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<br />A2,3.5, Imminent Failure Condition.
<br />
<br />There can still be a use in assessment and
<br />reporting for an IFF type term, In risk
<br />assessment it is in effect the condition
<br />which is estimated for a reasonable
<br />conditional probability offailure.
<br />
<br />There is a proposal to still use the IFF
<br />term but redefined in probability terms as:
<br />IFF: the flood event that could be
<br />reasonably expected to cause failure of the
<br />dam.
<br />
<br />With "reasonably expected" defined as an
<br />equal likelihood (0.5 probability) that the
<br />dam would survive the event, or fail
<br />during the event.
<br />
<br />The IFF so calculated could be greater
<br />than, or less than, the DCF, depending on
<br />the type of dam, the nature of the crest and
<br />the nature of the foundations.
<br />
<br />A2.4. SPILLWAY
<br />ARRANGEMENTS.
<br />(Refer. sub-section 4.4, SDF & Spillway
<br />Provisions)
<br />
<br />A2.4,!. Proposed Dams,
<br />
<br />A2.4.U, General Considerations.
<br />
<br />The initial considerations will relate to the
<br />proposed purpose for the dam and the
<br />storage and yield requirements. The dam
<br />arrangement will then relate to the site
<br />topography, hydrology, geology, potential
<br />for slips and seismic potential.
<br />
<br />The site conditions, particularly
<br />foundations, and availability of materials,
<br />rock, filters, core material and aggregates,
<br />will influence the selection of the type of
<br />dam,
<br />
<br />The required supply from the dam (water
<br />supply, andlor power or flood attenuation)
<br />will set the capacity relative to yield,
<br />which, in conjunction with reseIVoir basin
<br />capacity and site limitations, will then set
<br />the storage level.
<br />
<br />A2A.l.2. Spillway Location.
<br />
<br />The trend for some years in Australia has
<br />been to construct earth and rockfill dams
<br />because of available sites, materials,
<br />climate and the economies of placing rock
<br />and earth.
<br />
<br />A site is then required for the spillway/s.
<br />This may be combined with use of the
<br />excavated material from the spillway in
<br />the dam (Dartmouth, Victoria; Copeton,
<br />NSW; Googong, ACl). The location is
<br />preferably in an abutment, or nearby
<br />saddle, although tunnels have been used.
<br />
<br />At Crotty Dam, Tasmania, a concrete
<br />chute has been successfully constructed on
<br />the downstream face of a rockfill dam.
<br />
<br />Where sites are suitable for concrete, or
<br />roller compacted concrete dams, spillway
<br />provisions can be incorporated within the
<br />main dam structure (New Victoria,
<br />Western Australia, CacHa Dam, NSW).
<br />
<br />A2A.I.3. Gated Spillways,
<br />
<br />The trend throughout the world is now
<br />towards ungated spillways because of
<br />concerns on potential misoperation,
<br />leading to possible legal action by
<br />communities downstream on flood
<br />damages from release of floodwaters; or
<br />dam failure due to gate malfunction or
<br />misoperation.
<br />
<br />A2A.IA, Spillway Arrangement for AFC.
<br />
<br />A preliminary study of conditions
<br />downstream will indicate potential
<br />consequences and Hazard Rating. This
<br />information can be used, with a scoping
<br />risk assessment, or prescriptive standard,
<br />to indicate an order of AFC relative to the
<br />flood frequency estimate for the site.
<br />
<br />The setting of the dam crest level above
<br />FSL is then a balance of economics
<br />between spillway alternatives, with flood
<br />routing, to assess flood surcharge and
<br />possible "dry" freeboard for wave action.
<br />
<br />A2-6 ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams
<br />
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