628 SEDIMENTARY PROCESSES/Catastrophic Floods Catastrophic Floods A J Russell, University of Newcastle upon Tyne, Newcastle upon Tyne, UK ß 2005, Elsevier Ltd All Rights Reserved Introduction At present there is no widely recognized definition of a flood other than a ‘notable flow of water’ Since the diluvial theory of the biblical flood, cataclysms and catastrophic flooding have drawn the attention of geologists throughout the development of the subject and to the present day Catastrophic floods occur worldwide within a wide range of depositional environments and hydroclimatic settings Catastrophic floods represent a major natural hazard within riverine environments, generating substantial loss of life and infrastructure As a surface process, catastrophic floods are capable of generating immense landscape change and the transfer of huge sediment volumes Large meltwater outbursts associated with continental deglaciation have recently been invoked as a trigger mechanism for changes in ocean circulation and consequent climate changes Catastrophic floods have been defined in a number of ways From an applied perspective, catastrophic floods can be defined in terms of human and monetary loss Floods can also be judged truly catastrophic, depending upon their impact on channel morphology and sedimentology or the work they in transporting sediment The extent to which a flood is catastrophic also depends upon the time-scale over which one wishes to consider its impacts Preservation of erosional landforms and sedimentary successions associated with floods over geological time-scales are likely to reflect truly catastrophic events and the presence of appropriate depositional environments for preserving evidence of high-magnitude floods Over shorter time-scales, a flood may be judged catastrophic if its impacts result in an irreversible change in fluvial geomorphology Highenergy flows are known to result in a suite of ‘catastrophic processes’, which not operate under nonflood conditions and which achieve substantial and distinctive impacts Catastrophic processes can, however, also occur under nonflood flow conditions within, for example, steep bedrock fluvial systems and subglacial drainage systems characterized by high stream powers Causes of Catastrophic Floods Catastrophic floods require the sudden input of large volumes of water to a flood routeway Flood generation mechanisms provide a major control on flood magnitude, frequency, and characteristics (Figure 1) Traditional classification schemes of flood causes examine the relationship between floods and climatological variables More recently, attention has focussed on identifying the physical limits on the magnitude of floods generated by various mechanisms Rainfall is the most common cause of flooding in rivers around the globe Rainwater is a major input to the fluvial system, often having travelled through hillslopes or more rapidly as overland flow Rainfall intensities tend to be high over short time periods and on a relatively local scale Meteorological and climatological controls on flood-producing precipitation events can be identified over a range of spatial and temporal scales (Figure 2) This hydroclimatological approach uses understanding of the controls on flood-generating precipitation events to characterize the distribution of floods in space and time Other approaches have considered maximum flood discharges, which can be yielded from a certain catchment area Empirically derived ‘envelope curves’ illustrate major differences in rainfall–runoff relationships between different climatic regions Envelope curves not, however, consider the relative role of specific drainage basin characteristics in generating flood flows (Figure 3) Snowfall generally reduces the immediate potential for flooding by putting water into storage for more gradual release upon melting However, the progressive or rapid melt of a snow-pack can generate substantial floods, especially as a ‘nival’ flood in springtime Nival floods commonly affect rivers with large catchments, which allow synchronous water input from all parts of the catchment area Snowmelt rates are controlled by the amount of available energy Heavy rainfall onto a snow-pack will enhance peak flood discharges from a catchment, although modest precipitation onto a snow-pack may delay flows, resulting in lower peak discharges Nival flows are also associated with secondary storage-release floods resulting from the failure of temporary snow dams or river ice jams Melt of glacier ice is a major control on discharge within proglacial river channels Although melt-induced flows commonly exhibit discharge variations on diurnal and synoptic time-scales, they are not normally considered floods unless associated with unusually high ablation This may occur, for example, during subglacial volcanic eruptions or where there are enhanced levels of geothermal activity More commonly, however, glacial meltwater is stored