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Paleohydrology and Sedimentology of a Post–1.8 Ka Breakout Flood from Intracaldera Lake Taupo, North Island, New Zealand

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Paleohydrology and Sedimentology of a Post–1.8 Ka Breakout Flood from Intracaldera Lake Taupo, North Island, New Zealand Paleohydrology and sedimentology of a post–1.8 ka breakout flood from intracaldera Lake Taupo, North Island, New Zealand V. Manville* Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand, and Wairakei Research Centre, Institute of Geological and Nuclear Sciences, Private Bag 2000, Taupo, New Zealand J. D. L. White Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand B. F. Houghton Wairakei Research Centre, Institute of Geological and Nuclear Sciences, C. J. N. Wilson } Private Bag 2000, Taupo, New Zealand ABSTRACT INTRODUCTION The Taupo Volcanic Zone, in the central North Island of New Zealand, includes a high concen- Sudden releases of impounded water from Failures of natural or artificial dams have tration of calderas and composite cones, and lakes in volcanic regions constitute a major caused many of the largest known floods, and abundant volcanic lakes vulnerable to breakout and frequently repeated hazard. An outburst constitute a significant threat to life and property floods (Healy, 1975). We present evidence for the flood from Taupo caldera, New Zealand, (Costa, 1988; Costa and Schuster, 1988). Out- catastrophic release of ~20 km3 of water from the released ~20 km3 of water, within decades burst events have occurred in a variety of envi- Taupo caldera following blockage of the caldera- following an ignimbrite-emplacing eruption, ronments and settings, including the enormous lake outlet during the Taupo 1.8 ka eruption ca. 1.8 ka. Paleohydrologic reconstruction of Pleistocene outbursts from glacial and pluvial (Wilson and Walker, 1985). We reconstruct the the Taupo flood provides estimates of peak dis- lakes in North America (e.g., Baker, 1973; Baker paleohydraulic parameters associated with this charge at the outlet in the range 17 000–35 000 and Bunker, 1985; Lord and Kehew, 1987) and flood using dimensionless and physical models m3/s. Dimensionless analysis demonstrates Siberia (Baker et al., 1993), recent breaches of of outlet flow, empirical lake volume and dis- that (1) failure of the barrier was essentially glacier and moraine impoundments (Lliboutry charge relationships, computer-based dynamic instantaneous, (2) the event may be treated et al., 1977; Blown and Church, 1985; Walder flow routing, and estimates of flow velocity and hydraulically as a dam break, and (3) the peak and Costa, 1996), failures of landslide dams cross-sectional area. Such analysis of the peak discharge was a function of outlet geometry (Costa and Schuster, 1988; Reneau and Dethier, discharge and downstream effects of this flood is rather than lake volume or breach formation 1996), and collapses of artificial structures essential for considerations of volcanic hazards rate. Paleohydraulic reconstructions based on (Jarrett and Costa, 1986; Costa and Schuster, in New Zealand and other volcanic areas. In New empirical relations derived from historic dam 1988). Major floods in volcanic regions may be Zealand, the short repose period of the region’s breaches yield only order of magnitude esti- caused syneruptively by the melting of summit active calderas (Houghton et al., 1995), and evi- mates of peak discharge. Calculations based glaciers and snowfields or the ejection of crater dence that floods may occur repeatedly at a single on the physical dimensions of the outlet chan- lake waters (e.g., Major, 1997; Healy et al., volcano highlight the severity of the hazard. Dis- nel and hydraulic principles are likely to be 1978; Waitt et al., 1983; Waitt, 1989; Pierson turbances of hydrologic networks may be one of more accurate and are in close agreement with et al., 1990; Trabant et al., 1994; Neall, 1996; the most serious consequences of volcanic activ- computer-implemented dynamic-flow–routing Cronin et al., 1997; Gudmundsson et al., 1997). ity; delayed impacts may reach far beyond the models. The latter give peak discharges and Posteruptive floods may originate from break- zone directly affected by an eruption. However, maximum stage levels similar to constraints outs of lakes developed in craters and calderas unlike many other natural hazards, breakout imposed by field evidence and estimates of (e.g., Healy, 1975; Newhall et al., 1987; Newhall floods are amenable to mitigation by engineering flow depth and velocity. The long duration of and Dzurisin, 1988); drainages blocked by pyro- intervention (Glicken et al., 1989). the Taupo flood and the relatively narrow, clastic material (Buesch, 1991; Smith, 1991a, confined flood route resulted in minimal at- 1991b; Scott et al., 1996; Park and Schminke, GEOLOGIC SETTING tenuation of the flood wave compared with 1997; White et al., 1997); lava flows (Hamblin, modern dam breach events, and flood depos- 1994); and aggradational channel deposits Lake Taupo has a modern area of 620 km2 and its can be traced as far as 232 km downstream. (Kuenzi et al., 1979; Pierson et al., 1992; Umbal a volume of ~60 km3, and is the largest lake in the Caldera lake breakout floods may be among and Rodolfo, 1996). Releases of huge volumes Taupo Volcanic Zone (Fig. 1) (Lowe and Green, the most far-reaching hazards associated with of water from such impoundments can be sud- 1992), an area of intense Quaternary silicic vol- volcanism. den and lethal (e.g., Healy, 1954). Known canism in the central North Island of New breaches of intracaldera lakes include Aniakchak Zealand (Wilson et al., 1995). The area contains caldera in Alaska (McGimsey et al., 1994; numerous lakes occupying calderas, craters, and Waythomas et al., 1996) and Lake Tarawera in valleys dammed by lava flows or pyroclastic- *E-mail: [email protected]. New Zealand (White et al., 1997). flow deposits (Healy, 1975; Lowe and Green, GSA Bulletin; October 1999; v. 111; no. 10; p. 1435–1447; 13 figures; 2 tables. 1435 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/10/1435/3382963/i0016-7606-111-10-1435.pdf by guest on 01 October 2021 MANVILLE ET AL. 0 0 0 176 E 174 E 1780 E 176 15'E N Atiamuri NORTH ISLAND Quarry NEW ZEALAND Lake Atiamuri 360 S x P Lower Hamilton Hamilton Lake Ohakuri basin 380 30'S Arapuni Ohinewai Reporoa Huntly Orakei Basin Waikato x U Q River Korako T R O Lake Taupo S Area N of Fig. 1 400 S 100 km Aratiatia Cascade L M Lake Aratiatia Te Toke Broadlands Area of K 10 km Figure 4 F G H Area of D E J Figure 3 C I Huka Taupo B EXPLANATION township Falls 380 45'S EE A G measured cross section Modern lakes and rivers Waikato valley margin Lake Taupo Post–1800a highstand Hydroelectric dams shoreline 1750 45'E 1760 E Flood alluvium (fan deposit) Figure 1. Lake Taupo and the Waikato River, showing locations of measured cross sections and localities referred to in the text. 1992). The northern part of Lake Taupo owes power plants farther downstream, produce >25% nated all vegetation and destroyed surface hydro- much of its present form to caldera collapses dur- of New Zealand’s power. logic networks by filling and blocking off valleys ing the 26.5 ka Oruanui and 1.8 ka Taupo erup- and channels (Smith, 1991a). Caldera collapse tions (Self and Healy, 1987; Wilson et al., 1984; Taupo 1.8 ka Eruption formed a subrectangular basin within the older Wilson, 1993), whereas the southern part formed Oruanui collapse structure (Davy and Caldwell, by faulting and warping on regional structures. The most recent explosive eruption from the 1998) into which the remainder of the preerup- The lake overflows with a mean discharge of Taupo caldera occurred 1.8 ka (Wilson and tion lake drained (Wilson and Walker, 1985). A ~130 m3/s into the 320-km-long Waikato River. Walker, 1985). An estimated 35 km3 of magma sediment-buried bench 110 m below the present The river falls 337 m in the first 180 km down- was erupted, distributed as Plinian deposits more lake surface possibly marks an early posteruptive stream of the outlet, through a steep, narrow than 0.1 m thick over an area of 30 000 km2 east lake level (Northey, 1983). valley confined between late Pleistocene fluvial of the vent, and as 20 000 km2 of radially em- terraces and gorges incised in ignimbrite placed ignimbrite (Wilson and Walker, 1985; Eruption Aftermath (Thompson, 1958; Schofield, 1965). It then Wilson, 1985). The pumiceous, largely fine- crosses the lowlands of the Hamilton basin (Kear grained ignimbrite forms a thin drape over the The emplacement of 30–40 m of nonwelded and Schofield, 1978). Extensive hydroelectric preeruption landscape and fills valleys and de- ignimbrite blocked the caldera outlet channel via development has turned the first 180 km of the pressions to depths of 5–60 m (Walker et al., the Waikato River. Remnants of this fill survive Waikato River into a chain of eight reservoirs 1981; Wilson, 1985; Wilson and Walker, 1985). along the valley walls and in side tributaries at (Hill, 1975) which, coupled with two thermal Emplacement of this pyroclastic material elimi- elevations declining from ~390 m near the outlet 1436 Geological Society of America Bulletin, October 1999 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/111/10/1435/3382963/i0016-7606-111-10-1435.pdf by guest on 01 October 2021 BREAKOUT FLOOD, LAKE TAUPO, NORTH ISLAND, NEW ZEALAND EXPLANATION 420 VPI surface 1 highstand shoreline Lake spillway terraces 2 eroded IVD facies 400 Taupo highwater marks3 1 boulders slackwater deposits 2 boulders/flood terraces 380 4 incised Pueto delta knickpointsVPI surface 2 fan deposit modernmodern channel Waikato profile surface and cross sections 360 D A peak stage level 340 modern outlet Huka D 2 Reporoa Basin 320 Falls Figure 2. Schematic longitudinal elevation (m ASL) 3 4 Aratiatia channel profile along the upper Post–1800a 300 breach G 3 Waikato River showing geological O evidence for flooding.
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