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.

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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. Lower part 280 shows representative valley cross sec- -5 0 5 10 15 20 25 30 35 40 45 50 tions at various distances downstream downstream distance (km) as marked in Figure 1.

Section A: Breach (0.00 km) 420 Section D: Spillway (8.05 km) 400

380

360 Section G: Valley Section O: Reporoa Basin 340 (14.08 km) (37.01 km)

320 elevation (m ASL)

300

280 1000 2000 channel width (m)

to ~360 m at Aratiatia, 12 km downstream (Fig. FIELD EVIDENCE FOR FLOODING narrow trench cut through 40 m of ignimbrite 2). The lake basin subsequently refilled from pre- that filled the pre-1.8 ka channel and formed the cipitation runoff. The post–1.8 ka highstand of Evidence for catastrophic flooding along the syneruptive blockage (Figs. 3 and 4). This reach the lake is marked by a semicontinuous, but tec- Waikato River downstream of Lake Taupo (Fig. 2) shares many of the diagnostic features of spill- tonically warped, wave-cut bench and shoreline includes the presence of (1) a 12-km-long, verti- ways (i.e., fluvial channels, rather than fluvial deposits at elevations of 28–43 m above the mod- cal-walled spillway floored with thin, down- valleys, which were partially or wholly filled by ern lake level of 357 m above sea level (ASL) stream-dipping fans of coarse lithic gravel and transient floodwaters) identified in relation to (Clarkson, 1996; Wilson et al., 1997; Riggs and boulder lags; (2) streamlined landforms; (3) boul- Pleistocene outburst floods in North America. Ort, 1999). A mean highstand level of ~390 m dery fan deposits at the entrance to a broad valley Such features include relatively constant width, ASL, i.e., 34 m above the present level, is ob- downstream of the spillway reach; (4) fine- consistent bank heights, discrete cutbanks, tained from areas least affected by post–1.8 ka grained slack-water deposits in off-channel areas streamlined residual landforms, thin basal de- tectonic deformation. Estimates of the time nec- to 20 m above the modern channel; and (5) distal posits of coarse-grained sediments, and a termi- essary for the lake to refill to its highstand level erosional and aggradational features associated nal coarse-grained fan at the entrance to a down- based on hypsometry and estimated precipitation with abrupt and large-magnitude changes in the stream basin (Kehew and Clayton, 1983; Kehew runoff vary from 15 (Smith, 1991a, 1991b) to discharge and fluvial style of the river. Many of and Lord, 1986, 1987). The near vertical walls of 40 yr (Wilson and Walker, 1985). Higher esti- these features, although smaller in scale, are simi- this valley (Fig. 5) are cut into partially silicified mates include the time necessary to saturate the lar to those left by other large paleofloods (e.g., pre-Oruanui lacustrine deposits and 26.5–1.8 ka sublacustrine pumice-choked caldera. Malde, 1968; Baker, 1973; Kehew and Lord, pyroclastic and sedimentary deposits. Remnants Eventually the pyroclastic barrier failed, prob- 1986). River distances are measured relative to an of 1.8 ka ignimbrite are present along the trench ably because of overtopping by the rising lake. incised meander 2 km downstream from the margins and in tributaries. The floor of the spill- Once initiated, breach growth was probably modern outlet. This point marks the upstream way is partially covered by thin downstream- rapid. The absence of intermediate shoreline limit of the pyroclastic barrier and the down- dipping fans of pebble-cobble lithic gravel and terraces between the +34 m shoreline and a semi- stream limit of the highest lake shoreline terrace boulders that overlie exhumed silicified pre- continuous, postflood wave-cut bench at +2–5 m (Wilson et al., 1997). Oruanui sediment, pre-1.8 ka paleosols and indicates an ~32 m drawdown of the lake in a pyroclastic deposits, and locally, Plinian fall single phase (Wilson et al., 1997). The rapid Spillway Features beds from the early phases of the Taupo erup- release of the ~20 km3 of water impounded by tion. The modern Waikato River occupies a nar- the barrier caused a major flood down the Between Lake Taupo and Aratiatia, the row channel cut in the valley floor and is flanked Waikato River valley. Waikato River flows through a 12-km-long, by gravel- and boulder-capped terraces. About

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4 km from the outlet, at Huka Falls, lithified pre- Oruanui sediment forms a broad surface termi- EXPLANATION nated by a 13-m-high arcuate scarp through Huka Falls C which the river cuts a narrow slot. The spillway modern lake and river ends at Aratiatia, where the Waikato River enters silicified pre-Oruanui the wider valley of the Reporoa basin after flow- units Reids Farm ing through a gorge cut into a rhyolite dome. spillway terrace edge B Remnants of primary 1.8 ka fall deposits on the spillway floor indicate that the valley existed intermediate terraces streamlined forms before the eruption and was filled by ignimbrite up to the level marked by the marginal remnants. lower terrace edges The Taupo flood subsequently exhumed the central valley and left the thin, dense gravel lag. incised tributaries The intermediate-height marginal terraces are in- measured cross ferred to represent channel sections abandoned sections during deepening of the central channel during the flood, whereas the scarp at Huka Falls represents a headward-migrating knickpoint cut in a resistant horizon. view in figure 5 Streamlined Erosional Forms and Exhumed River Terraces inferred upstream limit of pyroclastic dam + 33.6 m Low whaleback-shaped features sculpted from pre-Oruanui sediment and Oruanui pyroclastic deposits are present along the spillway reach, A Post–1800a highstand shoreline particularly at Reids Farm (Fig. 3). These land- N forms vary from 10 to 500 m long and from 1 to + 36.6 m 10 m high, with length-to-width ratios of ~4:1, and are similar to streamlined forms developed modern shoreline 357m ASL by other outburst floods (Baker, 1978; Kehew and Lord, 1987; Komar, 1989). 1km Further indications of severe erosion are found along relatively narrow reaches 45–54 km down- Lake Taupo stream (between the outlet to the Reporoa basin + 34.8 m and the Orakei Korako gorge), and 68–75 km downstream (between the bedrock constrictions at Ohakuri and Atiamuri). Along these reaches, Figure 3. Summary map of flood features and deposits along the outlet Huka Falls spillway 1.8 ka pyroclastic deposits have been stripped to reach of the upper Waikato River. Dots indicate heights of post–1.8 ka highstand shoreline above expose older post-Oruanui sediment, Oruanui modern lake datum. pyroclastic beds, and silicified late Pleistocene lacustrine deposits and rhyolite lavas. Synflood and postflood deposits formed now-submerged (Fig. 6). The flood deposit is strongly bimodal, where the modern, underfit, Waikato River flows terraces that are visible on pre-hydroelectric- containing polymict boulders to 4 m in diameter through a narrow slot incised in the lowest part of scheme aerial photographs (Fig. 4). that decrease in size and frequency downstream the valley. The Taupo flood also eroded pre-flood (Fig. 7, A and B). Bank exposures along the sediment in the Reporoa basin (Fig. 2), cutting Bouldery Fan Deposits Waikato River show that boulders are sparsely through the Pueto delta that had accumulated in scattered throughout the thickness of the fan de- an earlier temporary lake ponded on the ignim- Downstream of the Aratiatia Cascade, a posit. Close to the fan head, randomly scattered brite surface (Manville, in press). 48 km2 fan-shaped flood deposit extends for boulders are concentrated on lower terrace treads We interpret the bouldery fan to have formed as more than 20 km into the southern Reporoa basin (Fig. 7A), whereas farther downstream they are the Taupo flood emerged from the confined spill- (Figs. 1 and 2). The proximal fan consists of exposed in broad shallow channels, either in iso- way reach into the broad, low-gradient Reporoa coarse-grained pumiceous and lithic gravel and lation or arranged into small clustered groups basin. Deceleration and shallowing of the flood pumice, crystal, and lithic sand (Fig. 6), orga- (Brayshaw, 1984). The average intermediate di- caused the deposition of material eroded from up- nized into crudely stratified low-angle tabular ameter of the boulders decreases downstream, stream. Similar deposits are recorded in associa- beds and steeply dipping foresets, which uncon- from 2.5 m at the fan apex to 0.5 m 5.4 km away. tion with relatively shallow, sediment-laden formably overlie primary 1.8 ka pyroclastic The distal fan in the Reporoa basin (Fig. 1) con- floods, i.e., the jökulhlaups of Iceland (Maizels, deposits. Multiple terraces step down to the mod- sists of cross-bedded pumice gravel and sand. A 1997), at Aniakchak (Waythomas et al., 1996), ern river, and have surfaces capped with 0.5–1 m series of horseshoe-shaped scarps and terraces to and at Lawn Lake (Blair, 1987). Although trans- of crystal- and lithic-rich coarse sand and gravel 6 m high are cut through the center of the fan ported together, we infer that simultaneous depo-

1438 Geological Society of America Bulletin, October 1999

N 1km

Aratiatia Cascade

Figure 4. Summary map of flood features and deposits along the Wairakei-Aratiatia spillway reach of the upper Waikato River, based on view in figure 7a field observations and pre–hydro- Wairakei F electric development aerial photographs. E EXPLANATION lower terrace edges 1941 river course

Aratiatia rhyolite dome incised tributaries

D spillway terrace edge measured cross sections

intermediate terrace edges boulders

sition of the two grain-size modes occurred by little or no segregation of particles in terms of trance to the Reporoa basin resulted in the rapid different mechanisms. The boulders were de- grain size and density. This fabric is also seen in deposition of an unsorted coarse sandy gravel, posited due to a fall in flow competence and the the deposits of hyperconcentrated flows (Pierson while a simultaneous fall in flow competence matrix material was deposited due to a decline in and Scott, 1985; Smith, 1986), where deposition caused the flood to drop its load of boulders, pro- flow capacity; competence and capacity are dif- occurs by grain-by-grain accretion from the base ducing the bimodal fan deposit. During the flood ferent measures of sediment transporting ability. of a shearing high-concentration sole layer at the peak, backwater effects produced by valley con- The former process is supported by the cluster base of the flow (Sohn, 1997). We infer that dur- strictions farther downstream (discussed later) bedforms (Brayshaw, 1984), which indicate that ing the rising limb of the Taupo flood, erosion of caused hydraulic ponding, reducing flow veloci- the boulders moved as tractional bedload, and by the ignimbrite barrier and spillway reach caused ties and preventing substantial erosion of the fan their downstream fining. Evidence for abruptly the entrainment of large volumes of sediment. An deposit. Stabilization of the spillway reach during diminished flow capacity is in the very poor sort- abrupt fall in flow capacity associated with shal- the flood peak reduced erosion farther upstream ing of the mixed-component sandy matrix, with lowing and deceleration of the flood at the en- and hence resulted in lower sediment yields during waning flow, further aided by a fall in stream power. Consequently, the later phases of the Taupo flood were underconcentrated with sediment (i.e., under capacity), enabling erosion and reworking of the upper part of the fan deposit. This produced the series of recessional terraces and knickpoints that step down to the modern river, and exposed the boulders on the terrace treads by removing the surrounding matrix. Pref- erential erosion of the low-density pumiceous component from the matrix produced the thin veneer of lithic- and crystal-rich sand and gravel on the surfaces of the lower terraces (Figs. 6 and 7).

Slack-Water Deposits

Fine-grained, laminated sediment is found in tributaries along the outlet–Reporoa basin reach. These deposits are similar to slack-water flood de- Figure 5. Spillway reach of the Waikato River in the area of the upstream limit of the pyro- posits that accumulated in low-energy environ- clastic barrier including the area of the breach. View looking upstream toward Taupo township. ments (Kochel and Baker, 1982; Baker et al.,

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Aggradation Recession 30 20 pumice 25 pumice crystal crystal 15 20 lithic lithic undiff. 15 undiff. 10 10 5 weight percent weight percent weight 5 Figure 6. Granulometric and compositional data for sediment 0 0 –8–7–6–5–4–3–2–101234pan –7 –6 –5 –4 –3 –2 –1 0 1 2 3 4 pan from the proximal flood fan de- phi phi posit. Aggradational-phase de- 99.99 posits are poorly sorted and show 99.9 a diverse mix of components, in- 99.5 cluding relatively low density 99 pumiceous material, indicative of 98 very rapid deposition from over- 95 ent 90 loaded flood waters. Recessional- 80 phase deposits show better sort- 70 ing, and low-density components 60 50 such as pumice have been largely 40 30 pumice gravel winnowed out (or were not de- 20 posited). 10 pumice/lithic gravel 5 (aggradational) 2

cumulative weight perc 1 pumice/lithic gravel 0.5 (recessional) 0.1

0.01 –8–7–6–5–4–3–2–10123456789 phi A B

Figure 7. (A) Boulder-strewn terraced fan surface downstream of the Aratiatia Cascade. Boulders of spherulitic rhyolite derived from the Aratiatia dome are as much as 4 m in diameter. (B) Aggradational Taupo flood deposits emplaced during the rising and/or peak limb of the out- break flood, overlain above a planar erosion surface by degradational phase sediments. Lithic boulders ~1 m in diameter are set in coarse, pebbly, pumiceous sands.

1983). The maximum elevation of these deposits Distal Flood Features sive erosional surfaces with boulder lags, coarse- declines from ~318 m just downstream of Aratiatia grained aggradational fans downstream of bed- to ~302 m in the Reporoa basin where 10–20-cm- Sedimentary and geomorphic features associ- rock constrictions, and sediment-buried forests thick deposits of massive pumiceous silt and fine ated with the Taupo flood persist along the adjacent to channels and on flood plains. sand, locally containing rounded pumice pebbles, Waikato River valley for more than 230 km Taupo flood deposits erosively terminate a drape a large low-lying area (Manville, in press). downstream of the outlet. These include exten- post–1.8 ka sedimentary sequence, consisting of

1440 Geological Society of America Bulletin, October 1999

horizontally and low-angle cross-bedded pumi- (Lowe and Green, 1992). At Ohinewai (Fig. 1), as an independent check to derive an estimate of ceous sand, 80 km downstream of the outlet a forest growing on the flood plain was buried the flow competence, average flow velocity, and (Fig. 8). Coarse pebbly lithic- and crystal-rich by 8 m of plane-bedded pumiceous sand, pre- peak discharge of the flood (O’Connor, 1993). sand containing subrounded boulders to 2 m in serving upright trees that have been truncated at diameter overlies a prominent erosion surface the modern land surface. This sequence indi- Dimensionless Analysis of Peak Discharge with as much as 8 m of relief. The boulders in- cates abrupt overbank sedimentation during clude blocks of 1–2-m-thick loessic paleosol flooding of the low-lying parts of the Hamilton Dimensionless analysis is a technique for in- and clasts from local lava domes and welded basin by the Taupo flood, following river-bed vestigating the relative importance of breach ignimbrites, whereas the sand fraction includes aggradation in response to previous decades of growth rate and lake volume and shape on the a significant component reworked from post- high sediment discharge. peak discharge of a dam breach flood (Walder Oruanui sediment. We interpret this sequence as and O’Connor 1997). In this method a dimen- recording a period of rapid posteruption aggra- PALEOHYDROLOGIC ANALYSIS sionless measure of peak discharge Qp*, is de- o dation followed by the Taupo flood, which in- fined relative to the dimensional discharge Qp : cised and widened the previously aggrading Hydrologic variables such as peak flood dis- Q o stream bed. Undercut talus aprons and uncon- charge, duration, and flow velocity were esti- Q * = p (1) solidated terrace gravels of post-Oruanui age mated from consideration of lake and barrier p gd05.. 25 along the valley margins contributed sand and characteristics and paleohydraulic analysis of pebble-grade material and blocks of paleosol. flood evidence along the Waikato River using a and is primarily controlled by the dimensionless Recessional flow then cut a series of terraces number of techniques. Dimensionless analysis of parameter η, where surfaced with lithic gravel lags that step down to the peak discharge based on lake and breach kV the modern river channel. parameters (Walder and O’Connor, 1997) sug- η=kV** = o , (2) At Arapuni, 130 km downstream of the outlet gests that breach growth was effectively instanta- o gd05.. 35 (Fig. 1), a fan composed of coarse-grained, neous and that a dam-break analogue is appropri- boulder-bearing, pumiceous and lithic gravel ate for modeling the Taupo flood. Very rapid in which k* is the dimensionless rate of breach 0.5 0.5 and sand extends for 8 km downstream of a breach growth is also supported by consideration growth (k* = k/g d ), Vo* is the dimensionless 3 100-m-wide bedrock gorge. We infer that this of field evidence. The adoption of a dam-breach excess lake volume (Vo* = Vo/d ), g is gravita- fan accumulated during the Taupo flood as the model allowed peak discharge at the outlet to be tional acceleration, k is the rate of breach erosion, sediment-laden flow shallowed and slowed at calculated from regression equations fit to his- Vo is the volume excess of the lake (volume of the valley expansion. torical dam breach floods (Clague and Mathews, water discharged), and d is the depth excess of Post–1.8 ka fluvial sediments along the 1973; Costa, 1988; Costa and Schuster, 1988). the lake (amount of lake lowering or breach Waikato River are confined within a paleovalley Similarly, the U.S. National Weather Service depth). Dimensionless measures of breach ero- between post-Oruanui terraces as far as Huntly, computer program SMPDBK (Wetmore and sion rate, breach geometry, and lake shape can be 230 km downstream (Fig. 1). Below Huntly, Fread, 1984) was used to model the Taupo flood. derived from dimensional estimates of: (1) the deposition of pumiceous material on the broad Model results of peak-stage profiles were com- volume and depth excess of the lake; (2) a hypso- flood plains of the lower Hamilton basin (Selby pared with field evidence of minimum stage metric function describing the relationship be- and Lowe, 1992) locally dammed tributaries to height along the Waikato valley. The size of the tween lake volume and water depth; (3) the ero- form shallow lakes behind post–1.8 ka levees largest boulders transported by the flood was used sion rate of the breach; and (4) the breach geometry. For plausible breach growth rates (Costa and Schuster, 1988), i.e., 0.2–100 m/hr, the Taupo breach would grow to its maximum size in 0.32 hr to 1 week. Values of η derived using appropriate parameters for Lake Taupo are between 1.9 and 1015. In practical terms η > 1 means that breaching of the dam is instantaneous, and that peak discharge is solely controlled by the geometry and dimensions of the outlet, rather than being affected by drawdown of the lake during breach development. In dimensional form the relationship becomes

o = 05.. 25 QCgdp (3)

and the similarity to the broad-crested weir equation (Table 1) reflects the shared under- lying assumption of critical flow through the Figure 8. Erosional unconformity between early fluvial aggradational deposits and high- breach. The coefficient C depends mainly on the energy Taupo flood deposits 80 km downstream. The pumiceous sand deposits become more breach width/depth ratio r, and to a lesser extent texturally and compositionally mature upward, changing from hyperconcentrated flow deposits on the slope of the breach walls θ and the ratio into trough cross-bedded sand and gravel indicative of deposition in shallow braided streams. of breach height to dam height ζ. Using repre- Scale bar = 1 m. sentative values of these parameters, i.e., r = 2.5,

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TABLE 1. EQUATIONS RELATING DAM-BREAK FLOODS TO RESERVOIR CHARACTERISTICS Reference Type of dam/analysis Equation Peak discharge (m3/s) Hydraulic analysis 0.5 2.5 Walder and O’Connor (1997) Dimensionless analysis* Qp = 1.94 g D 35 200 0.5 Fread (1996) (a) Broad-crested weir Qp′ = 3 H ′W ′(H′) 27 900 equation: rectangular breach† 0.5 1.5 Price et al. (1977); Costa (1988) (b) Broad-crested weir Qp = 0.3 (0.4 Bw + 0.6 Tw)g H 17 300 equation: trapezoidal breach§ Regression relation # 0.42 Costa and Schuster (1988) Constructed dam—earth Qp = 0.0184 Ep 79 300 # 0.41 Costa and Schuster (1988) Landslide dam Qp = 0.0158 Ep 47 300 # 0.60 Costa and Schuster (1988) Moraine dam Qp = 0.00013 Ep 390 000 # 0.42 Costa and Schuster (1988) Envelope curve—all types Qp = 0.063 Ep 271 000 0.46 Walder and O’Connor (1997) Constructed dam—all types** Qp = 1.6 Vo 87 500 0.46 Walder and O’Connor (1997) Landslide dam** Qp = 1.16 Vo 63 400 0.66 Walder and O’Connor (1997) Moraine dam** Qp = 0.045 Vo 282 000 Note: All units are SI unless indicated otherwise. *Qp is the peak discharge at the outlet; g = 9.8 m/s, the acceleration due to gravity; and D (depth at the dam face) = 32 m for the Lake Taupo highstand. † 3 3 Equation uses non-SI units. Input parameters are Qp′ = discharge, cubic feet per second (1 m /s = 35.3147 ft /s); H ′ (breach height) = 105 ft; W ′ (breach width) = 305 ft (1 m = 3.2808 ft). § Equation uses SI units. Input parameters are Tw (top width of breach) = 130 m; Bw (basal width of breach) = 60 m; H (breach height, the approximate water depth at face) = 32 m. # 3 3 Ep is the potential energy of the excess volume of water in the reservoir, i.e., the product of the density of the lake water, pw = 10 kg/m , the acceleration due to gravity, g = 9.8 m/s2, the water depth at the dam face, d = 32 m, and the volume excess of highstand Lake Taupo, 10 3 Vo = 1.99 × 10 m . 10 3 **Vo is the excess volume of water held in the lake at its highstand; for Lake Taupo, Vo = 1.99 × 10 m .

θ = 35°, and ζ = 1, and a hypsometric function from the outlet to the first major drop in the Mathews, 1973; Beget 1986; Costa 1988; Costa of m = 2.5, Walder and O’Connor (1997) ignimbrite surface at Huka Falls is 4 km; and and Schuster 1988). However, dam failures are showed that C = 1.94 for overtopping failures (3) the congruence between ignimbrite and high- complex and difficult phenomena to quantify, and where η » 1. At Lake Taupo, m ~ 1.5 (i.e., a flat stand elevations. The barrier may have already large uncertainties are inherent in fitting regres- floor and a mix of vertical and constant slope been partially destroyed by headward migrating sion relations to such widely scattered data. A walls) and r ~ 3.0 (breach top width of 95 m and channels initiated at natural steps in the valley landslide dam may be the most suitable analogue depth of 32 m); however, these parameters have long-profile such as at Huka Falls and Aratiatia, for the Taupo blockage, being typically very wide little influence on C for η > 5. Therefore, peak and by phreatic explosion pits formed along the with a low surface slope, and composed of poorly discharge estimated for the Taupo flood from buried former channel as a result of interactions sorted, easily eroded material (Costa and Schuster, this analysis is ~35 200 m3/s and a dam breach between buried water and hot ignimbrite (e.g., 1988). However, the resistant walls of the pre- model is appropriate. Rowley et al., 1981). We infer that surface flow 1.8 ka valley impose a limit on the maximum across the barrier following overtopping caused width of the breach compared with most natural Mode of Dam Failure rapid incision and the development of a head- dam breaches. For purposes of comparison, ward migrating gully at the downstream end of models based on forms of the broad-crested weir In dam-break floods, the discharges at, and the blockage, aided by the low density and un- equation for rectangular and trapezoidal sections downstream of, the breach largely depend on the consolidated nature of the ignimbrite. Breach are included in Table 1 (Costa, 1988). These uti- mode and rate of failure of the barrier. The formation at the upstream face began when the lize the geometry of the breach, represented by mechanism of failure is a function of the stability initial gully had cut back all the way through the the minimum cross section of the spillway, on the of the barrier, its height, width, and thickness, the barrier, possibly aided by piping flow as the assumption that flow is critical in or near the height of water behind it, and the size and volume headwall thinned. Once established, a through- breach. These relationships should hold unless the of the lake impoundment. Scouring and modifi- going breach would have grown rapidly by verti- breach height/breach length ratio is very low, in cation of the outlet channel by the Taupo flood cal erosion of the channel bed and lateral widen- which case the flow may remain subcritical has removed most of the original blockage, re- ing by undercutting and bank collapse. (Walder and Costa, 1996). quiring its reconstruction from indirect evidence. On the basis of the following evidence we infer Dam-Break Analogues Dynamic Hydraulic Flow Routing that the barrier failed by overtopping and that breach growth was rapid. Numerous empirical equations of power-law A flood wave, generated by a dam breach, The maximum height of 1.8 ka ignimbrite form have been developed relating the peak dis- changes as it moves downstream because of along the spillway reach is 390 m, close to the charge from dam failures to lake characteristics interactions with the channel. These include off- posteruptive highstand level of the lake. Thus, we such as depth and volume (Table 1). Relationships channel flow retention, frictional resistance to infer that failure of the barrier began by overtop- have been obtained for historical floods generated flow, acceleration and deceleration, flow losses, ping. Initiation of failure by piping flow is largely by the failure of: (1) artificial dams (Costa and back-water effects, and channel constrictions and discounted due to: (1) the fine-grained nature of Schuster, 1988); (2) landslide dams (Costa and expansions. The flood wave will also attenuate most of the ignimbrite; (2) the very low height/ Schuster, 1988); (3) moraine dams (Costa and and change speed and shape with increasing dis- thickness ratio of the barrier; i.e., the distance Schuster, 1988); and (4) glacier dams (Clague and tance of travel. Dynamic flood-routing tech-

1442 Geological Society of America Bulletin, October 1999

niques can most accurately simulate such a flood the ratio of lake volume to breach size is rela- 1996). Values of n vary with the magnitude of the wave and account for such downstream effects tively low, so that the total duration of the outflow flow because additional roughness elements are (Fread, 1996). Dynamic routing is based on the hydrograph is not much more than tb. At Lake submerged as the flow depth increases and the simultaneous solution of various equations for Taupo, however, the ratio was very high, so that a cross-sectional flow area relative to the wetted unsteady flow, which include terms for the con- high discharge could be maintained for a long perimeter (hydraulic radius) decreases. The model servation of mass and momentum (e.g., Fread, period without greatly reducing the surface level was run with n = 0.03 for active parts of the chan- 1996). Requirements for a solution are: (1) an ini- of the lake and hence the hydraulic head differ- nel, reflecting substantial flow depths in a bedrock o tial upstream condition such as a flood hydro- ence across the outlet. Consequently, Qp for the channel (O’Connor, 1993), and n = 0.5 for inactive graph; (2) a downstream boundary condition Taupo flood is relatively insensitive to tb when parts. Varying the value of n by ±15% altered cal- such as a rating curve, (3) knowledge of the chan- compared with typical dam breach floods from culated peak discharges and stage levels by only nel geometry; and (4) a coefficient of flow re- artificial and natural impoundments. Modeled 5%. It was assumed that the valley geometry at the sistance, usually represented by the Manning co- peak discharge varies by only 30% for values of time of the flood was the same as today (i.e., no efficient n. To apply the model, (1) and (4) need tb between 1 hr and 1 week (Fig. 9). erosion or deposition) without the narrow modern to be determined, and this can be very difficult. Various estimates of peak discharge were gen- channel of the Waikato River. Stage levels calcu- o The resulting suite of partial differential equa- erated for the Taupo flood using different values of lated by SMPDBK for different values of Qp and tions is solved numerically using an approximate tb. The resulting flood hydrographs were routed tb do not correspond well with field observations finite-difference algebraic method (Fread, 1996). down a 58.5-km-long channel reach defined by 21 of maximum flood height (Fig. 10). This is prob- The U.S. National Weather Service computer measured and 15 interpolated cross sections. The ably due to extensive scour and/or aggradation of program SMPDBK (Wetmore and Fread, 1984) latter were inserted along channel reaches subject the channel bed during the flood that is un- was used to model the Taupo flood. The applica- to abrupt variations in channel width or steep accounted for in the model. Hydraulic ponding is tion generates an input flood hydrograph based channel gradients to prevent numerical instabilities indicated by the model upstream of valley con- on lake, barrier, and breach characteristics, and in the model. Cross-section locations were selected strictions at Te Toke, Broadlands, and Orakei then routes it through a channel of specified at abrupt width or gradient changes along the Korako (Fig. 1), although supporting field evi- geometry to give stage heights, discharges, and valley. They were measured from aerial photos, dence exists only at the latter site. flood durations at specified points downstream. 1:50 000 scale topographic maps, and field sur- Numerous simplifying assumptions are used, in- veys, and were subdivided into an active part, Flow Competence and Velocity cluding inviscid Newtonian flow, straight chan- where flow was assumed to have occurred, and an nel reaches, no flow losses, and no lateral flow. inactive part, where flow velocity was assumed to Discharge can be estimated from flow-compe- The critical parameters in generating the peak be negligible. Inactive channel areas include lateral tence data because established relationships exist o discharge at the outlet, Qp , are the time for embayments, tributaries, and the broad flat flood between hydraulic flow parameters (i.e., velocity, breach growth to maximum size, tb, and its plain of the Reporoa basin that stored water during shear stress, stream power) and the size of parti- width, bw. The former is a function of the height, the rising flood and attenuated flood flows. Next to cles that can be transported (Baker and Ritter, composition, and degree of compaction of the channel geometry, the coefficient of flow resistance 1975; Costa, 1983; Komar, 1989; O’Connor, dam, and the volume of the overtopping flow exerts the greatest influence on flow routing. The 1993). In an analysis of the Bonneville flood, (Fread, 1996). At the Lake Taupo outlet, bw is Manning n represents all the variables that retard O’Connor (1993) compared boulder deposits constrained by local geological conditions to the flow, including bedforms, vegetation, river bends, with local flow conditions derived from step- minimum spillway width, 95 m, and can be taken and frictional roughness of the channel, and is a backwater modeling to obtain a regression rela- as constant. In most failures of constructed dams, major source of uncertainty in the model (Fread, tion between particle size Di (the mean intermediate diameter in cm of the 5 largest boulders at each site) and flow velocity v (m/s): 30000 50

vD= 029. 060. . (4) /s) i 3 duration 25000 40 Measurements of the mean intermediate diameter of the five largest spherulitic rhyolite boulders in a given area downstream of Aratiatia 20000 (Figs. 4 and 7A) decrease from 2.5 to 0.5 m over 30 a distance of 5.4 km (Fig. 11). Using equation 4, average flow velocities of 7.9–3.2 m/s were cal- 15000 culated from these dimensions. According to the Duration of floor (days)

Peak discharge at outlet (m discharge Peak 20 continuity equation for valley discharge (dis- discharge charge = flow width × flow depth × flow veloc- 10000 ity), a flood discharge between 22 100 and 8700 m3/s would be required to transport the boulders 10 0 4 8 121620242832 (Table 2). Although equation 4 was determined Time for breach formation (days) for basalt boulders, we do not consider that the slightly lower density of rhyolite will signifi- Figure 9. Peak discharge at the breach and flood duration versus the time for the breach in cantly modify its validity. This method assumes the pyroclastic barrier to grow to its maximum size. Calculated using model from U.S. National that the transported boulders were the largest that Weather Service computer program SMPDBK. Manning number = 0.03. could be transported by the flood, whereas they

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400

Lake Taupo highstand

380 modern channel Tb = 1 hour to 1 day 360 Tb = 1 week A Tb = 1 month B maximum highwater marks 340 C D minimum highwater marks E

Elevation (mASL) Elevation 320

300 F G H I J K L M N O P Q R S 280 T U –5 0 5 10 15 20 25 30 35 40 45 50 55 60 Distance downstream from outlet (km)

Figure 10. Comparison of field evidence for maximum stage height (ASL—above sea level) of the Taupo flood with modeled stage heights and surface-water slopes (U.S. National Weather Service computer program SMPDBK) for three different values of breach-formation time, tb, and o peak discharge at the outlet, Qp ; Manning number = 0.03. Flat sections of the water surface indicate areas of hydraulic ponding upstream of channel constrictions.

may have only been the largest available, thus compared with typical dam-breach floods (Costa models based on resulting flood hydrographs pro- yielding minimum estimates of flow velocity. and Schuster, 1988). This is due to the large ratio duce maximum water surface elevations consist- of excess water in the lake (~20 km3) to the size of ent with field observations. Discharge estimates DISCUSSION the breach (~ 3200 m2). SMPDBK derived values derived from flow-competence analysis of boulder o of Qp for tb = 1 day to 1 week vary between 25600 deposits downstream of Aratiatia provide an inde- Discharge estimates for the Lake Taupo out- and 17500 m3/s respectively, whereas varying the pendent test of the other methods (Fig. 12). These burst flood are summarized in Figure 12 and flow resistance parameter n by 15% alters the peak estimates indicate that the Taupo flood is com- Table 1. Dimensionless analysis indicates that outlet discharge by <5%. Downstream routing parable in terms of discharge and volume of water development of the outlet was virtually instantaneous compared with the duration of the flood. Consequently, peak discharge at the outlet was 450 primarily a function of its dimensions and geom- 400 largest boulder per site etry, rather than lake drawdown during its formation. Therefore, treatment of the Taupo flood as a 350 average intermediate diameter of five dam breach is appropriate and reconstructions largest boulders and one standard 300 deviation utilizing the physically based broad-crested weir equation and the minimum spillway cross sec- 250 3 tion, giving values of 17 300–27 900 m /s, are 200 likely to be the most reliable estimates of peak outflow discharge. Modeling the Taupo flood 150 source of boulders using physically based dam breach models based 100 on vertical breach growth rates of 1–100 m/hr for at Aratiatia Cascade artificial dam failures (Costa and Schuster, 1988) 50 rhyolite dome is problematic. The weakness arises from apply- Intermediate axis diameter (cm) 0 ing high rates of breach growth to the relatively thick, but narrow and elongate, Taupo blockage. 10 12 14 16 18 SMPDBK reconstructions of the Taupo flood Distance downstream from outlet (km) o show that peak outlet discharge (Qp ) is relatively independent of the time for breach formation, tb, Figure 11. Dimensions of spherulitic rhyolite boulders eroded from the Aratiatia dome for times between 1 day and 1 week (Fig. 9), when (shaded) and deposited downstream of the cascade, showing downstream decrease in size.

1444 Geological Society of America Bulletin, October 1999

TABLE 2. PALEOHYDRAULIC PARAMETERS FOR THE TAUPO FLOOD DOWNSTREAM OF ARATIATIA Distance downstream Intermediate axis diameter Flow velocity Active channel width* Estimated flow Discharge from outlet Largest boulder Average of five largest (m/s) (m) depth† (m3/s) 0.60 (km) (cm) boulders (cm) v = 0.29 D i w (m) Q = wdv Dmax Di d 11.4 395 248 7.93 310 6 14 700 13.0 240 226 7.50 460 6 20 600 13.5 190 160 6.09 540 6 19 700 14.3 210 192 6.80 650 5 22 100 14.6 185 130 5.38 610 5 16 400 15.0 200 144 5.72 710 5 20 300 15.4 220 143 5.70 770 5 21 900 15.8 95 89 4.29 740 5 15 900 16.3 220 142 5.67 610 5 17 300 16.8 75 54 3.18 550 5 8 700 Note: Parameters based on flow-competence methods of O’Connor (1993). *Active channel width measured from aerial photographs, 1:50 000 topographic maps, and field observations. †Flow depth taken as the elevation difference between high-water marks and the fan deposit surface.

released to some of the largest known examples of Korako contributed to the flooding of low-lying assume that other temporary impoundments of natural and artificial dam breaches anywhere in the parts of the Reporoa basin, leading to the deposi- water, such as Lake Reporoa, which formed be- world (Fig. 13). Errors in the paleohydraulic re- tion of a thin drape of fine-grained slack-water hind an ignimbrite barrier near Orakei Korako construction likely arise from considerable scour sediments over much of the basin floor. The Taupo (Manville, in press), and Lake Hinemaiaia (Smith, and widening along the spillway reach, and errors flood eroded 1.8 ka pyroclastic deposits along the 1991b), also generated floods when their tem- in the estimation of n. Attenuation of the flood Waikato River, cutting down to preeruption levels porary barriers were breached. Waythomas et al. wave is indicated by downstream decreases in and exhuming the preeruption valley. (1996) hypothesized that floods from caldera lakes peak discharge, most noticeably at channel expan- were a common phenomenon based on evidence sions (e.g., below the Aratiatia Cascade and at the Flood Hazards from Calderas from calderas in Alaska and Kamchatka, and our entrance to the Reporoa basin, Fig. 12). The effect work in the Taupo Volcanic Zone corroborates this is less pronounced than for well-studied historical In this paper, we have documented a large-mag- view. In the brief 150 yr historical record of New dam break floods because of the comparatively nitude flood from intracaldera Lake Taupo (Fig. Zealand, there are two examples of caldera long duration of the Taupo flood. Hydraulic pond- 13), caused by breaching of a temporary blockage and/or crater lake floods. In 1953 the crater lake of ing behind the valley constriction at Orakei at the outlet created during the 1.8 ka eruption. We Ruapehu breached a temporary barrier of ash and

40000 Paleohydraulic analysis

35000 dimensionless analysis

30000 broad-crested weir (trapezoidal breach)

25000 broad-crested weir (rectangular breach) /s) 3

flow competence 20000

SMPDBK model runs Discharge (m Discharge 15000

Tb = 1 hour, n = 0.030 10000 Tb = 1 day, n = 0.030 (upper crosses, n = 0.025: lower crosses, n = 0.035) 5000 Tb = 1 week, n = 0.030

0 –5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 Tb = 1 month, n = 0.030 Distance downstream from outlet (km)

Figure 12. Summary of discharge estimates for the Taupo flood plotted against distance downstream from the post–1.8 ka outlet. Many empirically derived values (see Table 1) do not plot on the chosen scale. Peak discharges modeled with the U.S. National Weather Service computer program SMPDBK for various values of breach-formation time (tb) and Manning number (n) = 0.030 decrease downstream, notably at the channel expansion below Aratiatia. Variation of n by ±0.05 has little effect on peak discharge. Estimates of discharge obtained from channel dimensions and flow-competence data from flood-transported boulders are also included.

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1013 10 10 Figure 13. Comparison of the Taupo x flood with other examples of volcanogenic Bonneville x 9 1012 Missoula x 10 outburst floods from New Zealand, Alaska, )

3 Grimsvotn 1996 11 8 and Iceland. Volcanogenic outburst floods

10 ) (m 10

Taupo 1800a 2 o Aniakchak compare in peak discharge and volume of V 10 10 7 water released to breakouts from natural 10 Mississippi x x Amazon and artificial dams. Some examples of pre- 9 x x 10 6 10 Kaharoa 700a cipitation-triggered river floods from major Tarawera 1904 x x Teton Dam 108 x 10 5 rivers are included, plotted as catchment x x volcanogenic dam breach floods area versus discharge. Data are from Healy 107 Waikato x x x 10 4 x x x artificial dam breach floods (1954), Baker and Costa (1987), Jarrett and NZ 1998 x x catchment area (km River Malde (1987), Costa and Schuster (1988), 106 10 3 Volume of water released, of water Volume x natural dam breach floods Mayo (1989), McKerchar and Pearson x x Haast NZ 105 10 2 (1989), O’Connor and Baker (1992), Baker x Ruapehu 1953 riverine floods et al. (1993), Waythomas et al. (1996), 4 1 10 10 Gudmundsson et al. (1997), and White et al., 1 2 3 4 5 6 7 8 10 10 10 10 10 10 10 10 (1997). All dates are in years A.D. 3 Peak discharge,Qp (m /s)

ice 8 yr after an eruption and caused a flood that Nummedal, D., eds., The Channeled Scabland: Washing- Cronin, S. J., Neall, V. E., LeCointre, J. A., and Palmer, A. S., ton, D.C., NASA, p. 81–115. 1997, Changes in Whangaehu river lahar characteristics claimed 151 lives (Healy, 1954). At Lake Tarawera Baker, V. R., and Bunker, R. C., 1985, Cataclysmic late Pleis- during the 1995 eruption sequence, Ruapehu Volcano, in 1904, a pyroclastic barrier emplaced during the tocene flooding from glacial Lake Missoula: A review: New Zealand: Journal of Volcanology and Geothermal 1886 eruption of Tarawera was swept away, caus- Quaternary Science Review, v. 4, p. 1–41. Research, v. 76, p. 47–61. Baker, V. R., and Costa, J. E., 1987, Flood power, in Mayer, L., Davy, B. W., and Caldwell, T. G., 1998, Gravity, magnetic and ing a flood that affected communities for 40 km and Nash, D., eds., Catastrophic flooding, the Binghamp- seismic surveys of the caldera complex, Lake Taupo, downstream to the sea (White et al., 1997). Deep- ton Symposia in Geomorphology, International Series, North Island, New Zealand: Journal of Volcanology and ening of the river channel during this flood reju- no. 18: Boston, Allen and Unwin, p. 1–21. Geothermal Research, v. 81, p. 69–89. Baker, V. R., and Ritter, D. F., 1975, Competence of rivers to Fread, D. L., 1996, Dam-breach floods, in Singh, V. P., ed., venated tributary streams in headwater areas. The transport coarse bedload material: Geological Society of Hydrology of disasters: Water Science and Technology resulting release of sediment from incising streams America Bulletin, v. 86, p. 975–978. Library, Volume 24: Dordrecht, Kluwer Academic Pub- Baker, V. R., Kochel, R. C., Patton, P. C., and Pickup, G., 1983, lishing, p. 85–126. triggered a period of downstream aggradation that Paleohydraulic analysis of Holocene flood slack-water Glicken, H., Meyer, W., and Sabol, M., 1989, Geology and was more damaging and long lasting than the sediments, in Collinson, J. D., and Lewin, J., eds., Mod- groundwater hydrology of Spirit Lake blockage, Mount flood. Floods are also inferred to have occurred ern and ancient fluvial systems: International Association St. Helens, Washington, with implications for lake reten- of Sedimentologists Special Publication 6, p. 229–239. tion: U.S. Geological Survey Bulletin 1789, 33 p. from Lake Tarawera following the ca. A.D. 1300 Baker, V. R., Benito, G., and Rudoy, A. N., 1993, Paleohydrol- Gudmundsson, M. T., Sigmundsson, F., and Björnsson, H., Kaharoa eruption and two previous events, and ogy of late Pleistocene superflooding, Altay Mountains, 1997, Ice-volcano interaction of the 1996 Gjálp subglacial from the Taupo caldera following the 26.5 ka Oru- Siberia: Science, v. 279, p. 348–350. eruption, Vatnajökull, Iceland: Nature, v. 389, p. 954–957. Beget, J. E., 1986, Outburst floods from glacial Lake Missoula: Hamblin, W. K., 1994, Late Cenozoic lava dams in the western anui eruption (Thompson, 1958). Consequently, Comment: Quaternary Research, v. 25, p. 133–138. Grand Canyon: Geological Society of America Memoir the likelihood of large-scale floods, possibly oc- Blair, T. C., 1987, Sedimentary processes, vertical stratification 183, 135 p. sequences and geomorphology of the Roaring River allu- Healy, J., 1954, Submission to the Tangiwai railway disaster curring decades to centuries after the volcanic ac- vial fan, Rocky Mountain National Park, Colorado: Jour- Board of Inquiry: Wellington, New Zealand, Report of the tivity that created the impoundments, must be con- nal of Sedimentary Petrology, v. 57, p. 1–18. Board of Inquiry, p. 1–31. sidered in any analysis of caldera volcanic hazards. Blown, I., and Church, M., 1985, Catastrophic lake drainage Healy, J., 1975, Volcanic lakes, in Jolly, V. H., and Brown, within the Homathko River basin, British Columbia: J. M. A., eds., New Zealand lakes: Auckland, New Canadian Geotechnical Journal, v. 22, p. 551–563. Zealand, Auckland University Press, p. 70–83. ACKNOWLEDGMENTS Brayshaw, A. C., 1984, Characteristics and origin of cluster Healy, J., Lloyd, E. F., Rishworth, D. E. H., Wood, C. P., Glover, bedforms in coarse-grained alluvial channels, in Koster, R. B., and Dibble, R. R., 1978, The eruption of Ruapehu, E. H., and Steel, R. J., eds., Sedimentology of gravels and New Zealand, on 22 June 1969: New Zealand Department Research for this study by Manville was con- conglomerates: Canadian Society of Petroleum Geolo- of Scientific and Industrial Research Bulletin 224, 80 p. ducted during an Otago University postdoctoral gists Memoir 10, p. 77–85. Hill, C. F., 1975, Impounded lakes of the Waikato River, in Buesch, D. C., 1991, Changes in depositional environments re- Jolly, V. H., and Brown, J. M. A., eds., New Zealand fellowship in association with the Institute of Geo- sulting from emplacement of a large-volume ignimbrite, lakes: Auckland, New Zealand, Auckland University logical and Nuclear Sciences. Funding was pro- in Fisher, R. V., and Smith, G. A., eds., Sedimentation in Press, p. 140–149. vided by the Foundation for Research Science and volcanic settings: SEPM (Society for Sedimentary Geol- Houghton, B. F., Wilson, C. J. N., McWilliams, M. O., ogy) Special Publication 45, p. 139–153. Lanphere, M. A., Weaver, S. D., Briggs, R. M., and Technology, New Zealand, contract C05516. The Clague, J. J., and Mathews, W. H., 1973, The magnitude of Pringle, M. S., 1995, Chronology and dynamics of a large manuscript benefited from thorough and construc- jökulhlaups: Journal of Glaciology, v. 12, p. 501–504. silicic magmatic system: Central Taupo Volcanic Zone, tive reviews by K. A. Hodgson, J. W. Vallance, Clarkson, R. A., 1996, Immediate post–c.1.8 ka Taupo eruption New Zealand: Geology, v. 23, p. 13–16. secondary deposits and shorelines [Master’s thesis]: Jarrett, R. D., and Costa, J. E., 1986, Hydrology, geomorphol- G. A. Smith, J. S. Walder, C. F. Waythomas, and Dunedin, New Zealand, University of Otago, 157 p. ogy, and dam-break modelling of the July 15, 1982 Lawn J. E. O’Connor. Costa, J. E., 1983, Paleohydraulic reconstruction of flash-flood Lake Dam and Cascade Lake Dam failures, Larimer peaks from boulder deposits in the Colorado Front County, Colorado: U.S. Geological Survey Professional Range: Geological Society of America Bulletin, v. 94, Paper 1369, 78 p. REFERENCES CITED p. 986–1004. Jarrett, R. D., and Malde, H. E., 1987, Paleodischarge of the late Costa, J. E., 1988, Floods from dam failures, in Baker, V. R., Pleistocene Bonneville flood, Snake River, Idaho, com- Baker, V.R., 1973, Paleohydrology and sedimentology of Lake Kochel, R. C., and Patton, P. C., eds., Flood geomorphol- puted from new evidence: Geological Society of America Missoula flooding in eastern Washington: Geological So- ogy: New York, John Wiley and Sons, p. 439–463. Bulletin, v. 99, p. 127–134. ciety of America Special Paper 144, 79 p. Costa, J. E., and Schuster, R. L., 1988, The formation and fail- Kear, D., and Schofield, J. C., 1978, Geology of the Ngaruawahia Baker, V. R., 1978, Large-scale erosional and depositional fea- ure of natural dams: Geological Society of America Bul- subdivision: New Zealand Geological Survey Bulletin 88, tures of the Channeled Scabland, in Baker, V. R., and letin, v. 100, p. 1054–1068. 168 p.

1446 Geological Society of America Bulletin, October 1999

Kehew, A. E., and Clayton, L., 1983, Late Wisconsin floods and lake in western Guatemala: Journal of Volcanology and Sohn,Y. K., 1997, On traction-carpet sedimentation: Journal of development of the Souris-Pembina spillway system in Geothermal Research, v. 33, p. 81–107. Sedimentary Research, v. 67, p. 502–509. Saskatchewan, North Dakota, and Manitoba, in Teller, Northey, D. J., 1983, Seismic studies of the structure beneath Thompson, B. N., 1958, The geology of the Atiamuri dam site: J. T., and Clayton, L., eds., Glacial Lake Agassiz: Geo- Lake Taupo [Ph.D. dissert.]: Wellington, New Zealand, New Zealand Journal of Geology and Geophysics, v. 1, logical Association of Canada Special Paper 26, Victoria University of Wellington, 281 p. p. 275–306. p. 187–209. O’Connor, J. E., 1993, Hydrology, hydraulics, and geomor- Trabant, D. C., Waitt, R. B., Jr., and Major, J. J., 1994, Disruption Kehew, A. E., and Lord, M. L., 1986, Origin and large-scale phology of the Bonneville flood: Geological Society of of Drift glacier and origin of floods during the 1989–1990 erosional features of glacial-lake spillways in the northern America Special Paper 274, 83 p. eruptions of Redoubt Volcano, Alaska: Journal of Vol- Great Plains: Geological Society of America Bulletin, O’Connor, J. E., and Baker, V. R., 1992, Magnitudes and im- canology and Geothermal Research, v. 62, p. 369–385. v. 97, p. 162–177. plications of peak discharges from glacial Lake Mis- Umbal, J. V., and Rodolfo, K. S., 1996, The 1991 lahars of Kehew, A. E., and Lord, M. L., 1987, Glacial-lake outbursts soula: Geological Society of America Bulletin, v. 104, southwestern Mount Pinatubo and evolution of the lahar- along the mid-continent margins of the Laurentide ice- p. 267–279. dammed Mapanuepe Lake, in Newhall, C. G., and sheet, in Mayer, L., and Nash, D., eds., Catastrophic Park, C., and Schminke, H.-U., 1997, Lake formation and cata- Punongbayan, R. S., eds., Fire and mud: Eruptions and flooding, the Binghampton Symposia in Geomorphology, strophic dam burst during the late Pleistocene Laacher lahars of Mount Pinatubo, Philippines: Washington, D.C., International Series, no. 18: Boston, Allen and Unwin, See eruption (Germany): Naturwissenschaften, v. 84, University of Washington Press, p. 951–970. p. 95–120. p. 521–525. Waitt, R. B., Jr., 1989, Swift snowmelt and floods (lahars) Kochel, R. C., and Baker, V. R., 1982, Paleoflood hydrology: Pierson, T. C., and Scott, K. M., 1985, Downstream dilution caused by great pyroclastic surge at Mount St. Helens vol- Science, v. 215, p. 353–361. of a lahar: Transition from debris flow to hyperconcen- cano, Washington, 18 May 1980: Bulletin of Volcanology, Komar, P. D., 1989, Flow-competence evaluations of the trated streamflow: Water Resources Research, v. 21, v. 52, p. 138–157. hydraulic parameters of floods: An assessment of the p. 1511–1524. Waitt, R. B., Jr., Pierson, T. C., MacLeod, N. S., Janda, R. J., technique, in Beven, K., and Carling, P., eds., Floods— Pierson, T. C., Janda, R. J., Thouret, J. C., and Borrero, C. A., Voight, B., and Holcomb, R. T., 1983, Eruption-triggered Hydrological, sedimentological, and geomorphologi- 1990, Perturbation and melting of snow and ice by the 13 avalanche, flood, and lahar at Mount St. Helens—Effects cal implications: Chichester, John Wiley and Sons, November 1985 eruption of Nevado del Ruiz, Colombia, of winter snowpack: Science, v. 221, p. 1394–1397. p. 107–134. and consequent mobilization, flow and deposition of Walder, J. S., and Costa, J. E., 1996, Outburst floods from Kuenzi, W. D., Horst, O. H., and McGehee, R. V., 1979, Effect lahars: Journal of Volcanology and Geothermal Research, glacier-dammed lakes: The effect of mode of lake drainage of volcanic activity on fluvial-deltaic sedimentation in a v. 41, p. 17–66. on flood magnitude: Earth Surface Processes and Land- modern arc-trench gap, southwestern Guatemala: Geo- Pierson, T. C., Janda, R. J., Umbal, J. V., and Daag, A. S., 1992, forms, v. 21, p. 701–723. logical Society of America Bulletin, v. 90, p. 827–838. Immediate and long-term hazards from lahars and excess Walder, J. S., and O’Connor, J. E., 1997, Methods for predict- Lliboutry, L., Arnao, B. M., Pautre, A., and Schneider, B., 1977, sedimentation in rivers draining Mt. Pinatubo, Philip- ing peak discharges of floods caused by failure of natural Glaciological problems set by the control of dangerous pines: U.S. Geological Survey Water-Resources Investi- and constructed earthen dams: Water Resources Re- lakes in Cordillera Blanca, Peru: I. Historical failures of gation Report 92–4039, p. 183–203. search, v. 33, p. 2337–2348. morainic dams, their causes and prevention: Journal of Price, J. T., Lowe, G. W., and Garrison, J. M., 1977, Unsteady Walker, G. P. L., Wilson, C. J. N., and Froggatt, P. C., 1981, An Glaciology, v. 18, p. 239–254. flow modelling of dam-break waves in Proceedings, Dam- ignimbrite veneer deposit: The trail-marker of a pyro- Lord, M. L., and Kehew, A. E., 1987, Sedimentology and paleo- break modelling workshop, October 1977, Bethesda, clastic flow: Journal of Volcanology and Geothermal Re- hydrology of glacial-lake outburst deposits in southeastern Maryland: Washington, D.C., United States Water Re- search, v. 9, p. 409–421. Saskatchewan and northwestern North Dakota: Geologi- sources Council Hydrology Committee, p. 89–130. Waythomas, C. F., Walder, J. S., McGimsey, R. G., and Neal, cal Society of America Bulletin, v. 99, p. 663–673. Reneau, S. L., and Dethier, D. P., 1996, Late Pleistocene land- C. A., 1996, A catastrophic flood caused by drainage of a Lowe, D. J., and Green, J. D., 1992, Lakes, in Soons, J. M., and slide-dammed lakes along the Rio Grande, White Rock caldera lake at Aniakchak Volcano, Alaska, and implica- Selby, M. J., eds., Landforms of New Zealand (second Canyon, New Mexico: Geological Society of America tions for volcanic-hazards assessment: Geological Society edition): Auckland, New Zealand, Longman Paul Ltd., Bulletin, v. 108, p. 1492–1507. of America Bulletin, v. 108, p. 861–871. p. 107–143. Riggs, N. R., and Ort, M., 1999, Marginal lacustrine sedimen- Wetmore, J. N., and Fread, D. L., 1984, The NWS simplified Maizels, J. K., 1997, Jökulhlaup deposits in proglacial areas: tation and the post-eruptive rise and fall of Lake Taupo, in dam break flood forecasting model for desk-top and Quaternary Science Reviews, v. 16, p. 793–819. White, J. D. L., and Riggs, N. R., eds., Volcanogenic sedi- hand-held microcomputers: Washington, D.C., Federal Major, J. J., and Newhall, C. G., 1989, Snow and ice perturba- mentation in lacustrine settings: International Association Emergency Management Agency (FEMA), 122 p. tion during historical volcanic eruptions and the forma- of Sedimentologists Special Publication (in press). White, J. D. L., Houghton, B. F., Hodgson, K. A., and Wilson, tion of lahars and floods: Bulletin of Volcanology, v. 52, Rowley, P. D., Kuntz, M. A., and MacLeod, N. S., 1981, Pyro- C. J. N., 1997, Delayed sedimentary response to the A.D. p. 1–27. clastic-flow deposits, in Lipman, P. W., and Mullineaux, 1886 eruption of Tarawera, New Zealand: Geology, v. 25, Malde, H. E., 1968, The catastrophic late Pleistocene Bonne- D. R., eds., The 1980 eruptions of Mount St. Helens, p. 459–462. ville flood in the Snake River Plain, Idaho: U.S. Geologi- Washington: U.S. Geological Survey Professional Paper Wilson, C. J. N., 1985, The Taupo eruption, New Zealand. cal Survey Professional Paper 596, 52 p. 1250, p. 489–512. II. The Taupo Ignimbrite: Royal Society of London Philo- Manville, V., in press, Sedimentology and history of Lake Schofield, J. C., 1965, The Hinuera Formation and associated sophical Transactions, ser. A, v. 314, p. 229–310. Reporoa: An ephemeral supra-ignimbrite lake, Taupo Vol- Quaternary events: New Zealand Journal of Geology and Wilson, C. J. N., 1993, Stratigraphy, chronology, styles, and dy- canic Zone, New Zealand, in White, J. D. L., and Riggs, Geophysics, v. 8, p. 772–791. namics of late Quaternary eruptions from Taupo volcano, N. R., eds., Volcanogenic sedimentation in lacustrine set- Scott, K. M., Janda, R. J., de la Cruz, E. G., Gabinete, E., Eto, New Zealand: Royal Society of London Philosophical tings: International Association of Sedimentologists Spe- I., Isada, M., Sexon, M., and Hadley, K. C., 1996, Chan- Transactions, ser. A, v. 343, p. 205–306. cial Publication (in press). nel and sedimentation responses to large volumes of 1991 Wilson, C. J. N., and Walker, G. P. L., 1985, The Taupo erup- Mayo, L. R., 1989, Advance of Hubbard Glacier and 1966 out- volcanic deposits on the east flank of Mount Pinatubo, in tion, New Zealand. I. General aspects: Royal Society burst of Russell Fiord, Alaska, USA: Annals of Glaciol- Newhall, C. G., and Punongbayan, R. S., eds., Fire and of London Philosophical Transactions, ser. A, v. 343, ogy, v. 13, p. 189–194. mud: Eruptions and lahars of Mount Pinatubo, Philip- p. 199–228. McGimsey, R. G., Waythomas, C. F., and Neal, C. A., 1994, pines: Washington, D.C., University of Washington Press, Wilson, C. J. N., Rogan, A. M., Smith, I. E. M., Northey, D. J., High stand and catastrophic draining of intracaldera Sur- p. 971–988. Nairn, I. A., and Houghton, B. F., 1984, Caldera volca- prise Lake, Aniakchak Volcano,Alaska, in Till, A. B., and Selby, M. J., and Lowe, D. J., 1992, The Middle Waikato Basin noes of the Taupo Volcanic Zone, New Zealand: Journal Moore, T. E., eds., High stand and catastrophic draining and hills, in Soons, J. M., and Selby, M. J., eds., Land- of Geophysical Research, v. 89, p. 8463–8484. of Surprise Lake, Aniakchak volcano, Alaska. Geological forms of New Zealand (second edition): Auckland, New Wilson, C. J. N., Houghton, B. F., McWilliams, M. O., studies in Alaska by the U.S. Geological Survey, 1993: Zealand, Longman Paul Ltd., p. 233–255. Lanphere, M. A., Weaver, S. D., and Briggs, R. M., 1995, U.S. Geological Survey Bulletin 2107, p. 59–71. Self, S., and Healy, J., 1987, Wairakei Formation, New Volcanic and structural evolution of Taupo Volcanic Zone, McKerchar, A. I., and Pearson, C. P., 1989, Flood frequency in Zealand: Stratigraphy and correlation: New Zealand Jour- New Zealand: A review: Journal of Volcanology and Geo- New Zealand: Christchurch, New Zealand, Hydrology nal of Geology and Geophysics, v. 30, p. 73–86. thermal Research, v. 68, p. 1–28. Centre Publication 20, 87 p. Smith, G. A., 1986, Coarse-grained nonmarine volcaniclastic Wilson, C. J. N., Riggs, N. R., Ort, M. H., White, J. D. L., and Neall, V. E., 1996, Hydrological disasters associated with vol- sediment: Terminology and depositional process: Geo- Houghton, B. F., 1997, An annotated atlas of post–1.8 ka canoes, in Singh, V. P., ed., Hydrology of disasters: Water logical Society of America Bulletin, v. 97, p. 1–10. shoreline features at Lake Taupo, New Zealand: Institute Science and Technology Library, Volume 24: Dordrecht, Smith, R. C. M., 1991a, Landscape response to a major ignim- of Geological and Nuclear Sciences Scientific Report Kluwer Academic Publishing, p. 395–425. brite eruption, Taupo Volcanic Centre, New Zealand, in 97/19, 35 p. Newhall, C. G., and Dzurisin, D., 1988, Historical unrest at Fisher, R. V.,and Smith, G. A., eds., Sedimentation in vol- large calderas of the world: U.S. Geological Survey Bul- canic settings: SEPM (Society for Sedimentary Geology) letin 1885, 1108 p. Special Publication 45, p. 123–137. Newhall, C. G., Paull, C. K., Bradbury, J. P., Higuera-Gundy, Smith, R. C. M., 1991b, Post-eruption sedimentation on the MANUSCRIPT RECEIVED BY THE SOCIETY SEPTEMBER 25, 1998 A., Poppe, L. J., Self, S., Sharpless, N. P., and Ziagos, J., margin of a caldera lake, Taupo Volcanic Centre, New REVISED MANUSCRIPT RECEIVED FEBRUARY 8, 1999 1987, Recent geologic history of Lake Atitlan, a caldera Zealand: Sedimentary Geology, v. 74, p. 89–138. MANUSCRIPT ACCEPTED FEBRUARY 24, 1999

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