Hydraulic Structures: Fourth Edition

7.2.2 Parameters in monitoring dam behaviour

The most significant parameters in monitoring dam behaviour are as follows:

1. seepage and leakage (quantity, nature (e.g. turbidity), location and
   source);


2. settlement and loss of freeboard in embankments (magnitude, rate);


3. external and internal deformation (magnitude, rate, location);


4. porewater pressures and uplift (magnitude, variation);

Certain key parameters are of primary concern regardless of the type of
dam considered, e.g. seepage and external movement or deflection; others
are relevant essentially to a specific type of dam, e.g. porewater pressures
in relation to earthfill embankments. The relative significance of individual
parameters may also reflect the nature of a problem under investigation,
e.g. the settlement of an old embankment, where progressive deformation
is suspected.
The desirable minimum provisions for monitoring and surveillance
on all dams, to be installed retrospectively if necessary, should account for
the measurement of seepage flows and crest deformations. The latter pro-
vision is of particular importance in relation to detecting settlement on
embankment crests as an indicator of possible internal distress and of local
loss of freeboard.
The supreme importance of seepage flow as a parameter cannot be
overstressed. Regular monitoring should be standard practice for all but
the smallest of dams. Serious problems are invariably preceded by a
detectable change in the seepage regime through or under the dam which
is unrelated to changes in the retained water level or to percolation of pre-
cipitation falling on the downstream slope. Direct observation of the
seepage quantity and turbidity is relatively simple, with internal drain
systems conducted to calibrated V-notch weirs. Ideally, a number of weirs
are each positioned to collect the flow from specific lengths of the dam,
permitting identification of the approximate location of any change in the
seepage régime.
The principal parameters are set out in greater detail in Table 7.2,
with identification of the instruments employed. The table also lists indica-
tive examples of defects or problems which may be associated with abnor-
mally high values or significant trends in each parameter.

7.2.3 Instruments: design principles

Monitoring instruments are required to function satisfactorily under very
harsh environmental conditions and for essentially indeterminate periods

INSTRUMENTATION 293