cone fields in sw elysium planitia hydrothermal venting on mars 1 we have studied a large cone field in sw elysium planitia that shows unique characteristics that have as yet not been described elsewhere on mars pitted cones and cone fields are quite common on mars and the elysium planitia region where they have been mainly interpreted as pseudocraters the cones in the study area differ from cones elsewhere on mars in morphology geologic setting and their assemblage with pitted ridges as well as their close association to aeolis planum medusae fossae formation mff material the cones appear almost exclusively along an extensive contact zone several hundred kilometers long where lava flows that emanated from the cerberus fossae flooded fine grained mff material of the aeolis planum complex particularly striking is the observation that several cones in the study area have developed on top of mesas and terraces of aeolis planum material that were embayed but not overflowed by lava and can thus not be explained as pseudocraters we propose that the striking assemblage of these structures was formed by a combination of pseudocrater formation and hydrothermal venting through mesas of aeolis planum mff material by a process similar to hydrothermal vent complexes on earth the voluminous cerberus fossae lava flows that flooded the study area covered large parts of aeolis planum material initiating the mobilization of volatiles and leading to hydrothermal activity along the lava mff contact furthermore our observations strongly support the hypothesis that the volatile content of the mff is or was high at least in that part of the mff constituting the aeolis planum complex

flow characteristics of large eruption triggered debris flows at snow clad volcanoes constraints for debris flow models abstract theoretical modelling is not yet adequate to predict the behavior of debris flows which can be an extremely hazardous hydrologic process commonly associated with volcanic eruptions particularly at snow clad stratovolcanoes to provide a realistic basis for modelling the behavior of large 1000 m3 s debris flows this paper summarizes kinematic volumetric and hydraulic characteristics of ten large historic volcanic debris flows from four different volcanoes although debris flows larger than these are known to have occurred in the past the ten summarized here define a practical upper range in magnitude of more typical flows to be considered for future hazard prediction peak flow velocities of the ten debris flows studied were indirectly measured to be between about 5 and 20 m s on gradients of 0 005 to 0 25 m m but locally they were as great as 40 m s hydraulic average depths were as great as 25 m but were more commonly between 5 and 15 m in channels up to 400 m wide computed peak discharges volumetric flow rates were as high as 105 m3 s and total volumes were as much as 108 m3 total flow volumes increased by as much as four times in relatively steep channels as eroded sediment was incorporated into the debris flows flows generally achieved supercritical flow and deposited minimal volumes of sediment on gradients steeper than 0 02 m m subcritical flow and active deposition predominated on gradients less than 0 01 m m although flows travelled tens of kilometers on such low gradients while laying down deposits total distances travelled as debris flows were as far as 120 km

rapid formation of river terraces in non welded ignimbrite along the hishida river kyushu japan abstract river terraces are well preserved in the middle to lower reaches of the hishida river that flows through the extensive ignimbrite field produced by the large scale ito eruption about 24 500 years ago from aira caldera in southern kyushu japan the terraces are erosional and formed during dissection of the initial topography of the ito ignimbrite these occur within the upper non welded zone of the ito ignimbrite about 20 60 m below the depositional surface and usually more than 20 m above the welded zone streams associated with terrace formation were ephemeral because perennial streams are considered highly unlikely within the porous non welded ignimbrite all tephra layers on the original ignimbrite depositional surface also cover the terraces this indicates that the formation of terraces preceded the accumulation of the oldest tephra the age of which is about 23 000 years old high erodibility of the non welded ito ignimbrite and absence of vegetation on the initial surface would have permitted a rapid dissection of the initial topography and the formation of the river terraces thus it is interpreted that the river terraces were formed very rapidly in a period of far less than 1500 years immediately after the deposition of the ito ignimbrite and before cover of new vegetation on the depositional surface

the formation and failure of natural dams of the numerous kinds of dams that form by natural processes dams formed from landslides glacial ice and late neoglacial moraines present the greatest threat to people and property landslide dams form in a wide range of physiographic settings the most common types of mass movements that form landslide dams are rock and debris avalanches rock and soil slumps and slides and mud debris and earth flows the most common initiation mechanisms for dam forming landslides are excessive rainfall and snowmelt and earthquakes landslide dams can be classified into six categories based on their relation with the valley floor type i dams 11 of 184 landslide dams from around the world that we were able to classify do not reach from one valley side to the other type ii dams 44 span the entire valley floor in some cases depositing material high on opposite valley sides type iii dams 41 move considerable distances both upstream and downstream from the landslide failure type iv dams 1 are rare and involve the contemporaneous failure of material from both sides of a valley type v dams 1 also are rare and are created when a single landslide sends multiple tongues of debris into a valley and forms two or more landslide dams in the same reach of river type vi dams 3 involve one or more failure surfaces that extend under the stream or valley and emerge on the opposite valley side many landslide dams fail shortly after formation in our sample of 73 documented landslide dam failures 27 of the landslide dams failed less than 1 day after formation and about 50 failed within 10 days over topping is by far the most common cause of failure the timing of failure and the magnitude of the resulting floods are controlled by dam size and geometry material characteristics of the blockage rate of inflow to the impoundment size and depth of the impoundment bedrock control of flow and engineering controls such as artificial spill ways diversions tunnels and planned breaching by blasting or conventional excavation glacial ice dams can produce at least nine kinds of ice dammed lakes the most dangerous are lakes formed in main valleys dammed by tributary glaciers failure can occur by erosion of a drainage tunnel under or through the ice dam or by a channel over the ice dam cold polar ice dams generally drain supraglacially or marginally by downmelting of an outlet channel warmer temperate ice dams tend to fail by sudden englacial or subglacial breaching and drainage late neoglacial moraine dammed lakes are located in steep mountain areas affected by the advances and retreats of valley glaciers in the last several centuries these late neoglacial dams pose hazards because 1 they are sufficiently young that vegetation has not stabilized their slopes 2 many dam faces are steeper than the angle of repose 3 these dams and lakes are immediately downslope from steep crevassed glaciers and near vertical rock slopes and 4 downstream from these dams are steep canyons with easily erodible materials that can be incorporated in the flow and increase flood peaks the most common reported failure mechanism is overtopping and breaching by a wave or series of waves in the lake generated by icefalls rockfalls or snow or rock avalanches melting of ice cores or frozen ground and piping and seepage are other possible failure mechanisms natural dams may cause upstream flooding as the lake rises and downstream flooding as a result of failure of the dam although data are few for the same potential energy at the dam site ownstream flood peaks from the failure of glacier ice dams are smaller than those from landslide moraine and structed earth fill and rock fill dam failures moraine dam failures appear to produce some of the largest downstream flood peaks for potential energy at the dam site greater than 1011 1012 joules differences in flood peaks natural dam failures appear to be controlled by dam characteristics and failure mechanisms

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 constitute a major and frequently repeated hazard an outburst flood from taupo caldera new zealand released 20 km 3 of water within decades following an ignimbrite emplacing eruption ca 1 8 ka paleohydrologic reconstruction of the taupo flood provides estimates of peak discharge at the outlet in the range 17 000 35 000 m 3 s dimensionless analysis demonstrates that 1 failure of the barrier was essentially instantaneous 2 the event may be treated hydraulically as a dam break and 3 the peak discharge was 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 channel and hydraulic principles are likely to be more accurate and are in close agreement with computer implemented dynamic flow routing models the latter give peak discharges and maximum stage levels similar to constraints imposed by field evidence and estimates of flow depth and velocity the long duration 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 associated with volcanism

distal fluvio lacustrine volcaniclastic resedimentation in response to an explosive silicic eruption the pliocene mushono tephra bed central japan the pliocene mushono tephra bed central japan is characterized by fluvio lacustrine volcaniclastic resedimentation at distal locations from the volcanic source sedimentary responses to an explosive eruption are well recorded in the resedimented mushono tephra bed immediately after the eruption large volumes of primary volcaniclastic material were reworked from proximal areas and discharged into the distal basin overloading of fluvial systems by pyroclastic debris resulted in the development of low sinuosity active fluvial channel systems that carried sediment laden hyperconcentrated flows the fluvial system flowed into a lake more than 150 km from the inferred source location resulting in delta formation and progradation primary pyroclastic fall deposits are only a few centimeters thick but are overlain by reworked volcaniclastic deposits more than 5 m thick which show coarsening upward successions representing fluvio lacustrine deltaic progradation the reworked succession consists of six sedimentary facies which can be interpreted as prodelta mouth bar distributary channel hyperconcentrated flow inter distributary lowland or floodplain and slump deposits rounded cobble sized pumice clasts in prodelta and distributary channel deposits and thick reworked deposits imply that the majority of resedimented volcaniclastic material was derived from voluminous proximal ignimbrite rather than thin fine grained distal fallout ash material vertical facies changes are concordant with a decrease in sediment yields with time lateral facies distributions correspond to episodic influxes of volcaniclastic sediments that controlled the morphology of discrete fluvio lacustrine deltas the distal sedimentary response to an explosive silicic eruption is thus clearly recorded in vertical and lateral facies distribution of reworked volcaniclastic sediments

impacts of explosive volcanism on distal alluvial sedimentation examples from the pliocene holocene volcaniclastic successions of japan abstract volcanic activities can create cataclysmic hazards to surrounding environments and human life not only during the eruption but also by hydrologic remobilisation lahar processes after the cessation of eruptive activity although there are many studies dealing with the assessment and mitigation of volcanic hazards these are mostly concentrated on primary eruptive processes in areas proximal to active volcanoes however the influence of volcaniclastic resedimentation may surpass the impacts of primary eruptive activity in terms of both extent and persistence and can ultimately result in severe hazards in downstream areas examination of the volcaniclastic successions of non marine pliocene holocene sedimentary basins in japan has revealed hydrological volcaniclastic sedimentation in fluvial and lacustrine environments hundreds of kilometres from the inferred source volcano impacts on these distal and often spatially separated basins included drastic changes in depositional systems caused by sudden massive influxes of remobilised pyroclastic material typical volcaniclastic beds comprise centimetre to decimetre thick primary pyroclastic fall deposits overlain by metre to 10s of metres thick resedimented volcaniclastic deposits intercalated in sedimentary successions of non volcanic provenance the relatively low component of primary pyroclastic fall deposits in the volcaniclastic beds suggests that 1 potential volcanic hazards would be underestimated on the basis of primary pyroclastic fall events alone and 2 the majority of resedimented material was likely derived from erosion of non welded pyroclastic flow deposits in catchment areas rather than remobilisation of local fallout deposits from surrounding hillslopes the nature distribution and sequence of facies developed by distal volcaniclastic sediments reflect the influence of 1 proximity to ignimbrite but not directly with the distance to the eruptive centre 2 ignimbrite nature non welded or welded and volume 3 temporal changes in sediment flux from the source area 4 the physiography and drainage patterns of the source area and the receiving basin and any intervening areas and 5 the formation of ephemeral dam lakes and intra caldera lakes whose potential catastrophic failure can impact distal areas models of the styles and timing of distal volcaniclastic resedimentation are thus more complicated than those developed for proximal settings of stratovolcanoes and their volcaniclastic aprons and hence present different challenges for hazard assessment and mitigation

depositional processes of primary to reworked volcaniclastics on an alluvial plain an example from the lower pliocene ohta tephra bed of the tokai group central japan abstract a change from primary to reworked volcaniclastic deposits in the lower pliocene ohta tephra bed has been identified using facies analysis the ohta tephra bed is a widespread marker tephra distributed over a distance of more than 130 km the source volcano of this tephra is inferred to lie 50 to 100 km north of the most proximal ohta tephra bed the ohta tephra bed shows a change in volcaniclastic facies in distal areas eight sedimentary facies are recognized these are interpreted as pyroclastic fall pyroclastic flow and reworked mudflow flood flow channel fill swamp or backswamp lacustrine and ephemeral pond deposits in underlying and overlying non volcaniclastic beds four sedimentary facies are recognized indicating a fluvio lacustrine depositional system on an alluvial plain from the successions of volcaniclastic sedimentary facies deposition of the ohta tephra bed can be divided into stages i to vi each depositional stage is intimately related to volcanic activity stage i is the first eruption during which only the northern part of the study area was covered with fallout ash the second eruption stage of stage ii involved fallout ash distributed over all the study area these two fallout ash beds are derived from precursor eruptions stage iii reflects a period of volcanic quiescence and development of widespread lacustrine conditions the fallout ash may have destroyed vegetation and prompted greater discharge to form the large lacustrine deposits of stage iii a pyroclastic flow stage in stage iv is associated with an explosive eruption and resulted in pyroclastic flow deposits reaching the northern part of the study area stage v is characterized by mudflow deposits widespread mudflow deposits also originated from high water and clastics discharge on the unvegetated surface mudflows extend more than 180 km from the source volcano abundant supply of volcaniclastic material and high water and clastic discharge enhanced a rapid progradation of alluvial plains final reworking occurred during stage vi when volcaniclastic sediments were deposited from a fluvio lacustrine system the reworking system comprises initial mudflows followed by a normal fluvial system normal stream flow the mudflow deposits of stage v are traceable with roughly constant thickness but the volcaniclastic content of the deposits decreases in distal areas the fluvial system deposits of stage vi are thicker and have less abundant volcaniclastics in distal areas fluvial systems were braided streams promoted by volcanic eruption while they were meandering streams during the pre eruption period some eruption induced braided streams remained for a while following the cessation of volcanic activity

the 26 may 1982 breakout flows derived from failure of a volcanic dam at el chichon chiapas mexico the eruptions of el chichon between 28 march and 4 april 1982 produced a variety of pyroclastic deposits the climactic phase on 3 april at 07 35 4 april at 01 35 gmt destroyed the central andesitic dome and fed pyroclastic surges and flows that dammed nearby drainages including the magdalena river by late april a lake had formed 4 km long and 300 400 m wide containing a volume of 26 10 6 m 3 of hot water at 01 30 on 26 may the pyroclastic dam was breached and surges of sediment and hot water soon inundated the town of ostuacan 10 km downstream this hot flood was finally contained at penitas hydroelectric dam 35 km downstream where one fatality occurred and three workers were badly scalded stratigraphic and sedimentologic evidence indicates that the rapidly draining lake initially discharged two debris flows followed by five smaller debris flows and water surges the main debris flows became diluted with distance and by the time they reached ostuacan they merged into a single hyperconcentrated flow with a sediment concentration of 30 vol deposits from this hyperconcentrated flow were emplaced for 15 km as far as the confluence with another river the mas pac below which the flow was diluted to sediment laden streamflow the minimum volume of the breakout flow deposits is estimated at 17 10 6 m 3 from high water marks flow profiles and simulations utilizing the dambrk code from the national weather service we calculated a maximum peak discharge of 11 000 m 3 s at the breach this maximum peak discharge occurred 1 h after initial breaching the calculations indicated that 2 h were required to drain the lake

breakout flood from an ignimbrite dammed valley after the 5 ka numazawako eruption northeast japan numazawa volcano in northeast japan erupted most recently at ca 5 ka forming a 2 km diameter caldera and emplacing at least 4 km 3 of valley confined ignimbrite the ignimbrite dammed the tadami river to a depth of 100 m temporarily impounding 1 6 km 3 of water overtopping of the barrier triggered breaching and catastrophic release of the dam lake pyroclastic material redeposited by the ensuing flood is widely distributed along the tadami and agano rivers in deposits tens of meters thick as far as the coastal niigata plain 150 km downstream of the volcano evidence for damming and flooding found along the rivers includes 1 fine grained thinly laminated lacustrine deposits upstream of the pyroclastic blockage 2 30 m thick pumiceous debris flow and hyperconcentrated flow deposits that show continuous sedimentation with no major hiatus downstream of the volcano 3 fine grained slackwater deposits at tributary river mouths and 4 large flood boulders that form lags on stripped ignimbrite surfaces and younger terrace surfaces or cropping out within hyperconcentrated flow deposits paleohydraulic estimation techniques indicate a peak discharge of 30 000 50 000 m 3 s at the breach point burial of neolithic jomon settlements by flood and volcaniclastic aggradational deposits at distal locations indicates that such indirect volcanogenic hazards are significant even where direct primary consequences of volcanic eruptions are minor or absent

paleoflood hydrology origin progress prospects from an origin in diverse studies of flood geomorphology and quaternary geology paleoflood hydrology emerged as a geophysical and an applied hydrological science during the 1970s and 1980s since acquiring its formal name in 1982 the most productive approach in paleoflood hydrology has become energy based inverse hydraulic modeling of discrete paleoflood events recorded in appropriate settings as slackwater deposits and other paleostage indicators swd psi or as various threshold indicators of non exceedence technological advances particularly in hydraulic modeling and geochronology were instrumental in moving the discipline to its present status the most recent advances include 1 new techniques for the accurate geochronology of flood sediments notably tams radiocarbon analyses and osl dating and 2 the phenomenal increase in computer power that allows complex hydraulic calculations to become feasible for routine studies from its initial demonstration in the southwestern united states swd psi paleoflood hydrology proved its widespread applicability to various landscape environments particularly important studies have been accomplished in australia china india israel south africa spain and thailand paleoflood hydrology has also generated its share of controversy in part because of the differing viewpoints and attitudes of the two scientific traditions from which it emerged quaternary geology geomorphology versus applied hydrologic hydraulic engineering nevertheless the future growth of the discipline is assured given the rapid pace of discoveries that it engenders indeed so many international studies exist that it is appropriate to pursue global syntheses to address interesting and timely questions of extreme flood phenomena in relation to global climatic change

modeling strong motion in the ashigara valley for the 1990 odawara japan earthquake finite difference seismograms are calculated for the 1990 odawara earthquake using a basin structure derived from a geotechnical survey and a double couple solution determined from the p first motion data a two dimensional sh finite difference calculation reproduces the observed spatial variation in amplitude and waveform of the transverse records at the ashigara valley blind prediction sites for frequencies lower than 2 hz comparison of the synthetics and the observed seismograms suggests that the observed amplitude variation of the transverse records in the ashigara valley is mainly due to the source radiation pattern rather than the basin structure several different double couples are used in the waveform modeling to show the sensitivity of ground motion to the source mechanism we thus conclude that it is important to use an appropriate focal mechanism to model near field ground motion

sediment transportation and deposition by rain triggered lahars at merapi volcano central java indonesia abstract a lahar is a general term for a rapidly flowing mixture of rock debris and water other than normal streamflow from a volcano and refers to the moving flow located in the populated area of central java the stratovolcano merapi 2965 m is prone to lahar generation due to three main factors 1 millions of cubic meters of pyroclastic deposits are the product of frequent pyroclastic flows which have occurred on 2 to 4 year intervals 2 rainfall intensity is high often 40 mm in 2 h on average during the rainy season from november to april and 3 drainage pattern is very dense following the 22 november 1994 eruption of merapi 31 rain triggered lahar events were recorded in the boyong river between december 1994 and may 1996 on merapi s slopes instantaneous sediment concentration at any given time of the lahars varies widely over time and space lahars are transient sediment water flows whose properties are unsteady so that the sediment load fluctuates during the flow the boundary between the flow types debris flow with sediment concentration 60 volume or hyperconcentrated flow with sediment concentration ranges from 20 to 60 volume may fluctuate within the flow itself grain size distribution physical composition of sediments shear stress yield stress and water temperature play each a role on this boundary natural self damming and rapid breakout are partly responsible for the sediment variations of the flows debris flow phases at merapi typically last a few minutes to 10 min and are often restricted to the lahar front debris flow surges are sometimes preceded and always followed by longer hyperconcentrated flow phases as a result mean sediment concentration of the lahars is low commonly from 20 to 50 volume besides transient normal streamflow phases sediment concentration low sediment load and frequent transient flows in the merapi channels may result from at least three factors 1 several breaks in slope along the channel increase the deposition rate of sediment and hinder the bulking capacity of the lahars 2 source material is mainly coarse debris of merapi type block and ash flows consequently the remobilization of coarse debris is more difficult and the clast deposition is accelerated 3 variations of rainfall intensity over time and space common in tropical monsoon rainfall also influence the sediment load variations of the lahars sedimentologic analyses of the lahar deposits in the boyong river at merapi encompass clast supported and matrix supported debris flow deposits hyperconcentrated flow deposits and streamflow deposits the stratigraphic succession of massive and stratified beds observed immediately after any given lahar event in the boyong river indicates that the sediment concentration varies widely over time and space during a single lahar event sedimentation rate varies from 3 to 4 5 cm min during relatively long lived surges to as much as 20 cm min during short lived surges these results indicate that the sediment load fluctuates during lahar flow further demonstrating that lahars are transient sediment water flows with unsteady flow properties

freshwater megaflood sedimentation what can we learn about generic processes abstract there is growing recognition that sedimentary deposits related to high energy large scale freshwater floods are widespread across the continents and in the main can be related to quaternary outbreak flows from ice dammed lakes such deposits may also occur in the neogene and earlier recognition of these megaflood deposits is hindered by the lack of well conditioned stratigraphic models of depositional successions however descriptions of successions from many locations often exhibit commonalities when compared carefully thus the primary purpose of this paper is to examine and condense the published stratigraphic and sedimentological evidence so as to identify the key signatures of megaflood successions the deposits often are the only record of a flood and so the secondary purpose is to interpret the sedimentary sequences in order to recreate the behaviour of floods through single and multiple events finally some pointers are provided as to those areas of sedimentological research that might be profitably explored in more depth to improve understanding of megaflood dynamics a key characteristic of megaflood deposits is the apparent massive appearance of single units which on closer inspection usually are amalgamated layers of similar grain size distribution individual beds and sets of beds are commonly near horizontal planar and laterally extensive successions evidently are not channelized and extensivity is indicative of simultaneous and coherent deposition over large areas localised large scale clinoforms or smaller scale cross beds are common associated with bar front or dune progradation respectively but otherwise extensivity implies that cross cutting erosional contacts notably steeply inclined examples and small scale scours are not common except immediately around large isolated clasts such as dropstones and ice blocks a common vertical stack of sequences in any one succession consists of 1 basal thick coarse parallel bedded units 2 large scale clinoforms 3 horizontally bedded thin laminated units 4 ripple and dune cross beds 5 silt beds and 6 succession capping debris flow deposits in some cases such a succession usually is indicative of a single cycle of waxing and waning flood flow generally dominated by high concentration suspension deposition onto plane beds the high power allows the transport of fine gravel and coarse sand in suspension leading upon deposition to relatively coarse beds from suspension fall out clay and silt is generally sparse or absent being transported further down the system at the scale of the hydrograph the successions indicate that flows initially accelerate and then decelerate with significant shorter period flow pulses also evident in the sedimentary signatures in parallel bedded sequences primarily during waxing flows bedload transport often was ineffective in sorting sediment due to the high rates of deposition from the highly sediment charged waters resulting in diffuse accretion surfaces at each location similar sequence stacks are common leading to the impression of rhythmic deposition from pulsing flow including from repetitive series of floods although often composed of coarse grains many sequence stacks excluding any debris flows are not dissimilar to turbidity flow sequences the bouma sequence in particular although there is limited experimental evidence for turbidity currents associated with high energy freshwater flows theory does not preclude the deposition of similar sedimentary motifs from high concentration flows such that an analogy between some megaflood sedimentary beds and turbidites is not unreasonable despite considerable variation in megaflood sedimentary successions commonalities occur which are indicative of similar processes of deposition from high energy floods at geographically different locations the changes in style of deposition throughout a succession provide clues as to the nature of the flood flows and this information could be used to improve estimates of flood hydrograph behaviour however hypotheses linking deposit characteristics to flow dynamics are currently ill conditioned and often poorly tested the majority of published studies describe the coarser scale stratigraphy with only passing reference to the fine scale structure of individual beds closer attention to lamination styles particle orientation and syn depositional deformation structures for example should provide information on the flow mechanics at the time of bed deposition thus better consideration of the sedimentology of megaflood deposit beds will lead to better understanding of the controlling hydraulics and should result in better models of flood deposition

snowline elevation and eolian dust flux in the japanese islands during isotope stages 2 and 4 abstract changes in snowline elevation and dust flux clearly contrast the paleoenvironments of the japanese islands during isotope stages 2 and 4 a lower snowline elevation and a smaller dust flux during isotope stage 4 suggests a climate with a cold and dry summer dominated by winter monsoonal precipitation between 70 and 55 ka a higher snowline elevation and a larger dust flux during isotope stage 2 indicates a climate with a cold and very dry summer with a minimized winter monsoon precipitation especially around 18 ka a drier climate during stage 2 can be explained by 1 a lowering of sea surface temperature in the japan sea by blocking the warm tsushima current during a sea level lowering 2 the extension of sea ice over the northern part of the japan sea and the sea of okhotsk and 3 weakening of the summer monsoon and the southern migration of the polar front the increase in dust flux in both stages 2 and 4 suggests an intensification of westerlies and winter monsoon during these stages a bigger dust flux in stage 2 than stage 4 reflects 1 stronger westerlies and winter monsoon and 2 the emergence of continental shelf which became an important dust source

formation and failure of volcanic debris dams in the chakachatna river valley associated with eruptions of the spurr volcanic complex alaska abstract the formation of lahars and a debris avalanche during holocene eruptions of the spurr volcanic complex in south central alaska have led to the development of volcanic debris dams in the chakachatna river valley debris dams composed of lahar and debris avalanche deposits formed at least five times in the last 8000 10 000 years and most recently during eruptions of crater peak vent in 1953 and 1992 water impounded by a large debris avalanche of early holocene age may have destabilized an upstream glacier dammed lake causing a catastrophic flood on the chakachatna river a large alluvial fan just downstream of the debris avalanche deposit is strewn with boulders and blocks and is probably the deposit generated by this flood application of a physically based dam break model yields estimates of peak discharge q p attained during failure of the debris avalanche dam in the range 10 4 q p 6 m 3 s 1 for plausible breach erosion rates of 10 100 m h 1 smaller short lived lahar dams that formed during historical eruptions in 1953 and 1992 impounded smaller lakes in the upper chakachatna river valley and peak flows attained during failure of these volcanic debris dams were in the range 10 3 q p 4 m 3 s 1 for plausible breach erosion rates volcanic debris dams have formed at other volcanoes in the cook inlet region aleutian arc and wrangell mountains but apparently did not fail rapidly or result in large or catastrophic outflows steep valley topography and frequent eruptions at volcanoes in this region make for significant hazards associated with the formation and failure of volcanic debris dams

quaternary paleolake formation and cataclysmic flooding along the upper yenisei river abstract a suite of geomorphological and sedimentological features in the catchment of the upper yenisei river in the sayan mountains of southern siberia testifies to the occurrence of cataclysmic floods that flowed down the river evidence of large scale high energy flood events includes 1 gravel dunes up to a few meters high and spaced 50 to 80 m apart in the kyzyl basin 2 landforms such as hanging valleys and paleochannels and 3 flood sediments in a tributary valley the origins of the yenisei floods were likely diverse due to complex hydrological processes operating in the sayan mountains the possibilities include failures of multiple variably impounded ice sedimentary tectonic scarp and lava flow dams paleolakes in the two large intermontane basins of darkhadyn khotgor and todza and other minor basins in the upper yenisei river catchment dating techniques applied to the paleolakes in the darkhadyn khotgor and todza basins revealed their formation during various periods in the middle late pleistocene and holocene flooding from the darkhadyn khotgor appears to explain many of the inferred flood features although contributions by flooding from other paleolake basins cannot be ruled out computer simulation of the flooding caused by a darkhadyn khotgor paleolake ice dam failure indicates a probable peak discharge of 3 5 10 6 m 3 s 1 approximately one fifth that of the floods that formed the channeled scabland in the u s a many of the outburst events probably occurred in the late quaternary but earlier floods could also have occurred

volcaniclastic resedimentation in distal fluvial basins induced by large volume explosive volcanism the ebisutoge fukuda tephra plio pleistocene boundary central japan the ebisutoge fukuda tephra plio pleistocene boundary central japan has a well recorded eruptive style history magnitude and resedimentation styles despite the absence of a correlative volcanic edifice this tephra was ejected by an extremely large magnitude and complex volcanic eruption producing more than 400 km 3 total volume of volcanic materials volcanic explosivity index 7 which extended more than 300 km away from the probable eruption centre remobilization of these ejecta occurred progressively after the completion of a series of eruptions resulting in thick resedimented volcaniclastic deposits in spatially separated fluvial basins more than 100 km from the source facies analysis of resedimented volcaniclastic deposits was carried out in distal fluvial basins the distal tephra 100 300 km from the source comprises two different lithofacies primary pyroclastic fall deposits and reworked volcaniclastic deposits the resedimented volcaniclastic succession shows five distinct sedimentary facies interpreted as debris flow deposits facies a hyperconcentrated flow deposits facies b channel fill deposits facies c floodplain deposits with abundant flood flow deposits facies d and floodplain deposits with rare flood deposits facies e resedimented volcaniclastic materials at distal locations originated from unconsolidated deposits of a climactic large ignimbrite forming eruption factors controlling inter and intrabasinal facies changes are 1 temporal change of introduced volcaniclastic materials into the basin 2 proximaldistal relationship and 3 distribution pattern of pyroclastic flow deposits relative to drainage basins thus studies of the ebisutoge fukuda tephra have led to a depositional model of volcaniclastic resedimentation in distal areas after extremely large magnitude eruptions an aspect of volcaniclastic deposits that has often been ignored or poorly understood

geomorphic and sedimentary evidence of a gigantic outburst flood from towada caldera after the 15 ka towada hachinohe ignimbrite eruption northeast japan abstract this study reports on geomorphic and sedimentary evidence for a gigantic breakout flood from towada caldera honshu island northeast japan as observed along the oirase river downstream from the outlet of the caldera lake a number of features of the oirase river are inconsistent with its present discharge including the occurrence of 1 hanging valleys and a horseshoe shaped waterfall in the upstream gorge area 2 boulder bars and scattered flood boulders 3 a dry valley at a high elevation in the gorge and 4 a large fan the sanbongi fan in the downstream area of the river the sanbongi fan is composed of thick lithic rich hyperconcentrated flow deposits that include pumice clasts derived from the 15 ka towada hachinohe ignimbrite and well rounded meter sized and outsized boulders derived from bedrock of welded ignimbrite the deposits are entirely aggradational with no major channels indicating the absence of a major hiatus during sedimentation the depositional facies also indicate that a single sheet like flood event deposited the sediment within the sanbongi fan area based on the age of the sanbongi fan the flood occurred between 15 and 12 ka after eruption of the towada hachinohe ignimbrite the most probable water source for the flood is towada caldera lake as suggested by landforms and sediments along the lake outlet a paleohydrological analysis indicates that at least 6 km 3 of water was released from the caldera at a peak discharge of 2 10 4 to 3 10 5 m 3 s 1 during the breakout flood although the sanbongi fan was previously considered to be a normal alluvial fan that formed during a stage of low stand sea level the present results show that the formation of the fan was closely related to a catastrophic flood from towada caldera rather than perennial fluvial activity climate change or a change in relative sea level

a catastrophic flood caused by drainage of a caldera lake at aniakchak volcano alaska and implications for volcanic hazards assessment aniakchak caldera located on the alaska peninsula of southwest alaska formerly contained a large lake estimated volume 3 7 10 9 m 3 that rapidly drained as a result of failure of the caldera rim sometime after ca 3400 yr b p the peak discharge of the resulting flood was estimated using three methods 1 flow competence equations 2 step backwater modeling and 3 a dam break model the results of the dam break model indicate that the peak discharge at the breach in the caldera rim was at least 7 7 10 4 m 3 s 1 and the maximum possible discharge was 1 1 10 6 m 3 s 1 flow competence estimates of discharge based on the largest boulders transported by the flood indicate that the peak discharge values which were a few kilometers downstream of the breach ranged from 6 4 10 5 to 4 8 10 6 m 3 s 1 similar but less variable results were obtained by step backwater modeling finally discharge estimates based on regression equations relating peak discharge to the volume and depth of the impounded water although limited by constraining assumptions provide results within the range of values determined by the other methods the discovery and documentation of a flood caused by the failure of the caldera rim at aniakchak caldera underscore the significance and associated hydrologic hazards of potential large floods at other lake filled calderas

on traction carpet sedimentation traction carpets are highly concentrated bedload layers that are developed beneath and driven by turbulent overlying flows they have a convex up velocity profile and comprise a lower frictional and an upper collisional region the frictional region having a particle concentration more than 80 of the packed bed is characterized by nearly continuous grain contacts low strain rate and hampered grain segregation on the other hand the collisional region has a particle concentration between 15 and 80 of the packed bed and is characterized by active grain collisions and higher strain rate deposition from the traction carpets occurs via progressive aggradation of the bed rather than via en masse freezing while the downward grain flux from the overlying flow maintains them the thickness of a traction carpet stratum is therefore determined by the cumulative amount of sediment settled during the lifespan of a traction carpet and can be much larger than the thickness of a moving traction carpet inverse grading can be produced in the collisional region by the vertical gradient of dispersive pressure which is related to nonuniform distribution of particles when a thick frictional region develops however the inverse size distribution in the collisional region is poorly recorded in the deposits depositional features of traction carpet strata are therefore determined by the duration of a traction carpet and the thickness ratio of collisional to frictional regions the thickness ratio is further controlled by the applied shear stress sediment fallout rate and grain size generally a collisional region is better developed beneath a highly competent and coarse grained gravelly flow whereas a thick frictional region is developed beneath a heavily sediment laden fine grained sandy flow this explains the more common occurrence of inverse grading in coarse grained deposits

coarse grained nonmarine volcaniclastic sediment terminology and depositional process modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent dilute stream flow or viscous debris flow the characteristics of these poorly sorted sediments suggest rapid deposition from high concentration dispersions but not en masse sedimentation thus appears related to high discharge flows intermediate in sediment water ratio between stream flow and debris flow the term hyperconcentrated flood flow is proposed for describing this intermediate condition hyperconcentrated flood flow deposits are distinguished from debris flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification these deposits are distinguished from normal dilute stream flow deposits by lack of cross stratification in sand facies and by very poor sorting poor imbrication and numerous clasts with long axes oriented parallel to flow direction in gravel facies the horizontal bedding that dominates sandy hyperconcentrated flood flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper flow regime and should not be confused with the more familiar thin graded laminae of fine to medium grained sand often associated with parting lineation hyperconcentrated flood flow deposits are not unique to volcanic settings they also occur in arid alluvial fan sequences debris flow and hyperconcentrated flood flow deposits however are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings in volcanic regions therefore these deposits have greater preservation potential show greater lateral variability and are more voluminous transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water recently observed at mount st helens is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow

changes in whangaehu river lahar characteristics during the 1995 eruption sequence ruapehu volcano new zealand abstract during the 1995 ruapehu eruptive sequence multiple lahars occurred in the whangaehu river which drains ruapehu s crater lake during the earlier phreatic and phreatomagmatic eruptions lahars were generated by expulsion of waters from the lake but once the lake had emptied lahars were formed by remobilisation of seasonal snowpack laden with saturated freshly erupted tephra four types of lahars occurred during the eruptive sequence 1 initial snow slurry lahars composed of granular snow and ice incorporated by eruptively expelled crater lake waters which left behind frozen deposits with 2 5 20 clastic sediment 2 large dilute lahars generated as the volumes of ejected lake water increased and removed much of the readily available snow at least one third of the pre eruption crater lake volume was expelled during one day producing the largest lahars of the series these lahars were hyperconcentrated flows for up to 84 km from source leaving extensive deposits along the channel margins 3 concentrated lahars smaller volume lahars generated as the frequency of eruptions and volumes of expelled water declined these lahars were able to maintain high sediment concentrations measured at 46 52 by volume suspended sediment at 42 km from source their high sediment concentrations were maintained by erosion and incorporation of sand from the deposits of earlier flows which were lining the channel margins 4 remobilised tephra lahars generated following the two largest tephra eruptions of the sequence seasonal snowpack was covered by water saturated tephra warmer spring temperatures and heavy rainfall events caused collapse and remobilisation of snow and tephra producing several lahars in catchments draining eastern ruapehu

new views of granular mass flows concentrated grain fluid mixtures in rock avalanches debris flows and pyroclastic flows do not behave as simple materials with fixed rheologies instead rheology evolves as mixture agitation grain concentration and fluid pressure change during flow initiation transit and deposition throughout a flow however normal forces on planes parallel to the free upper surface approximately balance the weight of the superincumbent mixture and the coulomb friction rule describes bulk intergranular shear stresses on such planes pore fluid pressure can temporarily or locally enhance mixture mobility by reducing coulomb friction and transferring shear stress to the fluid phase initial conditions boundary conditions and grain comminution and sorting can influence pore fluid pressures and cause variations in flow dynamics and deposits

sedimentary processes and facies of the waterlaid anvil spring canyon alluvial fan death valley california the anvil spring canyon fan of the panamint range piedmont in central death valley was built entirely by water flow processes as revealed by an analysis of widespread 2 to 12 m high stratigraphic cuts spanning the 9 7 km radial length of this 2 5 5 0 sloping fan two facies deposited from fan sheetfloods dominate the fan from apex to toe the main one 60 95 of cuts consists of sandy granular fine to medium pebble gravel that regularly and sharply alternates with cobbly coarse to very coarse pebble gravel in planar couplets 5 25 cm thick oriented parallel to the fan surface the other facies 0 25 of cuts comprises 10 to 60 cm thick wedge planar and wedge trough beds of pebbly sand and sandy pebble gravel in backsets sloping 3 28 both facies are interpreted as resulting from rare sediment charged flash floods from the catchment and were deposited by supercritical standing waves of expanding sheetfloods on the fan standing waves were repeatedly initiated enlarged migrated and then terminated either by gradually rejoining the flood or by more violent breakage and washout the frequent autocyclic growth and destruction of standing waves during an individual sheetflood resulted in the deposition of multiple coarse and fine couplet and backset sequences 50 250 cm thick across the active depositional lobe of the fan erosional intensity during washout of the standing wave determined whether early phase backset bed deposits or washout phase sheetflood couplet deposits were selectively preserved in a given cycle two minor facies are also found in the anvil fan pebble cobble gravel lags 0 20 of cuts are present above erosional scours into the sheetflood couplet and backset deposits they consist of coarse gravel concentrated through fine fraction winnowing of the host sheetflood facies by sediment deficient water flows this reworking occurred during recessional flood stage or from non catastrophic discharge during the long intervals between major flash floods this facies is common at the surface giving rise to a braided stream appearance however it is stratigraphically limited present as thin continuous to discontinuous beds or lenses that bound 50 to 250 cm thick sheetflood sequences the other minor facies of the anvil fan consists of clast supported and imbricated thickly stratified pebbly cobbly boulder gravel present in narrow radially aligned ribbons nested within sheetflood deposits this facies is interpreted as representing deposition in the incised channel of the fan a subenvironment characterized by greater flow competence resulting from maintained depth from channel wall confinement and by more frequent water flows and winnowing events caused by its direct connection with the catchment feeder channel

methods for predicting peak discharge of floods caused by failure of natural and constructed earthen dams floods from failures of natural and constructed dams constitute a widespread hazard to people and property expeditious means of assessing flood hazards are necessary particularly in the case of natural dams which may form suddenly and unexpectedly we revise statistical relations derived from data for past constructed and natural dam failures between peak discharge qp and water volume released v0 or drop in lake level d but assert that such relations even when cast into a dimensionless form are of limited utility because they fail to portray the effect of breach formation rate we then analyze a simple physically based model of dam breach formation to show that the hydrograph at the breach depends primarily on a dimensionless parameter kv0 gl 2d7 2 where k is the mean erosion rate of the breach and g is acceleration due to gravity the functional relationship between qp and takes asymptotically distinct forms depending on whether 1 relatively slow breach formation or small lake volume or 1 relatively fast breach formation or large lake volume theoretical predictions agree well with data from dam failures for which k and thus can be estimated the theory thus provides a rapid means of predicting the plausible range of values of peak discharge at the breach in an earthen dam as long as the impounded water volume and the water depth at the dam face can be estimated

origin characteristics and behaviour of lahars following the 1990 eruption of kelud volcano eastern java indonesia in contrast to most twentieth century eruptions of kelud volcano eastern java the 10 february 1990 plinian eruption was not accompanied by lake outburst lahars however at least 33 post eruption lahars occurred between 15 february and 28 march 1990 they swept down 11 drainage systems and travelled as far as 24 km at an estimated mean peak velocity in the range of 4 11 m s 1 the deposits volume 30 000 000 m3 were approximately 7 m thick 2 km from vent and 3 m thick 10 km from vent on the volcaniclastic apron surrounding the volcano subtle but significant sedimentological differences in the deposits relate to four flow types a early massive deposits are coarse poorly sorted slightly cohesive and commonly inversely graded they are inferred to record hot lahars that incorporated pumice and scoria from pyroclastic flow deposits probably by rapid remobilization of hot proximal pyroclastic flow deposits by rainfall runoff sedimentary features such as clasts subparallel to bedding and thick reversely to ungraded beds suggest that these flows were laminar b abundant very poorly sorted deposits include non cohesive clast supported inversely graded beds and ungraded finer grained and cohesive matrix supported beds these beds display layering and vertical segregation density stratification suggesting unsteady properties of pulsing debris flows they are interpreted as deposited from segments of flow waves at a middle distance downstream that incorporated pre eruption sediments sedimentological evidence suggests unsteady flow properties during progressive aggradation c fine grained poorly sorted and ungraded deposits are interpreted as recording late hyperconcentrated streamflows that formed in the waning stage of an overflow and transformed downcurrent into streamflows d ungraded crudely stratified deposits were emplaced by flows transitional between hyperconcentrated flows and streamflows that traveled farther downvalley as far as 27 km from the vent at kelud the transformation of flow and behavior occurs within only 10 km of the source at the apex of the alluvial fans the rapid change of flow behavior is attributed to the low fines content and to the unsteady flow regime which may be due to a the rapid deposition of bedload owing to the break in channel gradient close to the vent and to changes in channel cross section and roughness and b the very low silt clay content in the non cohesive deposits these deposits mix with water to produce streamflows