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I!~ I I I ., I I~ I I I' I I II II I I I I I I I I II I I , I I I I I 4.4.2. SpillwllY Economy. (Comments on Spillway arrangements are given in Appendix 2. sub-section A2. 4). 4.4.2,]' Existing Dams. The costs for remedial works can often be optimised by using a combination of lower cost auxiliary and emergency spillways in conjunction with the existing spillway to provide the additional flood capacity from SDF to the AFC. The main, operational spillway should normally be capable of passing the selected SDF with minimal damage. 4.4.2.2, Proposed Dams. The site topography, selected dam type and construction economies can be assessed for an optimum spillway arrangement. 4.4.2.3. Hydraulic Design. In cases where one spillway is provided to pass all flows up to the AFC, or for the main spillway with supplementary spillways, it may not be economic to select the extreme flood for the hydraulic design of the spillway crest, chute and dissipator. A check can be made to assess whether a lesser design flow can be adopted for the spillway chute structure which, while resulting in damage at higher flows, would not endanger the integrity of the dam. 4.4.3. Combined Spillways. A combination of main spillway plus supplementary and emergency spillways can be considered to meet the overall AFC requirement. (i) Supplementary spillways may be unlined, depending on site conditions, provided that scour will not threaten the integrity of the dam, as they will only operate infrequently. Consideration may be required to possible environmental impacts of scour and deposition of sediment. (ii) Fuse plugs are other alternatives that provide additional flood capacity. The fuse plug can be set at stepped levels to fail incrementally. If there is likely to be persistent wave action, which could prematurely start fuse plug failure, it may be necessary to construct a baflle/s in front of the fuse plug trigger section/so The possibility should be considered of flood- surge effects that may occur when the fuse plug fails, and the possible loss of storage if active storage is held against the fuse plug. The flood surge can increase flood releases above inflows - contrary to normal operating rules - and could lead to legal action. Some authorities accept the increased effects if the estimated incremental surge wave is within 300 mm of the estimated flood level before the fuse plug operates. 4.5 Freeboard. (Fig 4.1). 4.5.1. Freeboard When selecting the AFC for a proposed dam, or for upgrading an existing dam, the DCF stage, which includes flood surcharge (wet freeboard), can be considered initially without additional "dry" freeboard for wind wave run-up and set-up. Embankment dams are vulnerable to damage and failure from overtopping, and in deterministic design are generally assumed' at the limit of safety when the reselVoir level is at the crest level. Concrete and meshed rockfill dams generally can tolerate some overtopping. 4.5.2. Freeboard Considerations. (Refir Appendix 2.sub- section A2.5). Each case should be assessed in regard to the possible combination of overtopping depth and duration, the assessment of dam ability to resist some overtopping and the consequences of failure, It may be prudent to provide freeboard in particular cases, depending on the circumstances, such as:- . embankment dams where, during an extreme flood event, the concurrent dominant winds relative to reselVoir reach could resuh in persistent, strong ANCOLD Guidelines on Selection of an Acceptable flood Capacity for Dams 9