the prediction of hillslope flow paths for distributed hydrological modelling using digital terrain models the accuracy of the predictions of distributed hydrological models must depend in part on the proper specification of flow pathways this paper examines some of the problems of deriving flow pathways from raster digital terrain data in the context of hydrological predictions using topmodel distributed moisture status is predicted in topmodel on the basis of spatial indices that depend on flow path definition the sensitivity of this index to flow path algorithm and grid size is examined for the case where the surface topography is a good indicator of local hydraulic gradients a strategy for the case where downslope subsurface flow pathways may deviate from those indicated by the surface topography is described with an example application

comparative lahar hazard mapping at volcan citlaltepetl mexico using srtm aster and dted 1 digital topographic data abstract in this study we evaluated and compared the utility of spaceborne srtm and aster dems with baseline dted 1 bald earth topography for mapping lahar inundation hazards from volcan citlaltepetl mexico a volcano which has had a history of producing debris flows of various extents in particular we tested the utility of these topographic datasets for resolving ancient valley filling deposits exposed around the flanks of the volcano for determining their magnitude using paleohydrologic methods and for forecasting their inundation limits in the future we also use the three datasets as inputs to a gis stream inundation flow model laharz and compare the results in general all three datasets with spatial resolution of 90 m or better were capable of resolving debris flow and lahar deposits at least 3 106 m3 in volume or larger canopy and slope related height errors in the aster and srtm dems limit their utility for measuring valley filling cross sectional area and deriving flow magnitude for the smallest deposits using a cross sectional area to volume scaling equation height errors in the aster and srtm dems also causes problems in resolving stream valley hydrography which controls lahar flow paths and stream valley morphology which controls lahar filling capacity however both of the two spaceborne dem datasets are better than dted 1 at resolving fine details in stream hydrography and erosional morphologies of volcaniclastics preserved in the valleys around the more humid eastern flanks of the volcanic range the results of laharz flow inundation modeling using all three dems as inputs are remarkably similar and co validate one another for example at citlaltepetl all lahar simulations show that the city of orizaba is the most vulnerable to flows similar in magnitude to or larger than one that occurred in 1920 many of the other cities and towns illustrated are built higher up on terrace deposits of older debris flows and are safe from all but the largest flows which occur less frequently finally asters 60 km swath width and 8 duty cycle presents a challenge for mapping lahar inundation hazards at e w oriented stream valleys in low latitude areas with persistent cloud cover however its continued operations enhances its utility as a means for updating the continuous but one time coverage of srtm and for filling voids in the srtm dataset such as those that occur along steep sided valleys prone to hazards from future lahars

chemical stability of the atmosphere and ocean abstract the earth system processes most of the chemical components of its atmosphere and oceans in geologically short periods of time it does this in a regulated way one that maintains a remarkably constant surface environment this we know primarily from the fossil record of uninterrupted complex life on earth that extends over the last billion years we are only beginning to understand the feedbacks that control the chemistry of the oceans and atmosphere numerical models of biogeochemical cycles are most stable when there is feedback between the amount of a chemical component in the ocean or atmosphere and its transfer to or from that reservoir coupling of subsystem especially of those operating on different time scales enhances stability an example of the role of feedback in stabilizing earth s chemical environment is the mechanism of control of atmospheric oxygen there appears to be no strong relationship between oxygen level and oxygen consumption however oxygen production may be a function of oxygen level the burial rate of organic carbon oxygen production in marine sediments may be sensitive to bottom water oxygenation levels also combustion may be an effective mechanism of transferring nutrients namely phosphorus from efficient terrestrial ecosystems to less efficient marine ecosystems when o2 rises fires become more frequent and p is transferred to the ocean stimulating marine organic carbon burial but depressing global burial rates global o2 production rates decline as does the o2 level a negative feedback models of ocean chemical composition are presently incapable of reproducingthe temporal constancy indiated by geological observations these models do not incorporate ion exchange equilibria as important processes in marine geochemical cycles when included these equilibria significantly damp the fluctuations in ion ratios calculated by the extant models

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

diagenetic stabilization of the upper permian dalan formation persian gulf basin abstract the upper permian dalan formation is a supergiant gas reservoir in the persian gulf basin reservoir quality is controlled by the original facies type dolomitization and anhydrite cementation this study focuses on the texture petrographic analysis bulk stable isotope values and trace element composition in order to identify the original mineralogy of carbonate grains and cements and to assess the synsedimentary and diagenetic processes including the timing and setting of dolomitization the limestone data suggest that the initial precipitation of isopachous acicular to fibrous aragonite cements in the marine phreatic environment was followed by dissolution of aragonite grains and cements in the meteoric phreatic zone and neomorphism and low mg calcite cementation blocky and drusy crystals syntaxial overgrowths in the active freshwater phreatic zone the relatively pristine 18o and 13c values low mn and relatively high sr and na contents indicate that this meteoric diagenesis was relatively limited the remaining porous space was cemented by blocky calcite later during burial four types of dolomites are identified within a complex mixture of texturally and isotopically different dolomites dolomicrite dolomicrosparite dolosparite and saddle dolomite dolomicrite formed syndepositionally from the evaporated permian seawater or by reflux of dense evaporated brines during shallow burial the second major episode of dolomitization dolomicrosparite and dolosparite was driven by reflux of warmer and more saline fluids at greater depth which affected both previously non dolomitized and dolomitized sediments the final diagenetic stabilization during deep burial is associated with extensive pressure solution fracturing vein filling of calcite and dolomite cements and crystallization of saddle dolomite

prediction of debris flow inundation areas using empirical mobility relationships abstract a method proposed for delineating lahar hazard zones in volcanic valleys is adapted to debris flows the original method is based on two predictive equations that relate the lahar volume to the cross sectional inundated area and to the planimetric inundated area these predictive equations were derived by combining a scaling analysis of lahar kinematics with the statistical analysis of data for 27 historical lahars this paper tests the applicability of this approach to debris flow fans forty debris flow basins in the italian alps were surveyed and data on 27 historical debris flow events were collected in the field or by means of aerial photo analysis collected data reveal that the flow area a and the planimetric area b of a debris flow are linearly correlated to the debris flow volume v on a logarithmic scale with high statistical significance moreover the computed regression equations are very similar to those obtained from other published datasets on debris flows leading to the global fitting equations a 0 08 v 2 3 and b 17 v 2 3 these predictive equations are then implemented in an automated code dflowz that delineates the inundated area on a debris flow fan on the basis of a user specified debris flow volume taking the statistical uncertainty in the prediction into account dflowz is simple reproducible and it can handle both confined and unconfined flow the model is tested against 10 historical debris flow events providing a satisfactory agreement between observed and computed inundated areas in spite of the limitations due to dem accuracy and statistical uncertainty of the regression parameters test results show that the method can be useful for practical applications

two dimensional water flood and mudflow simulation flo 2d is a two dimensional finite difference model that simulates clear water flood hazards mudflows and debris flows on alluvial fans and urban floodplains interactive flood or mudflow routing between channel street and floodplain flow is performed using a uniform grid system to describe complex floodplain topography a quadratic rheological model developed from field and laboratory mudflow data enables appropriate simulations of flooding conditions ranging from clear water to hyperconcentrated sediment flows computer aided design cad graphics of predicted time sequenced flood depths automates the delineation of flood hazards replication of the 1983 rudd creek mudflow in utah demonstrates the capability of the model

pelagic carbonate chemostratigraphy sr mg 18o 13c abstract a synthesis of the elementary sr and mg and isotopic 18 o and 13 c geochemistry of mesozoic to recent bulk pelagic carbonates suggests that the influence of late burial diagenesis is considerably less than was previously thought and that bulk carbonate geochemistry could be important for both paleoceanography and stratigraphy numerous chemical fluctuations in sr ca mg ca and 13 c 12 c ratios have occurred in the ocean during the last 140 ma the main factors responsible are 1 variations in submarine hydrothermal activity and 2 sea level fluctuations the validity of the chemostratigraphic methodology can be demonstrated by the synchroneity and the worldwide occurrence of various geochemical shifts

dolomite formation and biogeochemical cycles in the phanerozoic summary based on present knowledge of the purely chemical controls on the kinetics of massive dolomite formation the abundance and distribution of dolomite throughout the phanerozoic remains an enigma sometimes referred to as the dolomite problem comparing dolomite abundance to secular variation in seawater chemistry indicates that some changes in seawater chemistry are more likely to have resulted from extensive dolomitization rather than to have caused it the recently formulated microbial dolomite model provides the opportunity to view the geological history of dolomite formation from a new perspective a biogeochemical approach to the dolomite problem reveals a plausible connection between phanerozoic geochemical cycles and dolomite formation in particular periods of more extensive dolomitization broadly correlate with diverse indicators of decreased oxygen levels in the atmosphere and oceans lowered oxygen levels would have fostered a more active community of anaerobic microbes including sulphate reducing bacteria which in turn could have led to more extensive dolomitization of marine carbonates

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

ground deformation at soufriere hills volcano montserrat during 1998 2000 measured by radar interferometry and gps abstract we examine the motion of the ground surface on the soufriere hills volcano montserrat between 1998 and 2000 using radar interferometry insar to minimise the effects of variable atmospheric water vapour on the insar measurements we use independently derived measurements of the radar path delay from six continuous gps receivers the surfaces providing a measurable interferometric signal are those on pyroclastic flow deposits mainly emplaced in 1997 three types of surface motion can be discriminated firstly the surfaces of thick valley filling deposits subsided at rates of 150 120 mm year in the year after emplacement to 50 30 mm year two years later this must be due to contraction and settling effects during cooling the second type is the near field motion localised within about one kilometre of the dome both subsidence and uplift events are seen and though the former could be due to surface gravitational effects the latter may reflect shallow

a comparison of pyroclastic flow and debris avalanche mobility many pyroclastic flows have runout distances longer than expected from basic frictional arguments in these cases the ratio of the height descended h and the runout l are as small as 0 2 a feature which has been attributed to fluidization however significant fluidization by gases is now considered unlikely during emplacement of debris avalanches where h l and volume are inversely related we determine whether pyroclastic flows are more mobile than debris avalanches of the same volume and if different emplacement mechanisms are needed to describe their runout h l and volume data were compiled for 34 nonvolcanic and 40 volcanic debris avalanche deposits as well as 15 pyroclastic flow deposits systematic differences in the volume versus h l plots were explored by statistical analysis results show an inverse relationship between h l and volume for both nonvolcanic and volcanic debris avalanche deposits as expected and also for pyroclastic flow deposits regression lines for plots of pyroclastic flow and volcanic debris avalanche data are statistically indistinguishable and the non volcanic debris avalanche line is parallel to the other two we argue that the nonvolcanic and volcanic debris avalanche differences are due to material properties rather than emplacement mechanisms therefore since pyroclastic flows and volcanic debris avalanches have indistinguishable h l versus volume it is unlikely that pyroclastic flows have a significantly different primary emplacement mechanism based on the data in hand we conclude that it is unnecessary to invoke gas fluidization to explain the mobility of the types of pyroclastic flows examined or the excess runout of large volume debris avalanches

meteorological monitoring of an active volcano implications for eruption prediction abstract rainfall data collected on and around the soufriere hills volcano montserrat between 1998 and 2003 were analysed to assess the impact on primary volcanic activity defined here as pyroclastic flows dome collapses and explosions fifteen such rainfall triggered events were identified if greater than 20 mm of rain fell on a particular day the probability of a dome collapse occurring on that day increased by a factor of 6 3 to 9 2 compared to a randomly chosen day similarly the probability of observing pyroclastic flows and explosions on a day with 20 mm of rainfall increased by factors of 2 6 and 5 4 respectively these statistically significant links increased as the rainfall threshold was increased seventy percent of these rainfall induced dome collapse episodes occurred on the same calendar day most within a few hours as the onset of intense rainfall but an extra 3 occurred one or two calendar days later the state of the volcano was important with the rainfall volcanic activity link being strongest during periods of unstable dome growth and weakest during periods of no dome growth or after a recent major collapse over 50 of the heavy rain days were associated with large scale weather systems that can potentially be forecast up to a few days ahead however the remaining heavy rain days were associated with small scale essentially unpredictable weather systems there was significant variability in the amount of rainfall recorded by different rain gauges reflecting topographic variations around the volcano but also the inherent small scale variability within an individual weather system hence any monitoring warning program is recommended to use a network rather than just a single gauge the seasonal cycle in rainfall was pronounced with nearly all the heavy rain days occurring in the may december wet season hence the dome was at its most vulnerable at the beginning of the wet season after a period of uninterrupted growth interannual variability in rainfall was related to tropical pacific and atlantic sea surface temperature anomalies and holds out the prospect of some limited skill in volcanic hazard forecasts at even longer lead times

recent lahars at volcan de colima mexico drainage variation and spectral classification abstract volcan de colima is the most active volcano in mexico and represents a high risk for more than 500 000 people in 1998 the volcano renewed its activity with the extrusion of a lava dome and subsequent lava and block and ash flows during the recent period of activity pyroclastic products did not directly affect villages around the volcano however several lahars did we used lidar topographic coverage aster and landsat images for the recognition of morphological changes in the drainage system and lahar detection for lahar delineation we applied principal components analysis and canonical classification tasseled cap in order to perform a supervised image classification using the maximum likelihood rule algorithm laharz objective delineation of distal debris flow hazard zones has been used and tested using two topographic datasets with different resolutions which provided evidence of the importance of high resolution topographic coverage in hazard assessment finally a hazard map for lahars is presented showing that several villages and ranchos can be affected in contrast due to morphological changes produced by products of the intense explosive activity other populated areas such as la yerbabuena are outside of the high risk zone they could be affected by lahars only in case of cataclysmic eruptions such as the 1913 plinian event

scales and uncertainties in using models and gis for volcano hazard prediction practical decision making in civil protection based on predicting volcano hazards often involves using process models linked with geographic information systems gis optimum use of these techniques for such decision support requires careful and coordinated consideration of process data and model scales and their related uncertainties to avoid wasting resources and time on inappropriate data collection improper model use and resultant poor decision making there is a pressing need for a scientific and functional framework within which to examine implementation and use of geo spatial assessment tools to be useful for researchers and decision makers volcano hazard simulation approaches must consider the spatial and temporal variability in volcano processes and the data collected representing those the successful application and implementation of a geo spatial distributed volcano hazard model at variable scales requires explicit or implicit use of some form of scaling theory applied to the tasks of selection and transformation of appropriate data and use of results in general there are five consecutive scaling steps that demonstrate how data and model scale as well as the methods for information transformation between these play key roles in controlling whether prediction results have been produced efficiently and are appropriate at the scale of interest for a civil protection manager s decision making process this new scaling theory can be used as a framework to construct practical procedures for applying gis model based volcano models that allow effective model application based on realistic data availability and environmental settings

contrasts between debris flows hyperconcentrated flows and stream flows at a channel of mount semeru east java indonesia abstract in order to differentiate between different types of sediment laden flows in volcanic channels we carried out observation of debris flows hyperconcentrated flows and stream flows in the curah lengkong river on the southeast slope of mount semeru in east java indonesia the aims of this study are 1 to provide quantitative data for these flows in motion 2 to compare the data for different types of flow that occur in the same river 3 to assess the influence of rainfall on debris flows hyperconcentrated flows and streamflow generation the curah lengkong river transports large volumes of sediment in the range of 1 10 5 to 5 10 5 m 3 per debris flow and 10 3 to 10 5 m 3 per hyperconcentrated flow and stream flow large sediment discharges result from the following factors continuous and voluminous sediment supply of fine juvenile material by daily explosions of the semeru volcano pyroclastic source material emplaced on steep slopes strong erosion of weathered river banks and strong rainfall intensities the occurrence of the flows focuses in the period from november through april and the daily timing of it is the mid to late afternoon nearly all debris flows are triggered by stationary rainfall confined to the upper slopes of mount semeru whereas hyperconcentrated flows and stream flows are mainly generated by migratory or regional rains driven upwards on the eastern slope this slope receives its maximum of annual rainfall 3800 mm at 800 m asl the peak surface velocity of debris flows is always greater than the peak frontal velocity the peak discharge of debris flows occurs several minutes after the passage of the flow front volumetric concentrations of sediment are high 48 to 69 between the debris flow front and the point of peak discharge after the peak discharge it usually decreases gradually contrary to the case of debris flows high concentration of sediment appears in various portion of hyperconcentrated flows and stream flows near the front in the mid point or at the back of flows although the concentration of sediments in them is much less than in of debris flows relationships between sediment discharge total volume and peak discharge show a very strong positive correlation for debris flows and hyperconcentrated flows therefore sediment discharge can be roughly estimated from direct visual observation

evaluation of the titan2d two phase flow model using an actual event case study of the 2005 vazcun valley lahar abstract titan2d is a depth averaged thin layer computational fluid dynamics cfd code suitable for simulating a variety of geophysical mass flows titan2d output data include flow thickness and flow momentum at each time step for all cells traversed by the flow during the simulation from this information the flow limit run out path flow velocity deposit thickness and travel time can be calculated results can be visualized in the open source grass gis software or with the built in titan2d viewer a new two phase titan2d version allows simulation of flows containing various mixtures of water and solids the purpose of this study is to compare simulations by the two phase flow version of titan2d with an actual event the chosen natural flow is a small ash rich lahar volume 50 000 m3 70 000 m3 that occurred on 12 february 2005 in the vazcun valley located on the north east flank of volcan tungurahua ecuador lahars and pyroclastic flows along this valley could potentially threaten the 20 000 inhabitants living in and near the city of banos a variety of data sources exist for this lahar including post event meter scale topography and photographic video seismic and acoustic flow monitoring afm records from during the event these data permit detailed comparisons between the dynamics of the actual lahar and those of the titan2d simulated flow in particular detailed comparisons are made between run up heights flow velocity inundation area and flow thickness simulations utilize a variety of data derived from field observations such as lahar volume solid to pore fluid ratio and pre event topography titan2d is important in modeling lahars because it allows assessment of the impact of the flows on buildings and infrastructure lifelines located near drainages that descend from volcanoes

field observations rheological testing and numerical modelling of a debris flow event debris flows generated from landslides are common processes and represent a severe hazard in mountain regions due to their high mobility and impact energy we investigate the dynamics and the rheological properties of a 90 000 m3 debris flow event triggered by a rapid regressive landslide with high water content field evidence revealed a maximum flow depth of 7 8 m with an estimated peak discharge of 350 400 m3 s 1 depositional evidence and grain size distribution of the debris pose the debris flow in an intermediate condition between the fluid mud and grain flow behaviour the debris flow material has silt clay content up to 15 per cent the rheological behaviour of the finer matrix was directly assessed with the ball measuring system the measurements performed on two samples at 45 63 per cent in sediment concentration by volume gave values of 3 5 577 pa for the yield strength and 0 6 27 9 pa s for the viscosity based on field evidence we have empirically estimated the yield strength and viscosity ranging between 4000 200 pa and 108 134 pa s respectively we used the flo 2d code to replicate the debris flow event we applied the model with rheological properties estimated by means of direct measurements and back analyses the results of these models show that the rheological behaviour of a debris flow mass containing coarse clasts can not be assessed solely on the contribution of the finer matrix and thus neglecting the effects of direct grain contacts for debris flows composed by a significant number of coarse clasts a back analysis estimation of the rheological parameters is necessary to replicate satisfactorily the depositional extent of debris flows in these cases the back estimated coefficients do not adequately describe the rheological properties of the debris flow copyright 2006 john wiley sons ltd

the sensitivity of a volcanic flow model to digital elevation model accuracy experiments with digitised map contours and interferometric sar at ruapehu and taranaki volcanoes new zealand abstract a growing trend in the field of volcanic hazard assessment is the use of computer models of a variety of flows to predict potential areas of devastation these can be compared against historic and geologic evidence of past events for model calibration or used to construct hazard zone maps for mitigation and planning purposes the accuracy of these computer models depends on two factors the nature and veracity of the flow model itself and the accuracy of the topographic data set over which it is run all digital elevation models dems contain innate errors the nature of these depends on the accuracy of the original measurements of the terrain and on the method used to build the dem in this paper we investigate the effect that these errors have on the performance of a volcanic flow model designed to delineate areas at risk from lahar inundation the model was run over two dems of southern ruapehu volcano derived from 1 digitised 1 50 000 topographic maps and 2 airborne c band synthetic aperture radar sar interferometry obtained using the nasa airsar system on steep slopes of 4 or more drainage channels are more likely to be incised deeply and flow paths predicted by the model are generally in agreement for both dems despite the differing nature of the source data over shallow slopes

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

analyzing debris flows with the statistically calibrated empirical model laharz in southeastern arizona usa abstract hazard zone delineation for extreme events is essential for floodplain management near mountain fronts in arid and semiarid regions on 31 july 2006 unprecedented debris flows occurred in the santa catalina mountains of southeastern arizona following extreme multiday precipitation recurrence interval 1000 years for 4 day precipitation most mobilized sediment contributing to debris flows was derived from shallow seated failures of colluvium on steep slopes a total of 435 slope failures in the southern santa catalina mountains released 1 34 million mg of sediment into the channels of 10 drainage basins five drainages produced debris flows that moved to the apices of alluvial fans on the southern edge of the mountain front damaging infrastructure and aggrading channels to reduce future flood conveyance using the statistically calibrated empirical debris flow model laharz and modified model coefficients developed to better match conditions in southeastern arizona we predicted the approximate area of deposition and travel distance in comparison to observed depositional areas and travel distance for seven debris flows two of the modeled debris flows represented single slope failures that terminated downslope with no additive influence of other debris flows or streamflow flooding five of the simulated debris flows represented the aggregation of multiple slope failures and streamflow flooding into multiple debris flow pulses because laharz is a debris flow hazard zone delineation tool the complexity of alternating transport and deposition zones in channels with abrupt expansions and contractions reduces the applicability of the model in some drainage basins

depletion of 13c in cretaceous marine organic matter source diagenetic or environmental sigal geochemical studies of cretaceous strata rich in organic carbon oc from deep sea drilling project dsdp sites and several land sections reveal several consistent relationships among amount of oc hydrocarbon generating potential of kerogen measured by pyrolysis as the hydrogen index hi and the isotopic composition of the oc first there is a positive correlation between hi and oc in strata that contain more than about 1 oc second percent oc and hi often are negatively correlated with carbon isotopic composition 13c of kerogen the relationship between hi and oc indicates that as the amount of organic matter increases this organic matter tends to be more lipid rich reflecting the marine source of the organic matter cretaceous samples that contain predominantly marine organic matter tend to be isotopically lighter than those that contain predominantly terrestrial organic matter average 13c values for organic matter from most cretaceous sites are between 26 and 28 and values heavier than about 25 occur at very few sites most of the 13c values of miocene to holocene oc rich strata and modern marine plankton are between 16 to 23 values of 13c of modern terrestrial organic matter are mostly between 23 and 33 the depletion of terrestial oc in 13c relative to marine planktonic oc is the basis for numerous statements in the literature that isotopically light cretaceous organic matter is of terrestrial origin even though other organic geochemical and or optical indicators show that the organic matter is mainly of marine origin a difference of about 5 in 13c between modern and cretaceous oc rich marine strata suggests either that cretaceous marine planktonic organic matter had the same isotopic signature as modern marine plankton and that signature has been changed by diagenesis or that oc derived from cretaceous marine plankton was isotopically lighter by about 5 relative to modern plankton oc diagenesis does not produce a significant shift in 13c in miocene to holocene sediments and therefore probably did not produce the isotopically light cretaceous oc this means that cretaceous marine plankton must have had 13c values that were about 5 lighter than modern marine plankton and at least several per mil lighter than cretaceous terrestrial vegetation the reason for these lighter values however is not obvious it has been proposed that concentrations of co2 were higher during the middle cretaceous and this more available co2 may be responsible for the lighter 13c values of cretaceous marine organic matter

tectonic cycles in southern africa abstract the recurrence of major geological events within the 3600 ma stratigraphic record of southern africa suggests a periodicity of320 25ma for at least the last 3200 ma this periodicity is reflected in major episodes of continental volcanism granitoid plutonism orogeny reciprocity of terrestrial and marine dominated sequences platform carbonate sedimentation glaciation and first order changes in sea level these serve to define 11 completed megacycles each introduced by a major rifting event accompanied by listric faulting alluvial sedimentation in restricted fault bound troughs volcanism and a type granitoid plutonism prior to 3200 ma continental rift magmatism occurred subaqueously due to very high early archean sea levels resulting in pillow lavas and a lack of interbedded terrigenous siliciclastics the long term reciprocity of terrestrial and marine dominated sequences records eustatic first order changes in sea level with highstands characterized by generally humid warm continental climates low relief and tropical afforestation on the other hand lowstand climates were hot and arid with high relief and widespread desertification thick marine sequences accumulate in areally extensive epeirogenic basins without significant volcanism or faulting and commonly are divided into an early phase of open shelf sedimentation followed by deposition in a brackish marine foreland basin within openshelf environments an early phase of biochemical precipitation and or siliciclastic reworking along linear shorelines is separated from a later phase involving rapid deposition of immature siliciclastics along lobate shorelines evolution of the foreland basin is related to orogenic activity and the isolation of an epeiric sea which sometimes developed the characters of a barred basin marine dominated sequences display a deterministic stacking of lithofacies important glacial epochs commonly occur midway through first order regression sometimes with lesser events closely following peak transgression potentially they display 320 ma periodicity but with an unexplained mid proterozoic gap glaciation is related to the combined effect of climatic cooling associated with first order regression and depletion of atmospheric co2 by biological uptake associated with platform carbonate development terrestrial volcanism is believed to be the main contributor of atmospheric co2 climatic change is accompanied by cyclical patterns of cratonic weathering and pedogenesis

two phase diagenetic alteration of carbonate matrix middle ordovician limestones taiyuan city area china both pseudospar and micrite are present in carbonate matrix of middle ordovician wackestones in the taiyuan city area china under the scanning electron microscope pseudospar crystals display unpitted surfaces and an absence of aragonite relics in polished and slightly etched sections micritic crystals show a similar feature with slightly pitted surfaces and absence of aragonite relics this possibly indicates that both pseudospar and micrite have a similar original mineralogy i e as calcites this is further supported by their low sr contents 417 ppm the geochemistry of pseudospar and micrite suggests that they were the products of two distinct episodes of diagenesis affecting the matrix micrite was possibly formed in near surface meteoric environments but pseudospar which is characterized by a population of more depleted 18o values 9 3 to 10 2 pdb than that of micrite 7 4 to 8 7 pdb is explained to have formed during shallow burial diagenesis because its 18o composition is too light to be accounted for by near surface meteoric origin the lower sr contents 417 ppm of both pseudospar and micrite in conjunction with their more depleted 18o values 7 4 to 10 2 pdb are interpreted to be the results of meteoric diagenesis when compared with the geochemistry of marine diagenetic calcites the more negative and wider range of 13c values of pseudospar and micrite 0 3 to 4 7 in comparison with those of other coeval meteoric calcites was attributed to the influence of the decay of organic matter in carbonate matrix as well as to meteoric diagenesis although the contents of fe and mn between pseudospar and micrine overlap widely an inverse relationship exists between the concentrations of fe and mn and the 13c values in both pseudospar and micrite this can be accounted for by variable redox by variable redox conditions during diagenesis the 18o values of void fill sparry calcite cements in micrite 8 2 to 8 6 pdb fall into the category of near surface meteoric diagenesis possibly indicating that the majority of micrites were cemented earlier and consequently were resistant to any further alterations so that only minor amount of pseudospars were formed during shallow burial diagenesis by recrystallization

an overview of pre devonian petroleum systems unique characteristics and elevated risks abstract the radiation of land plants at the onset of the devonian resulted in a change in the character of available biomass which in turn introduced changes to the nature of petroleum systems this study examines the nature of pre devonian petroleum systems prior to this significant evolutionary event conventional hydrocarbon resources have been associated with pre devonian petroleum systems across the globe including the neoproterozoic infracambrian huqf supergroup of the oman basin the riphean vendian sequences of east siberia the cambrian of the murzuq basin the ordovician of the tarim basin and the silurian qusaiba of saudi arabia although individual accumulations can be quite significant e g hassi messaoud has ultimate recoverable reserves of 10 billion barrels the relative importance of pre devonian derived oils is thought to be limited compared to the global conventional resource base with growing interest in unconventional resources the relative importance of these systems is expected to increase as resource plays such as the ordovician utica shale of the appalachian basin the cambrian of china and possibly the silurian of central and eastern europe develop the advanced ages of these systems result in some unique properties and amplify risks that exist in younger petroleum systems differences in the nature of the biomass contributing to pre devonian source rocks may give rise to oils that display unique geochemical characteristics for example there are some precambrian oils where c 29 steranes dominate even though land plants were absent there are also ordovician source rocks dominated by gloeocapsamorpha prisca a primitive prokaryote which yield oils containing limited amounts of c 20 components and nearly lack pristane and phytane the potential for unconventional reservoirs in pre devonian systems may also be highly dependent upon the age of the system as biological evolution influences the availability and nature of biogenic silica an important factor controlling brittleness and fracability literature has shown that biogenic silica from different sources displays varying degrees of resistance to diagenesis which allows the formation of a silica network and influences brittleness for example radiolaria present during the cambrian are more resistant to diagenesis than diatoms which did not evolve until the jurassic risks associated with preservation of hydrocarbons are also amplified in pre devonian petroleum systems many such systems have been exposed to significant thermal stresses resulting in the cracking of oil and wet gas others have had complex tectonic histories potentially resulting in the breaching of seals or changes in pvt conditions that can result in gas loss gas loss may also occur through diffusion from these older reservoirs as a result of long residence times

rainfall triggered lahars at volcan de colima mexico surface hydro repellency as initiation process article volcan de colima is currently the most active volcano in mexico since 1998 intermittent activity has been observed with vulcanian eruptions lava flows and growing domes that have collapsed producing several block and ash flow deposits during the period of heightened activity since 1998 at volcan de colima pyroclastic flows from dome or column collapse have not reached long distances most of the time less than 6k m from the crater in contrast rain induced laharswere more frequent and have reached relatively long distances upto 15 km causing damage to infrastructureand affecting small villages in 2007 tworain gauge stationswereinstalledonthesouthern flankofthevolcanoregisteringeventsfromjunethroughtooctober the period when rains are intense and lahars frequent by comparing lahar frequency with rainfall intensity and the rainfall accumulated during the previous 3 days lahars more frequently occur at the beginning of the rainfall season with low rain accumulation 10 mm and triggered by low rain intensities 20 mm h during the months with more rainfall july and august lahars are less frequent and higher peak intensities up to 70 mm h are needed to trigger an event in both cases lahars were initiated as dilute sediment laden streamflows which transformed with entrainment of additional sediment into hypercon centratedanddebris flows withalternationsbetweenthesetwo flowtypes ahydro repellencymechanism in highly vegetated areas i e evergreen tree types with considerable amount of resins and waxes such as pines with sandy soils can probably explain the high frequency of lahars at the beginning of the rain season during low rainfall events under hydrophobic conditions infiltration is inhibited and runoff is facilitated at more highly peaked discharges that are more likely to initiate lahars

toward a model for the bulking and debulking of lahars 1 the incorporation and loss of material sediments or water during the advance of a lahar can significantly influence the advance rate mobility rheology and inundation limits of the flow and therefore these processes have consequences for destructive potential we have developed a theoretical treatment based on the conservation of flow volume and mass to describe the influence of such processes on the depth discharge and density during transit we apply our model to a common scenario of lahar generation and evolution the formation of an initially dilute flow by catastrophic breaching or ejection of a volcanic summit crater lake followed by bulking due to incorporation of substrate materials and later debulking due to sediment deposition the key features of our model are time dependence and the inclusion of two terms describing the influx or efflux of sediments to or from the flow for lahars triggered by crater lake water we derive the time evolution of flow behavior for comparison with the well documented 1953 lahar of mount ruapehu new zealand our model demonstrates the importance of these processes on observable quantities such as flow depth density and deposit profiles hydrograph evolution and transformations between rheologic density regimes

relationship between eustatic sea level changes and basement subsidence geohistory reconstructions based on well data provide a direct and quantitative method of estimating basement subsidence geohistories in addition to responding to rates of sediment accumulation and to local and regional subsidence mechanisms also reflect eustatic sea level changes we have determined quantitatively that component of subsidence that is common to a worldwide set of wells after compensating for the response of the crust to sedimentation we find some correlations between the global rate of subsidence and low frequency changes in the height of eustatic sea level as determined from the stratigraphic interpretation of seismic data these results suggest that basement subsidence data which are independent of the techniques used previously may be used to re ognize variations in eustatic sea level however we are unable to determine the size of the eustatic sea level excursions because of difficulties in determining and separating variations in lithospheric rigidity thermal subsidence and sediment supply independent of tectonic subsidence and sea level variation we compare subsidence curves in an empirical fashion with published eustatic sea level curves thus avoiding the application of geologic models involving crustal properties we cannot quantify geohistory analysis is also a powerful means of studying subsidence on a regional scale and so of deducing the regional tectonic history we illustrate this point by subsidence maps through time for the north sea

petrology of late eocene basaltic lavas at cascade head oregon coast range abstract cascade head is one of three prominent middle to late eocene volcanic centers in the oregon coast range that erupted alkalic basalt at cascade head 300 600 m of submarine to subaerial volcanic rocks are interbedded with thin bedded tuffaceous brackish water marine siltstone of the nestucca formation the intercalated basalts in the nestucca formation are informally named yachats basalt of cascade head this volcanic sequence consists of basal submarine basaltic breccia and lapilli tuff ankaramitic basalt aphyric submarine to subaerial alkali basalt lavas which account for 75 of the volcanic pile hornblende trachyandesite lavas and dikes and an uppermost basaltic sandstone unit the nestucca formation also contains andesitic and more silicic ash deposits major and trace element trends and mass balance modeling of the cascade head lavas are compatible with fractional crystallization of the observed phenocryst phases olivine clinopyroxene plagioclase apatite ilmenite the suite is characterized by enrichments in high field strength elements hfse and by steeply negatively sloping rare earth element ree patterns la lu n 15 20 transition metal contents are low ni 5 36 ppm cr 3 87 ppm sc 1 22 ppm indicating that none of the lavas are primitive elemental abundance diagrams are typical of continental alkalic basalt in that they show k depletion and nb and ta enrichment element abundance diagrams of interbedded andesitic ash are distinct from those of the basalts are depleted in nb ta and ti and indicate an arc source the geochemical signature of the yachats basalt of cascade head is consistent with a fore arc tectonic setting that was undergoing extension during late eocene time the basalts may be related to nearby hot spot activity but they could also have erupted through a slab window formed during plate reorganization during a decrease in the rate of convergence between the farallon and north american plates

microbialite formation in seawater of increased alkalinity satonda crater lake indonesia abstract the crater lake of the small volcanic island satonda indonesia is unique for its red algal microbial reefs thriving in marine derived water of increased alkalinity the lake is a potential analogue for ancient oceans sustaining microbialites under open marine conditions current reef surfaces are dominated by living red algae covered by non calcified biofilms with scattered cyanobacteria and diatoms minor caco3 precipitates are restricted to the seasonally flooded reef tops which develop biofilms up to 500 m thick dominated by the cyanobacteria pleurocapsa calothrix phormidium and hyella microcrystalline aragonite patches form within the biofilm mucilage and fibrous aragonite cements grow in exopolymer poor spaces such as the inside of dead lysed green algal cells and reef framework voids cementation of lysed hadromerid sponge resting bodies results in the formation of wetheredella like structures hydrochemistry data and model calculations indicate that co2 degassing after seasonal mixis can shift the carbonate equilibrium to cause caco3 precipitation increased concentrations of dissolved inorganic carbon limit the ability of autotrophic biofilm microorganisms to shift the carbonate equilibrium therefore photosynthesis induced cyanobacterial calcification does not occur instead passive diffusion controlled eps mediated permineralization of biofilm mucus at contact with the considerably supersaturated open lake water takes place in contrast to extreme soda lakes the release of ca2 from aerobic degradation of extracellular polymeric substances does not support caco3 precipitation in satonda because the simultaneously released co2 is insufficiently buffered subfossil reef parts comprise green algal tufts encrusted by microstromatolites with layers of fibrous aragonite and an amorphous unidentified mg si phase the microstromatolites probably formed when lake satonda evolved from seawater to ca2 depleted raised alkalinity conditions because of sulfate reduction in bottom sediments and pronounced seasonality with deep mixing events and strong co2 degassing the latter effect caused rapid growth of fibrous aragonite while mg si layers replaced the initially mg calcite impregnated biofilms this could be explained by dissolution of siliceous diatoms and sponge spicules at high ph followed by mg calcite dissolution and mg silica precipitation at low ph due to heterotrophic activity within the entombed biofilms

the coalescence and organization of lahars at semeru volcano indonesia we present multi parameter geophysical measurements of rainfall induced lahars at semeru volcano east java using two observation sites 510 m apart 11 5 km from the summit our study site in the curah lengkong channel is composed of a 30 m wide box valley with a base of gravel and lava bedrock representing an ideal geometry for high density measurements of active lahars instrumentation included pore pressure sensors stage a broad band seismograph arrival times vibrational energy and turbulence video footage and direct bucket sampling a total of 8 rainfall induced lahars were recorded with durations of 1 3 h heights 0 5 2 m and peak velocities 3 6 m s flow types ranged from dilute to dense hyperconcentrated flows these recorded flows were commonly composed of partly coalesced discrete and unsteady gravity current packets represented by multiple peaks within each lahar these packets most likely originate from multiple lahar sources and can be traced between instrument sites those with the highest concentrations and greatest wetted areas were often located mid lahar at our measured reach accelerating towards the flow front as these lahars travel downstream the individual packets thus coalesce and the flow develops a more organised structure observations of different degrees of coalescence between these discrete flow packets illustrate that a single mature debris flow may have formed from multiple dynamically independent lahars each with different origins

gis assisted modelling for debris flow hazard assessment based on the events of may 1998 in the area of sarno southern italy part i maximum run out based on the debris flow events that occurred in may 1998 in the area of sarno southern italy this paper presents an approach to simulate debris flow maximum run out on the basis of the flow source areas and an average thickness of 1 2 m of the scarps we estimated debris flow volumes of the order of 104 and 105 m3 flow mobility ratios h l derived from the x y z coordinates of the lower most limit of the source areas i e apex of the alluvial fan and the distal limit of the flows ranged between 0 27 and 0 09 we performed regression analyses that showed a good correlation between the estimated flow volumes and mobility ratios this paper presents a methodology for predicting maximum run out of future debris flow events based on the developed empirical relationship we implemented the equation that resulted from the calibration as a set of gis macros written in visual basic for applications vba and running within arcgis we carried out sensitivity analyses and observed that hazard mapping with this methodology should attempt to delineate hazard zones with a minimum horizontal resolution of 0 4 km the developed procedure enables the rapid delineation of debris flow maximum extent within reasonable levels of uncertainty it incorporates sensitivities and it facilitates hazard assessments via graphic user interfaces and with modest computing resources copyright 2007 john wiley sons ltd

genesis of the eastern paratethys manganese ore giants impact of events at the eocene oligocene boundary abstract the giant mn ore deposits of the paratethys are located on the slopes and or margins of deep basins the eocene ocean and associated basins were strongly influenced by large scale submarine hydrothermal activity which supplied prodigious amounts of ca co2 sio2 mn and other components to the oceans this caused a substantial shift in seawater composition and resulted in the deposition of carbonates as well huge amounts of mn the giant mn ore deposits of the eastern paratethys were formed as a consequence of the early oligocene global transgression in shelf environments of marginal and epicontinental seas frequently with anoxic regime of the bottom water

evaluation of aster and srtm dem data for lahar modeling a case study on lahars from popocatepetl volcano mexico abstract lahars are among the most serious and far reaching volcanic hazards in regions with potential interactions of lahars with populated areas and human structures the assessment of the related hazards is crucial for undertaking appropriate mitigating actions and reduce the associated risks modeling of lahars has become an important tool in such assessments in particular where the geologic record of past events is insufficient mass flow modeling strongly relies on digital terrain data availability of digital elevation models dems however is often limited and thus an obstacle to lahar modeling remote sensing technology has now opened new perspectives in generating dems in this study we evaluate the feasibility of dems derived from the advanced spaceborne thermal emission and reflection radiometer aster and the shuttle radar topography mission srtm for lahar modeling on popocatepetl volcano mexico two gis based models are used for lahar modeling laharz and a flow routing based debris flow model modified single flow direction model msf both predicting areas potentially affected by lahars results of the lahar modeling show that both the aster and srtm dems are basically suitable for use with laharz and msf flow path prediction is found to be more reliable with srtm data though with a coarser spatial resolution errors of the aster dem affecting the prediction of flow paths due to the sensor geometry are associated with deeply incised gorges with north facing slopes laharz is more sensitive to errors of the aster dem than the msf model lahar modeling with the aster dem results in a more finely spaced predicted inundation area but does not add any significant information in comparison with the srtm dem lahars at popocatepetl are modeled with volumes of 1 10 5 to 8 10 6 m 3 based on ice melt scenarios of the glaciers on top of the volcano and data on recent and historical lahar events as regards recently observed lahars the travel distance of lahars of corresponding volume modeled with laharz falls short by 2 to 4 km an important finding is that the travel distance of potential lahar events modeled with laharz may differ by about 2 km when using srtm or aster data because of varying lateral flow volume distribution as a consequence verification and sensitivity analysis of the dem is fundamental to deriving hazard maps from predicted modeled inundation areas because of the global coverage of this type of remote sensing data the conclusion that both srtm and aster derived dems are feasible for lahar modeling opens a wide field of application in volcanic hazards studies

carbon isotope excursions in precambrian cambrian boundary beds morocco the precambrian cambrian boundary is one of the most significant points on the geological timescale because it marks the change in the fossil record from soft bodied organisms to skeletonized faunas the cambrian explosion or radiation event1 saw the evolution of many new life forms especially various molluscs and archaeocyathids from the base of the cambrian tommotian stage others including the trilobites and bivalves evolved a little later in the succeeding atdabanian stage2 3 a massive increase in the biomass of the photic zone through phytoplankton blooms has also been postulated for this time4 particularly in the late riphean early vendian and in the early cambrian5 there has been much discussion concerning where precisely to place the boundary6 7 and here are several contenders in the world for the type section8 i present here a carbon isotope profile from the precambrian cambrian boundary beds in morocco where two distinctive positive 13c excursions are revealed these are interpreted as records of increased organic productivity as part of the precambrian cambrian explosion of life 13c excursions could be useful points for correlation between boundary sections

sedimentary carbonates through phanerozoic time plate tectonic processes play a critical role in the origin and distribution of sedimentary carbonates through phanerozoic time the phanerozoic age distribution of sedimentary properties like calcite dolomite ratio inferred ooid and cement mineralogy and survival rate of continental carbonates is cyclic the cycles appear to be coupled to plate tectonic processes that give rise to global sea level change and changes in the properties of the ocean atmosphere system first order changes in sea level are driven by the accretion of mid ocean ridges high accretion rate high sea level low accretion rate low sea level although correlations between sea level and sedimentary carbonate properties are not strong high sea level over an extended period of time appears to be correlated with low calcite dolomite ratios lack of inferred aragonite ooids and cements and maxima in the survival rate of continental carbonates the opposite is true for extended periods of low mid ocean ridge accretion rates and global sea levels the lack of strong correlations may reflect an insufficient data base and the possibility of lags between sea level change and change in carbonate properties furthermore the survival rate of continental carbonates appears to be affected by differential cycling and therefore may not be directly related to accumulation rate it appears that the environmental conditions for early dolomitization and calcite ooid and cement formation are best met during extended times of high sea level when atmospheric co2 levels are high and the saturation state of seawater with respect to carbonate minerals relatively low during low sea levels early dolomitization is less favored and aragonite precipitates are more abundant because of low atmospheric co2 levels and enhanced seawater carbonate saturation states differential cycling has modified the phanerozoic sedimentary carbonate mass age distribution because of erosion of younger units within continental carbonate cycles it may be difficult to derive an unequivocal record of the partitioning of carbonate between the deep sea and shallow water realms of deposition during the phanerozoic this difficulty must be considered in further quantification of geochemical models describing the geologic history of atmospheric co2 and climate change

dynamic interactions between lahars and stream flow a case study from ruapehu volcano new zealand three lahars were sampled in the whan gaehu river on the eastern flank of ruapehu volcano new zealand at 23 5 and 42 km from the source as they were flowing on september 27 and 29 and october 6 1995 the lahars were generated by water explosively ejected from the highly mineralized crater lake we used the chemical contrast between the lahars and resident stream water in their paths to describe a four phase model of a noncohesive lahar wave in a river channel 1 ambient stream water pushed ahead of the lahar in a process of miscible displacement due to hydrodynamic dispersion 2 a zone of mixing between the stream water and the lahar that increases in length with distance from source 3 a remnant of the original lahar least diluted by stream water that decreases in length and dilutes downstream 4 the tail of the lahar surge peak discharge occurs at the end of phase 1 as water is pushed ahead of the lahar peak sediment concentration occurs at the end of phase 2 where debris entrainment by the flow is at its greatest i e in front of the lahar proper deposits record only phases 2 and 3 of the lahar wave phase 1 flow left only a tide line of organic debris and phase 4 deposits were rapidly eroded by later streamflow downstream dilution by stream water eventually caused transformation of phases 2 and 3 of the lahars from debris flow to hyperconcentrated streamflow and then to normal streamflow as the flows became progressively finer grained and more turbulent

evaluation of approaches to calculate debris flow parameters for hazard assessment abstract many different runout prediction methods can be applied to estimate the mobility of future debris flows during hazard assessment the present article reviews the empirical analytical simple flow routing and numerical techniques all these techniques were applied to back calculate a debris flow which occurred in 1982 at la guingueta catchment in the eastern pyrenees a sensitivity analysis of input parameters was carried out while special attention was paid to the influence of rheological parameters we used the voellmy fluid rheology for our analytical and numerical modelling since this flow resistance law coincided best with field observations the simulation results indicated that the basal friction coefficients rather affect the runout distance while the turbulence terms mainly influence flow velocity a comparison of the velocity computed on the fan showed that the analytical model calculated values similar to the numerical ones the values of our rheological parameters calibrated at la guingueta agree with data back calculated for other debris flows empirical relationships represent another method to estimate total runout distance the results confirmed that they contain an important uncertainty and they are strictly valid only for the conditions which were the basis for their development with regards to the simple flow routing algorithm this methods could satisfactorily simulate the total area affected by the 1982 debris flow but it was not able to directly calculate total runout distance and velocity finally a suggestion on how different runout prediction methods can be applied to generate debris flow hazard maps is presented taking into account the definition of hazard and intensity the best choice would be to divide the resulting hazard maps into two types final hazard maps and preliminary hazard maps only the use of numerical models provided final hazard maps because they could incorporate different event magnitudes and they supplied output values for intensity calculation in contrast empirical relationships and flow routing algorithms or a combination of both could be applied to create preliminary hazard maps the present study only focussed on runout prediction methods other necessary tasks to complete the hazard assessment can be looked up in the guidelines for landslide susceptibility hazard and risk zoning included in this special issue

developing a simplified geographical information system approach to dilute lahar modelling for rapid hazard assessment in this study we present a geographical information system gis based approach to enable the estimation of lahar features important to rapid hazard assessment including flow routes velocities and travel times our method represents a simplified first stage in extending the utility of widely used existing gis based inundation models such as laharz to provide estimates of flow speeds laharz is used to determine the spatial distribution of a lahar of constant volume and for a given cell in a gis grid a single direction flow routing technique incorporating the effect of surface roughness directs the flow according to steepest descent the speed of flow passing through a cell is determined from coupling the flow depth change in elevation and roughness using manning s formula and in areas where there is little elevation difference flow is routed to locally maximum increase in velocity application of this methodology to lahars on montserrat west indies yielded support for this gis based approach as a hazard assessment tool through tests on small volume 5 000 125 000 m3 dilute lahars consistent with application of manning s law dominant flow paths were mapped and for the first time in this study area velocities magnitudes and spatial distribution and average travel times were estimated for a range of lahar volumes flow depth approximations were also made using modified laharz and these refined the input to manning s formula flow depths were verified within an order of magnitude by field observations and velocity predictions were broadly consistent with proxy measurements and published data forecasts from this coupled method can operate on short to mid term timescales for hazard management the methodology has potential to provide a rapid preliminary hazard assessment in similar systems where data acquisition may be difficult

palaeoatmospheres in the phanerozoic except in the recent geological past palaeoatmospheric composition cannot be measured but only inferred from palaeontological or geochemical observations the likely controls on atmospheric composition are reviewed and the role of organisms stressed as monitors recorders motors and moderators of palaeoatmospheric changes whereas there is a good general case for changes in po 2 and pco 2 during phanerozoic time proposals for timing and mechanism in the earlier phanerozoic are speculative at present it seems probable that the late palaeozoic spread of land plants raised o 2 and reduced co 2 levels in the atmosphere the end cretaceous crisis had a transient impact on the atmosphere whose general constancy through geological time is one of the most remarkable features of earth history

the sulphur isotopic composition of trace sulphates in carboniferous brachiopods implications for coeval seawater correlation with other geochemical cycles and isotope stratigraphy the carboniferous 34s record based on the analysis of structurally substituted sulphate sss in brachiopod shells is characterized by a significant decline from values around 21 1 in the tournaisian to 12 1 in the gzelian superimposed on this broad evolution is a higher order structure with a relative minimum in the late visean followed by a maximum in the bashkirian on the 107 year time scale 34s is positively correlated with 13ccarb in contrast to the long term negative linkage between sulphur and carbon observed for the entire phanerozoic this indicates that mechanisms other than the redox balance control the carbon and sulphur cycles on this time scale the 87sr 86sr and 34s records display parallel trends during the tournaisian to moscovian this positive correlation suggests that tectonics may be of importance for the evolution of 34s on time scales of 107 years a possible scenario to explain the co evolution of the isotope systems and their linkage to tectonism and palaeoclimate invokes an increased nutrient supply to the oceans during periods of enhanced uplift and continental weathering this nutrient flux may have resulted in increased burial of organic matter and enhanced bacterial sulphate reduction

sediment charged flash floods on montserrat the influence of synchronous tephra fall and varying extent of vegetation damage on 20th may 2006 the soufriere hills volcano on the caribbean island of montserrat experienced a large dome collapse and intense rainfall generated flash floods the floods had very high loads of volcanic debris derived both from this and previous eruptions and can thus be classified as lahars the floods reached unusually high water levels and caused substantial geomorphic change in the belham valley detailed rainfall and geomorphological data coupled with the precise timing of events and yewitness accounts have facilitated an assessment of the relative importance of rainfall volume and intensity older volcanic debris pre and syn flood tephra fall and the extent of pre flood vegetation damage for the behavior of this and subsequent sediment laden floods in this setting the change in runoff behavior was controlled by preexisting vegetation damage and synchronous tephra fall and this was critically important in controlling the impact of these flash floods although rainfall intensity and volume have some control on flood occurrence they are not the critical control on flash flood impact on the geomorphology in the belham valley a significant conclusion of this study is that the extreme nature of the flash floods was not caused by extreme rainfall as is commonly believed to be the primary cause of flash floods but rather it was the result of changed runoff behaviour caused by the widespread syn flood tephra deposition and importantly the widespread vegetation damage by volcanic associated acid rain in the preceding weeks

objective delineation of lahar inundation hazard zones a new method of delineating lahar hazard zones in valleys that head on volcano flanks provides a rapid objective reproducible alternative to traditional methods the rationale for the method derives from scaling analyses of generic lahar paths and statistical analyses of 27 lahar paths documented at nine volcanoes together these analyses yield semiempirical equations that predict inundated valley cross sectional areas a and planimetric areas b as functions of lahar volume v the predictive equations a 0 05v 2 3 and b 200v 2 3 provide all information necessary to calculate and plot in

numerical simulation of some lahars from mount st helens abstract an unsteady open channel model is used to simulate lahars discharge flow depth peak arrival time etc under the restriction of negligible volume and solid fraction variations and of channelled flow the model is able to deal with the propagation of steep fronts and transitions from a supercritical to a subcritical flow regime and vice versa conditions frequently occurring on account of the rapidity of lahar triggering mechanisms large water debris volumes are often mobilized in a very short time and of topographical characteristics the model is based on equations governing balance of mass and momentum while the energy dissipation term is expressed as a power function of the flow depth and discharge two past lahars at mount st helens the pine creek and muddy river lahars of may 18 1980 are simulated and the results obtained are discussed for these lahars the triggering mechanism was the melting of snow and ice due to pyroclastic flows and surges many millions of cubic meters of water mostly from the melting of ice and snow and sediments eroded material and pyroclastic products were mobilized in only a few minutes the simulation begins at some kilometers from the vent where lahar volumes reach an almost constant value and the flow is channelled the triggering mechanism plays a fundamental role in the downvalley destructive power of a lahar initial hydrographs characterized by steep slopes such as those occurring as a consequence of pyroclastic flows and surges rapidly flatten downvalley as predicted by the model conversely hydrographs with a prolonged peak discharge even if the peak value is smaller undergo little changes over distances of lens of kilometers with disastrous consequences as in the case of the 1980 north fork lahar at mount st helens triggered by the seismic shaking of an avalanche debris

geographical information system approaches for hazard mapping of dilute lahars on montserrat west indies many research tools for lahar hazard assessment have proved wholly unsuitable for practical application to an active volcanic system where field measurements are challenging to obtain two simple routing models with minimal data demands and implemented in a geographical information system gis were applied to dilute lahars originating from soufriere hills volcano montserrat single direction flow routing by path of steepest descent commonly used for simulating normal stream flow was tested against laharz an established lahar model calibrated for debris flows for ability to replicate the main flow routes comparing the ways in which these models capture observed changes and how the different modelled paths deviate can also provide an indication of where dilute lahars do not follow behaviour expected from single phase flow models data were collected over two field seasons and provide 1 an overview of gross morphological change after one rainy season 2 details of dominant channels at the time of measurement and 3 order of magnitude estimates of individual flow volumes modelling results suggested both gis based predictive tools had associated benefits dominant flow routes observed in the field were generally well predicted using the hydrological approach with a consideration of elevation error while laharz was comparatively more successful at mapping lahar dispersion and was better suited to long term hazard assessment this research suggests that end member models can have utility for first order dilute lahar hazard mapping

paleoclimatic and tectonic control on the accumulation of north american cratonic sediment variation in accumulation rate as a function of depositional latitude has been determined for north american limestone dolostone sand stone gray black shale red shale and evaporite rocks from tabulations by ronov and co workers from data in lexicons of united states and canadian stratigraphy and from isopach and lithofacies maps published in 1975 by cook and bally phanerozoic sediment is similar to that on other continents consisting of 63 terrigenous elastic facies 23 sandstone 40 shale 36 carbonate and 1 chert evaporite and other minor rock types latitudinal variation in accumulation rate is influenced primarily by past positions of foreland and intracratonic basins additional variation related to paleoclimate is recorded as strong dependence of limestone dolostone red shale and evaporite deposition on latitudes of accumulation a relation reflecting the importance of ambient temperature and meteoric precipitation on the formation of various authigenic phases poleward increase in dolostone total carbonate ratio in both hemispheres implicates the importance of syndepositional diagenesis in the formation of most dolostone sequences the ratio of sandstone total terrigenous clastic sediment exhibits little variation with latitude a constancy indicating that most sand and mud is derived from older sediment rather than from the weathering of crystalline rock as a result terrigenous lithofacies record little climatic influence in the proportional delivery of mechanically versus chemically derived weathering products to basins of accumulation

the fast response of volcano seismic activity to intense precipitation triggering of primary volcanic activity by rainfall at soufriere hills volcano montserrat abstract one minute resolution time series of rainfall and seismic data from the soufriere hills volcano montserrat are analysed to explore the mechanism of external forcing of volcanic eruptions by rainfall over three years of activity the real time seismic amplitude rsam shows a narrow statistically significant peak within 30 min after the start of intense rainfall events and a much broader peak with a lag of 6 40 h the classified seismic events indicate that the volcanic response to rainfall begins at the surface and gradually penetrates deeper into the dome as there is an increase in the pseudo magnitude of surface rockfall events including pyroclastic flows with lags from the first 30 min to 40 h long period rockfalls from shallow degassing at lags of 4 and 14 h and long period and hybrid events source depth approximately 1 km with lags at 14 and 24 h after the start of rainfall events there was no rainfall related change in deeper volcano tectonic activity there was no change in the frequency of any type of classified event indicating that the rainfall acts to modulate existing internal processes rather than generating new events itself these robust results are due to many 229 different rainfall events and not just to a few large magnitude cases the rainfall triggered volcanic activity examined here is consistent with a model of fast shallow interactions with rainfall at the dome surface after which a deeper dome collapse follows

tephra deposits associated with a large lava dome collapse soufriere hills volcano montserrat 12 15 july 2003 abstract the 12 13 july 2003 dome collapse at soufriere hills volcano montserrat was the largest event of its kind during the eruption thus far 1995 2005 involving the removal of 210 million m 3 of the lava dome complex over 18 h less than 2 of the total volume of material involved in the dome collapse was deposited on land a pyroclastic density current deposit alongshore and inland from the tar river fan was generated from a single blast originating at the shoreline the blast was caused by the interaction of pyroclastic flows with seawater we propose that at the peak of the lava dome collapse a sharp increase in the volume flux of pyroclastic flows caused substantial displacement of seawater from the shoreline followed by inrush of seawater when the flux decreased a few minutes later the tsunami allowed penetration of seawater into the main body of the pyroclastic flow at the shoreline which led to explosive fragmentation of pyroclastic blocks tephra fall deposits accumulated at a high rate on montserrat causing extensive damage to vegetation and buildings their stratigraphy recorded fallout from high co pyroclastic flow clouds from a vulcanian explosion cloud at the peak in collapse rate caused by the fragmentation of degassed lava dome and from four vulcanian explosion clouds after the dome collapse caused by fragmentation of bubbly magma in the conduit the total tephra fall volume is estimated at 10 20 million m 3

tectonic controls of phanerozoic sedimentary rock cycling the cyclic nature of the phanerozoic sedimentary rock distribution carbon sulphur coupling and material transfer among sedimentary reservoirs appears to be controlled by tectonic factors the distribution of preserved sedimentary mass in terms of rock mass remaining u geologic age shows a minimum c 300 350 ma ago which separates two subcycles of erosion and deposition of sedimentary rocks the older subcycle was interrupted because of the major continental collisions of the devonian and late carboniferous these collisions resulted in a reduction of outcrop areas of rocks of the older cycle relative to their masses leading to a decline in the probability of destruction and an increase in half life of these older sediments a strong correlation exists between the long term cyclicity in the phanerozoic global sea level curve and the distribution of carbon and sulphur among their major exogenic reservoirs this correlation is related to two principal tectonic modes of the phanerozoic oscillatory and submergent it is postulated that the submergent mode of active plate convergence obduction and subduction of sediments large ridge volume and high sea level gave rise to low erosion and sedimentation rates less restricted environments of carbonate deposition and relatively high atmospheric co 2 levels high temperatures resulting from an increased rate of production of co 2 from diagenetic and metamorphic reactions at subduction zones as sea level rose carbon was transferred from the sedimentary reservoir of reduced organic carbon to that of oxidized inorganic carbon in limestones whereas sulphur moved from the oxidized sulphate reservoir to the reduced sulphide reservoir as sea level fell reservoir transfers were opposite to those above culminating in the oscillatory mode of generally elevated continental interiors these reservoir transfers are consistent with secular changes observed in the distribution of 13 c and 34 s in sedimentary materials during the phanerozoic petrographic examination of phanerozoic oolite formations shows that ooids with preserved calcitic relict textures are characteristic of pre carboniferous carbonate rocks whereas ooids with relict textures indicative of initial aragonite mineralogy are dominant in rocks of younger age these changes in ooid mineralogy may be interpreted as reflecting changes in co 2 levels of the ocean atmosphere system consistent with the above tectonic considerations atmospheric co 2 levels were higher prior to carboniferous time favouring formation of calcitic ooids and skeletal parts after the carboniferous co 2 levels fell and aragonite and mg calcites of greater than 8 mol mg increased in abundance as precipitates

ooids in purbeck limestones lowermost cretaceous of the swiss and french jura ooids occurring in the shallow water purbeckian carbonate sediments of the jura mountains can be grouped into six types gradations from one type to another and coexistence of the various types are common type 1 ooids are small and well rounded they display fine concentric micritic laminae in many cases their cortices are dissolved and replaced by void filling spar microsparitic neomorphic replacement occurs locally type 2 ooids are large and have irregular shapes they show fine micritic laminae and occasional layers of fine radial crystals they commonly evolve into oncoids ooids of type 3 display many fine radial cortical laminae and are patchily micritized they are medium in size and mostly well rounded this type of ooid may pass into large irregularly shaped coated grains type 4 ooids have 1 to 4 cortical laminae with a fine radial structure and patchy micritization they are medium in size and well rounded type 5 ooids have only one lamina with a coarse radial structure they are small and well rounded associated are spherical grains containing bundles of elongate crystals ooids of type 6 show superpositions of two or more different radial and or fine micritic laminae the cortical structure may also change laterally in the same lamina the preferential dissolution of type 1 ooid cortices to form oomoulds indicates a primary composition of unstable carbonate sedimentological features and comparison with modern ooid occurrences point to formation on high energy sandbars in normal marine waters type 2 ooids grew in marine lagoonal environments with quiet water and abundant cyanobacteria the radially structured ooid cortices of types 3 4 and 5 show no dissolution features this implies that they were originally composed of stable carbonates or that an unstable carbonate phase was transformed into a stable one at an early stage of diagenesis type 3 ooids occur together with marine faunas and indicate high water energy ooids of type 4 and type 5 originated probably from relatively quiet water of variable salinity coexistence of different ooid types and mixed forms of type 6 implies gradual or rapid changes in hydrodynamic geochemical and microbiological conditions which were a feature of the purbeckian depositional environments

lahar hazard assessment using titan2d for an alluvial fan with rapidly changing geomorphology whangaehu river mt ruapehu abstract rapid changes in small areas at the apex of alluvial fans may have devastating consequences by directing downstream flood or lahar impacts into catchments of widely varying population or infrastructure vulnerability during a series of lahars in 1995 at mt ruapehu new zealand aggradation of the whangaehu fan apex draining the eastern edifice caused the onset of avulsion of flows northward into the highly vulnerable tongariro catchment an earth training dike or bund was constructed to protect this catchment by retaining flows on the southern side and the normal lahar outlet path to the south surveys in 2001 late 2005 and following a major lahar in march 2007 now show net degradation of a channel in the whangaehu fan apex bordered by the bund this indicates a net increase in the channel capacity at this site and shows that the bund remains at its effective design capacity past hydrological modelling used for the bund design provided a large range of discharge estimates but lacked precise constraints on the size and nature of lahars from eruption and lake breakout events new modelling has been carried out using titan2d to examine the impacts of a 6 106 m3 volume granular flow down this catchment this simulates either an eruption or a lake breakout induced lahar with a historically typical volumetric bulking factor of 4 these simulations predict minimum discharges between 1800 and 2100 m3 s at the bund site by comparison the largest 1995 flow at this site was estimated at around 1200 m3 s further any single modelled flow from the normal outlet channel of crater lake could not be induced to overtop the bund because discharge appears to be limited by the narrow upper reaches of the whangaehu gorge theoretical discharge levels required to overtop the bund are estimated to be 6800 m3 s assuming no aggradation of the channel by the decelerating flow maximum potential discharge at the bund site is additionally modified by potential bifurcation of lahars above a certain size threshold at a point 2 5 km upstream of the bund a major side channel the chute effectively diverts part of the flows away to the southern side of the whangaehu fan this potential for splitting of the flow appears to have increased since 1995 from aggradation of lahar deposits and reworked sediment in the area immediately upstream of the divide despite remaining at design capacity for lahar events in the whangaehu erosion of new channels following the 2007 lahar renders the bund vulnerable to undercutting

calcitic aragonitic and mixed calcitic aragonitic ooids from the mid proterozoic belt supergroup montana a great variety of ooid types occurs within the siyeh and snowslip formations of the mid proterozoic belt supergroup montana cortical layers are inferred to have been composed either of calcite in a radial concentric or radial with dark rays fabric or aragonite in a radial or concentric fabric the calcitic cortical layers record their original fabrics but the originally aragonitic cortical layers have been replaced by calcite in a range of textures and by quartz and dolomite some formerly aragonitic cortical layers are replaced by calcite spar which contains relics of the original cortical structure others consist of calcite spar without inclusions or columnar calcite which grew radially from the nucleus commonly a calcitic ooid some ooids were wholly composed of calcite others were of aragonite but two phase ooids were common mostly consisting of an inner calcitic part and an outer aragonitic part probable microdolomite inclusions suggest a high mg content of the calcitic cortical layers the depositional environment of these oolites was probably analogous to baffin bay texas where a similar range of ooid types is forming today

active ooid growth driven by sediment transport in a high energy shoal little ambergris cay turks and caicos islands ooids are a common component of carbonate successions of all ages and present significant potential as paleoenvironmental proxies if the mechanisms that control their formation and growth can be understood quantitatively there are a number of hypotheses about the controls on ooid growth each offering different ideas on where and how ooids accrete and what role if any sediment transport and abrasion might play these hypotheses have not been well tested in the field largely due to the inherent challenges of tracking individual grains over long timescales this study presents a detailed field test of ooid growth hypotheses on little ambergris cay in the turks and caicos islands british overseas territories this field site is characterized by westward net sediment transport from waves driven by persistent easterly trade winds this configuration makes it possible to track changes in ooid properties along their transport path as a proxy for changes in time ooid size shape and radiocarbon age were compared along this path to determine in which environments ooids are growing or abrading ooid surface textures petrographic fabrics stable isotope compositions 13 c 18 o and 34 s lipid geochemistry and genetic data were compared to characterize mechanisms of precipitation and degradation and to determine the relative contributions of abiotic e g abiotic precipitation physical abrasion and biologically influenced processes e g biologically mediated precipitation fabric destruction through microbial microboring and micritization to grain size and character a convergence of evidence shows that active ooid growth occurs along the transport path in a high energy shoal environment characterized by frequent suspended load transport median ooid size increases by more than 100 m and bulk radiocarbon ages decrease by 360 yr westward along the 20 km length of the shoal crest lipid and 16s rrna data highlight a spatial disconnect between the environments with the most extensive biofilm colonization and environments with active ooid growth stable isotope compositions are indistinguishable among samples and are consistent with abiotic precipitation of aragonite from seawater westward increases in ooid sphericity and the abundance of well polished ooids illustrate that ooids experience subequal amounts of growth and abrasion in favor of net growth as they are transported along the shoal crest overall these results demonstrate that in the ambergris system the mechanism of ooid growth is dominantly abiotic and the loci of ooid growth is determined by both carbonate saturation and sediment transport mode microbes play a largely destructive rather than constructive role in ooid size and fabric

rainfall induced lahars in the belham valley montserrat west indies rain falling on loose volcanic debris over the soufriere hills volcano montserrat generates hazardous floods in the belham valley these rainfall induced lahars vary greatly in discharge and sediment concentration in space and time they differ from examples documented on other volcanoes in that 1 the eruption has been continuing since july 1995 generating repeated pulses of excess sediment 2 rainfall is the only significant trigger 3 the system is small with short distance to the sea and relatively low altitude at the catchment top repeat mapping and comparison with pre eruption data demonstrate significant geomorphological change with c 120 m shoreline progradation and c 0 4 m a 1 mean aggradation rate in the middle to lower valley the nature of the hazard and area of risk have changed as the valley has aggraded the channel widened and the runoff efficiency increased as a result of rilling and vegetation removal lahars in the belham valley correlate with days when 10 mm rain fell in 24 h with more events triggered in the late rainy season the flows are mainly newtonian but one non newtonian flow event has been demonstrated and is described in detail 20 march 2000 this flow is explained by direct volcanic ash input to the runoff

a fluid dynamics approach to modelling the 18th march 2007 lahar at mt ruapehu new zealand lahars are water sediment mass flows from a volcanic source they can be triggered by a variety of mechanisms and span a continuum of flow rheology and hydraulic properties even within the same event lahars are extremely powerful landscaping agents and represent a considerable hazard potential however this highly dynamic character and a lack of direct measurements has made modelling lahars difficult this study therefore applies a fluid dynamics model delft3d to analyse the 18th march 2007 dam break lahar at mount ruapehu new zealand the modelled lahar routed through the whangaehu gorge in 30 min crossed the whangaehu fan in 60 min and then over a further 3 h travelled an additional 22 km distance along the whangaehu river to the tangiwai bridge the modelled mean frontal velocity was 6 5 m s 1 along the gorge although peak velocity reached up to 19 6 m s 1 the modelled lahar flow front progressively slowed across the fan but along the river it accelerated from 2 1 3 3 m s 1 calculated peak velocity along the river was 4 5 m s 1 these results generally compare well with gauged records with historical records and with other modelling approaches however discrepancies in frontal velocity and time to peak stage arise due to 1 specifying roughness which arises from slope variations between adjacent computational nodes and which is stage dependant and 2 due to rapid topographic changes that produce frequent hydraulic jumps which are inadequately accommodated in the numerical scheme the overall pattern of discharge attenuation and of relationships between topographic and hydraulic variables is similar to that calculated for lahars on other volcanoes this modelling method could be applied at other similar sites where a likely source hydrograph and high resolution topographic data are available these results have important implications for hazard management at ruapehu and for examining geomorphic and sedimentary impacts of this lahar

unit hydrograph derived from a spatially distributed velocity field a unit hydrograph model is proposed in which the watershed is decomposed into subareas which are individual cells or zones of neighbouring cells the unit hydrograph is found for each subarea and the response at the outlet to excess rainfall on each subarea is summed to produce the watershed runoff hydrograph the cell to cell flow path to the watershed outlet is determined from a digital elevation model a constant flow velocity is assigned to each cell and the time lag between subarea input and response at the watershed outlet is found by integrating the flow time along the path from the subarea to the outlet the response function for a subarea is modelled as a lagged linear reservoir in which the flow time is equal to the sum of a time of translation and an average residence time in the reservoir it is shown that the assumption of a spatially varying but time invariant velocity field underlying this model produces a linear system model for all subareas whose outputs can be summed in the manner indicated an example application is presented for the 8 70 km2 severn watershed at plynlimon in wales using a 50 m digital elevation model in which the cell velocity is calculated by modifying an average watershed velocity according to the terrain slope and the drainage area of each cell the resulting model reasonably reproduces the observed unit hydrograph