clearly identified. Within the operating plan the frequency of monitoring
should be determined on a rational basis, reflecting the objectives and the
individual parameters under scrutiny. It is, in any event, subject to amend-
ment in the context of the information retrieved.
Detailed prescription of periodicity is a question of common sense
allied to engineering judgement. An excess of data will prove burdensome
and may confuse important issues; too little information will raise more
questions than it resolves. Excessive complexity in a system, whether in
terms of equipment or the operating skills required, similarly diminishes
its utility. A judicious balance is therefore always required, and care must
be taken to ensure that the ‘system’ remains sufficiently responsive and
flexible. The monitoring routine should provide for observations at the dif-
ferent seasons and with significant changes in retained water level.
Representative monitoring frequencies for embankments may range as
indicated in Table 7.3.
Routines for prompt processing of field data must be established,
giving careful consideration to the optimum form of presentation. Charts
and overlays are generally the most satisfactory method, with parameters
plotted against retained water levels and precipitation as on Fig. 7.7. Illustra-
tive schematic diagrams are shown in Johnston et al.(1999). It may some-
times prove useful to superimpose predetermined ‘safe limit’ envelopes for
certain key parameters (e.g. porewater pressures) on such plots.
7.3 Surveillance
Dams of all types require regular surveillance if they are to be maintained
in a safe and operationally efficient state. As with all structures, they are
subject to a degree of long-term but progressive deterioration. Some of the
latter may be superficial in relation to structural integrity, but the possibil-
ity of concealed and serious internal deterioration must be considered.
Older dams will have been designed and constructed to standards which
may no longer be considered adequate, e.g. in terms of their spillway dis-
charge capacity or structural stability.
304 DAM SAFETY: INSTRUMENTATION AND SURVEILLANCE
Table 7.3 Representative monitoring frequencies
Parameter Frequency
Water level Daily wherever possible
Seepage Daily or weekly
Piezometers Once or twice weekly (e.g. construction), extending
to three to six monthly (routine)
Settlement–deformation Daily (e.g. suspected serious slip), extending to three
to six monthly (routine)
