Paleohydrology and sedimentology of a post-1.8 ka breakout flood from intracaldera Lake Taupo, North Island, New Zealand

Sudden releases of impounded water from lakes in volcanic regions constitut e a major and frequently repeated hazard. An outburst flood from Taupo cald era, New Zealand, released similar to 20 kin(3) of water, within decades fo llowing an ignimbrite-emplacing eruption, ca. 1.8 ka. Paleohydrologic recon struction of the Taupo flood provides estimates of peak discharge at the ou tlet in the range 17000-35000 m(3)/s. Dimensionless analysis demonstrates t hat (1) failure of the barrier was essentially instantaneous, (2) the event may be treated hydraulically as a dam break, and (3) the peak discharge wa s a function of outlet geometry rather than lake volume or breach formation rate. Paleohydraulic reconstructions based on empirical relations derived from historic dam breaches yield only order of magnitude estimates of peak discharge. Calculations based on the physical dimensions of the outlet chan nel and hydraulic principles are likely to be more accurate and are in clos e agreement with computer-implemented dynamic-flow-routing models. The latt er give peak discharges and maximum stage levels similar to constraints imp osed by field evidence and estimates of flow depth and velocity. The long d uration of the Taupo flood and the relatively narrow, confined flood route resulted in minimal attenuation of the flood wave compared with modern dam breach events, and flood deposits can be traced as far as 232 km downstream . Caldera lake breakout floods may be among the most far-reaching hazards a ssociated with volcanism.