,
not just for statics purposes but also to ensure that it does
not leak and to create effective protection against undesirable
infiltrations (see Fig. 4). There will be a small channel along
the centre of the bottom of the tunnel designed to collect and
remove deposits of sand during periodical cleaning to be performed
by jets of water provided from special tubing. The arch of the
tunnel will contain a power line for lighting and any electrical
equipment that may be required during maintenance, cables for
operating and controlling equipment and for data transmission
and piping for high pressure water supplies and for compressed
air where this is considered necessary. Drainage piping will be
place along the lower part of the tunnel outside the lining. This
will be indispensable for removing water during construction work
and will be used, when the tunnel is in service to drain off water
outside the tunnel which is not up to the appropriate standards
and water which must in any case be either permanently or temporarily
removed. The vertical geometry of the route is horizontal sloping
slightly upwards to inlet points. The final route will be carefully
defined at the final design stage in order to make it pass through
the most interesting areas in terms of the quality of the material
excavated and the availability of surface and groundwater.
7. Principal data determining the dimensions of the project
The main data is as follows:
- the underlying catchment basin: . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . .40,000,000 sq. m. approx.
- minimum annual rainfall forecast: . . . .. . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . .500 mm.
- minimum total annual volume of water: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20,000,000 cu. m. divided as follows:
…volume of rainwater lost due to evaporation and transpiration 58%: . . 11,600,000 cu. m
.…volume of surface run off 29%: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5,800.000 cu. m. volume
…volume of underground run off 13%:
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 2.600.000 cu. m.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .
. . . . . . ______________
...Total: . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . .20,000,000 cu. m.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . ==============
- annual volume collected by the tunnel : 55,800,000 cu. m.+ 2,600,000
cu. m. = 8,400,000 cu. m.
- quarterly average volume: 8,400,000 cu. mm. / 4 = . . . . . .
. . . . . . . . . . . . . . . . . . . . .2,100,000 cu. m.
- population equivalent to be supplied: 250,000 inhabitants in the major tourist periods and 50,000 inhabitants in the off season
- daily requirement on peak days: 250,000
x 0.300= 75.000 cu. m.;
- on other days: 50,000 x 0.300 = 15,000 cuu. m.
- volume necessary for quarterly compensatiion: 90 days x 75.000
cu. m. x 0.22 = 1,500,000 cu. m.
- capacity of tunnel-reservoir: 25,600 m. xx 76.20 sq. m. = 1,950,000
cu. m. corresponding approximately to the average quarterly inflow
of rainwater (2,100,000 cu. M.)
8. Conclusions
The water supply for a very particular and delicate geographical
area like that of the Isle of Elba, still affected by an intolerably
poor supply, cannot be provided without resort to special solutions
which take account of the equally special environment and integrate
perfectly with it. There are valid reasons to consider that the
works proposed here, consisting brasically of a large tunnel-reservoir,
are suitable to satisfy this primary requirement rationally and
also to contain environmental damage caused by construction and
operation to a minimum considering that they are totally underground.
The island is freed from all external dependence on water supplies
by means of plant that is very economical to construct and run
because the expenses to be incurred for construction are to a
large extent paid for by the use the granite excavated, the operating
supply cost are relieved of the high costs of external supply,
because local water only is used and consumption of electrical
power for pumping is considerably diminished because the operation
exploits the natural fall of the water.
The works planned are based essentially on the use of a tunnel
excavated in rock (using a construction technique which has made
formidable leaps forward in recent years), different from those
generally known but effective nonetheless. The function that the
tunnel is called upon to fulfill for the whole of its 25 km. length,
is not in fact the usual function of road or rail transport, but
that of containing an element whose existence underground is an
established rule of nature. A total of two million cu. m. of drinking
water can be stored underground for a long time, in the cool,
in the dark and sheltered from the sun, from the intrusion of
vegetation or animals and lastly, but not to be overlooked, treating
it at an altitude sufficiently high to be able to supply it directly
by gravity to most of the consumers without the need for many
of the pumping stations currently employed.They are undoubtedly
complex and important works requiring careful analysis and study
at the design stage in many sectors. These include rainfall and
geological, hydrological and hydraulic aspects of the area, the
routes and the vertical geometry to be decided, the final capacity
of the reservoir, methods of collecting water, the channelling
of water into the reservoir both directly and through channelling
works, the reduction of floods, excavation techniques and the
construction of the underground installations in general, the
best use of the water capacity etc., subjects which it is obviously
impossible to summarize in these few pages. The intention here
was to simply give a general idea of the work and above all to
point out how useful it is to transfer services underground which
can cause serious damage on the surface. This possibility, often
ignored on mainland areas which must put up with the presence
of enormous and space consuming installations, becomes essential
on the Isle of Elba which is already too built-up and could not
stand the impact of a surface reservoir like that necessary to
solve its water supply problems.
Further information on the characteristics of the tunnel-reservoir
and the relative management of it can be found at the web site
http://altratecnica.3000.it where the general project is described
complete with a report and drawings.
environmental damage caused by construction
and operation to a minimum considering that they are totally underground.
The island is freed from all external dependence on water supplies
by means of plant that is very economical to construct and run
because the expenses to be incurred for construction are to a
large extent paid for by the use of the granite excavated, the
operating supply costs are relieved of the high costs
Bibliography
BENCINI A., GIARDI M., PRANZINI G.,TACCONI B.M., (1985) Le risorse
idriche dell'Isola d'Elba, Tac
chi Editore, Pisa.
MEGALE PG., Uso degli acquiferi locali
per la regolazione delle risorse idriche dell'Isola d'Elba,
World Water Day "Acque sotterranee: risorsa invisibile",
vol. 1955, pp. 115-120, Rome 1998CIGRISPA
Consorzio Intercomunale per la Gestione delle Risorse Idriche, II Piano di risanamento, October 2000.
BRACCESI G., PRANZINI G., (1997) Carta della Vulnerabilità all'Inquinamento degli Acquiferi dell'Isola d'Elba, EL.LC.A. Meeting, Portoferraio, September 22-24, 1997.
MARINELLO G., Carta geologica dell'Isola d'Elba alla scala 1:25000