mobility of pyroclastic flows and surges at the soufriere hills volcano montserrat the soufriere hills volcano on montserrat has produced avalanche like pyroclastic flows formed by collapse of the unstable lava dome or explosive activity pyroclastic flows associated with dome collapse generate overlying dilute surges which detach from and travel beyond their parent flows the largest surges partially transform by rapid sedimentation into dense secondary pyroclastic flows that pose significant hazards to distal areas different kinds of pyroclastic density currents display contrasting mobilities indicated by ratios of total height of fall h run out distance l area inundated a and volume transported v dome collapse flow mobilities characterised by either l h or a v2 3 resemble those of terrestrial and extraterrestrial cold rockfalls dade and huppert 1998 in contrast fountain fed pumice flows and fine grained secondary pyroclastic flows travel slower but for comparable initial volumes and heights can inundate greater areas

flow of variably fluidized granular masses across three dimensional terrain 2 numerical predictions and experimental tests numerical solutions of the equations describing flow of variably fluidized coulomb mixtures predict key features of dry granular avalanches and water saturated debris flows measured in physical experiments these features include time dependent speeds depths and widths of flows as well as the geometry of resulting deposits three dimensional 3 d boundary surfaces strongly influence flow dynamics because transverse shearing and cross stream momentum transport occur where topography obstructs or redirects motion consequent energy dissipation can cause local deceleration and deposition even on steep slopes velocities of surge fronts and other discontinuities that develop as flows cross 3 d terrain are predicted accurately by using a riemann solution algorithm the algorithm employs a gravity wave speed that accounts for different intensities of lateral stress transfer in regions of extending and compressing flow and in regions with different degrees of fluidization field observations and experiments indicate that flows in which fluid plays a significant role typically have high friction margins with weaker interiors partly fluidized by pore pressure interaction of the strong perimeter and weak interior produces relatively steep sided flat topped deposits to simulate these effects we compute pore pressure distributions using an advection diffusion model with enhanced diffusivity near flow margins although challenges remain in evaluating pore pressure distributions in diverse geophysical flows riemann solutions of the depth averaged 3 d coulomb mixture equations provide a powerful tool for interpreting and predicting flow behavior they provide a means of modeling debris flows rock avalanches pyroclastic flows and related phenomena without invoking and calibrating rheological parameters that have questionable physical significance

laboratory measurements of the vertical structure of rip currents 1 this paper describes the three dimensional variation of rip currents through the use of experimental measurements of rip currents in a directional wave basin the rip currents are unstable appearing sporadically at any given location the vertical profile of the rip current is found to vary from depth uniform inside the channel to depth varying further offshore offshore from the channel the rip current has much stronger velocities at the surface than near the bottom the instantaneous profiles twist rapidly over depth farther offshore and are fairly uniform in the channel the depth variations are shown to be sensitive to the total volume flux in the rip

an experimental study of rip channel flow a laboratory study of the flow over a bar with a single rip channel has been performed first the well known pattern of a bar circulation cell with a strong offshore directed current out through the rip channel and a weaker onshore directed return flow over the bar is documented then measurements of the three dimensional structure of the flow in the area where the rip channel the bar and the trough meet and well inside the rip channel are presented these measurements reveal that 3d effects play an important role and that a depth integrated viewpoint may not always be sufficient for predicting the flow in the near bed region particle tracking experiments illustrate the near bed flow pattern over the entire area these demonstrate how the overall trajectory pattern changes as a function of the distance of wave breaking from the bar crest for some conditions the rip current is fed from the trough and for other conditions it is fed directly from the bar both the 3d measurements and the trajectory tests show the existence of a weaker onshore directed near bed drift in the area where the rip current ceases finally in a series of sensitivity tests measurements of the rip current intensity for different wave climate and water level conditions reveal a strong correlation between the rip current intensity and the wave height both normalized

vorticity and divergence of surface velocities near shore abstract the nearshore environment is complex with many competing dynamical elements surface waves and edge waves a form of surface wave trapped to the shore can generally be separated from other forms of motion because of their fast propagation speeds however other motions such as internal waves shear waves density flows and isolated vortex pairs can move at comparable speeds a tool to help separate these dynamical elements is decomposition of the surface 2d flow into two parts one nondivergent and the other irrotational solenoidal and potential flows respectively here an efficient algorithm for this separation is developed and applied and two examples are examined from data taken at duck north carolina in 1997 as part of the sandyduck experiment the first example is a fresher water density flow propagating downcoast probably from the chesapeake bay it is seen that 1 the wave driven alongshore flow leads the flow generating a surge of offshore surface flow in its wake 2 the i

evaluation of geophysical mass flow models using the 2006 block and ash flows of merapi volcano java indonesia towards a short term hazard assessment tool abstract the dynamics and depositional processes associated with block and ash flows bafs are most commonly inferred to be a function of granular or inertial grain flow similar to debris flows and cold rock avalanches existing geophysical mass flow models are either based on frictional mohr coulomb behavior the titan2d model developed at the state university of new york at buffalo usa or another rheological law i e a constant retarding stress eventually adding some viscous and turbulent components the volcflow model developed at the laboratoire magmas et volcans clermont ferrand france the 2006 bafs of merapi present a rare opportunity to test these two well established models against a well constrained field example integration of high resolution field based data into numerical simulations allows the validity of these models to be tested and rapid quantification of best fit input parameters we first show that with the incorporation of spatially varying bed friction angles titan2d is capable of reproducing the paths runout distances areas covered and deposited volumes of the 2006 merapi flows over highly complex topography however some discrepancies with field data are noted and the velocity and travel time of the flows do not match entirely using a single free parameter a constant retarding stress simulations obtained with the volcflow model also reproduce the morphology and distribution of the natural deposits as well as the time of emplacement and velocities of the flows the results suggest that the performance of these models in simulating actual events is critically dependent on 1 the calibration of the model by using extensive field based data such as deposit distribution and processes of flow generation transport and deposition 2 the incorporation of a suitable numerical topographic dataset i e high resolution digital elevation model and 3 the choice of input parameters such as location and volume of the initial pile of material and source characteristics single or multiple dome collapse dome collapse duration and total volume of collapsed material sensitivity analyses and inundation maps based on the probability of impact were used to produce a suite of potentially inundated areas from future gravitational dome collapse events affecting the gendol valley and adjacent areas on the southern flank of the volcano our results provide the basis for defining hazard zonations of key areas at risk from bafs which will be generated during future comparable eruptions at merapi

surf zone diffusivity on a rip channeled beach absolute and relative diffusivity are measured on a rip channeled beach using 30 position tracking drifters released in clusters 4 12 drifters deployed on 7 days with different wave forcing and tidal elevations at sand city monterey bay california diffusivity and dispersion were found to be larger on days with rip current flow patterns and larger waves rip currents cause material to diffuse quickly for t 0 9 two independent methods are used to measure the small scale turbulent diffusion contribution kxy which are found significantly correlated r2 0 95 with each other and calculated surf zone wave breaking induced turbulent eddy viscosity here kxy is small relative to the total dispersion ke kxy 3 30 indicating that the shear flow is the primary mechanism responsible for dispersion in a rip current system

explosive activity and generation mechanisms of pyroclastic flows at arenal volcano costa rica between 1987 and 2001 explosive activity at arenal and associated tephra fall that has occurred over the 14 year period from 1987 2001 is described explosions have been notably variable in both frequency and size a marked decrease in both frequency and quantity of tephra fallout occurred in early 1998 until the end of 2001 grainsize distributions of cumulative tephra samples collected once a month are typically bimodal aggregation causing premature fallout of fine ash and possibly fallout from ash plumes produced by pyroclastic flows are considered responsible for the bimodality of fallout scanning electron microscopy of the glass component of tephra from single explosions show predominantly blocky and blocky fluidal clast types interpreted as being the product of vulcanian type explosions fragmentation of a mainly rigid degassed magma body and a minor molten component is inferred for these explosions pyroclastic flows were produced either associated with the larger explosions by a mechanism of column collapse 1987 1990 or unrelated to explosions by partial collapse of the crater wall 1993 1998 2000 2001 pyroclastic flow activity has migrated from west to north during the period reported pyroclastic flow deposits are variable in the quantity of juvenile material and any associated surge component large juvenile blocks were partially molten on emplacement and many have a typical cauliform texture blocks with both juvenile and lithic textures indicate that at the summit magma was in intimate contact with the pre existing edifice rather than as a simple open crater or lava pool crater wall collapse may have been promoted by the reduction in explosive activity which has increased the lava accumulation at the summit and in turn increased instability of the summit region thus although explosive activity has waned if the lava output is maintained the hazard of pyroclastic flows is likely to continue

dynamics and deposits generated by the kos plateau tuff eruption controls of basal particle loss on pyroclastic flow transport the explosive eruption of voluminous silicic magmas often produces widespread and massive deposits formed from pyroclastic density currents while these punctuated events dramatically alter the landscape and have potential climate altering impact our understanding of the internal structure and transport dynamics of these eruptions is hampered by a lack of direct observations we utilize the natural boundary conditions encountered by the eruption of the kos plateau tuff to probe its internal structure as well as constrain the neotectonic activity in the region and eruption duration of this moderate to large 60 km3 event at the time of the eruption 161 ka the lower sea level in the mediterranean may have resulted in flows that traversed mostly land to the north of the eruptive vent while flows to the south may have encountered an expanse of water steep topography and overwater transport can impede the transport of the dense basal portions of the flow where particles make multiple or sustained contact with the bed we use an eulerian eulerian lagrangian computational approach coupled with overwater and overland boundary conditions including topography to determine the role of bed load versus suspended load in the transport of these flows we find that a ring vent structure and eruptive fluxes greater than 2 106 m3 s are required for the spatial distribution of the kpt the maximum grain size and deposit locations of the first voluminous ignimbrite unit d can be explained by suspended flow to the south consistent with overwater transport and bed load and suspended load transport to the north consistent with overland transport however the maximum lithic size for the largest and last ignimbrite unit e requires some bed load transport in all directions we propose that the boundary conditions were significantly altered during the course of the eruption through either the in filling of a shallow sea to the south or the development of a thick pumice raft to aid saltation on the basis of the inferred eruptive flux we estimate that the duration of the eruption climax in which most of the material was erupted likely only lasted from a few hours to a day

parallel adaptive numerical simulation of dry avalanches over natural terrain high fidelity computational simulation can be an invaluable tool in planning strategies for hazard risk mitigation the accuracy and reliability of the predictions are crucial elements of these tools being successful we present here a new simulation tool for dry granular avalanches using several new techniques to create a highly accurate tool with a combination of good models and numerical methodologies highlights of our methodology are the use of a depth averaged model of the conservation laws and an adaptive grid godunov solver to solve the resulting equations the software is designed to run on distributed memory supercomputers and make use of digital elevation data dynamically i e refine the grid and input data to finer resolutions to better capture flow features as the flow evolves our simulations are validated using quantitative and qualitative comparisons to tabletop experiments and data from field observations our software is freely available and can be used on a range of hardware from inexpensive desktop computers to the largest supercomputers

effect of grain size on mobility of dry granular flows of angular rock fragments an experimental determination abstract rapid mass movements of rock fragments are among the most hazardous natural phenomena they are too dangerous to be approached and studied directly and for this reason experiments are necessary to understand their flow mechanisms here we present the results of experiments on granular flow mobility obtained releasing batches of angular rock fragments down a curved chute our experiments demonstrate that the finer the grain size of the flow the larger its mobility this is accompanied by a decrease of fragments agitation when grain size decreases as shown by particle image velocimetry measurements of high speed video camera images the smaller the agitation the smaller is also the energy dissipated by flows per unit of travel distance this explains the effect of finer grain size on flow mobility these results are an important step towards the prediction of runout distances as a function of the features that characterize the different geophysical flows

nearshore circulation over transverse bar and rip morphology with oblique wave forcing there is a paucity of field data to describe the transition in nearshore circulation between alongshore meandering and rip current systems a combination of in situ current meters and surf zone drifters are used to characterize the nearshore circulation over a transverse bar and rip morphology at pensacola beach florida in the presence of relatively low energy oblique waves current speeds vary in response to the relative wave height ratio hs h which defines the degree and extent of breaking over the shoal in the absence of wave breaking the nearshore circulation was dominated by an alongshore current driven by the oblique waves as waves begin to break across the shoal 0 2 0 5 the meandering current is replaced by an unsteady rip circulation that moves offshore between the shoals before turning alongshore in the direction of wave advance outside the surf zone the increase in wave dissipation is associated with an increase in very low frequency vlf variations in the current speed across the shoal and in the rip channel that caused the circulation to oscillate between an offshore and an alongshore flow the unsteady nature of the nearshore circulation is responsible for 55 of all surf zone exits under these more dissipative conditions in contrast only 29 of the drifters released from the shoal exited the surf zone and bypassed the adjacent shoal with the alongshore meandering current while the currents had a low velocity maximum of 0 4 m s 1 and would not pose a significant hazard to the average swimmer the results of this study suggest that the transverse bar and rip morphology is sufficient to create an alongshore variation in wave dissipation that forces alongshore meandering and low energy rip circulation systems under oblique wave forcing copyright 2013 john wiley sons ltd

the anatomy of a pyroclastic density current the 10 july 2015 event at volcan de colima mexico pyroclastic density currents pdcs represent one of the most dangerous phenomena occurring in explosive volcanic eruptions and any advance in the physical understanding of their transport and sedimentation processes can contribute to improving their hazard assessment the 10 11 july 2015 eruption at volcan de colima provided a unique opportunity to better understand the internal behaviour of pdcs based on seismic monitoring data on 10 july 2015 the summit dome collapsed producing concentrated pdcs that filled the main channel of the montegrande ravine a lahar monitoring station installed 6 km from the volcano summit recorded a pdc before being completely destroyed real time data acquisition from a camcorder and a geophone that were part of the station along with field observations and grain size data of the pyroclastic deposits are used here to interpret the internal flow structure and time variant transport dynamics of low volume valley confined concentrated pdcs the pdc that reached the monitoring station moved at a velocity of 7 m s and filled a 12 m deep channel the outcrops show massive block and ash flow deposits with trains of coarse clasts in the middle and towards the top of the depositional units the seismic record gathered with the geophone was analysed for the time window when the flow travelled past the sensor the geophone record was also compared with the recordings of a broadband seismic station located nearby two main frequency ranges were recognised which could be correlated with the basal frictional forces exerted by the flow on the channel bed 10 20 hz and a collisional regime 20 40 hz interpreted to be associated with a clast segregation process i e kinematic squeezing this latter regime promoted the upward migration of large blocks which subsequently deviated towards the margin of the flow where they interacted with the sidewall of the main channel the energy calculated for both seismic components shows that the collisional regime represents 30 of the total energy including an important sidewall stress component these results gathered directly from a moving flow contribute to unravelling the internal behaviour of concentrated pdcs providing information on energy partitioning and particle particle interactions this confirms previous assumptions inferred from field observations and tested by analogue or numerical modelling the nature of the contact between grains is still poorly documented in natural pdcs and there is still much uncertainty and discussion about dominant forces in such currents data reported here may thus be useful to better constrain the physics of low volume valley confined concentrated pdcs and our findings need to be considered in theoretical models in parallel this study shows how geophones can provide a cheap alternative for pdc detection

geometry and state of stress of the downgoing cocos plate in the isthmus of tehuantepec mexico the seismicity in the isthmus of tehuantepec mexico is anomalously active compared with other parts of the middle american subduction zone the results of a microearthquake study conducted in this region show that the slab changes dip from an almost subhorizontal geometry west of longitude 96 w to a dip of about 45 to 50 to the east of this longitude the change in dip occurs gradually in a broad flexure of the subducting slab that takes place over 150 km this geometry of the slab is reminiscent of that in southern peru the change in dip however takes place more gently in tehuantepec in addition to the change in the dip of the subducted cocos plate a rapid increase is observed in the maximum depth extent of the subducted slab in the region of subhorizontal subduction focal depths are consistently shallower than about 80 km whereas earthquakes as deep as 200 km are found where the subducted slab dips at a steeper angle both the change in the dip of the slab and of the maximum depth of intermediate depth earthquakes appear to correlate with the presence of the tehuantepec fracture zone which separates two distinct provinces of the oceanic cocos plate the age and the crustal thickness of the cocos plate increase to the southeast of the tehuantepec fracture zone relative to its northwestern counterpart in tehuantepec the dip and maximum depth of the subducted slab appears to be correlated with the age of the oceanic plate as has been observed in other subduction zones of the world

pyroclastic flows generated by gravitational instability of the 1996 97 lava dome of soufriere hills volcano montserrat numerous pyroclastic flows were produced during 1996 97 by collapse of the growing andesitic lava dome at soufriere hills volcano montserrat measured deposit volumes from these flows range from 0 2 to 9 106 m flows range from discrete single pulse events to sustained large scale dome collapse events flows entered the sea on the eastern and southern coasts depositing large fans of material at the coast small runout distance 1 km flows had average flow front velocities in the order of 3 10 m s while flow fronts of the larger runout distance flows up to 6 5 km advanced in the order of 15 30 m s many flows were locally highly erosive field relations show that development of the fine grained ash cloud surge component was enhanced during the larger sustained events periods of elevated pyroclastic flow productivity and sustained dome collapse events are linked to pulses of high magma extrusion rates

flow and deposition of pyroclastic granular flows a type example from the 1975 ngauruhoe eruption new zealand small volume pyroclastic density currents pdcs are generated frequently during explosive eruptions with little warning assessing their hazard requires a physical understanding of their transport and sedimentation processes which is best achieved by the testing of experimental and numerical models of geophysical mass flows against natural flows and or deposits to this end we report on one of the most detailed sedimentological studies ever carried out on a series of pristine small volume pdc deposits from the 1975 eruption of ngauruhoe volcano whose emplacement were also witnessed during eruption using high resolution gps surveys a series of lateral excavations across the deposits and bulk sedimentological analysis we constrained the geomorphology internal structure and texture of the deposits with respect to laterally varying modes of deposition deposition from these pdcs began only on slopes at or around the material s angle of repose c 30 in unconfined settings the granular pdcs are interpreted to have been quasi steady forming sheets and lobes around the angle of repose where flows were confined sheet like proximal facies made up around 10 of the deposit volume at the angle of repose but 90 of the material was deposited from apparently unsteady inertial granular pdcs as a distal levee and channel facies on slopes well below the repose angle hence confined pdcs were able to travel up to 50 farther than unconfined flows in the distal facies the deposit width is inversely correlated to the local slope and the height of the levees above the deposit centreline is positively correlated with slope internally the deposits comprise three parts a coarse grained fines free sole layer that laterally connects to levees zone i an ashy matrix supported central body zone ii and an overlying coarse plaster of clasts zone ii trends in grain size data suggests these zones derive from a continuous un mixing of coarse particles from the initial bulk material by granular segregation that preferentially drives large particles to the upper free surface of the flow where they are concentrated at the front of flow before being deposited and overrun by comparison to analogue experiments we suggest a model of flow and deposition where the temporally and spatially varying mode of deposition is determined by the flow velocity the local slope the vertical velocity gradient the velocity gradient at the free surface and the vertical deposition rate using this model estimated vertical deposition rates of c 5 cm s 1 from the ngauruhoe pdcs agree with those determined in laboratory experiments on inertial granular flows

observations of surfzone alongshore pressure gradients onshore of an ebb tidal delta abstract the relative importance of radiation stress gradients and alongshore pressure gradients to surfzone dynamics is investigated using observations of water levels waves and flows measured onshore of a large ebb tidal delta incident wave heights measured along the 11 m depth contour varied about 10 over a 1 2 km alongshore transect resulting in alongshore wave setup differences on the order of 10 cm over the 600 m extent of the surfzone instrument array in 1 5 m depth despite the moderate alongshore variability in wave heights the southerly alongshore pressure gradient associated with the alongshore variability of wave driven set up was typically twice as large as the northerly radiation stress gradient forcing consistent with the observed southerly currents during the week long experiment the magnitude of the alongshore forcing and resulting alongshore velocity is reproduced by the two dimensional depth averaged numerical model of shi et al jgr oceans 2011 these observations together with the numerical results indicate that moderate alongshore wave height gradients o 10 4 outside the surfzone owing to alongshore variations in the offshore bathymetry can result in alongshore pressure gradients that are larger than radiation stress gradients

relation between dry granular flow regimes and morphology of deposits formation of levees in pyroclastic deposits experiments on dry granular matter flowing down an inclined plane are performed in order to study the dynamics of dense pyroclastic flows the plane is rough and always wider than the flow focusing this study on the case of laterally unconfined free boundary flows we found that several flow regimes exist depending on the input flux and on the inclination of the plane each flow regime corresponds to a particular morphology of the associated deposit in one of these regimes the flow reaches a steady state and the deposit exhibits a levee channel morphology similar to those observed on small pyroclastic flow deposits the levees result from the combination between lateral static zones on each border of the flow and the drainage of the central part of the flow after the supply stops particle segregation features are created during the flow corresponding to those observed on the deposits of pyroclastic flows moreover the measurements of the deposit morphology thickness of the channel height of the levees width of the deposit are quantitatively related to parameters of the dynamics of the flow flux velocity height of the flow leading to a way of studying the flow dynamics from only measurements of the deposit some attempts to make extensions to natural cases are discussed through experiments introducing the polydispersity of the particle sizes and the particle segregation process

exploring the factors that influence the perception of risk the case of volcan de colima mexico abstract volcan de colima has increased its activity considerably since 1998 with four periods of effusion and since 2003 daily vulcanian explosions during 2005 the magnitude of the explosivity increased producing many pyroclastic flows two of which reached over 5 km from the volcano making them the largest events since the last plinian eruption in 1913 a significant risk is also presented by the lahar hazard with various examples of damage to infrastructure during the last few years and 23 people killed in 1955 nearly 5000 people live in small settlements within 15 km of the volcano since 1997 six studies have been carried out in the region in an attempt to define the relationship that exists between the population and its neighbour although the methodologies used were different each study considered the results of the previous and attempted to contribute further data to define geographical variation in the perception of risk the results of the studies highlight the minor role of increasing activity on risk perception and distance from the volcano was shown to not directly influence risk perception in most cases it is the combination of various socio cultural historical and political factors that defines the perception of volcanic risk within these villages by studying the social representation of the risk such complexity could be better understood the 1997 2000 information campaign evidently improved risk perception however it is clear that this type of program needs to be continued to achieve a long lasting impact increased knowledge however does not guarantee willing participation in official risk mitigation strategies

preliminary report on the july 10 11 2015 eruption at volcan de colima pyroclastic density currents with exceptional runouts and volume abstract on july 10 11 2015 an eruption occurred at colima volcano produced 10 5 km long pyroclastic density currents pdcs along the montegrande and 6 5 km long along the san antonio ravines the summit dome was destroyed and a new crater excavated and breached to the south this new breach connects to a narrow channel that descends along colima s southern flank and was used by a subsequent lava flow the montegrande pdcs represent the longest and hottest flow of this type recorded during the past 30 years but are still smaller in comparison to the 15 km long pdcs produced during the 1913 plinian eruption data obtained from field reconnaissance lahar monitoring stations and satellite imagery suggest that at least six pdcs occurred the two largest pdcs h l 0 2 were able to surmount topographic barriers or bends based on field reconnaissance and digital elevation models extracted from spot satellite imageries we estimate a minimum volume for the valley pond and distal fan deposits of 4 5 106 m3 after one week the deposits were still hot with burning trees on the surface and millimeter sized holes from which fumes were emanating the juvenile components of the deposits consist of gray dense blocks and vesicular dark gray blocks and bombs with bread crust textures and cooling joints the mineral association of these rocks consists of plagioclase clinopyroxene orthopyroxene feti oxides olivine and resorbed hornblende in a dark glassy matrix that corresponds to an andesitic composition

stratigraphic reconstruction of two debris avalanche deposits at colima volcano mexico insights into pre failure conditions and climate influence abstract throughout its history colima volcano has experienced numerous partial edifice collapses with associated emplacement of debris avalanche deposits of contrasting volume morphology and texture a detailed stratigraphic study in the south eastern sector of the volcano allowed the recognition of two debris avalanche deposits named san marcos 28 000 cal yr bp v 1 3 km 3 and tonila 15 000 16 000 cal yr bp v 1 km 3 this work sheds light on the pre failure conditions of the volcano based primarily on a detailed textural study of debris avalanche deposits and their associated pyroclastic and volcaniclastic successions furthermore we show how the climate at the time of the tonila collapse influenced the failure mechanisms the 28 000 cal yr bp san marcos collapse was promoted by edifice steep flanks and ongoing tectonic and volcanotectonic deformation and was followed by a magmatic eruption that emplaced pyroclastic flow deposits in contrast the tonila failure occurred just after the last glacial maximum 22 000 18 000 cal bp and in addition to the typical debris avalanche textural characteristics angular to sub angular clasts coarse matrix jigsaw fit it shows a hybrid facies characterized by debris avalanche blocks embedded in a finer homogenous and partially cemented matrix a texture more characteristic of debris flow deposits the tonila debris avalanche is directly overlain by a 7 m thick hydromagmatic pyroclastic succession massive debris flow deposits often more than 10 m thick and containing large amounts of tree trunk logs represent the top unit in the succession fluvial deposits also occur throughout all successions these represent periods of highly localized stream reworking all these lines of evidence point to the presence of water in the edifice prior to the tonila failure suggesting it may have been a weakening factor the tonila failure appears to represent an anomalous event related to the particular climatic conditions at the time of the collapse the presence of extensive water at the onset of deglaciation modified the mobility of the debris avalanche and led to the formation of a thick sequence of debris flows the possibility that such a combination of events can occur and that their probability is likely to increase during the rainy season should be taken into consideration when evaluating hazards associated with future collapses at colima volcano

coupling of turbulent and non turbulent flow regimes within pyroclastic density currents the internal dynamics of pyroclastic density currents are not easily observed experiments reveal how the underflow and turbulent ash cloud regimes within pyroclastic flows are dynamically coupled through a zone of intermediate turbulence volcanic eruptions are at their most deadly when pyroclastic density currents sweep across landscapes to devastate everything in their path1 2 the internal dynamics underpinning these hazards cannot be directly observed3 here we present a quantitative view inside pyroclastic density currents by synthesizing their natural flow behaviour in large scale experiments the experiments trace flow dynamics from initiation to deposition and can explain the sequence and evolution of real world deposits we show that inside pyroclastic density currents the long hypothesized non turbulent underflow and fully turbulent ash cloud regions4 5 are linked through a hitherto unrecognized middle zone of intermediate turbulence and concentration bounded by abrupt jumps in turbulence the middle zone couples underflow and ash cloud regions kinematically inside this zone strong feedback between gas and particle phases leads to the formation of mesoscale turbulence clusters these extremely fast settling dendritic structures dictate the internal stratification and evolution of pyroclastic density currents and allow the underflows to grow significantly during runout our experiments reveal how the underflow and ash cloud regions are dynamically related insights that are relevant to the forecasting of pyroclastic density current behaviour in volcanic hazard models

can dispersive pressure cause inverse grading in grain flows grain flows in the collisional regime are self regulated in such a way that dispersive pressure at every level tends to balance static pressure and so cannot promote the upward segregation of large particles in a steady flow le roux adds new insights to this analysis by pointing out that dispersive and static pressures would not balance in unsteady flows when shear strain rate increases in a grain flow dispersive pressure which is positively correlated to shear strain rate increases too and the gradient of dispersive pressure becomes higher than the gradient of static pressure as a consequence the grain flow expands thus lowering the dispersive pressure until the two gradients reequilibrate during the

modeling the alongshore current on barred beaches mean alongshore currents observed on two barred beaches are compared with predictions based on the one dimensional time and depth averaged alongshore momentum balance between forcing by breaking waves wind and 10 100 km scale alongshore surface slopes bottom stress and lateral mixing the observations span 500 hours at egmond netherlands and 1000 hours at duck north carolina and include a wide range of conditions with maximum mean currents of 1 4 m s including rollers in the wave forcing results in improved predictions of the observed alongshore current structure by shifting the predicted velocity maxima shoreward and increasing the velocity in the bar trough compared with model predictions without rollers for these data wave forcing balances the bottom stress within the surfzone with the other terms of secondary importance the good agreement between observations and predictions implies that the one dimensional assumption holds for the range of conditions examined despite the presence of small alongshore bathymetric nonuniformities with stronger bathymetric variations the model skill deteriorates particularly in the bar trough consistent with earlier modeling and laboratory studies

nonlinear shear instabilities of alongshore currents over barred beaches the nonlinear dynamics of finite amplitude shear instabilities of alongshore currents in the nearshore surf zone over barred beach topography are studied using numerical experiments these experiments extend the recent study of allen et al 1996 which utilized plane beach constant slope topography by including shore parallel sandbars the model involves finite difference solutions to the nonlinear shallow water equations for forced dissipative initial value problems and employs periodic boundary conditions in the alongshore direction effects of dissipation are modeled by linear bottom friction forcing for the alongshore currents is specified using a model formulated by thornton and guza 1986 t g distinct classes of flows develop depending on the dimensionless parameter q the ratio of an advective to a frictional timescale for q greater than a critical value qc the flows are linearly stable for q qc q 0 the flow is unstable for small values of q equilibrated shear waves develop that propagate alongshore at phase speeds and wavelengths that are in agreement with predictions from linear theory for the most unstable mode at intermediate values of q unsteady vortices form and exhibit nonlinear interactions as they propagate alongshore occasionally merging pairing or being shed seaward of the sandbar at the largest values of q examined the resulting flow field resembles a turbulent shear flow a net effect of the instabilities at large aq is to distribute the time averaged alongshore momentum from local maxima of the t g forcing located over the sandbar and near the shore into the region of the trough the across shore structure of the time averaged alongshore current is in substantially better qualitative agreement with observations than that given by a steady frictional balance with t g forcing the results point to the possible existence in the nearshore surf zone of an energetic eddy field associated with instabilities of the alongshore current

aeromechanic analysis of pyroclastic density currents past a building an aeromechanic analysis of pyroclastic density currents pdcs past a building is carried out on the results of a computer simulation the analysis shows that pdcs strongly interact with buildings resulting in turbulent boundary layer separation and recirculation these results could be used to better assess the hazard of pdcs impacting urban areas and be of service to civil protection authorities and urban planners who work in active volcanic areas

undertow over a barred beach the spatial distribution of the mean cross shore flow undertow over a barred beach is examined with field data obtained on three energetic wave days during the duck94 experiment the vertical structure of the undertow is modeled using a turbulent eddy viscosity closure and includes the important effects of wave breaking described using the roller concept and convective acceleration of the current other than a more realistic description of observed turbulence variations a depth dependent eddy viscosity compared with a constant does not improve the agreement between predicted and observed undertow profiles the effect of using different boundary conditions is investigated by extending the formulations of stive and wind 1986 and svendsen et al 1987 to include random waves by ensemble averaging over the wave height distribution the contribution of breaking wave rollers to the surface mass flux can be of the same order or greater than the contribution associated with the organized wave motion the largest discrepancies between model predictions and observations occur over the sandbar where the mass transport of the breaking waves appears to be underestimated

emplacement of pyroclastic flows during the 1998 1999 eruption of volcan de colima mexico after three years of quiescence volcan de colima reawakened with increasing seismic and rock fall activity that reached its peak on november 20 1998 when a new lava dome forced its way to the volcano s summit the new lava rapidly reached the s sw edge of the summit area beginning the generation of merapi type pyroclastic flows that traveled down la lumbre and the el cordoban western and eastern ravines reaching distances of 3 4 5 and 3 km respectively on december 1 1998 the lava flow split into three fronts that in early 1999 had reached 2 8 3 1 and 2 5 km in length advancing down the el cordoban ravines the lava flow fronts disaggregated into blocks forming pyroclastic flows one of the best examples occurred on december 10 1998 as the lava flow ceased moving in early 1999 activity became more explosive strong blasts were recorded on february 10 may 10 and july 17 1999 the last event developed a 10 km high eruptive column from which a pyroclastic flow developed from the base traveling 3 3 km sw from the summit into the san antonio montegrande ravines regardless of the mechanism of pyroclastic flow generation each flow immediately segregated into a basal avalanche that moved as a granular flow and an upper ash cloud in which particles were sustained in turbulent suspension when the basal avalanche lost velocity and eventually stopped the upper ash cloud continued to move independently as a dilute pyroclastic flow that produced a massive pyroclastic flow deposit and an upper dune bedded surge deposit the dilute pyroclastic flow scorched and toppled maguey plants and trees and sandblasted vegetation in the direction of the flow at the end of the dilute pyroclastic flow path the suspended particles lifted off in a cloud from which a terminal ash fall was deposited the basal avalanche emplaced block and ash flow deposits up to 8 m thick that filled the main ravines and consisted of several flow units each flow unit was massive monolithologic matrix supported and had a clast supported steep front ca 1 5 to 2 m thick composed of boulders up to 1 7 m in diameter the juvenile lithic clasts had an average density of 1800 kg m3 the dilute pyroclastic flow emplaced overbank deposits found on valley margins or beyond the tip of block and ash flow deposits they consist from bottom to top of a massive medium to coarse sand size flow layer 2 4 cm thick a dune bedded surge layer 2 10 cm thick and a massive silt size layer 0 5 cm thick the total estimated volume of the pyroclastic flow deposits produced during the 1998 1999 eruption is 24 105 m3

eyewitness stratigraphy chemistry and eruptive dynamics of the 1913 plinian eruption of volcan de colima mexico abstract based on the stratigraphic record of the deposits analysis of previous works historic archives and eyewitness accounts the 1913 eruption of volcan de colima was reconstructed the eruption started in 17 january and peaked in 20 january 1913 it occurred in three main phases 1 an opening phase with the generation of merapi type pyroclastic flows units f1 f2 f3 and a pyroclastic surge s1 2 a vent clearing phase with strong explosions that produced vulcanian soufriere type pyroclastic flows f4 and a pyroclastic surge s2 which destroyed the summit dome decompressing the magma system and 3 a plinian phase with the establishment of a 23 km high column dispersing a fallout c1 to the ne followed by the collapse of the column that generated a pyroclastic surge s3 and 15 km long pumice rich pyroclastic flows f5 after the eruption remobilization of the pyroclastic material generated lahars in main gullies around the crater fallout c1 blanketed an area of 191 318 km2 covering the cities of guzman guadalajara and saltillo coahuila located at 725 km from the source it had a volume of 1 4 km3 0 57 km3 dre dense rock equivalent the total volume of pyroclastic flow and surge deposits was 0 26 km3 0 07 km3 dre giving a total volume of the eruption of 1 66 km3 0 64 km3 dre the plinian column lasted 4 6 h with a total mass of 1 5 1012 kg and a mass eruption rate of 9 02 107 kg s the column height and the ejected magma volume indicate that the 1913 eruption had a vei 5 being the largest event in the historical record of colima volcano juvenile scoria and pumice consisted of pl opx cpx hbl accessory titanomagnetite ap and ol with reaction rims chemistry of juvenile scoria and pumice samples is andesitic very homogeneous 58 3 0 5 wt sio2 and similar to the 1818 juvenile products 58 9 0 2 wt sio2 the presence of banded scoria olivine phenocrysts with reaction rims and trace element variations strongly suggest that the 1913 eruption was caused by a magma mixing event

can dispersive pressure cause inverse grading in grain flows discussion the concept of dispersive pressure was originally proposed by bagnold 1954 who showed experimentally that sheared granular material develops a dispersive pressure that is proportional to the shear stress and the particle concentration legros 2002 presented a theoretical analysis suggesting that dispersive pressure cannot account for the inverse grading commonly observed in grain flows modified grain flows or traction carpets contrary to what has been accepted for decades by many sedimentologists he pointed out that if the dispersive pressure in a sheared granular mass increases and exceeds the applied normal stress or static pressure the net upward force acting upon the bed causes it to expand as has in fact been observed in experiments bed expansion immediately causes a decrease in particle concentration and consequently in dispersive pressure until the latter balances the static pressure and expansion stops if the dispersive pressure becomes less than the static pressure the net downward force acting upon the granular bed causes compaction thus increasing the particle concentration and dispersive pressure this is an important concept enhancing our understanding of an as yet poorly understood process however legros 2002 went further to state that the dispersive pressure must always equal the applied normal shear stress at every level emphasis mine through the shearing granular bed this implies that the whole sheared bed is in constant equilibrium which is probably not the case in this discussion a modified process is proposed that accounts for the bipartite traction carpet structure proposed by sohn 1997 traction carpets are moving layers of highly concentrated bedload material developed beneath and driven by an overlying turbulent flow dzulynski and sanders 1962 sohn 1997 on the basis of numerical analyses computer simulations field measurements and experimental studies bagnold 1955 1963 hanes and bowen 1985 wan

kinematic characteristics of pyroclastic density currents at merapi and controls on their avulsion from natural and engineered channels we herein report an example of pyroclastic density current avulsion on 14 june 2006 at merapi indonesia four discrete series of multiple dome collapses led to the generation of four individual block and ash flows into kali gendol valley all four pyroclastic density currents locally overflowed the channel margins to devastate cultivated terraces along each side of the box shaped canyon while propagating as much as 2 2 km within adjacent tributaries the largest destruction was caused by the second and third pyroclastic density currents both of these flows partially spilled out of the gendol valley at a travel distance of 4 9 km bypassing a sabo dam just upstream of the village of kaliadem and leaving the village almost completely destroyed and buried by several meters of overbank deposits the main mechanism of flow avulsion on june 14 was the overflow of up to 20 vol of the dense basal part of the pyroclastic density currents onto interfluves the relative proportions of valley escaped material increased dramatically in the succession of each of the four pyroclastic density currents the main geometric parameters controlling flow avulsion and their critical values were quantified through high resolution real time kinetic global positioning system rtk gps data of the pre event topography for each of the 14 june flows this case highlights the way in which a sabo dam can significantly increase the potential of flow avulsion key lessons are derived for the future hazard mitigation in valleys subject to volcanic mass flows flow observational and geometric data are combined into a model to derive the kinematic characteristics of the basal valley ponding avalanche and the valley escaping veneer flows which are otherwise hidden by overriding clouds of elutriated ash

modeling of pyroclastic flows of colima volcano mexico implications for hazard assessment the 18 24 january 1913 eruption of colima volcano consisted of three eruptive phases that produced a complex sequence of tephra fall pyroclastic surges and pyroclastic flows with a total volume of 1 1 km 3 0 31 km 3 dre among these events the pyroclastic flows are most interesting because their generation mechanisms changed with time they started with gravitanional dome collapse block and ash flow deposits merapi type changed to dome collapse triggered by a vulcanian explosion block and ash flow deposits soufriere type then ended with the partial collapse of a plinian column ash flow deposits rich in pumice or scoria the best exposures of these deposits occur in the southern gullies of the volcano where heim coefficients h l were obtained for the various types of flows average h l values of these deposits varied from 0 40 for the merapi type similar to the block and ash flow deposits produced during the 1991 and 1994 eruptions 0 26 for the soufriere type events and 0 17 0 26 for the column collapse ash flows additionally the information of 1991 1994 and 1998 1999 pyroclastic flow events was used to delimit hazard zones in order to reconstruct the paths velocities and extents of the 20th century pyroclastic flows a series of computer simulations were conducted using the program flow3d with appropriate heim coefficients and apparent viscosities the model results provide a basis for estimating the areas and levels of hazard that could be associated with the next probable worst case scenario eruption of the volcano three areas were traced according to the degree of hazard and pyroclastic flow type recurrence through time zone 1 has the largest probability to be reached by short runout b5 km merapi and soufriere pyroclastic flows that have occurred every 3 years during the last decade zone 2 might be affected by soufriere type pyroclastic flows 9 km long similar to those produced during phase ii of the 1913 eruption zone 3 will only be affected by pyroclastic flows 15 km long formed by the collapse of a plinian eruptive column like that of the 1913 climactic eruption today an eruption of the same magnitude as that of 1913 would affect about 15 000 inhabitants of small villages ranches and towns located within 15 km south of the volcano such towns

deposits and physical properties of pyroclastic density currents during complex subplinian eruptions the ad 472 pollena eruption of somma vesuvius italy small scale pyroclastic density currents pdcs associated with the ad 472 pollena eruption of somma vesuvius italy were generated by both magmatic and phreatomagmatic explosive fragmentation the resulting deposits were emplaced under flow boundary conditions dominated by varying combinations of grain interaction fluid escape and traction processes stratigraphic and lithofacies analysis of these pdcs offers a new perspective on the en masse versus progressive aggradation debate for pdc deposition in particular the analyses indicate that pdcs were density stratified with a basal underflow dominated by grain interactions the underflows comprised trains of self organized granular pulses of variable thickness and magnitude depending on the overall particle concentration and fluid turbulence a change in gradient between the upper and lower slopes of the volcano promoted deposition and the different pulses aggraded sequentially stepwise in this model each pulse stops en masse and the whole deposit aggrades progressively particle concentration density mean velocity and flow height were assessed for the studied pdcs using differaent methods for massive and stratified deposits the calculated mobility of the flows was 0 2 to 0 3 in the expected range for small scale pdcs

numerical simulations of block and ash flows using the titan2d flow model examples from the 2006 eruption of merapi volcano java indonesia we present titan2d simulations of two well characterized block and ash flow baf events of the 2006 eruption of merapi java indonesia that affected the gendol valley on the volcano s southern flank and adjacent densely populated interfluve non valley areas 1 a single dome collapse event to the south that generated one of the smaller post june 14 flows and 2 a sustained multiple dome collapse event also directed to the south that produced the largest flows of the 2006 eruption emplaced in the afternoon of june 14 using spatially varying bed friction angles titan2d is capable of reproducing the paths velocities runout distance areas covered and deposited volumes of these flows over highly complex topography the model results provide the basis for estimating the areas and levels of hazards associated with bafs generated during relatively short as well as prolonged dome collapse periods and guidance during future eruptive crises at merapi

transient rip currents and nearshore circulation on a swell dominated beach 1 eulerian and lagrangian measurements of nearshore circulation on a longshore uniform beach with perpendicularly incident swell waves are described both the eulerian and lagrangian data show a variable rotational part of the current field that is distinct from but coexists with that due to infragravity waves a well defined feature of the flow field includes transient rip currents which occur in varying locations and are not topographically controlled drifter trajectories show that the rips behave like a shallow plane jet consisting of a constrained neck region that flows into a spreading head region there is continuous modulation of the flow within the rip neck estimates of the length scale across the rip neck using two particle statistics are 20 30 m well defined discrete vortex features are observed at the initiation point of one of the necks and in one of the rip heads estimated dispersion values suggest that mixing is greatly enhanced outside the surf zone by the intermittent presence of the rip heads the dispersion calculated from both inside and outside the surf zone appears to show scale dependence with a power law close to 4 3 in a similar way to oceanic turbulence at much larger scales

quasi three dimensional modeling of rip current systems 1 the focus of the paper is the analysis of the flow in rip current systems generated by channels in longshore bars on a beach the horizontal variations of rip current systems are described through the use of the quasi three dimensional nearshore circulation model shorecirc model predictions are compared to laboratory measurements of waves and current velocities throughout the entire rip current system and show reasonable agreement the rips in the two channels are found to behave differently because of the depth variation across the basin it is found that higher bottom stress leads to more stable flow where the rip current meanders less and fewer eddies are generated the wave current interaction creates forcing which reduces the distance rip currents flow offshore and can lead to a slow pulsation of the rip current this pulsation is in addition to the instabilities of a jet which can also be present in rip currents the three dimensionality of the rip current system is found to have a significant effect on the overall circulation patterns index terms 4255 oceanography general numerical modeling 4512 oceanography physical currents 4546 oceanography physical nearshore processes keywords rip currents nearshore circulation numerical modeling waves

mean lagrangian flow behavior on an open coast rip channeled beach a new perspective article i nfo the accepted view of rip currents is that they are an efficient mechanism for transporting material out of the surf zone previous rip current campaigns on natural beaches have focused on eulerian measurements with sparse in situ pressure and current meter arrays here for the first time spatially synoptic estimates of rip current flow patterns vorticity and lagrangian transport behavior are measured in the field using a fleet of 30 position tracking surfzone drifters during multiple rip current occurrences on an open coast beach in monterey ca contrary to the classic view shepard et al 1941 the rip current flow field consisted of semi enclosed large scale vortices that retained the drifters and resulted in a high number of lagrangian observations that are temporally and spatially repeated approximately 19 of the drifters deployed in the rip currents exited the surf zone per hour on average during the experiments the observed surf zone retention of drifters is consistent with measurements from different open coast beach rip current systems 14 at meso macrotidal truc vert france and 16 at macrotidal perranporth united kingdom the three hour average cross shore rip current velocity at monterey was 30 cm s with peak time averaged velocities of 40 60 cm s depending on wave and tidal conditions drifters that episodically exited the surf zone were transported approximately 2 surf zone widths offshore at 20 cm s published by elsevier b v

evaluation of the impact of the 2010 pyroclastic density currents at merapi volcano from high resolution satellite imagery field investigations and numerical simulations the 2010 pyroclastic density currents pdc at merapi have presented a rare opportunity to collect a uniquely detailed dataset of the source extent lateral variations and impact of various pdc deposits on a densely populated area using traditional volcanological field based methods and a multi temporal dataset of high resolution satellite imagery a total of 23 pdc events have been recognized including 5 main channeled flows 15 overbank flows derived from overspill and re channelization of the main pdcs into adjacent tributaries and two main surge events the 2010 pdc deposits covered an area of 22 3 km2 unequally distributed between valley filling 6 9 overbank 22 4 and surge and associated fallout deposits 71 7 their total estimated non dre volume is 36 3 106 m3 with 50 2 of this volume accounting for valley filling deposits 39 3 for overbank deposits and 10 5 for surge and associated fallout deposits more than 70 of the total volume was deposited during the third eruptive phase 4 5 november and only 16 6 11 5 and 0 9 during the first 26 29 october second 30 october 3 november and fourth phase 6 23 november respectively the internal architecture and lithofacies variations of the 2010 pdc deposits were investigated using data collected from 30 stratigraphic sections measured after one rainy season of erosion the results show that complex local scale variations in flow dynamics and deposit architectures are apparent and that the major factors controlling the propagation of the main flows and their potential hazards for overbanking were driven by 1 the rapid emplacement of several voluminous pdcs associated with the steady infilling of the receiving landscape after the two first phases of the eruption 2 longitudinal changes in channel capacity following increased sinuosity in the valley and decreased containment space and 3 the effects of varying source mechanisms gravitational dome collapse vertical or lateral dome explosions and column collapse and source materials involved during individual pdc forming events integration of these data into numerical simulations of the channeled and overbank pdcs of the third eruptive phase 4 5 november using two well established geophysical mass flow models titan2d and volcflow allow us to evaluate the ability of these models to reproduce the main features of the natural deposits and some of the flow overbanking processes observed in the field using such a multi technique approach the dataset obtained in this study not only characterizes the pdcs and related hazards at merapi but will allow comparisons with similar events at other volcanoes around the globe

evidence for welding of a block and ash pyroclastic flow deposit the case of cerro bravo volcano colombia cerro bravo is an active composite volcano located in the san diego cerro machin volcano tectonic province scvtp the northernmost volcanic chain in the colombian andes among the products associated with the volcano there is a peculiar indurated deposit emplaced between 7 and 2 9 ka which resembles a block and ash pyroclastic flow deposit through field observations physical property analyses petrography and scanning electron microscopy sem we demonstrate that this induration is the result of welding processes high density deformed glassy fragments with a common orientation low porosity fiamme and sintering are the most visible characteristics the evidence indicates that welding reached up to rank iv in a i to vi classification scheme we suggest that welding is associated with 1 high emplacement temperature related to the syneruptive destruction of a growing endogenous dome and 2 a low glass transition temperature related to the influence of upward water diffusion after emplacement of the flow over wet ground thus with emplacement temperatures between 670 and 540 c and water contents between 0 5 and 1 2 wt the welding process was unusually efficient calculations made indicate that for the deposit to present the observed characteristics of welding the emplacement temperature needed to exceed the threshold of the glass transition temperature for a time of 5 6 days and have an effective viscosity of 1012 pa s as a result of the welding the deposit decreased its thickness by between 15 and 7 m this work contributes to the knowledge of welding processes and stresses the unusual occurrence of welding in this type of pyroclastic flow deposit which thus requires special emplacement conditions in terms of temperature and water content

lessons in reducing volcano risk since 1980 volcanologists have confronted more volcanic crises than any time since the mont pelee catastrophe in 1902 good science alone will not do the job of reducing volcano risk

investigation of variable aeration of monodisperse mixtures implications for pyroclastic density currents the high mobility of dense pyroclastic density currents pdcs is commonly attributed to high gas pore pressures however the influence of spatial and temporal variations in pore pressure within pdcs has yet to be investigated theory suggests that variability in the fluidisation and aeration of a current will have a significant control on pdc flow and deposition in this study the effect of spatially heterogeneous gas pore pressures in experimental pdcs was investigated sustained unsteady granular currents were released into a flume channel where the injection of gas through the channel base was controlled to create spatial variations in aeration maximum current front velocity results from high degrees of aeration proximal to the source rather than lower sustained aeration along the whole flume channel however moderate aeration i e 0 5 minimum static fluidisation velocity umf st sustained throughout the propagation length of a current results in greater runout distances than currents which are closer to fluidisation i e 0 9 umf st near to source then de aerating distally additionally although all aerated currents are sensitive to channel base slope angle the runout distance of those currents where aeration is sustained throughout their lengths increases by up to 54 with an increase of slope from 2 to 4 deposit morphologies a primarily controlled by the spatial differences in aeration where there is a large decrease in aeration the current forms a thick depositional wedge sustained gas aerated granular currents are observed to be spontaneously unsteady with internal sediment waves travelling at different velocities

inferring pyroclastic density current flow conditions using syn depositional sedimentary structures the processes occurring in the basal region of concentrated pyroclastic density currents pdcs influence the mobility runout distance and damage potential of a current but directly observing these processes is extremely difficult instead we must investigate the deposits to glean information regarding the conditions of sediment transport and deposition the pdc deposits of the may 18 1980 eruption of mount st helens wa usa contain sedimentary structures consisting of bed material reworked into undulose structures and recumbent flame structures the structures vary over two orders of magnitude in size with lengths ranging from 8 cm to 18 m and heights ranging from 4 cm to 1 8 m despite the large range in sizes the structures remain self similar in form suggesting a common mechanism for formation the structures are interpreted as the record of granular shear instabilities similar to kelvin helmholtz instabilities formed at the interface between a shearing high concentration flow and the substrate in the moments just prior to deposition the morphology of the structures suggests that the basal region of pdcs must be both highly concentrated and also highly mobile in the moments before final deposition likely a result of elevated pore fluid pressures we use a modified instability growth criterion to estimate pdc flow velocities at the time of formation for the mount st helens pdcs the velocity estimates range from 0 2 to 7 5 m s 1 with larger structures requiring higher flow velocities combining the velocity estimates with the dimensions of the structures suggests deposition rates of 4 to 32 cm s 1 such high deposition rates indicate that the deposits likely accumulated in a stepwise manner rather than either progressively or en masse the structures suggest that sections of the deposit accumulated during punctuated periods of high deposition lasting at most a few seconds followed by periods of bypassing i e non deposition or erosion lasting minutes to tens of minutes our findings motivate continued experimental and numerical work to understand how the formation of recumbent flame and similar structures affects subsequent flow behavior in terms of runout distance and hazard potential

testing the suitability of frictional behaviour for pyroclastic flow simulation by comparison with a well constrained eruption at tungurahua volcano ecuador we use a well monitored eruption of tungurahua volcano to test the validity of the frictional behaviour also called mohr coulomb which is generally used in geophysical flow modelling we show that the frictional law is not appropriate for the simulation of pyroclastic flows at tungurahua with this law the longitudinal shape of the simulated flows is a thin wedge of material progressively passing over several hundreds of metres from an unrealistic thickness at the front 1 mm to some tens of centimetres simulated deposits form piles which accumulate at the foot of the volcano and are more similar to sand piles than natural pyroclastic deposits finally flows simulated with a frictional rheology are not channelised by the drainage system but affect all the flanks of the volcano in addition their velocity can exceed 150 m s 1 allowing pyroclastic flows to cross interfluves at bends in the valley affecting areas that would not have been affected in reality and leaving clear downstream areas that would be covered in reality instead a simple empirical law a constant retarding stress i e a yield strength involving only one free parameter appears to be much better adapted for modelling pyroclastic flows a similar conclusion was drawn for the socompa debris avalanche simulation kelfoun and druitt j geophys res 110 b12202 2005

caldera formation at volcan colima mexico by a large large holocene volcanic debris avalanche abstract about 4 300 years ago 10 km 3 of the upper cone of ancestral volcan colima collapsed to the southwest leaving a horseshoe shaped caldera 4 km in diameter the collapse produced a massive volcanic debris avalanche deposit covering over 1550 km 2 on the southern flanks of the volcano and extending at least 70 km from the former summit the avalanche followed a steep topographic gradient unobstructed by barriers resulting in an unusually high area volume ratio for the colima deposit the apparent coefficient of friction fall height distance traveled for the colima avalanche is 0 06 a low value similar to those of other large volume deposits the debris avalanche deposit contains 40 75 angular volcanic clasts from the ancestral cone a small proportion of vesicular blocks that may be juvenile and in distal exposures rare carbonate clasts plucked from the underlying surface by the moving avalanche clasts range in size to over 20 m in diameter and are brecciated to different degrees pulverized and surrounded by a rock flour matrix the upper surface of the deposit shows prominent hummocky topography with closed depressions and surface boulders a thick coarse grained compositionally zoned scoria fall layer on the upper northeastern slope of the volcano may have erupted at the time of collapse a fine grained surge layer is present beneath the avalanche deposit at one locality apparently representing an initial blast event most of the missing volume of the ancestral volcano has since been restored at an average rate of 0 002 km 3 yr through repeated eruptions from the post caldera cone as a result the southern slope of volcan colima may again be susceptible to collapse over 200 000 people are now living on primary or secondary deposits of the debris avalanche and a repetition of this event would constitute a volcanic disaster of great magnitude ancestral volcan colima grew on the southern trenchward flank of the earlier and larger volcano nevado de colima trenchward collapse was favored by the buttressing effect of nevado the rapid elevation drop to the south and the intrusion of magma into the southern flank of the ancestral volcano other such trenchward younging paired volcanoes are known from mexico guatemala el salvador chile and japan the trenchward slopes of the younger cones are common sites for cone collapse to form avalanche deposits as occurred at colima and popocatepetl in mexico and at san pedro volcano in chile

flow of variably fluidized granular masses across three dimensional terrain 1 coulomb mixture theory rock avalanches debris flows and related phenomena consist of grain fluid mixtures that move across three dimensional terrain in all these phenomena the same basic forces govern motion but differing mixture compositions initial conditions and boundary conditions yield varied dynamics and deposits to predict motion of diverse grain fluid masses from initiation to deposition we develop a depth averaged three dimensional mathematical model that accounts explicitly for solid and fluid phase forces and interactions model input consists of initial conditions path topography basal and internal friction angles of solid grains viscosity of pore fluid mixture density and a mixture diffusivity that controls pore pressure dissipation because these properties are constrained by independent measurements the model requires little or no calibration and yields readily testable predictions in the limit of vanishing coulomb friction due to persistent high fluid pressure the model equations describe motion of viscous floods and in the limit of vanishing fluid stress they describe one phase granular avalanches analysis of intermediate phenomena such as debris flows and pyroclastic flows requires use of the full mixture equations which can simulate interaction of high friction surge fronts with more fluid debris that follows special numerical methods described in the companion paper are necessary to solve the full equations but exact analytical solutions of simplified equations provide critical insight an analytical solution for translational motion of a coulomb mixture accelerating from rest and descending a uniform slope demonstrates that steady flow can occur only asymptotically a solution for the asymptotic limit of steady flow in a rectangular channel explains why shear may be concentrated in narrow marginal bands that border a plug of translating debris solutions for static equilibrium of source areas describe conditions of incipient slope instability and other static solutions show that nonuniform distributions of pore fluid pressure produce bluntly tapered vertical profiles at the margins of deposits simplified equations and solutions may apply in additional situations identified by a scaling analysis assessment of dimensionless scaling parameters also reveals that miniature laboratory experiments poorly simulate the dynamics of full scale flows in which fluid effects are significant therefore large geophysical flows can exhibit dynamics not evident at laboratory scales

late miocene volcanism and intra arc tectonics during the early development of the trans mexican volcanic belt abstract the early stage of the trans mexican volcanic belt hereafter tmvb is marked by widespread mafic to intermediate volcanism emplaced between 11 and 7 ma from the pacific coast to the longitude of mexico city to the north of the modern volcanic arc petrological and geochronological data support the hypothesis that this volcanism made up a unique late miocenic central mexican comagmatic province mafic lavas at the mouth of the gulf of california and along the northwestern sector of the tmvb made up the nayarit district which includes calc alkaline to transitional varieties the central sector of the tmvb is characterized by two basaltic districts the jalisco guanajuato and the queretaro hidalgo which are distinguished from the westernmost ones by their lower nb la and generally lower hfse lile values as well as by spider diagrams characterized by larger negative spikes at th ta nb and ti the surface occurrence of the late miocene basalts appears to be controlled by pre existing zones of crustal weakness that channeled the mafic magmas field observations suggest that these structures have been reactivated in a transtensional fashion induced by differential tectonic motion of crustal blocks to the south and to the north of the tmvb starting from 12 ma the tmvb separates a northern tectonic domain subject to the developing divergent pacific north america plate boundary from a southern tectonic domain characterized by oblique subduction of the rivera and cocos plates apparently far field stresses related to these complex plate boundaries reactivated older suture zones allowing rapid uprise of mantle derived magmas the subduction related signature shown by miocene mafic lavas of the jalisco guanajuato district argues against the existence of mantle plumes beneath this sector of the north america plate on the other hand the occurrence in the western tmvb and in the guadalajara region of a large volume of mafic magmas which sometimes show characteristics transitional to ocean island basalts could be due to passive upwelling of the subslab asthenosphere that might have interacted with subduction related magmas and continental lithosphere to produce the observed basaltic varieties subslab magmas may have flowed through slab free areas along the northern and eastern edges of the subducting rivera plate which in the late miocene was already detached from the farallon plate remnants and diverging from the cocos plate

the cohesive naranjo debris flow deposit 10 km3 a dam breakout flow derived from the pleistocene debris avalanche deposit of nevado de colima volcano mexico abstract mass movement processes on volcanic terrains such as landslides and debris avalanches can cause the obstruction of main drainages producing the formation of temporary dams a good example of this occurred 18 5 ka ago when the eastern flank of the nevado de colima volcano collapsed producing a debris avalanche deposit that was previously described as one of the largest in the world the deposit extended from the volcanic summit as far as the pacific coast 120 km away new stratigraphic sedimentological and componentry data suggest that the volcanic collapse of nevado de colima resulted in a debris avalanche that traveled 20 km southeast to the naranjo river there it crashed against a topographic barrier consisting of cretaceous limestones cerro la carbonera and the flow direction was diverted to the south down the naranjo river channel for another 25 km before the avalanche came to a halt the obstruction of the drainage produced a temporary dam that stored ca 1 km3 of water and deposited fluvial and slack water sediments some time after the damming the accumulated water sediment load was able to overtop the obstructing material and to release a breakout flow with a calculated initial flow discharge of 3 5 million m3 s the resulting flood cohesive debris flow followed the channel of the naranjo river and due to the high erodibility of the channel and introduction of substrate material the debris flow progressively increased its volume up to 10 km3 six times its initial volume this study highlights the relevance of evaluating the potential remobilization of debris avalanche deposits to initiate large magnitude cohesive debris flows therefore the hazard and risk analysis of future potential events of this nature must consider the pre eruption conditions and the topography surrounding a volcano

the merapi 2010 eruption an interdisciplinary impact assessment methodology for studying pyroclastic density current dynamics abstract the large explosive eruption of merapi volcano indonesia in 2010 presented a key and rare opportunity to study the impacts of a major explosive eruption in a densely populated area pyroclastic density currents pdcs produced throughout the 2010 eruption were unusually destructive causing near complete devastation across a 22 km 2 swath of the densely populated southern flanks and casualties to the end of their runout at 15 5 km from the volcano the majority 120 of the more than 200 fatalities occurred more than 12 km from the volcano where many people were caught in pdcs as they were evacuating the 2010 eruption vei 4 exhibited a range of pdc behaviour in a complex multi stage event that marked a change in eruption behaviour at merapi being the first eruption of this magnitude and style since 1872 this shift in style may mark a change in regime and so understanding the potential impact of such large explosive eruptions is essential for future risk assessment at merapi we describe a new impact assessment methodology that allowed us to collect important empirical geological damage and casualty information and reconstruct impact dynamics associated with the pdcs in contrast to previous pdc impact studies we combined remote field laboratory and gis assessments and were able to enter the affected areas safely and before their disturbance by rains or human activity by integrating the results of our geological damage and medical studies we could reconstruct the spatial and temporal dynamics of the pdcs and their main hazard characteristics our interdisciplinary methods and preliminary findings are discussed here in the areas damaged by pdcs we used empirical damage data and calculations of material and structural resistance to lateral force to estimate approximate dynamic pressures dynamic pressures associated with the 5 november paroxysm exceeded 15 kpa more than 6 km from source and rapidly attenuated over a distance of less than 1 km at the end of the pdc runouts analysis of thermal indicators such as deformed plastic and correlation with information on burns injuries and fires provided estimates of ambient temperatures associated with the pdcs even at the relatively low temperatures estimated for the pdcs 200 300 c they were lethal to people inside as well as outside buildings in part because of the building design that enabled the pdcs to rapidly infiltrate inside such detailed quantitative data can be used to support numerical pdc and impact modelling and risk assessment at dome forming volcanoes providing an improved understanding of the complexity of pdcs and their associated impacts on exposed populations

post emplacement dynamics of andesitic lava flows at volcan de colima mexico revealed by radar and optical remote sensing data abstract we used optical and radar remote sensing datasets to map estimate the volume and measure the surface displacements of lava flows emplaced on the flanks of volcan de colima mexico by extrusion of lava dome material from the end of 2014 to early 2016 our main result is that the flow motion of the lava contributes significantly to the recorded displacements several months after its emplacement first we mapped the deposits and estimated their volumes using two digital elevation models dem one derived from radar data acquired before the peak of activity and one derived from optical images acquired just after this peak of activity coherence information derived from the radar dataset added some temporal constraints on the timing of emplacement of various deposits we thus estimated a mean extrusion rate of 1 2 m3 s 1 between november 2014 and february 2015 we then used a new approach to reconstruct the 3d displacement field taking advantage of images acquired by the same satellite on both ascending and descending tracks and using a physical a priori on the direction of horizontal displacements our results show that about 2 cm yr 1 of horizontal motion is still recorded a few months after the emplacement on the sw lava flow which is the only one covered by the two acquisition geometries in order to differentiate the potential causes of the observed displacements we modeled the thermal contraction of the lava flow using a finite element numerical method removing the contribution of thermoelastic contraction from the measured displacements enable to infer both the viscoelastic loading and flow motion effects from the residuals results show that thermal contraction flow motion and viscoelastic loading contribute significantly to the displacements recorded

insights into the october november 2010 gunung merapi eruption central java indonesia from the stratigraphy volume and characteristics of its pyroclastic deposits abstract the 2010 eruption of merapi was the second most deadly in the historic record of this volcano claiming over 380 lives by relating the observations of this eruption with detailed examination of deposit distribution stratigraphy and sedimentology a reconstruction of the properties of the pyroclastic density currents pdcs is presented including the valley controlled block and ash flows bafs and widespread energetic pyroclastic surges the distribution volume and mobility characteristics of all types of pdc during the eruption sequence show evidence for levels of intensity unseen since the large scale 1872 and 1930 eruption phases especially during the climactic events of october 26 and november 5 many tephra falls interbedded with pdc units show that most dome collapse events occurred along with and between explosive vulcanian eruptions the 2010 eruption produced very long runout bafs reaching 16 1 km in the kali gendol on november 5 this runout could be explained by its large volume 20 million m 3 around 10 times that of previous merapi bafs during the last 130 years major avulsion of these dense bafs to form overbank deposits became more common through the eruptive sequence as the valley was progressively filled with successive pdc deposits spreading avulsed bafs were a particular hazard downstream of 10 km where the landscape is less dissected less clear however is why pyroclastic surges extended up to 10 km from the vent on november 5 and 6 4 km on october 26 these expanded much farther from baf margins 2 km than ever seen before at merapi in one location they were decoupled from valley centered bafs with high momentum traveling initially laterally across steep valley systems before draining downslope at this site on the western side of the upper gendol at around 3 km from source surge decoupling was apparently exacerbated by upstream collision and deflection of high flux hot and gas rich bafs against the cliffs of gunung kendil the 1 4 km long cliff face was impacted directly for the first time in 2010 events and may have been responsible for the formation of larger than normal turbulent ash rich clouds above bafs these results imply that future eruption events under the present summit and upper flow path configuration are also highly likely to generate wide dispersal pyroclastic surges and extreme hazard especially now that dense forest has been destroyed on the upper southern slopes of the volcano

experimental study of nearshore dynamics on a barred beach with rip channels 1 wave and current measurements are presented from a set of laboratory experiments performed on a fixed barred beach with periodically spaced rip channels using a range of incident wave conditions the data demonstrate that the presence of gaps in otherwise longshore uniform bars dominates the nearshore circulation system for the incident wave conditions considered for example nonzero cross shore flow and the presence of longshore pressure gradients both resulting from the presence of rip channels are not restricted to the immediate vicinity of the channels but instead are found to span almost the entire length of the longshore bars in addition the combination of breaker type and location is the dominant driving mechanism of the nearshore flow and both are found to be strongly influenced by the variable bathymetry and the presence of a strong rip current the depth averaged currents are calculated from the measured velocities assuming conservation of mass across the measurement grid the terms in both the cross shore and longshore momentum balances are calculated and their relative magnitudes are quantified the cross shore balance is shown to be dominated by the cross shore pressure and radiation stress gradients in general agreement with previous results however the rip current is shown to influence the wave breaking and the wave induced setup in the rip channel analysis of the longshore balance shows that the overall strength of the longshore feeder currents is governed by a balance between longshore pressure gradient forcing and an opposing radiation stress gradient in addition the longshore feeder currents show maxima in the bar trough region providing experimental evidence that longshore pressure gradients can shift longshore current maxima shoreward from the bar crest finally since the longshore radiation stress gradient in the surf zone is governed by the amount of wave dissipation on the rip current there exists a positive feedback mechanism whereby a strong rip current can weaken the radiation stress gradient opposing the feeder currents and lead to even stronger feeder currents and rips

nature and velocity of pyroclastic density currents inferred from models of entrainment of substrate lithic clasts abstract deposits of pyroclastic density currents pdcs often contain accidental lithic clasts of typical size of 0 1 1 m captured from an underlying substrate by the parent flows at distances up to several tens of kilometers from the eruptive vent in order to gain insights into the nature of pdcs this study investigates the conditions required for entrainment of particles from a granular substrate by a gas particle density current with special emphasis to ignimbrite forming currents whose dynamics are controversial the two types of physics of emplacement of pdcs proposed in literature are considered the first model deals with a hydraulically rough dilute turbulent pdc of bulk density of 1 10 kg m 3 and considers that entrainment through both traction and saltation is controlled by a shield criterion at high 10 4 particle reynolds number the second model considers entrainment by a pdc consisting of a dense basal flow of bulk density of the order of 10 3 kg m 3 and with high interstitial gas pore pressure this model involves uplift of substrate particles caused by an upward pressure gradient at the flow substrate interface and then transport and deposition on the aggrading basal deposit of the flow as demonstrated by recent laboratory experiments results show that a dilute pdc can entrain blocks of maximum size of 10 15 cm for a block density of 2000 3000 kg m 3 if maximum current velocities up to 100 m s are taken into account this in turn suggests that larger heavier blocks found in deposits were captured by pdcs if these had a dense basal flow the dense flow model predicts that pdcs have the potential to entrain metric blocks whose maximum size up to 2 5 m decreases with decreasing atmospheric pressure i e increasing altitude application of the model considering published data on the characteristics of accidental blocks in several well studied ignimbrites indicates that the velocity of the parent dense pdcs was up to 25 30 m s

field observations and surface characteristics of pristine block and ash flow deposits from the 2006 eruption of merapi volcano java indonesia abstract a series of pristine block and ash flow deposits from the may june 2006 eruption of merapi represent an exceptional record of small volume pyroclastic flows generated by gravitational lava dome collapses over a period of about two months the deposits form nine overlapping lobes reaching 7 km from the summit in the gendol river valley on the volcano s southern flank which were produced by successive flows generated during and after the major dome collapse event on june 14 both single pulse post june 14 events and multiple pulse pyroclastic flows generated by sustained dome collapses on june 14 are recognised and three types of deposits spread over an area of 4 7 km are distinguished totalling 13 3 10 6 m 3 1 valley confined basal avalanche deposits 11 7 10 6 m 3 in the gendol river valley 2 overbank pyroclastic flow and associated surge deposits 1 4 10 6 m 3 where parts of the basal avalanche spread laterally onto interfluves and were subsequently channeled into the surrounding river valleys and 3 dilute ash cloud surge deposits 0 2 10 6 m 3 along valley margins variations in the distribution surface morphology and lithology of the deposits are related to the source materials involved in individual pyroclastic flow forming events and varying modes of transport and deposition of the different flows inferred flow velocities of the largest block and ash flows generated on june 14 vary from 43 8 13 5 m s for the basal avalanche and from 62 6 24 2 m s for the ash cloud surge the minimum temperatures range from 400 c for the basal avalanche to 165 c for the overlying ash cloud due to the potential of being re channeled into adjacent river valleys and flowing laterally away from the main river channel the overbank pyroclastic flows are considered the most hazardous part of the block and ash flow system the conditions that lead to their development during flow transport and deposition must be taken into account when assessing future pyroclastic flow hazards at merapi and similar volcanoes elsewhere

pyrite framboid study of marine permian triassic boundary sections a complex anoxic event and its relationship to contemporaneous mass extinction size analysis of pyrite framboids has been undertaken on epicontinental permian triassic boundary sections throughout the world in order to evaluate the intensity and duration of anoxia mid paleolatitude sections from the margins of the boreal spitsbergen greenland and neotethyan oceans western australia reveal intense anoxia throughout the permian triassic boundary interval with euxinic conditions frequently developing and dysoxia encountered even in relatively shallow water settings above storm wave base at equatorial paleolatitudes weakly oxygenated dysoxic conditions are widely developed in a broad range of water depths including those shallow enough to produce oolite deposition although euxinia was rare western and eastern tethyan locations re veal a complex and unstable redox history anoxia in the hindeodus praeparvus zone was replaced by oxygenated facies in the permian triassic boundary interval h changxingensis to h parvus zones oxygen poor deposition returned during the succeeding isarcicella isarcica zone the more persistent and intense development of oxygen restriction in cooler water mid paleolatitude sections argues against warming and dissolved oxygen decline as the key cause of permian triassic bound ary anoxia in higher paleolatitudes the ben thic invertebrate extinctions occurred during a prolonged phase of oxygen poor deposition while in equatorial tethyan locations benthic losses occurred at the end of the fi rst anoxic phase in the late h praeparvus zone

volcan de colima dome collapse of july 2015 and associated pyroclastic density currents during july 10th 11th 2015 volcan de colima mexico underwent its most intense eruptive phase since its subplinian plinian 1913 ad eruption production of scoria coincident with elevated fumarolic activity and so2 flux indicate a significant switch of upper conduit dynamics compared with the preceding decades of dome building and vulcanian explosions a marked increase in rockfall events and degassing activity was observed on the 8th and 9th of july on the 10th at 20 16 h local time utm 6 h a partial collapse of the dome generated a series of pyroclastic density currents pdcs that lasted 52 min and reached 9 1 km to the south of the volcano the pdcs were mostly channelized by the montegrande and san antonio ravines and produced a deposit with an estimated volume of 2 4 106 m3 nearly 16 h after the first collapse a second and larger collapse occurred which lasted 1 h 47 min this second collapse produced a series of pdcs along the same ravines reaching a distance of 10 3 km the total volume calculated for the pdcs of the second event is 8 0 106 m3 including associated ashfall deposits the two episodes produced a total of 14 2 106 m3 of fragmentary material the collapses formed an amphitheater shaped crater open towards the south we propose that the dome collapse was triggered by arrival of gas rich magma to the upper conduit which then boiled over and sustained the pdcs a juvenile scoria sample selected from the second partial dome collapse contains hornblende yet at an order of magnitude less abundant 0 2 than that of 1913 and exhibits reaction rims whereas the 1913 hornblende is unreacted at present there is no compelling petrologic evidence for imminent end cycle activity observed at volcan de colima

rip current review rip currents are shore normal narrow seaward flowing currents that originate within surf zone extend seaward of the breaking region rip head and can obtain relatively high velocities within the last decade there have been a significant number of laboratory and field observations within rip current systems an overview of rip current kinematics based on these observations and the scientific advances obtained from these efforts are synthesized rip current flows are partitioned into mean infragravity very low frequency vorticity and tidal contributions and it is found that each contributes significantly to the total data from the laboratory and the field suggest that the rip current strength increases with increasing wave energy and decreasing water depths the maximum mean current occurs inside the surf zone where the maximum forcing is present owing to the dissipation of waves

surf zone currents over irregular bathymetry drifter observations and numerical simulations 1 surf zone currents over irregular bathymetry were observed with gps tracked drifters and modeled with the depth integrated nonlinear shallow water equations trajectories of drifters released in 1 2 m depth sometimes defined rip currents and surf zone eddies features that have been difficult to resolve with fixed instruments the drifter delineated surf zone circulation evolved as the tidal level changed during each 4 6 hour deployment in one case as the tide dropped a shore normal rip current present for the first 2 hours evolved to a more shore parallel flow in a second case on a rising tide a well developed bifurcated rip current was replaced by a weak amorphous circulation rip current velocities were strongest near the surf zone edge and decayed by an order of magnitude within 2 surf zone widths from the shoreline an eddy was observed to persist over a bathymetric depression within the surf zone for at least 1 hour observed and numerically simulated drifter trajectories define similar flow features but the observed and modeled velocities differed by roughly a factor of 2 and flow evolution observed during roughly 0 5 m changes in tidal level were reproduced only if the simulated water levels varied by about 1 m the alongshore momentum balance was spatially variable with wave forcing balanced by pressure gradient friction and advective terms the cross shore momentum balance was everywhere dominated by wave forcing and pressure gradient terms of about equal magnitude in the vicinity of a simulated rip current the residual of the cross shore wave forcing and pressure gradient increased and was balanced by advection terms

pyroclastic flow deposits of the 1991 eruption of volcan de colima mexico the april 16 1991 eruption of volcan de colima represents a classical example of partial dome collapse with the generation of progressively longer runout merapi type pyroclastic flows that traveled up to 4 km along the el cordoban gullies east central and west the flows filled the gullies with block and ash flow deposits up to 10 m thick of which after 7 years of erosion only remnants remained in the el cordoban west and east gullies the el cordoban central gully however provided a well preserved and incised longitudinal section of the 1991 deposits the deposits were emplaced as proximal and distal facies separated by a change in slope angle from 30 to 20 the proximal facies consists of massive clast supported flow units up to 1 m thick with andesite blocks locally supported by a matrix of coarse ash and devoid of segregation structures or grading the distal facies consists of a massive matrix supported deposit up to 8 m thick which contains dispersed andesite blocks in a fine ash matrix in the distal facies a train of blocks marks flow unit upper boundaries and although sorting is poor some grading is present thin finely stratified or dune bedded layers of fine ash material are locally present above or below units of both facies sedimentologic parameters show that the size or fraction of large pyroclasts larger than 1 decreases from proximal to distal facies as the percentage of matrix 0 to 4 increases especially immediately beyond the break in slope we propose that the propagation of the colima pyroclastic flows is critically dependent on local slope angle the presence of erodible slope debris and the decrease in grain size with distance from the vent the progressive fining is probably caused by some combination of erosion clast breakup and deposition of larger pyroclasts and is itself influenced by the slope angle in the proximal region the flows moved as granular avalanches in which interacting grains ground each other and erosion occurred to produce an overriding dilute ash cloud the maximum runout distance of the avalanches was controlled by the angle of repose of the material and the volume and grain size of source and eroded material because the slope angle is close to the repose angle for this debris granular avalanches were not able to propagate far beyond the change in slope if however an avalanche had enough mass in finer grain size fractions at least part of the flow continued beyond the break in slope and across the volcano apron propagating in a turbulent state and depositing surge layers or in an otherwise settling modified state and depositing block and ash flow layers

a conceptual model for block and ash flow basal avalanche transport and deposition based on deposit architecture of 1998 and 1994 merapi flows the 1998 block and ash flow baf deposits on the western flanks of merapi were described both immediately following emplacement and after one and two wet seasons of erosion this afforded a unique combination of data on deposit morphology and internal structure from proximal to distal reaches the deposits of the basal avalanches from individual baf events were classified into three major and two minor facies in channelised reaches primarily resulting from effects of channel confinement and slope termination reaches were classified into three facies related to preevent topography the deposit facies architecture was interpreted to understand particle transport and deposition within the framework of granular flow systematics the transport model we propose for these basal avalanches involves unsteady flow conditions generated by pulses of collapse as the source and or by development of kinetic waves during the granular flow the coarsest clasts along with low density material migrated to the wave pulse fronts within these flows whereas in bulk flow grain dispersive pressures and kinetic sieving result in coarsest clasts being buoyed to the flow top with this tendency being greatest on higher slopes greater kinetic energy and with greater flow confinement maximising particle collisions deposition begins as the flow reaches near its full extent an initial pulse or flow wave freezes rapidly from its base upward and its rear forward as it looses kinetic energy and dispersive pressure such that an upper higher energy part of the flow is still able to move downstream over the frozen base later waves or pulses then accrete progressively horizontally upslope banking up onto or overtopping earlier deposited sediment hence the overall flow freezes from its front to its tail at termination reaches in unconfined areas the overflowing upper part of flow gradually deposits from its base upward resulting in a slightly tapering sheet formed deposit marked by a front made up of wood fragments and large boulders on sudden slope breaks in termination areas piling up of sediment occurs in flat reaches producing whaleback type forms as the flows rapidly lost competence one implication of this model is that each kinetic wave or flow pulse crest developed within the basal avalanche represents a portion of greater flow depth or stage height hence development of these crests control the ability of parts of the basal avalanche to escape from confined channels and generate unexpected hazards

stratigraphy sedimentology and inferred flow dynamics from the july 2015 block and ash flow deposits at volcan de colima mexico abstract the july 2015 block and ash flow baf events represent the first documented series of large volume and long runout bafs generated from sustained dome collapses at volcan de colima this eruption is particularly exceptional at this volcano due to 1 the large volume of baf material emplaced 0 0077 0 001 km3 2 the long runout reached by the associated bafs max 10 km and 3 the short period 18 h over which two main long sustained dome collapse events occurred on 10 and 11 july respectively stratigraphy and sedimentology of the 2015 baf deposits exposed in the southern flank of the volcano based on lithofacies description grain size measurements and clast componentry allowed the recognition of three main deposit facies i e valley confined overbank and ash cloud surge deposits correlations and lithofacies variations inside three main flow units from both the valley confined and overbank deposits left from the emplacement of the second series of bafs on 11 july provide detailed information about 1 the distribution volumes and sedimentological characteristics of the different units 2 flow parameters i e velocity and dynamic pressure and mobility metrics as inferred from associated deposits and 3 changes in the dynamics of the different flows and their material during emplacement these data were coupled with geomorphic analyses to assess the role of the topography in controlling the behaviour and impacts of the successive baf pulses on the volcano flanks finally these findings are used to propose a conceptual model for transport and deposition mechanisms of the july 2015 bafs at volcan de colima in this model deposition occurs by rapid stepwise aggradation of successive baf pulses flow confinement in a narrow and sinuous channel enhance the mobility and runout of individual channelized baf pulses when these conditions occur the progressive valley infilling from successive sustained dome collapse events promote the overspill and lateral spreading of the upper and marginal regions of the main flow body generating highly mobile overbank flows that travel outside of the main valley volume and distance dependent critical channel capacities for the generation of overbank flows are used to better estimate the inundation area of these hazardous unconfined pyroclastic flows these results highlight the importance of including and correctly assessing the hazards posed by large volume and long runout bafs associated with frequent small vei sustained dome collapse eruptions

long period seismicity during magma movement at volcan de colima during the period from february to september 2005 volcan de colima produced 30 vulcanian explosions of sufficient magnitude to produce pyroclastic flows of variable size with a total volume of at least 2 5 106 m3 swarms of long period events were associated with each event their duration ranging from about 6 h to 3 days and each swarm containing up to 886 events the characteristics of the swarms have been studied to understand the source mechanism and their relationship with the vulcanian explosions in total 12 548 long period events were analysed using various comparative and statistical methods patterns were not apparent in the data with no correlation between different properties of the swarms duration magnitude or frequency of occurrence of lp events and the magnitude of the associated vulcanian explosion whether recorded by seismicity volume of pyroclastics or altitude of the eruption column this along with other characteristics of the swarms such as the continuation of the swarm after the explosion with an increase in long period event amplitude in some cases suggests that the mechanism is not merely associated with the pressurization under an impermeable cap and resulting pressure differentials between adjacent volumes within the system it is more likely that the production of long period events is dominated by brittle fracturing on the margins of an ascending magma body a model is proposed whereby the unloading above the ascending magma column produced by a vulcanian explosion resulted in an increase in ascent rate reflected in the increasing amplitude of long period events the results reflect the complexity of non linear processes involved during magma ascent degassing crystallization and rupture of the impermeable plug during the vulcanian process at volcan de colima as at many volcanoes long period events represent a useful precursor for eruptive activity for monitoring this paper highlights some useful analyses that can be carried out which could illustrate certain characteristics of an eruptive episode a preliminary model is presented of the conduit processes at work during the cyclic extrusive and explosive activity during 2005

giant debris avalanches from the colima volcanic complex mexico implications for long runout landslides 100 km and hazard assessment at least two giant volcanic debris avalanche deposits are associated with the colima volcanic complex located in the western part of the trans mexican volcanic belt one avalanche originated from volcan de colima 3820 m probably 4300 yr ago a much larger avalanche originated from nevado de colima 4240 m 18500 yr ago it traveled more than 120 km from its source and covered an area of at least 2200 km 2 almost twice the area of any previously described avalanche deposit this older debris avalanche is the second largest by volume known has the longest travel distance yet reported and has one of the lowest height length ratios 0 04 the avalanche material was probably hot during emplacement and after initial slope failure may have behaved similarly to a pyroclastic flow such large long runout landslides from volcanic or nonvolcanic constructs present a serious geologic hazard that must be considered in risk assessment

the 17 july 1999 block and ash flow baf at colima volcano new insights on volcanic granular flows from textural analysis abstract on july 17 1999 a strong explosion occurred at colima volcano mexico that produced a 10 km high eruptive column the partial column collapse originated a block and ash flow baf that flowed to the south along the san antonio and montegrande ravines travelling 3 3 km from the volcano summit the flow filled the ravines with a volume estimated at 7 9 105 m3 the erosion of these deposits occurred between 1999 and 2002 time of sampling providing excellent longitudinal outcrops that allowed their detailed textural study the study was carried out by means of quantitative textural analysis 1 rosiwal intersections for carrying out vertical granulometric profiles 2 total grain size analysis from 11 to 9 and 3 fourier and fractal analysis of the particle morphology grain size and morphometric parameters obtained with these methods were used to identify vertical and longitudinal variation patterns in the baf deposit the grain size variations allowed to infer the main particle segregation mechanisms that acted during transport and deposition of the studied bafs the two methods used for studying the particle shape morphologies yielded results with different accuracy and reliability in particular fractal analyses have been found to be the most effective in describing the particle support mechanisms that acted during transport and deposition of the studied bafs the results highlight the importance of the information obtained by means of these techniques and provide new insights in transportation and deposition mechanisms of bafs

modeling surf zone tracer plumes 2 transport and dispersion 1 five surf zone dye tracer releases from the hb06 experiment are simulated with a tracer advection diffusion model coupled to a boussinesq surf zone model funwavec model tracer is transported and stirred by currents and eddies and diffused with a breaking wave eddy diffusivity set equal to the breaking wave eddy viscosity and a small 0 01 m2 s 1 background diffusivity observed and modeled alongshore parallel tracer plumes transported by the wave driven alongshore current have qualitatively similar cross shore structures although the model skill for mean tracer concentration is variable from negative to 0 73 depending upon release cross shore integrated tracer moments normalized by the cross shore tracer integral have consistently high skills 0 9 modeled and observed bulk surf zone cross shore diffusivity estimates are also similar with 0 72 squared correlation and skill of 0 4 similar to the observations the model bulk absolute cross shore diffusivity is consistent with a mixing length parameterization based on low frequency 0 001 0 03 hz eddies the model absolute cross shore dispersion is dominated by stirring from surf zone eddies and does not depend upon the presence of the breaking wave eddy diffusivity given only the bathymetry and incident wave field the coupled boussinesq tracer model qualitatively reproduces the observed cross shore absolute tracer dispersion suggesting that the model can be used to study surf zone tracer dispersion mechanisms

the controls and consequences of substrate entrainment by pyroclastic density currents at mount st helens washington usa evidence in the deposits from the may 18 1980 eruption at mount st helens demonstrates that pyroclastic density currents pdcs produced during the afternoon of the eruption became intermittently erosive using detailed componentry and granulometry we constrain the sources for lithic blocks in the deposits and identify deposits from pdcs that became locally erosive the componentry of the lithics in the fall deposits is used as a proxy for vent erosion and assumed to represent the starting componentry for pdcs prior to entrainment from any other source we find little evidence in the pdc deposits nearest to the base of the volcano for entrainment from the steep flanks however significant evidence indicates that pdcs eroded into the debris avalanche hummocks suggesting that entrainment is favored as pdcs interact with highly irregular topography evidence for locally entrained material downstream from debris avalanche hummocks decreases with height in the outcrop suggesting that less entrainment occurs as local relief decreases and upstream topography is buried the prevalence of lithofacies containing locally entrained material at the base of unit contacts and only 10s of meters downstream from debris avalanche hummocks suggests that the majority of entrainment occurs at or near the head of the current occasionally entrained material is located high above unit contacts and deposited well after the initial head of the current is inferred to have passed indicating that entrainment can occur during periods of non deposition either from the semi sustained body of the current or from a pulsating current additionally self channelization of pdcs either by levee deposition or scouring into earlier pdc deposits occurs independently of interaction with topographic obstacles and can affect carrying capacity and runout distance while we begin to explore the mechanisms and effects of erosion on current dynamics additional laboratory and numerical studies are necessary to fully understand these processes

high spatial resolution imagery helps map deposits of the large vei 4 2010 merapi volcano eruption and their impact the 26 october 23 november 2010 eruption is merapi s largest event vei 4 over the past 140 years we used high spatial resolution hsr imagery from geoeye pleiades ikonos and spot5 satellites to assess the extent and effects of the pyroclastic density currents pdcs and subsequent lahars we have tracked the geomorphic and structural fracturing changes of merapi s summit crater and dome between 2008 and 2012 the 4 september 2011 geoeye image shows that due to the explosive eruption the summit area lost about 10 106 m3 the eruption enlarged the sse trending gendol breach to be 1 3 0 3 0 2 km the 2010 tephra and pdc deposits covered about 26 km2 in the two catchments of gendol and opak rivers on merapi s south flank i e 60 75 of the total pdc deposit area with a total bulk volume of 45 106 m3 the tephra fall deposit covered an area of about 1300 km2 with a range in volume of 18 21 106 m3 supervised and object oriented classification on hsr imagery enables us to map in detail the pdc deposits across the gendol opak catchment we delineated 16 spectrally and or texturally distinct units of pdc deposits and compared them with previously published results they encompass high energy surge deposits within ca 8 km of the summit valley confined pdc deposits channeled as far as 16 5 km in the gendol river and widespread overbank pdc with ash cloud surge deposits on valley margins

field observations of pristine block and ash flow deposits emplaced april 16 17 1991 at volcan de colima mexico abstract for the first time since the effusive activity of 1981 1982 volcan de colima extruded scoriaceous andesitic lava on march 1 1991 this followed a period of increasing seismicity summit dome deformation and fumarolic activity that began in february of 1991 growth of the new lava dome proceeded until april 16 at 16 00 hrs local time gmt 6 hours when a series of gravitational failures of the upper edifice and the new lava dome flow triggered the emplacement of successive small volume non glowing block and ash flows to a distance of 4 km within the upper reaches of barranca el cordoban south southwest of the present summit aerial photos taken at the time of the event clearly show the lack of explosions at the summit dome efficient elutriation of the finest silt and clay sized fraction from the main body of the avalanching material produced large reddish yellowish and grayish ash clouds which rose to an altitude of 1 5 2 5 km for several hours and were carried southeast to northeast by predominant winds vegetation in the path of the debris was totally stripped sand blasted and bent in the sense of flowage but was not burnt no fatalities were reported we present a summary of our field observations of the pristine deposits which we studied prior the rainy season on april 20 27 between 2 200 and 2 500 m elevation

sector collapse of the sw flank of volcan de colima mexico the 3600 yr bp la lumbre los ganchos debris avalanche and associated debris flows abstract some 3600 years ago the sw flank of volcan de colima collapsed generating a debris avalanche that travelled 25 km to the armeria river and stopped against the cerro grande limestone escarpment the debris avalanche deposit covers an area of 48 km2 has an average thickness of 35 m and a volume of 1 7 km3 with an h l ratio of 0 13 along the la lumbre ravine the deposit forms a series of flat terraces up to 65 m thick a pyroclastic flow deposit overlies the debris avalanche deposit suggesting that the colima collapse was accompanied by a magmatic component the debris avalanche deposit dammed the armeria river forming a temporary lake that accumulated a water volume of 0 19 km3 the subsequent breaking of the dam formed a debris flow that travelled at least 20 km along the armeria river emplacing 15 70 m thick deposits the debris flow deposits cover a surface of 6 4 km2 and have a volume of 0 2 km3 the avalanche deposit contains fragments of carbonized wood that where dated at 3600 120 yr bp by komorowski et al 1997 this deposit underlies a younger debris avalanche 2505 45 yr bp and a debris flow deposit with pottery shards and charcoal dated at 1940 90 yr bp these pottery shards likely related to the capacha people 820 300 yr bc indicating that the region has been inhabited during the last 3000 years a future collapse of volcan de colima in the same direction may affect several towns for a total population of circa 40 000

eruptive and magmatic cycles at fuego de colima volcano mexico the fuego de colima volcano displays a pattern of eruptive cyclicity characterized by lava flows and or slow effusions in an open crater alternating with short explosive events historical accounts field investigations and petrological data allow us to refine our understanding of the relationships between recurrent acidic acidic andesite and mafic andesitic products and this eruptive behaviour the second to last 1818 1913 and present 1913 present day cycles are instructive in this respect the short explosive events in 1818 and 1913 correspond to the initial mixing stage of a differentiated magmatic body with a new mafic input mathematical modeling shows that the pyroclastic products of 1913 result from the mixing between an acidic andesite 61 5 sio2 and an olivine andesite 56 sio2 in various ratios the final andesite mixture is made of 38 basaltic andesite and 62 acidic andesite the long effusive extrusive phases correspond to the ensuing differentiation stage of the new magmatic body for example the 1961 lava flows were derived from the final mixed and homogenized magma after the eruption of 1913 by fractionation of 7 plagioclase 9 pyroxene clinopyroxene orthopyroxene and 0 7 titanomagnetite the fractionation of 11 plagioclase 9 pyroxene and 1 4 titanomagnetite is required to produce the composition of the 1986 summit lava dome from the same parent in conclusion at fuego de colima an eruptive cycle begins with a short violent explosive event related to a mixing process and continues with a long effusive phase characterized by magmatic differentiation this scheme differs from others that have been proposed which consider that the eruptive cycles end with an explosive event

pyroclastic density currents at etna volcano italy the 11 february 2014 case study abstract on 11 february 2014 a considerable volume 0 82 to 1 29 106 m3 of unstable and hot rocks detached from the lower eastern flank of the new southeast crater nsec at mt etna producing a pyroclastic density current pdc this event was by far the most extensive ever recorded at mt etna since 1999 and has attracted the attention of the scientific community and civil protection to this type of volcanic phenomena usually occurring without any clear volcanological precursor and especially toward the mechanisms which led to the crater collapse the pdc flow dynamics and the related volcanic hazard we present here the results of the investigation carried out on the 11 february 2014 collapse and pdc events data were obtained through a multidisciplinary approach which includes the analysis of photograph images from visible and thermal surveillance cameras and the detailed stratigraphic textural and petrographic investigations of the pdc deposits results suggest that the collapse and consequent pdc was the result of a progressive thermal and mechanical weakening of the cone by repeated surges of magma passing through it during the eruptive activity prior to the 11 february 2014 events as well as pervasive heating and corrosion by volcanic gas the collapse of the lower portion of the nsec was followed by the formation of a relatively hot up to 750 c dense flow which travelled about 2 3 km from the source stopping shortly after the break of the slope and emplacing the main body of the deposit which ranges between 0 39 and 0 92 106 m3 this flow was accompanied a relatively hot cloud of fine ash that dispersed over a wider area the results presented may contribute to the understanding of this very complex type of volcanic phenomena at mt etna and in similar volcanic settings of the world in addition results will lay the basis for the modeling of crater collapse and relative pdc events and consequently for the planning of hazard assessment strategies aimed at reducing the potential risks to scientists and tens of thousands of tourists visiting etna s summit areas every year

coupled sandbar patterns and obliquely incident waves 1 in double sandbar systems the alongshore variability in the inner bar often resembles that of the outer bar suggesting that the outer bar acts as a morphological template for the inner bar earlier observations have indicated that this resemblance also termed coupling may take several forms here we apply a nonlinear 2dh morphodynamic model with time invariant forcing to show that the angle of wave incidence is crucial for the alongshore variable morphodynamic evolution of the inner bar for a given crescentic outer bar in contrast to previous modeling efforts of double barred systems which mostly used highly idealized boundary conditions we force our model with realistic hydrodynamics and bathymetrical data derived from video observations at the double barred gold coast australia the results show that for small angles of wave incidence 10 over a crescentic outer bar cell circulation patterns govern the flow at the inner bar giving rise to rip channels that incise the inner bar at the locations of the landward perturbations in the outer bar horns on the other hand for obliquely incident waves 10 20 over a crescentic outer bar the circulatory nature of the flow disappears and gives way to a meandering alongshore current the offshore directed sections of this meandering alongshore current erode the inner bar downdrift of the outer bar horns leading to landward perturbations of the inner bar that are coupled to the outer bar horns an observed coupling type that had not been reproduced as yet oblique wave incidence thus proves to be crucial to the development of this type of sandbar coupling as previously hypothesized from sandbar coupling observations at the gold coast additional simulations including tidal water level variations and bar depth variations demonstrate the robustness of our findings

emplacement temperature estimation of the 2015 dome collapse of volcan de colima as key proxy for flow dynamics of confined and unconfined pyroclastic density currents abstract the recent 10th 11th of july 2015 volcan de colima eruption involved the collapse of the summit dome that breached to the south generating pyroclastic density currents pdcs along the montegrande ravine on the southern flank of the volcano trees within the valley were buried uprooted and variably transported by the pdcs while the trees on the edges of the valley and on the overbanks were mainly burned and folded the emplacement temperature of valley confined and overbank pdc deposits were reconstructed using partial thermal remanent magnetization ptrm analysis of lithic clasts and charcoal reflectance analysis ro applied to the charred wood a total of 13 sites were sampled for the ptrm study and 39 charcoaled wood fragments were collected for the charcoal optical analysis along the entire deposit length in order to detect temperature variation from proximal to distal zone the result overlap from both data sets display a t max from 345 385 c in valley confined area from 3 5 to 8 5 km from the vent and 170 220 c from 8 0 to 10 5 km from the vent in unconfined distal area the emplacement temperature pattern along the 10 5 km long deposit appears related to the degree of topography confinement valley confined and unconfined in particular the valley confined setting is very conservative in terms of temperature while the major drop occurs in a very narrow space where the pdc expanded over unconfined flat topography just at the exit of the main valley this study represents the first attempt in determining the relationship between pdcs flow dynamics variation and topographic confining using deposit emplacement temperature as key proxy

deposits associated with the 1990 1995 eruption of unzen volcano japan abstract the 1990 1995 eruption of unzen volcano in japan was characterized by lava dome growth and block and ash flows triggered by dome collapse the repeated dome collapse events produced block and ash flow ash cloud surge and fine grained ash fall deposits block and ash flow deposits which are confined to valleys are massive to reversely graded very poorly sorted and contain non vesiculated to poorly vesiculated blocks up to 10 m across the deposits often display finer grained basal parts coarse clast depleted flow units which have intermediate grain size distribution between main flow and surge deposits occur as marginal part of block and ash flow deposits ash cloud surge deposits which spread more widely are finer grained and better sorted than block and ash flow deposits ash fall deposits which were derived from ash clouds of block and ash flows and were dispersed around the volcano are very fine grained and are as well sorted as the surge deposits lahars resulting from remobilization of pyroclastic debris devastated the base of the volcano and their deposits are moderately fines depleted and occasionally display two different lahar facies debris flow and hyperconcentrated flow units

predicting the block and ash flow inundation areas at volcan de colima colima mexico based on the present day february 2010 status abstract maps of areas potentially affected by block and ash flows and associated ash clouds are here presented for the volcan de colima titan2d 2 0 1 code has been used to simulate block and ash flows using as an input volume that of the actual summit dome assessed at 2 10 6 m 3 while the energy cone model has been used to delimit the possible inundated area from associated ash clouds both merapi and soufriere type block and ash flows were generated using different basal friction angles and maintaining fixed the volume and the internal friction angle the setting of the input parameters takes into account some flow characteristics such as the stepwise aggradation of different pulses that piled up to form the total thickness of the block and ash flow deposits the outputs of the computational routines are reported as two maps describing the total thickness of the final deposits they predict that thick deposits will engulf the ravines descending from the main cone to the west south and southeast with expected maximum runouts between 4 5 and 7 km the associated ash clouds have slightly longer runouts and the model predicts they will inundate some higher grounds that are not affected by the concentrate underflows the presented maps represent useful tools for managing the current block and ash flow hazard at volcan de colima

evaluating titan2d mass flow model using the 1963 little tahoma peak avalanches mount rainier washington the titan2d geophysical mass flow model is evaluated by comparing its simulation results and those obtained from another flow model flow3d with published data on the 1963 little tahoma peak avalanches on mount rainier washington the avalanches totaling approximately 10 10 6 m 3 of broken lava blocks and other debris traveled 6 8 km horizontally and fell 1 8 km vertically h l 0 246 velocities calculated from runup range from 24 to 42 m s and may have been as high as 130 m s while the avalanches passed over emmons glacier titan2d is a code for an incompressible coulomb continuum it is a depth averaged dshallow watert granular flow model the conservation equations for mass and momentum are solved with a coulomb type friction term at the basal interface the governing equations are solved on multiple processors using a parallel adaptive mesh godunov scheme adaptive gridding dynamically concentrates computing power in regions of special interest mesh refinement and coarsening key on the perimeter of the moving avalanche the model flow initiates as a pile defined as an ellipsoid by a height z and an elliptical base defined by radii in the x and y planes flow parameters are the internal friction angle and bed friction angle results from the model are similar in terms of velocity history lateral spreading location of runup areas and final distribution of the little tahoma peak deposit the avalanches passed over the emmons glacier along their upper flow paths but lower in the valley they traversed stream gravels and glacial outwash deposits this presents difficulty in assigning an appropriate bed friction angle for the entire deposit incorporation of variable bed friction angles into the model using gis will help to resolve this issue d 2004 elsevier b v all rights reserved

setup and swash on a natural beach wave setup and swash statistics were calculated from 154 runup time series measured on a moderately steep beach under incident waves varying from 0 4 to 4 0 m significant wave height when scaled by the incident wave height setup swash height and total runup the sum of setup and half the swash height were found to vary linearly with the surf zone similarity parameter 0 h0 l0 1 2 the foreshore slope appeared the appropriate value for the calculation of 0 although the setup data showed some influence of an offshore bar at low tide for low irribaren numbers the swash height in the incident frequency band becomes saturated while for high irribaren numbers no such signs of saturations were seen thus the infragravity band appears to become dominant in the swash below some value of 0 for these data that value is approximately 1 75 although there is considerable scatter associated with that estimate

alongshore currents over variable beach topography the nonlinear dynamics of unstable alongshore currents in the nearshore surf zone over variable barred beach topography are studied using numerical experiments these experiments extend the recent studies of allen et al 1996 and slinn et al 1998 which utilized alongshore uniform beach topographies by including sinusoidal alongshore variation to shore parallel sandbars the model involves finite difference solutions to the nonlinear shallow water equations for forced dissipative initial value problems and employs periodic boundary conditions in the alongshore direction effects of dissipation are modeled by linear bottom friction forcing for the alongshore currents is provided by gradients in the radiation stress which are specified using linear theory and the dissipation function for breaking waves formulated by thornton and guza 1983 distinct flows develop depending on the amplitude and wavelength of the topographic variability and the dimensionless parameter q the ratio of an advective to a frictional timescale for q greater than a critical value qc the flows are linearly stable for q qc q 0 the flow can be unstable for small values of q the effect of increasing is to stabilize or regularize the flows and to cause the mean flow to approximately follow contours of constant depth equilibrated shear waves develop that propagate along the mean current path at phase speeds and wavelengths that are close to predictions for the most unstable mode from linear theory applied to alongshore averaged conditions at intermediate values of q unsteady vortices form and exhibit nonlinear interactions as they propagate along the mean current path occasionally merging pairing or being shed seaward of the sandbar eddies preferentially form in the mean current when approaching alongshore troughs of the sandbar and break free from the mean current when approaching alongshore crests of the sandbar at the largest values of q examined the resulting flow fields resemble a turbulent shear flow and are less strongly influenced by the alongshore variability in topography as the amplitude of the alongshore topographic variability increases alongshore wavenumber frequency spectra of the across shore velocity show a corresponding increase in energy at both higher alongshore wavenumbers and over a broader frequency range with significant energy at wavenumbers of topographic variability and harmonics across shore fluxes of mass and momentum generally increase with increasing topographic amplitude and increasing q time and space lagged correlations of the across shore velocity show that correlation length scales decrease as topographic perturbation amplitudes increase terms from the vorticity equation show that the alongshore variation of the radiation stresses and the value of q are of importance to the flow behavior hybrid experiments separating effects of spatially variable forcing and the dynamic influence of topography on time averaged currents show that the effects are generally comparable with the relative importance of each effect a function of q the results show that topographic variability has a significant influence on nearshore circulation

deposit architecture and dynamics of the 2006 block and ash flows of merapi volcano java indonesia the internal architecture of the 2006 block and ash flow deposits of merapi volcano java indonesia was investigated using data collected from 27 stratigraphic sections measured immediately after flow emplacement and after one and two rainy seasons of erosion identification of different depositional units and their longitudinal and lateral facies variations provide detailed information about i the distribution volumes and sedimentological characteristics of the different units ii flow types and mobility as inferred from associated deposits and iii changes in the dynamics of the different flows and their material during emplacement two main types of block and ash flows short runout to medium runout block and ash flows and long runout block and ash flows are defined based on flow generation mechanism flow volume travel distance deposit morphology distribution lithology and grain size distribution conceptual models for the transport and depositional mechanisms of these two types of block and ash flows are presented variations in the runout distances observed for short runout to medium runout block and ash flows are linked directly to different initial flow volumes degree of fragmentation and material properties of the moving mass during transport with the largest and finer grained flows having the greatest mobility deposition occurs only over a narrow range of basal inclinations close to the angle of repose for pyroclastic material indicating that such flows behave in a similar way to granular free surface flows on unconfined planes the flow mechanisms of long runout block and ash flows at merapi are interpreted to be similar in many respects to unsteady cohesionless grain flows with an inertial flow regime where collisional forces largely overcome frictional forces flow unsteadiness causes the main body to be segmented into different pulses that run closer to each other as the flow moves downslope deposition occurs stepwise with rapid aggradation of stacked sub units from different parts of the major flow pulses in such a model the arrival of each flow pulse front at selected sites in the main river valley controls the generation and development of highly mobile unconfined pyroclastic flows outside valley regions and their associated overbank deposits

paroxysmal dome explosion during the merapi 2010 eruption processes and facies relationships of associated high energy pyroclastic density currents an 11 minute sequence of laterally directed explosions and retrogressive collapses on 5 november 2010 at merapi indonesia destroyed a rapidly growing dome and generated high energy pyroclastic density currents pdcs spreading over 22 km2 with a runout of 8 4 km while contemporaneous co genetic valley confined pdcs reached 15 5 km this event formed stage 4 of the multi stage 2010 eruption the most intense eruptive episode at merapi since 1872 the deposits and the widespread devastating impact of associated high energy pdcs on trees and buildings show striking similarities with those from historical volcanic blasts montagne pelee martinique bezymianny russia mount st helens usa soufriere hills montserrat we provide data from stratigraphic and sedimentologic analyses of 62 sections of the first unequivocal blast like deposits in merapi s recent history we used high resolution satellite imagery to map eruptive units and flow direction from the pattern of extensive tree blowdown the stratigraphy of stage 4 consists of three depositional units u0 u1 u2 that we correlate to the second third and fourth explosions of the seismic record both u1 and u2 show a bi partite layer stratigraphy consisting each of a lower l1 layer and an upper l2 layer the lower l1 layer is typically very coarse grained fines poor poorly sorted and massive and was deposited by the erosive waxing flow head the overlying l2 layer is much finer grained fines rich moderately to well sorted with laminar to wavy stratification l2 was deposited from the waning upper part and wake of the pdc field observations indicate that pdc height reached 330 m with an internal velocity of 100 m s 1 within 3 km from the source the summit s geometry and the terrain morphology formed by a major transversal ridge and a funneling deep canyon strongly focused pdc mass towards a major constriction thereby limiting the loss of kinetic energy this favored elevated pdc velocities and high particle concentration promoted overspilling of pdcs across high ridges into other river valleys and generated significant dynamic pressures to distances of 6 km that caused total destruction of buildings and the forest the merapi 2010 eruption highlights that explosive and gravitational disintegration of a rapidly growing dome can generate devastating high energy high velocity pdcs this constitutes a credible high impact scenario for future multi stage eruptions at merapi and at other volcanoes that pose particular monitoring crisis response and risk reduction challenges