<br />.i"
<br />.'
<br />II
<br />,I
<br />. I
<br />I
<br />I'
<br />. I
<br />I,
<br />II
<br />I
<br />I
<br />I
<br />I
<br />I
<br />I
<br />I
<br />I
<br />I
<br />I
<br />I
<br />
<br />This will indicate the relative criticality of the
<br />flood load cases and the order of risk reduction
<br />to bring the cumulative risk to life within the
<br />criteria, (Refer the indicative examples in the
<br />Risk Supplement, Appendix 3, A3.5.).
<br />
<br />It may not be practicable in all cases, due to
<br />design, construction and cost limitations, to
<br />propose remedial risk reduction options for
<br />existing dams which will meet the ANCOLD
<br />risk to life criteria, including in some cases
<br />even the Limit.
<br />
<br />The options then have to be considered for the
<br />specific case using an ALARP type principle,
<br />with the risk criteria as a reference rather than
<br />a prescriptive requirement, in conjunction with
<br />deterministic standards.
<br />
<br />Alternatively, the question of legal
<br />responsibility should be considered if options
<br />based on risk to life criteria would pose a
<br />higher risk than the established deterministic
<br />standards.
<br />
<br />7.3. Warning and Evacuation Plans, &
<br />Non-structural options.
<br />
<br />7.3.1. General
<br />Flood warning plans should be developed,
<br />with the relevant emergency authority and
<br />the community, for all dams. Dambretrk
<br />warning and evacuation plans should be
<br />arranged within these plans, not as stand
<br />alone plans, (AEMI, 1995).
<br />
<br />The warning system may include radio and TV
<br />Bulletins, direct telephone, or radio contact,
<br />house-to house contact, public alarms such as
<br />fixed sirens and mobile loud speakers, and in-
<br />home alarms (NWS, USA, 1997).
<br />
<br />7.3.2. Dam break Warning & Evacuation
<br />Plans.
<br />
<br />For effective warning and evacuation, plans
<br />need to consider number, distribution and
<br />elevation of the populated areas for effective
<br />warning and evacuation, noting the dambreak
<br />dynamic flood wave and debris could impact
<br />to metres vertically above the natural flood
<br />levels.
<br />
<br />The plan should in<:lude identification of
<br />available escape routes under flood conditions
<br />prior to dam break and under dambreak; this is
<br />critical for evacuation.
<br />
<br />In many cases significant natural flooding will
<br />have already taken place before the flood
<br />reaches the dam failure stage, and phased
<br />emergency actions should have started, which '
<br />has advantages, but may lead to panic and
<br />chaos, making the further dam break warning
<br />and evacuation more difficult,
<br />
<br />However, with small catchment dams, the
<br />flood rise, and time to peak, can be so short as
<br />to preclude practical pre-warning and effective
<br />evacuation.
<br />
<br />7.3.3. Warning Time.
<br />
<br />The warning time is the time elapsed between
<br />initiation of a formal warning by public
<br />safety officials through radio, TV, direct
<br />warning, or otherwise, for a selected PAR to
<br />evacuate, and the time at which the leading
<br />wave of the failure flood reaches the PAR
<br />(USBR,1989).
<br />
<br />The warning time is measured from the time at
<br />which the official dissemination to the PAR of
<br />the warning begins, not the time when a
<br />potential dam break is recognised, or the
<br />emergency authority receives an alert from the
<br />dam operator.
<br />
<br />It relates to the time at which the first section
<br />of the PAR receives the warning, not the time
<br />to when all individuals are warned,
<br />
<br />7.3.4. Warning Lead Times.
<br />
<br />When assessing waming time for loss of life
<br />estimates, include effective lead time in regard
<br />to:
<br />. The operator/owner threat recognition
<br />time, from rain, inflow and reservoir
<br />level information, of a potential
<br />dambreak flood;
<br />. time to make a decision to alert the
<br />emergency centre (already under
<br />stress);
<br />. emergency centre decision time to
<br />decide if the alert is correct, and the
<br />
<br />ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams 19
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