VGB POWERTECH 7 (2020) - International Journal for Generation and Storage of Electricity and Heat

A journey through 100 years VGB | Hydropower | VGB POWERTECH · Issue 10 (2006)
Hydro-power in Europe
New
flap gate
Existing
gates
Usual, confined water path
New, multiple water path
Figure 3. “Piano Keys”: an efficient way to
improve spillway capacity.
Figure 2. Increase of spillway capacity at the barrage of Crescent.
a) Permanent measures involving:
– implementing appropriate maintenance
measures so as to maintain maximum reliability
of flood prevention means,
– carrying out regular tests to check that these
means are working,
– drawing up and implementing flooding
advice and instructions for the operator,
– carrying out ongoing training of operators,
in particular using flood passage simulation
tools.
b)Specific measures to re-evaluate extreme
flood flow rates, which were determined
at the time the facilities were designed by
simple extrapolation of the largest known
flood value. New hydrological studies carried
out by EDF using the GRADEX method,
which is recognised by the French
authorities, sometimes lead to higher results
than those of the original dimensions.
Detailed analysis of incidences of this deficit
is thus carried out so as to adapt the
solutions to the specific requirements of
each facility. As a last resort, it may prove
necessary to increase the capacity of the
flood prevention means.
For example, at the Crescent sur la Cure dam,
EDF has added an additional channel of
10.40 m equipped with a flap gate (height 4.5
m) to the two existing channels of the spillway
(segment gates L x H = 8.0 x 4.5 m).
The evacuable flow rate has thus been increased
from 275 to 430 m 3 /s for the same flood
level ( F i gure 2). Total cost of the operation:
€ 3,7 million.
“Piano Keys” Increase
for Flow Rate of Spillways
Faced with the problem of preventing floods,
one innovative solution which is much more
economical than creating a new gated channel
consists in installing “piano key” spillways
on the crest of the dam (Piano Key
Weir, PKW). This involves an arrangement
of rectangular channels which looks like the
keys of a piano, these being designed and arranged
in such a way that they increase the
overflow linear of the crest and multiply by
two to four the usual conveyance of a standard
weir. The result is that these are simple
and economical structures which can easily
be installed on the crest of many concrete
dams in order to remedy any deficits in terms
of evacuating floods. The innovative concept
of PKWs has already been used by EDF. It
should be noted that, due to a significant rise
in the operating level, PKWs can also be
used to increase at a reduced cost the storage
capacity of dams with an overflow crest ( F i -
gure 3).
Seepage Detection on Dykes
Using Distributed Optic Fibre
Temperature Measurements
Today, internal erosion is the main pathological
risk confronted by hydraulic structures
with large linear embankments in Europe. It
is a phenomenon that internal erosion tends
to develop very slowly for many years and
then suddenly it develops very rapidly thus
ruining the structure. That means that the ability
to detect such phenomena in good time is
a priority for the companies in charge.
It is difficult to detect this pathology. This is
why it is today one of the main preoccupations
of the plant owners, who must ensure the
safety of their structures and at the same time
optimise maintenance costs.
Until now, plant owners have had only two
main types of method for detecting leaks in
dykes:
– geotechnical or geophysical methods,
– visual inspections.
None of these current industrial methods
make it possible to detect leaks over a large
distance (typically around one to several kilometres)
continuously over time and automatically.
One promising method currently being developed
at EDF, and for which industrial installations
already exist in Sweden and in Germany,
consists in detecting leaks using a system
of distributed optic fibre temperature
measurements.
The principle of detecting leaks in a structure
by means of temperature measurements is
based on the fact that the temperature of the
medium passed through by a leak, in which
the transfer of heat is dominated by convection,
will be different from the temperature of
the embankment outside the leakage zone, in
which the transfer of heat is dominated by
conduction. By comparing the change in temperature
in different parts of the embankment,
it is thus possible to detect zones of
“abnormal” change, which are sites of particular
leaks.
This technology makes it possible to carry
out temperature measurements with a precision
of around 0.1 °C, which makes it possible
to detect leaks of around 1 l/min/m.
Developing New Projects
Hydro-power development projects can be
classified into three categories:
– New facilities
– Providing additional equipment for existing
structures.
This type of project makes it possible to
achieve a significant increase in power
(and sometimes energy) without having a
major impact on the environment 1 .
– Turbining of instream flows (residual
water flow).
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