Environmental Engineering Reference
In-Depth Information
(l) The seepage monitoring system needs to be capable of being calibrated to separate out
the effects of rainfall or snowmelt e.g. by prior observation and monitoring and cou-
pling seepage measurements to rainfall and snowmelt measurements at the dam. It
should however be recognised that there are many situations where it will be unlikely
that seepage will be detected, e.g. at night when visual surveillance is being relied upon;
if the toe of the dam is submerged; if seepage occurs high on the dam abutments and
bypass measuring weirs or in winter when the dam is covered by snow.
(m) There are clearly many dams which may have progression and breach times of the
order of hours. These dams particularly include some older dams which have no filter
or transitions, dams without downstream rockfill zones to stop or slow the erosion
process, and/or those dams on erodible foundations without well designed and con-
structed cutoffs or filters to intercept the foundation seepage. For these dams, an effec-
tive seepage monitoring program would require virtually continuous monitoring.
Daily, or even twice daily inspections, or measurements may be inadequate.
20.3
WHAT INSPECTIONS AND MONITORING IS REQUIRED?
20.3.1
General principles
As discussed in ICOLD (1989) and ANCOLD (2003), the inspection and monitoring
appropriate for a dam depends on a number of factors:
(a) The consequences of failure. Dams with a higher consequence of failure in terms of poten-
tial lives lost, economic and environmental damage, require a higher level of inspection
and monitoring;
(b) The type of dam. For example, dams which have no filters or drainage zones, may
require greater inspection and monitoring than a well designed and constructed dam
with filters, assuming the same consequences of failure;
(c) The type of dam foundation. A dam on permeable soil foundations would usually
require more inspection and monitoring than the same dam on a rock foundation;
(d) The size of dam. This is usually linked to (a) and (b), but in general larger dams have
a higher level of inspection and monitoring than small dams;
(e) Known deficiencies or deterioration of the dam. If there are identified safety issues,
such as marginal slope stability or larger than normal seepage or pore pressures, there
will usually be a need for enhanced inspection and monitoring until the deficiencies
are rectified;
(f) Identified potential failure modes;
(g) The age of the dam. There is a need for enhanced inspection and monitoring during first
filling of the reservoir, because it is known that many failures and accidents occur in this
period.
To this the authors would add:
(h) The reservoir level, compared to historic high levels. There is strong evidence that
failures and accidents from internal erosion and piping occur above or near historic
high reservoir levels. At such times the dam is also at greatest risk from a slope insta-
bility point of view. For dams with known deficiencies in internal erosion and piping,
there should be a significantly increased level of inspection and monitoring under
these reservoir conditions.
(i)
The reservoir level, compared to historic low levels. Many dams with marginal
upstream slope stability, or subject to internal deformations due to poorly compacted
