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I I I I I I I I I I I I t I ~ I I I I I I DRAFT D Book VI - Estimation of Large to Extreme Floods R J Nathan and P E Weinmann 1 INTRODUCTION 1.1 Scope and Intention of the Guidelines I , In the past decade, there has been an increasing focus of attention on the derivation of floods with very low probabilities of exceedance, Information on these floods is required in many aspects of civil engineering, including floodplain management and the design of major infrastructure (e.g. roads, bridges, and railways). The assessment of flood risk is of particular importance to the safe design, maintenance and operation of dams. The safety and economic implications of these flood estimates accentuate the desirability of using similar or compatible procedures by all Australian authorities and designers. The flODds under consideration in this book are events with an annual exceedance probability (AEP) of between 1 in 50 and 1 in 10'. The upper limit of flood magnitude under consideration is the probable maximum flood (PMF), which is a design concept that cannDt be readily assigned an AEP. These guidelines are intended to provide a clear statement of what constitutes best practice in sufficient detail to enable the procedures to be applied to practical problems. Material is also provided on the sources of information used to justify adoption of some procedures, particularly where they are based on the outcomes of recent research. These guidelines are not intended to be a training document fDr hydrologists, but rather it is intended that it be understood by practitioners with some experience in the field. Worked examples are provided in a somewhat didactic manner that may help practitioners with less relevant experience, but overall the thrust is to state What best practice is, not to explain in detail how to achieve it. 1.2 Applications Covered by These Guidelines extreme flood, Where the structure is located in a populated area and if failure could endanger lives or property, This may apply particularly where a series of structures is constructed on a watercourse and progressive failure could occur. Detention basins are discussed further in Book VIII Section 1, . Urban trunk drainage. While these drains generally are not designed to carry extreme floods, good practice requires that the effects of an extreme flood should be checked where lives and property could be endangered, as discussed in Book VIII Section 1. . Floodplain management FDr floodplai~ management or flood protection schemes it may be necessary to consider the potential flood damage arising from large to extreme floods, particularly where such schemes are based on risk management principles rather than standards-based protection. . Major bridges. AUSTROADS guide to the hydraulic design of bridges, culverts and f100dways (AUSTROADS, 1994) has adopted a limit slate approach. FDr the Ultimate Limit State Floods, it is necessary to estimate either the 1 in 500 AEP or 1 in 2000 AEP events. . other major WOtKs. In some cases, it may be desirable to at least check the effects of extreme floods, even if a smaller flood is used for design, Examples would be water supply intakes and sewage treatment plants where flood damage could cause severe disruption to a community, flat rDofs where blockage Df roof drains could cause collapse, or floodplain management studies where national heritage buildings or other irreplaceable items are endangered, An overview of the applicability of different parts of these guidelines tD specific investigations or design tasks is prDvided in Section 2.3. 1.3 Overall Design Approach Adopted Applications of estimates of the large tD extreme floods The procedures recommended here are based on the considered here include: recognition that the uncertainties involved with the flood . Spillways. The Australian National Committee on large1- estim~tion process i~crease with increasing size of flood (or Dams (AN COLD) has published "Selection ot redUCIng ,level of nsk). The procedures can be loosely Acceptable Flood Capacity for Dams" (ANCOlD, 1998), grouped Into three c1asse~ of ~ood magnitude ove~ the which gives recommendations on most aspects of ~nge of AEPs under conslder~tlon. The type of available spillway provision and satety levels for all potentially informatIOn, degree of uncertainty, and, hence n~ture Dt hazardous structures which store waler or other liquids, procedure that can be. used In the analYSIS vanes w~th flo~d including flood retarding basins service ba .ns d magnltude. The nollonal classes are summansed In tailings dams. The recommended 'design floodSS;ang:~n Fi~ure 1. This figure broadly divides the floods and rainfall~ AEP from 1 in 100 to 1 in 10'. The ANCOlD Guidelines of Interest Into large, Rare, and Extreme ranges, though .t recommend that these floods should be estimated by should be stres.sed that the adopted classes repre~ent a the procedures in this book. ~ontlnuum of Increasing uncertainty and not discrete Intervals. . Detention basins. large structures of this type may tall . .' . within one of the AN COLD (1998) referable dam . Recognitl~n of the Impacts o~ uncertamty should be c tegori - d th b b' ct t .t IncorpDrated Into management deCISions. It IS Intended that r:comm:~dati~~s. Eve~SWhen ethiS sd~(~: not a~PIY: : t~ese guidelines provide practitioners with an approach that be d . bl t h k th rt. f Yields esllmates In the mId-range of the uncertainty band. It dmateYnt'Io beasS"ar etho c ecqU e pe r.ormancle 0 a is not recommended that the practitioner adopt a value on e n In or e conse ences 0 . a very arge or ,