634 SEDIMENTARY PROCESSES/Catastrophic Floods Figure Flood hydrographs with contrasting shapes (A) Rapid rise to peak discharge associated with sudden dam failure and glacier outburst floods associated with sheet flow and hydrofracturing (B) Exponential rise to peak discharge is typical of floods where rising stage is controlled by progressive enlargement of ice tunnels (C) ‘Heartbeat’ event where temporary channel blockage results in a reduction in discharge followed immediately by a flood peak associated with dam failure (D) Composite flood hydrograph characterized by the superimposition of one flood wave on top of another (E) Jokulhlaup tunnel inlet generated by the drainage of a small ice dammed lake in West Greenland joă kulhlaups may fill ice-marginal basins within minutes, creating short-lived ice-dammed lakes, which in turn drain to produce a composite flood hydrograph where one flood peak is superimposed upon another (Figure 7D) Catastrophic floods are often characterized by a dearth of direct process measurements Most hydraulic variables are extrapolated from geomorphological and sedimentary evidence using hydraulic modelling techniques developed for lower magnitude flows Calculated velocities are often extremely high (