ice moraine and landslide dams in mountainous terrain we review recent work on ice moraine and landslide dams in mountainous terrain thus complementing several comprehensive summaries on glacier dams in intracontinental and arctic areas of low relief we discuss the roles of tectonic and climatic forcing on ice moraine and landslide dam formation and sudden drainage and focus on similarities and differences between their geomorphic impacts on confined valleys drained by steep bedrock and gravel bed rivers despite numerous reported failures of natural dams in mountain belts throughout the world their relevance to long term dynamics of mountain rivers remains poorly quantified all types of dams exert local base level controls thus trapping incoming sediment and inhibiting fluvial bedrock incision pervasive geomorphic and sedimentary evidence of outburst events is preserved even in areas of high erosion rates suggesting that sudden dam failures are characterized by processes of catastrophic valley floor aggradation active channel widening and downstream dispersion of sediment during which little bedrock erosion seems to be achieved we find that in the absence of direct evidence of former dams a number of similarities among the geomorphic and sedimentologic characteristics of catastrophic outburst flows may give rise to ambiguous inferences on the dam forming process this is especially the case for tectonically active mountain belts where there is ample and comparable potential for the formation and failure of ice moraine landslide and polygenetic dams concomitant with climatic oscillations or earthquake disturbance hence the palaeoclimatic implications of erroneously inferring the cause of dam formation may be significant we recommend that future research on natural dams in mountainous terrain addresses a climate and earthquake controlled systematics in the pattern of formation and failure b quantification of response of mountain rivers to catastrophic outburst events and their concomitant process sequences c elaboration of a comprehensive classification of natural dams in mountainous terrain with special attention to polygenetic dams d physical based modelling of dam formation failure and routing of water and sediment outbursts and e quantitative controls on the contribution of natural dams to sediment budgets in mountainous terrain

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

gradient irregularity in the herbert gorge of northeastern australia the seventy kilometre long herbert gorge of northeastern australia preserves a record of past floods in slackwater deposits and palaeostage indicators step backwater modelling of water surface profiles indicates that discharges ranging from 11000 to 17000 m3s 1 have occurred six times in the gorge during the last 900 years these flood reconstructions provide insight into the role of extreme flows in shaping bedrock channel morphology in particular the hydraulics of extreme flows can be related to boulder transport and to the location of large boulder bars large boulder bars occur throughout the herbert gorge being best developed at loci of stream power minima along the inside of bends at tributary junctions and at obstructions in the channel caused by bedrock highs only the flows exceeding approximately 8000 m3 s 1 are competent to transport the boulders which constitute the bars in the straight channel reaches the boulder accumulations and bedrock highs have a fairly regular spacing which appears to be independent of lithologic or structural controls the bars provide an efficient means of energy dissipation and they are interpreted as a result of the inherent high turbulence of flow in a steep channel the regular spacing of the bars and their correspondence with the hydraulics of large flows suggest that the bars and associated bedrock highs may represent a self regulating mechanism akin to the pool riffle sequence of alluvial channels it may therefore be appropriate to view bedrock channels as deformable on the timescale of extreme discharges

effects of an extreme flood on river morphology case study karoon river iran abstract an extreme flood occurred on 14 april 2016 in the karoon river iran the occurred flood discharge was the highest discharge recorded over the last 60 years in the karoon river using the oli landsat images taken on 8 april 2016 before the flood and 24 april 2016 after the flood the geomorphic effects were detected in different land cover types within the 155 km long study reach the results show that the flood significantly affected the channel width and the main effect was high mobilization of channel sediments and severe bank erosion in the meandering reaches according to field surveys the flood occupied the channel corridor and even the floodplain parts however the channel pattern was not significantly altered although the results show that the average channel width increased from 192 to 256 m statistical results indicate a significant change for active channel width and sinuosity index at 99 confidence level for both indexes the flood induced morphological changes varied significantly for different land cover types along the karoon river specifically the channel has widened less in residential areas than in other land cover types because of the occurrence of bank protection structures however the value of bank retreat in residential and protected sides of the karoon river is more than what we expected during the study of extreme flood

response and recovery of a subalpine stream following a catastrophic flood the july 15 1982 lawn lake flood in rocky mountain national park colorado was caused by the failure of a 79 yr old earthen dam peak discharges of the flood far exceeded naturally occurring flows and it caused severe channel disturbance along most of roaring river and some parts of fall river this study documents the geomorphic response of a 5 km reach of fall river in the 5 yr following the flood in 1983 the first year after the lawn lake flood snowmelt flows were well above average these high flows together with very high sediment yields from roaring river resulted in significant geomorphic changes on reaches of fall river downstream during the 1983 snowmelt runoff 15 5 x 106 kg of bed load sediment was eroded from the upper part of the study area these loads were at least 1 000 times higher than before the lawn lake flood most of this sediment was then deposited in a highly sinuous reach of fall river in the lower part of the study area this reach had not been much affected by the lawn lake flood but sedimentation during the period of high flow in 1983 completely filled in the channel resulting in the formation of a continuous 2 3 km long depositional zone in 1984 sediment yield from roaring river declined dramatically and this trend continued for the next 3 yr by 1987 the bed load sediment yield in the upper reaches of fall river was only about 0 4 x 106 kg yr the decline in sediment loads resulted in progressive erosion and recovery of the original channel of fall river in the depositional zone reach recovery in the upstream part of the depositional zone was complete by 1985 recovery in the downstream part of the depositional zone took longer because of the continued supply of sediment and because the sediment was mobile less of the time as of 1987 about 80 of the material initially stored in the sedimentation zone had been eroded bed load sediment yields at a sampling site 1 km downstream of the terminus of the depositional zone ranged from 5 x 106 to 11 x 106 kg yr but showed no significant decline over the 5 yr study period the average rate of bed load transport through this reach was at least 100 times greater than before the lawn lake flood but few discernible channel changes resulted from the higher loads

an improved method to represent dem uncertainty in glacial lake outburst flood propagation using stochastic simulations modelling glacial lake outburst floods glofs or jokulhlaups necessarily involves the propagation of large and often stochastic uncertainties throughout the source to impact process chain since flood routing is primarily a function of underlying topography communication of digital elevation model dem uncertainty should accompany such modelling efforts here a new stochastic first pass assessment technique was evaluated against an existing gis based model and an existing 1d hydrodynamic model using three dems with different spatial resolution the analysis revealed the effect of dem uncertainty and model choice on several flood parameters and on the prediction of socio economic impacts our new model which we call mc lcp monte carlo least cost path and which is distributed in the supplementary information demonstrated enhanced stability when compared to the two existing methods and this stability was independent of dem choice the mc lcp model outputs an uncertainty continuum within its extent from which relative socio economic risk can be evaluated in a comparison of all dem and model combinations the shuttle radar topography mission srtm dem exhibited fewer artefacts compared to those with the advanced spaceborne thermal emission and reflection radiometer global digital elevation model aster gdem and were comparable to those with a finer resolution advanced land observing satellite panchromatic remote sensing instrument for stereo mapping alos prism derived dem overall we contend that the variability we find between flood routing model results suggests that consideration of dem uncertainty and pre processing methods is important when assessing flow routing and when evaluating potential socio economic implications of a glof event incorporation of a stochastic variable provides an illustration of uncertainty that is important when modelling and communicating assessments of an inherently complex process

morphological impacts of flow events of varying magnitude on ephemeral channels in a semiarid region abstract flows in ephemeral channels in semiarid areas are only occasional and channel changes are episodic but the flash floods can be devastating and have major geomorphological impacts data on morphological impacts of flows in semiarid areas are needed to increase understanding of the dynamics and variability of geomorphic responses in such channels for this purpose nine reaches of river channel in three catchments in se spain the nogalte torrealvilla and salada have been sites for measurement of flows and their effects over the period 1997 2012 the sites encompass a range of channel size channel morphology substrate vegetation and position within the catchments a major difference is between schist and marl bedrock areas peak flow stage has been recorded and topography surveyed at frequent intervals and after major flow events over the 16 year period an average of 0 5 flow events per year has been recorded at the schist sites and an average of one per year at the marl sites but occurrence has been highly variable from year to year threshold daily rainfall for channel flow is mostly 15 20 mm but higher rainfalls do not always produce flow one to two major floods have occurred in each of the catchments in the period including the extreme flood event of september 2012 in the nogalte catchment measured morphological changes have occurred between 2 and 10 times at the monitored sites the same size flow can have differing effects depending on the state of the system low flow can mobilise sediment without producing much morphological change the long term trajectory of the reaches and the sediment substrate has a major influence on response to events channel change is governed by threshold values of hydraulic conditions the measurements provide an indication of the scale of maximum erosion and deposition that occurs within the channels and on the floodplains over a range of flow magnitudes and the flow impacts that need to be considered in management

response of the ha ha river to the flood of july 1996 in the saguenay region of quebec large scale avulsion in a glaciated valley modifications to valley form due to extreme flooding in the lower 34 km of the ha ha river were analyzed heavy regional precipitation in july 1996 triggered extreme runoff and the catastrophic drainage of ha ha lake producing discharges of 1100 m3 s 8 times the 100 year flood dominant valley modifications revealed by comparing preflood and postflood topographies derived photogrammetrically were related to two large scale avulsions in particular a deep retrogressive incision which bypassed the 30 m high perron falls exporting 6 106 m3 of glacial stratified drift from a 2 km section of valley and producing massive sedimentation in the reach downstream reconstructed maximum flow power values support the existence of a 300 w m2 threshold for major scouring of the alluvial valley bottom the evidence highlights the potential for massive scour and fill and reorganization of the long profile with potentially catastrophic effects on infrastructure during extreme floods in glaciated valley settings

reconstructing historic glacial lake outburst floods through numerical modelling and geomorphological assessment extreme events in the himalaya westoby m j glasser n f hambrey m j brasington j reynolds j m hassan m a a m 2014 reconstructing historic glacial lake outburst floods through numerical modelling and geomorphological assessment extreme events in the himalaya earth surface processes and landforms 39 12 1675 1692

flow hydraulics and geomorphic effects of glacial lake outburst floods in the mount everest region nepal glacial lake outburst floods glofs on 3 september 1977 and 4 august 1985 dramatically modified channels and valleys in the mount everest region of nepal by eroding transporting and depositing large quantities of sediment for tens of kilometres along the flood routes the glof discharges were 7 to 60 times greater than normal floods derived from snowmelt runoff glacier meltwater and monsoonal precipitation referred to as seasonal high flow floods shffs specific stream power values ranged from as low as 1900 w m 2 in wide low gradient valley segments to as high as 51 700 w m 2 in narrow high gradient valley segments bounded by bedrock along the upper 16 km of the glof routes the reach averaged specific stream power of the glofs was 3 2 to 8 0 times greater than the reach averaged specific stream power of the shffs the greatest geomorphic change occurred along the upper 10 to 16 km of the glof routes where the ratio between the glof specific stream power and the shff specific stream power was the greatest there was an abundant supply of sediment and channel valley boundaries consisted primarily of unconsolidated sediment below 11 to 16 km from the source area the geomorphic effects of the glofs were reduced because of the lower specific stream power ratio between the glofs and shffs more resistant bedrock flow boundaries reduced sediment supply and the occurrence of past glofs copyright 2003 john wiley sons ltd

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

bedrock benches and boulder bars floods in the burdekin gorge of australia the burdekin gorge of northeastern australia lies within the seasonal tropics and is characterized by high discharge variability slackwater sediments and paleostage indicators in the gorge record seven large floods that have occurred during the past 1 200 yrs these floods range in magnitude from 11 000 to 30 000 m 3 s 1 and are characterized by large downstream variations in hydraulics the floods generate high values of velocity boundary shear stress and stream power per unit area downstream fluctuations in these variables help to explain the location of boulder bars high flood levees small scale erosional features in the bedrock and the formation of inner channels boulder bars and flood levees form where shear stress and stream power decrease due to channel widening small scale erosional features such as potholes and troughs are best developed at sites of channel constriction and associated increases in shear stress and stream power the development of the inner channel appears to be controlled by complex interactions between bedrock lithology and structure and by flow hydraulics all of the channel features are related to the hydraulics of large floods which therefore are the dominant controls on many aspects of channel morphology in this bedrock gorge

the theoretical basis of ace an age calculation engine for cosmogenic nuclides abstract we present the theory behind ace an age calculation engine for cosmogenic nuclides ace is a theoretical development environment for cosmogenic nuclide dating and contains novel features such as the ability to work with any cosmogenic nuclide calibrate production rates from user supplied calibration databases and examine the sensitivity of computed sample ages to theoretical uncertainties the default algorithms for calibration and dating in ace are described so that users can interpret results within a scientific context sensitivities of the dating algorithms to assumptions regarding geomagnetic and atmospheric scaling independently dated calibration databases and secular variability are shown to identify key sources of uncertainty in the cosmogenic nuclide dating method

thresholds and the spatial variability of flood power during extreme floods abstract analysis of the effects of extreme events on overall landscape form has shifted geomorphic attention away from a focus on the magnitude and frequency relationship of discharge to a focus on flood power this paper analyzes the variation in flood power both at a station and downstream in a basin of differing lithologies and discusses the geologic and geomorphic controls on flood power design floods ranging from the 2 year discharge tothe 500 year discharge are routed trough a drainage basin and channel boundary shear stress and unit stream power are calculated using hec 2 along 21 cross valley transects because of the strong lithologic control on valley width and channel slope an irregular downstream pattern of flood power emerges which reflects primarily the pattern in valley width narrow valleys correspond to a greater percent of channel conveyance during large magnitude floods and increasing rates of depth increases thus maximizing flood power in these zones the opposite occurs in anomalously wide valleys where flood power is minimized a downstream spatial pattern emerges reflecting both local and general controls for a given probability event flood power varies at least threefold within the basin with a maximum occuring in a specific downstream section holding flood power constant produces an extremely wide range in flow probabilities in order to establish a link between critical flood power magnitude and flow frequency minimum flood powers are developed from an analysis of previous work on catastrophic flooding for humid alluvial channels these minimum thresholds tentatively correspond to 100 n m2 or 300 w m2 for shear stress and unit stream power respectively discharges required to attain these critical flood powers vary from events roughly twice the 100 year flood to a maximum of roughly 18 times the discharge of the 100 year flood

applications of grain pivoting and sliding analyses to selective entrapment of gravel and to flow competence evaluations models of sediment threshold by grain pivoting or sliding over underlying particles are examined in order to explore their application to evaluations of selective entrainment of gravel by flowing water of special interest is whether such process based models provide satisfactory evaluations of flow competence and the movement of large clasts by floods a detailed derivation is undertaken focusing first on the fluid flow and forces at the particle level the resulting threshold equation for the particle level velocity is then modified to yield the mean entrainment stress for the flow as a whole this approach is appropriate for considerations of selective entrainment of grains of varying sizes within a deposit the sorting being due to their relative projection distances above the bed and the dependence of their pivoting angles on grain size and shape the resulting threshold equations contain a number of coefficients e g drag and lift whose values are poorly known but can be constrained by requiring agreement with the shields curve for the threshold of grains in uniform deposits if pivoting coefficients based on laboratory measurements with tetrahedral arrangements of particles are used in the models smaller degrees of selective sorting are predicted than found in the field measurements of gravel entrainment however if reasonable modifications of those coefficients are made for expected field conditions then the models yield good agreement with the data sliding models where sorting is due entirely to projection distances of the grains above the bed yield somewhat poorer agreement with the field data however the sliding models may have support from laboratory experiments on gravel entrainment in that the data and theoretical curves have similar concave trends the existing measurements lack documentation of the mechanisms of grain movement so it is not possible to conclusively determine the relative importance of grain pivoting versus sliding in spite of such uncertainties the results are encouraging and it is concluded that pivoting and sliding models for grain entrainment do have potential for field computations of selective entrainment and flow competence

selective gravel entrainment and the empirical evaluation of flow competence the concept of flow competence is generally employed to evaluate velocities and bed stresses of river floods from the sizes of the largest sediment particles transported for the most part this evaluation has been empirical combining data from a number of separate flood events in different river systems those data are re examined and compared with empirical equations for the selective entrainment of gravel from deposits of mixed sizes it is found that the competence relationships trend counter to those obtained for selective entrainment indicating that the competence evaluations are affected by varying degrees of selective size entrainment individual data sets which have been employed to establish the flow competence relationships either show no trend on their own or yield a trend which runs counter to the competence equation instead being more compatible with the selective entrainment relationships in most instances the empirical competence equations greatly overestimate the hydraulics of flood flows and it is suggested that the better established selective entrainment equations be used for competence evaluations as well empirical equations are available for this purpose relating the dimensionless shields entrainment function or the bed shear stress to the diameter of the largest grain moved and to the median diameter of the deposit as a whole

frequency and characteristics of glacier floods in the swiss alps damage due to glacier floods in the swiss alps occurs about once every two years at present despite the pronounced retreat of glaciers during the twentieth century and the installation of many water reservoirs which act as flood retention basins over half 60 to 70 of the observed floods are caused by outbursts of marginal glacier lakes or sudden breaks of ice dams and 30 to 40 by ruptures of water pockets in a glacierized mountain region as densely populated as the swiss alps even debris flows triggered by outbursts of very small water masses may be dangerous historical information about glacier floods in the swiss alps although incomplete and heterogeneous is used as an empirical basis for an attempt to recognize potential hazards at an early stage by considering outburst processes volumes of water involved potential peak discharge values lithology and inclination within the reach of glacier streams

the 2010 mw 8 8 maule megathrust earthquake of central chile monitored by gps large earthquakes produce crustal deformation that can be quantified by geodetic measurements allowing for the determination of the slip distribution on the fault we used data from global positioning system gps networks in central chile to infer the static deformation and the kinematics of the 2010 moment magnitude m w 8 8 maule megathrust earthquake from elastic modeling we found a total rupture length of 500 kilometers where slip up to 15 meters concentrated on two main asperities situated on both sides of the epicenter we found that rupture reached shallow depths probably extending up to the trench resolvable afterslip occurred in regions of low coseismic slip the low frequency hypocenter is relocated 40 kilometers southwest of initial estimates rupture propagated bilaterally at about 3 1 kilometers per second with possible but not fully resolved velocity variations

geomorphic implications of glacier outburst flooding noeick river valley british columbia an outburst flood from ape lake in october 1984 severely affected channel and floodplain morphology of noeick river this event estimated to have exceeded 103 m3 s 1 was as much as 2 4 times greater than the probable maximum meteorological flood storage of water in ponds lakes and floodplain margins resulted in attenuation of the flood peak as it progressed downstream as a result flood effects were most evident in the alluvial reaches of upper noeick river below ape lake between km 10 and km 23 here the channel zone widened from a preflood average of 75 m to almost 200 m the area of eroded valley bottom increased by about 2 5 times and a cumulative length of 2 5 km of valley side colluvial fans was eroded transported sediment stored in the floodplain and along valley margins resulted in aggradation of up to 0 75 m except in restricted reaches where limited sediment supply and increased stream power caused degradation of up to 3 m a second and fractionally larger outburst flood in august 1

evidence of episodically accelerated denudation on the tibetan namche barwa massif by megafloods abstract megafloods cause widespread mobilization of sediments that have strong potential to control the geodynamic evolution of the namche barwa nb massif by changing the landscape morphology within a short time however the impact of catastrophic events on the denudation of the massif over the longer time scale has not been explored in detail this contribution assesses the impact of catastrophic floods on the denudation style of the namche barwa nb massif over millennial time scales towards this 21 optical ages and 1700 detrital thermochronometric ages of zircon and apatite minerals extracted from paleoflood sediments downstream of the massif were integrated with decadal sediment load and aggradation data to estimate the minimum denudation rate during paleoflood events the results indicate that the 7 ka flood eroded 71 m of material from the nb massif this denudation is equivalent to the material that would be produced over 8000 years of erosion at a rate of 9 mm a the 3 5 ka flood eroded 24 m of material equaling 2500 years worth of erosion at a rate of 9 mm a such relatively instantaneous denudations are three orders of magnitude higher than averaged denudation rates over million year time scale this study therefore provides the first evidence of the strong interplay of catastrophic flood events and tectonics for the rapid exhumation of the namche barwa massif megafloods from the failure of dammed lakes provide valuable information to understand the flood surge risk for human settlements in the siang and the assam valleys

threshold of coarse sediment transport in broad and narrow natural streams the threshold of coarse sediment transport has been examined in natural streambeds in an upland pennine u k area threshold values of the total boundary shear stress t0 for a given grain size in a narrow natural stream w d 11 efficiency in the entrainment process is related not only to the overall channel geometry but also varies as a function of discharge in channels characterized by compound roughness empirical curves relating t0 and a mean grain size d5 are presented but are limited in application to streams of similar physical and hydraulic characteristics as the ones examined in this investigation considerable divergence is noted between these empirical functions and a summary empirical function for general application obtained from a published source the reasons for this divergence are discussed the influence of grain shape was found not to be important in the initiation of motion criterion this conclusion may reflect the limited range of natural grain shapes in the study streams but might reasonably apply to other field investigations of similar streams modifications of the shields and yalin diagrams are suggested for practical applications in shallow streams with poorly graded bed material the shields parameter may be regarded as an inverse function of the relative protrusion of individual grains in the shallow flow depth d5 d the increased importance of augmented drag forces in the entrainment process in shallow flows is suggested as the physical explanation for the reduced values of the shields parameter however the relationships presented should not be applied to laboratory experiments concerned with well graded sediments therefore beds with little deviation in level in which the shields parameter may be regarded as constant at high reynolds grain numbers consistent estimated field values of a threshold sediment transport parameter might be used to compare field data to threshold values derived from statistical arguments and laboratory experiments reported in the literature

catastrophic lake drainage within the homathko river basin british columbia three distinct phenomena related to sudden lake drainage which occurred in a restricted area within the southern coast mountains of british columbia are described i moraine breaching by solita

wood mediated geomorphic effects of a jokulhlaup in the wind river mountains wyoming a jokulhlaup burst from the head of grasshopper glacier in wyoming s wind river mountains during early september 2003 five reaches with distinct sedimentation patterns were delineated along the dinwoody creek drainage this paper focuses on a portion of the jokulhlaup route where erosion of the forested banks created 16 large logjams spaced at longitudinal intervals of tens to hundreds of meters aggradation within the main channel upstream from each logjam created local sediment wedges and the jams facilitated overbank deposition during the jokulhlaup field surveys during 2004 and 2006 documented logjam characteristics and associated erosional and depositional features as well as initial modification of the logjams and flood deposits within the normal seasonal high flow channel overbank deposits have not been altered by flows occurring since 2003 field measurements supported three hypotheses that i logjams present along the forested portions of the jokulhlaup route are larger and more closely spaced than those along adjacent otherwise comparable stream channels that have not recently experienced a jokulhlaup ii logjams are not randomly located along the jokulhlaup route but instead reflect specific conditions of channel and valley geometry and flood hydraulics and iii the presence of logjams facilitated significant erosional and depositional effects this paper documents a sequence of events in which outburst floodwaters enhance bank erosion and recruitment of wood into the channel and thus the formation of large logjams these logjams sufficiently deflect flow to create substantial overbank deposition in areas of the valley bottom not commonly accessed by normal snowmelt peak discharges and through this process promote valley bottom aggradation and sediment storage changes in the occurrence of glacier outburst floods thus have the potential to alter the rate and magnitude of valley bottom dynamics in these environments which is particularly relevant given predictions of worldwide global warming and glacial retreat processes observed at this field site likely occur in other forested catchments with headwater glaciers

floods in mountain environments a synthesis floods are a crucial agent of geomorphic change in the channels and valley floors of mountains watercourses at the same time they can be highly damaging to property infrastructure and life because of their high energy mountain watercourses are highly vulnerable to environmental changes affecting their catchments and channels many factors have modified and frequently still tend to modify the environmental conditions in mountain areas with impacts on geomorphic processes and the frequency magnitude and timing of floods in mountain watercourses the ongoing climate changes vary between regions but may affect floods in mountain areas in many ways in many mountain regions of europe widespread afforestation took place over the twentieth century considerably increasing the amounts of large wood delivered to the channels and the likelihood of jamming bridges at the same time deforestation continues in other mountain areas accelerating runoff and amplifying the magnitude and frequency of floods in foreland areas in many countries in channel gravel mining has been a common practice during recent decades the resultant deficit of bed material in the affected channels may suddenly manifest during flood events resulting in the failure of scoured bridges or catastrophic channel widening during the past century many rivers in mountain and foreland areas incised deeply the resultant loss of floodplain water storage has decreased attenuation of flood waves hence increasing flood hazard to downstream river reaches on the other hand a large amount of recent river restoration activities worldwide may provide examples of beneficial changes to flood risk attained as a result of increased channel storage or reestablished floodplain water storage relations between geomorphic processes and floods operate in both directions which means that changes in flood probability or the character of floods e g increased wood load may significantly modify the morphology of mountain rivers but morphological changes of rivers can also affect hydrological properties of floods and the associated risk for societies this paper provides a review of research in the field of floods in mountain environments and puts the papers of this special issue dedicated to the same topic into context it also provides insight into innovative studies methods or emerging aspects of the relations between environmental changes geomorphic processes and the occurrence of floods in mountain rivers

energy expenditure and geomorphic work of the cataclysmic missoula flooding in the columbia river ggorge usa cataclysmic releases from the glacially dammed lake missoula producing exceptionally large floods have resulted in significant erosional processes occurring over relatively short time spans erosional landforms produced by the cataclysmic missoula floods appear to follow a temporal sequence in many areas of eastern washington state this study has focused on the sequence observed between celilo and the john day river where the erosional features can be physically quantified in terms of stream power and geomorphic work the step backwater calculations in conjunction with the geologic evidence of maximum flow stages indicate a peak discharge for the largest missoula flood of 10 10 6 m 3 s 1 the analysis of local flow hydraulics and its spatial variation were obtained calculating the hydrodynamic variables within the different segments of a cross section the nature and patterns of erosional features left by the floods are controlled by the local hydraulic variations therefore the association of local hydraulic parameters with erosional and depositional flood features was critical in understanding landform development and geomorphic processes the critical stream power required to initiate erosion varied for the different landforms of the erosional sequence ranging from 500 w m 2 for the streamlined hills up to 4500 w m 2 to initiate processes producing inner channels erosion is possible only during catastrophic floods exceeding those thresholds of stream power below which no work is expended in erosion in fact despite the multiple outbursts which occurred during the late pleistocene only a few of them had the required magnitude to overcome the threshold conditions and accomplish significant geomorphic work 1997 by john wiley sons ltd

geomorphological response to an extreme flood a case from southeast spain on 28 29 september 1980 an extreme storm of 150 mm of rain in c 5 hours caused a flash flood near tabernas in a semi arid area in southern spain with the rainfall total approximating the mean annual rainfall the recurrence interval is estimated to be 25 100 years flood peak discharges have been estimated from a mountain source area across an alluvial fan and the geomorphological response to the storm described channel adjustment differed between channels in the fanhead trench and those unconfined channels on the fan surface the implications of the spatial pattern of erosional an depositional feaures for landform adjustment in semi arid regions are discussed

rules of the road a qualitative and quantitative synthesis of large wood transport through drainage networks abstract to effectively manage wood in rivers we need a better understanding of wood mobility within river networks here we review primarily field based and some numerical studies of wood transport we distinguish small medium large and great rivers based on wood piece dimensions relative to channel and flow dimensions and dominant controls on wood transport we suggest further identification and designation of wood transport regimes as a useful way to characterize spatial temporal network heterogeneity and to conceptualize the primary controls on wood mobility in diverse river segments we draw analogies between wood and bedload transport including distinguishing eulerian and lagrangian approaches exploring transport capacity and quantifying thresholds of wood mobility we identify mobility envelopes for remobilization of wood with relation to increasing peak discharges stream size and dimensionless log lengths wood transport in natural channels exhibits high spatial and temporal variability with discontinuities along the channel network at bankfull flow and when log lengths equal channel widths although median mobilization rates increase with increasing channel size maximum mobilization rates are greatest in medium sized channels most wood is transported during relatively infrequent high flows but flows under bankfull can transport up to 30 of stored wood we use conceptual models of dynamic equilibrium of wood in storage and of spiralling wood transport paths through drainage networks as well as a metaphor of traffic on a road to explore discontinuous wood movement through a river network the primary limitations to describing wood transport are inappropriate time scales of observation and lack of sufficient data on mobility from diverse rivers improving models of wood flux requires better characterization of average step lengths within the lifetime travel path of a piece of wood we suggest that future studies focus on i continuous or high frequency monitoring of wood mobility ii monitoring changes in wood storage iii using wood characteristics to fingerprint wood sources iv quantifying volumes of wood buried within river corridors v obtaining existing or new data from unconventional sources such as citizen science initiatives and vi creating online interactive data platforms to facilitate data synthesis

peak discharge estimates of glacial lake outburst floods and normal climatic floods in the mount everest region nepal abstract glacial lake outburst floods glofs in the mount everest region of nepal on 3 september 1977 and 4 august 1985 dramatically modified channels and valleys in the region by eroding transporting and depositing large quantities of sediment for tens of kilometers along their flood routes prior to this research the glof discharges had not been determined and the hydrology of normal climatic floods shffs seasonal high flow floods was not known a one dimensional step backwater flow model was utilized in conjunction with paleostage indicators to estimate the peak discharges of the glofs and shffs and to reconstruct the hydrology and hydraulic conditions of the glofs at 10 reaches and shffs at 18 reaches the most reliable glof and shff peak discharge estimates were upstream from constrictions where there was critical depth control the peak discharge of the 1977 glof at 8 6 km from the breached moraine was approximately 1900 m3 s at 7 1 km downstream from the breached moraine the 1985 glof discharge was estimated at 2350 m3 s at 27 km downstream from the breached moraine the 1985 glof attenuated to an estimated discharge of 1375 m3 s the peak discharges of shffs ranged from 7 to 205 m3 s and were positively correlated with increasing drainage area the glof discharges were 7 to 60 times greater than the shff discharges with the greatest ratios occurring near the breached moraines the downstream decline in the ratio between the glof discharge and shff discharge is the result of the downstream attenuation of the glof and the increased discharge of the shff because of increased contributing drainage area and the increased effects of monsoonal precipitation at lower elevations

paleohydrology of pool and riffle pattern development boulder creek utah the low flow channel morphology of boulder creek is characterized by a well developed pool and riffle pattern the riffles consist of accumulations of basaltic boulders deposited from upstream source areas during extremely large flows paleoflood water surface profiles defined by high water indicators such as slack water sediments and silt lines indicate that discharges of up to 400 50 m 3 s have affected the lower reaches of this bedrock stream system stratigraphic relationships and archaeologic and radiometric age constraints indicate that at least four large magnitude low frequency flow events have occurred within the past 500 to 1 000 radiocarbon years b p step backwater hydraulic reconstructions of these large flows suggest that the positions of the boulder comprised riffles are controlled by spatial variations in large flow stream power boulder deposition occurs where channel stream power drops below critical power thresholds necessary for boulder transport high discharge stream power minima occur in reaches immediately upstream of canyon bends and constrictions and downstream of canyon expansions the low flow riffles occur at these sites comparison of calculated stream power values and measured boulder sizes with established coarse particle transport relationships indicates that a 400 m 3 s flow is approximately the minimum discharge competent to affect this pool and riffle pattern