SEDIMENTARY PROCESSES/Catastrophic Floods 633 Figure The jokulhlaup cycle The upper diagram illustrates the advance and retreat of a glacier during the Little Ice Age Ice positions 17 in the upper diagram correspond with the timescale illustrates in the lower diagram The cycle of jokulhlaup drainage occurs as the glacier is thinning and retreating from its advance Jokulhlaup magnitude within the cycle decreases as ice dammed lake volumes decrease progressively during glacier retreat After Evans SG and Clague JJ (1994) Recent climatic change and catastrophic geomorphic processes in mountain environments Geomorphology 10: 107 128 failure of caldera Lake Taupo generated a peak discharge of 1.7–3.5  105 m3 s 1, draining a total of 20 km3 of water The failure in Alaska of Aniakchak caldera (3.7 km3) generated a peak discharge of  106 m3 s Deposition of alluvial fans at tributary junctions within ephemeral systems can block subsequent flood flows, resulting in enhanced flood peaks due to sudden dam failure Catastrophic Flood Characteristics The discharge hydrograph is the most important characteristic of any flood as it defines flood duration and magnitude Flood magnitude can be described by the peak discharge and total flood volume Hydrograph shape reflects flood generation mechanisms and drainage basin characteristics Different flood generation mechanisms result in different hydrograph types (Figure 7) Dam failure due to rapid breach development generates very rapid rises to peak discharge (Figure 7A) Volcanically generated (volcanoglacial) joă kulhlaups also produce extremely rapid rises to peak discharge due to extremely high ice melt rates and glacier hydrofracturing Rainfallgenerated floods in ephemeral river-systems also display rapid rises to peak discharge followed by more sedate waning flow stages Many joă kulhlaup hydrographs exhibit gradual rising stages characterized by an exponential shape (Figure 7B) These joă kulhlaup hydrographs are controlled by a positive feedback between meltwater flow and tunnel enlargement (Figure 7B and 7E) The rate of exponential discharge increase is controlled by ice-dammed lake water temperature, the length of the tunnel routeway, and height difference between the lake surface and the outlet Flood hydrographs with multiple peaks can represent either nonsynchronous response of various subcatchments, complex channel networks to runoff, or temporary channel blockage and subsequent release For volcanoglacial joă kulhlaups, multiple peaks may be generated by peaks in volcanic activity Similarly ‘heartbeat’ events associated with temporary tunnel blockage are characterized by a sudden fall and then rise in discharge (Figure 7C) Sudden onset