Environmental Engineering Reference
In-Depth Information
(g) Although modern information processing can help with huge masses of raw data,
experience has shown that it is a good policy to read often a limited number of key
instruments very well situated in the dam;
(h) The first filling of a reservoir is a stage in its life that is both important and delicate. It
is in fact the proof that a dam can fulfill its design functions. Of course, measurements
must start earlier so that the structure's initial state is known and some instruments are
read during construction so that the reaction of dam and foundation to loading are
known;
(i) Continuous reading of some instruments is conceivable during first filling to obtain a
large amount of information during this fairly short period. During normal operation
the raw data is processed by computer to give useful information. However complete
automation - the automatic transmission of the state of safety of a dam and possibly of
danger signs - is not used by EDF (Electricite de France). The delicate task of fine
interpretation belongs to the engineer.
The authors would add:
(a) The use of instruments that can be easily rechecked if readings differ significantly
from previous readings or are well outside the expected range;
(b) If a reading does not change for some time that is no reason to stop reading that
instrument;
(c) Results should be quickly recorded within the owner's storage system. Wherever pos-
sible the immediately gathered data should be presented in simple, easily understood
graphical plots and the data should be reviewed by an experienced dam engineer and
not left for 5 years or so until the next main review is planned.
20.4.2
Seepage measurement and observation
Seepage data is one of the best indicators of a dam's performance. By observing the loca-
tion, quantity and quality of seepage emerging from the dam embankment and its foun-
dation, and particularly the changes which occur, one can get early warning of problems
which may be developing, particularly in the important problem areas of internal erosion
in the dam and its foundation and in increased pore pressures.
It is emphasized that routine observation of where seepage is emerging can be as useful a
guide as the actual measurement. However, as indicated by most of the reporting countries
in ICOLD (1989) and ANCOLD (1983), it is usual to measure the quantity of seepage.
As shown in
Figure 20.9
,
it is preferable to collect the seepage close to the downstream
toe of the impervious zone, and to isolate areas from each other so the readings are not
influenced excessively by flow through rockfill zones and runoff from abutments.
However, as pointed out by ANCOLD (1983), for smaller dams it is acceptable to
measure total seepage downstream of the toe of the dam and for larger dams, it is often
impractical to have a system such as that shown in Figure 20.9. The authors know of
cases where drainage trenches to subdivide the foundation into collection areas under
rockfill were filled rapidly with fines from the rockfill, making the system ineffective. In
other instances the seepage from individual areas simply by-passes the collector drains -
the cost to seal below the drain is far too high. Where seepage is collected downstream,
the influence of rainfall runoff on the readings has to be determined and separated from
the base seepage flow.
Measurement of seepage may be made by:
-V-notch or similar measuring weirs. This may include continuous recording and teleme-
tering of data for more important dams. Level switches may be provided to give an
independent alarm;
