geomorphic response to seasonal variations in rainfall in the southwest united states the interaction of the north american monsoon with watershed hydrology and landscape response is evaluated by observing geomorphic characteristics of hillslopes hydrology and stream channels in two mountain ranges with contrasting intensity of precipitation the study compares watersheds in the hualapai and santa catalina mountains in arizona which are similar in lithology elevation tectonic setting vegetation and annual precipitation but differ in the proportion of precipitation received in summer thunderstorms the hualapai mountains receive most of their precipitation in winter whereas rainfall in the santa catalina mountains occurs mostly in summer drainages in the santa catalinas are more variable in local relief and exhibit much more exposed bedrock and higher drainage density the trunk channel of a major drainage in the santa catalina mountains has measured discharges several orders of magnitude greater than a channel draining a comparable area in the hualapai mountains the alluvial segment of the channel in the santa catalina piedmont additionally displays greater concavity smaller width to depth ratios and a larger caliber of bed material in contrast to conventional interpretation summer monsoonal precipitation is not primarily responsible for generating most of the discharges that modify channels in large scale drainages in the monsoonal climate regime of the santa catalinas small basins flood most often in summer whereas larger drainages exhibit peak discharges in response to low intensity winter precipitation we attribute the paradoxical discharge response of larger drainages to the small spatial scale of summer thunderstorms which fail to deliver enough precipitation to generate floods in these larger basins but do prime hillslopes by stripping colluvium and lowering hillslope infiltration rates making the large drainages more responsive to areally extensive winter storms

geomorphological and sedimentological analysis of a catastrophic flash flood in the aras drainage basin central pyrenees spain abstract on august 7th 1996 an intense and short duration convective storm occurred over the 18 6 km 2 aras drainage basin central pyrenees spain this high relief basin is composed of three subbasins aso betes and la selva and feeds the aras alluvial fan in the gallego river valley this alluvial fan had been drained by an artificial channel about 125 m 3 s at bank full capacity more than 30 check dams in its feeder channel the aras barranco had been previously filled by earlier sediments the heaviest rain was over the betes subbasin total rainfall 178 4 mm maximum rainfall intensity of 153 mm h for a 10 min time interval was estimated most of the rainfall fell in a 70 min period this storm resulted in high runoff causing catastrophic damage and significant geomorphic changes in the drainage basin especially in the betes subbasin the high discharge concentrated in the aras barranco destroyed most of the check dams flushing out a great amount of debris major channel trenching and widening occurred in this barranco when the confined sediment laden flash flood reached the basin mouth it sheet flooded the southern sector of the aras fan depositing a massive amount of debris on this fan 87 people lost their lives and the direct physical damage has been estimated at 55 million dollars two stages in the development of the flood have been differentiated from the sedimentological and morphological analysis of the flooded fan lobe a first stage peak discharge of sheet flooding deposited a coarse boulder lobe burying the artificial channel at the fan head and causing a darnming effect on the water flood during the second stage discharge decline the flood made its way through the fan head incising the previous debris accumulation and splitting into two main flow paths

lichenometric application to historical and subrecent dynamics and sediment transport of a corsican stream figarella river france abstract lichenometry has been applied to blocks canyon walls and terrace deposits a growth curve specific to corsica was made based on information obtained from tombstones old monuments from the genoese occupation of corsica and roman megaliths this allowed us to create a curve going as far back as 2000 years the curve is similar to those developed in mediterranean environments cevennes and eastern pyrenees but is very different from those developed in alpine and arctic regions lichens present on boulders of the bedload allow the time at which the boulders were laid down to be determined the lichenometric study of the figarella shows firstly that there is high stream competence blocks approximately 100 cm wide are moved during the 5 year flood recent important flood events were identified in 1973 and 1869 these correspond well with historical sources other older and much more important floods were also identified this analysis has allowed us to extend the specific streampower curves proposed by costa geol soc am bull 94 1983 986 and williams geogr ann 65a 1983 227 to blocks of more than 2 m wide lichens present on the canyon walls and on terrace sides in the plain allowed the last main incision phase to be dated to the little ice age aerial photograph analysis has shown that the present braided system in the lower course of the river is no longer active

what was the transport mode of large boulders in the campine plateau and the lower meuse valley during the mid pleistocene abstract the campine plateau in northeastern belgium a remnant of an alluvial fan deposited by the meuse river during the mid pleistocene is characterised by the presence of boulders with maximum dimensions of up to 2 m embedded in a gravel matrix these boulders originated in the ardennes region and are generally assumed to have been transported by ice rafting processes this paper investigates for the first time quantitatively the possibility of purely hydraulic transport of the boulders taking into account channel and flow characteristics in the boulder provenance area during the mid pleistocene empirical transport relations that describe incipient motion thresholds in nonuniform river beds as a function of the relative grain size or the ratio between the grain size of interest and the median grain size of the channel bed are applied in order to calculate critical water depths for transport of boulders of various sizes results indicate that hydraulic transport of boulders with intermediate diameters

river terrace development by concurrent fluvial processes and climatic changes abstract along the usui river in central japan three well developed terrace levels are observed each terrace is subdivided into two segments a filltop terrace along the upper reach and a strath terrace along the lower one three valley fillings with coarse gravel beds in the upper reach correspond to the last three glacial ages the positions of these valleysfills migrated upstream from the older terrace to the younger one to clarify the response of fluvial systems to climatic change terrace sediments were interpreted from the viewpoint of paleohydraulics using the grain diameter of terrace sediments paleo tractive forces were evaluated based on the correlation between tractive force and grain diameter of the sediment on the present river bed the valley filling was caused by the diminution in tractive force related to the decrease in heavy rain produced by typhoons during the glacial ages during the interglacial ages the reverse responses occurred due to the increased incidence of typhoons from another aspect of the development of river landforms the decrease in the liver channel gradient during the interglacial ages caused the transportational and or depositional area of coarse gravels to migrate upstream in the subsequent glacial ages

10be dating of former glacial lake naskaupi quebec labrador and timing of its discharges during the last deglaciation abstract the last deglaciation of the laurentide ice sheet in northern quebec and labrador led to the formation of several glacial lakes that drained into ungava bay and the nearby labrador sea assessing the potential impact of their drainage on ocean surface conditions and climate however is limited by the few existing age constraints on ice retreat and associated evolution of these lakes here we report 21 10be ages from shorelines and an outburst flood landform formed by lake naskaupi one of the largest glacial lakes in this region the results indicate that the lake drained from its full extent at 8300 300 a suggesting that it may have contributed to the freshwater forcing initiated by the drainage of lake agassiz ojibway that caused the 8 2 ka cold event additionally the results provide important constraints on the position of the ice margin of the labrador sector during its retreat across this region

coarse sediment distribution as evidence of an elevation limit for flash flooding bear creek colorado bear creek is a tributary of the south platte river in central colorado the stream flows east from an elevation of 4348 m at the continental divide to the mountain front at 1670 m it thus encompasses the 2300 m elevation limit for substantial rainfall flooding in the colorado front range proposed by jarrett maximum paleoflood discharges estimated from flood deposits at four sites along bear creek demonstrate a consistent decrease in unit discharge with increasing elevation and support the hypothesis of an upper elevation limit for rainfall floods the unit discharge values were used to explain coarse sediment distribution along bear creek measurements of coarse grained channel sediment at 19 sites along the creek indicate a decrease in particle size in flood deposits with increasing elevation as well as a decrease in the size of clasts introduced to the main channel along tributaries these changes in grain size are hypothesized to reflect changes in the competence of channel transport as a result of snowmelt dominated versus rainfall dominated discharge regimes above and below 2100 m elevation calculations of flow competence versus entrainment thresholds for the deposits may support this interpretation one of the geomorphic implications of the elevation limit on flash flooding is a reversal of the usual downstream fining trend in coarse channel sediments

laterally extensive modified placer gold deposits in the paleoproterozoic mississagi formation clement and pardo townships ontario local gold concentrations are found in laterally extensive pyrite bearing framework supported cobble and boulder conglomerates in the basal 30 m of the mississagi formation in the south eastern part of the huronian outcrop belt these were deposited as part of a valley fill succession in shallow gravel bed braided rivers with local hyperconcentrated flows the basal contact with underlying archean psammites is typically highly irregular and shows no obvious sign of weathering suggesting that deposition took place soon after retreat of the glaciers responsible for deposition of the ramsey lake formation highest gold concentrations are associated with moderately well sorted medium to large pebble conglomerate that show some signs of reworking during low flow events depositional elements are typically lenticular and of limited lateral extent unlike the older pyritic quartz pebble conglomerates at the base of the matinenda formation in the elliot lake and blind river areas these conglomerates contain no uraninite and are polymict with material derived from a highly restricted catchment area with marked local and regional topographic relief porous detrital allogenic pyrite and euhedral post depositional pyrite have overlapping generally positive delta 34 s values indicating a closed system during diagenesis and metamorphism the presence of biotite enriched rims on many of the metavolcanic and metasedimentary clasts in the conglomerates suggests that gold was partly leached from the allogenic pyrite grains at the peak of the penokean orogeny at 1 85 1 5 ga the potential source of the au bearing detrital allogenic pyrite appears to be an as yet undiscovered archean volcanic hosted massive sulfide vhms deposit located 1 to 8 km north of the deposit

interpreting flash flood palaeoflow parameters from antidunes and gravel lenses an example from montserrat west indies the wavelength of stationary water surface waves and their associated antidune bedforms are related to the mean velocity and depth of formative flow in sand bed flume experiments alexander et al 2001 found that lens structures were preserved during antidune growth and change and the dimension of the lenses were empirically related to antidune wavelength and thus could be used to estimate flow velocity and depth this study is the first to compare observations of formative flow conditions and resulting sedimentary structures in a natural setting testing the alexander et al 2001 relationship at a field scale trains of stationary and upstream migrating water surface waves were prevalent during the flash flood in october 2012 in the belham valley montserrat west indies wave positions and wavelengths were assessed at 900 s intervals through the daylight hours of the event within a monitored reach the wave data indicate flow depths up to 1 3 m and velocity up to 3 6 m s 1 sedimentary structures formed by antidune growth and change were preserved in the event deposit these structures include lenses of clast supported gravel and massive sand with varying internal architecture the lenses and associated low angle strata are comparable to sand bed structures formed from stationary and upstream migrating waves in flume experiments confirming the diagnostic value of these structures using mean lens length in the event deposit underestimated peak flow conditions during the flood and implied that the lenses were preserved during waning flow

paleohydrology of arid southeastern maui hawaiian islands and its implications for prehistoric human settlement abstract arid slopes on the southeastern side of maui are densely covered with archaeological remains of hawaiian settlement from the late prehistoric to early postcontact period ca a d 1500 1860 permanent habitation sites agricultural features and religious structures indicate perennial occupation and farming in a subregion called kahikinui yet there is presently no year round water source we explore the possibility that postcontact deforestation led to the loss of either 1 perennial channel flow or 2 perennial springs or seeps to investigate the first possibility we estimated ancient peak flows on 11 ephemeral channels in kahikinui using field measurements and paleohydrology peak flow estimates 3 230 m3 s for a given drainage area are smaller than those for current perennial maui streams but are equivalent to gauged peak flows from ephemeral and intermittent streams in the driest regions of hawai i and maui islands this is consistent with the long term absence of perennial channel flow in kahikinui on the other hand others have shown that canopy fog drip in hawai i can be greater than rainfall and thus a large part of groundwater recharge using isolated live remnants and snags we estimate the former extent of the forest upstream from archaeological sites we use rough estimates of the loss of fog drip recharge caused by deforestation and apply a simple steady state hydrologic model to calculate potential groundwater table fall these order of magnitude estimates indicate that groundwater could have fallen by a minimum of several meters abandoning perennial seeps this is consistent with archaeological evidence for former perennial seeps such as stonewalls enclosing potential seeps to protect them although longer term reductions in rainfall cannot be ruled out as a factor deforestation and loss of fog drip recharge are obvious and more immediate reasons for a recent loss of perennial water in kahikinui maui

the storage of bed material in mountain stream channels as assessed using optically stimulated luminescence dating abstract a detailed understanding of channel forming and maintenance processes in mountain streams requires some measurement and or prediction of bed load transport and sediment mobility traditional field based measurements of such processes are problematic because of the high formative discharges characteristic of such streams the application of optically stimulated luminescence osl dating is proposed here as a new way of determining actual residency times of fine sediments and consequently validating selected predictions for the entrainment of sediment in these streams model predictions of sediment mobility for selected step pool and plane bed channels in a mountain catchment in south eastern australia are initially calculated using equations of hydraulic competence and the one dimensional hec ras model results indicate that floods exceeding bankfull with recurrence intervals up to 13 years are competent to mobilise the maximum overlying surface grain sizes at both sites osl minimum age model results from 7 samples of well bleached quartz in the fine matrix particles indicate general agreement with selected competence equations the apparent long 100 1400 y burial age of most of the mineral quartz however suggests that competent flows are not able to flush all subsurface fine bed material the depth of maximum bed load exchange flushing was limited to twice the depth of the overlying d 90 grain size application of osl in this study provides important insight into the nature of storage and flushing of matrix material in mountain streams

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

flooding and geomorphic impacts in a mountain torrent raise beck central lake district england raise beck is a mountain torrent located in the central lake district fells northern england drainage area of 1 27 km2 the torrent shows evidence of several major flood events the most recent of which was in january 1995 this event caused a major channel avulsion at the fan apex diverting the main flood flow to the south blocking the a591 trunk road and causing local flooding the meteorological conditions associated with this event are described using local rainfall records and climatic data records show 164 mm of rainfall in the 24 hours preceding the flood the peak flood discharge is reconstructed using palaeohydrological and rainfall runoff methods which provide discharge values of 27 74 m3 s 1 and 4 6 m3 s 1 respectively the flood transported boulders with b axes up to 1400 mm these results raise some important general questions about flood estimation in steep mountain catchments the geomorphological impact of the event is evaluated by comparing aerial photographs from before and after the flood along with direct field observations over the historical timescale the impact and occurrence of flooding is investigated using lichenometry long term rainfall data and documentary records two major historical floods events are identified in the middle of the nineteenth century the deposits of the recent and historical flood events dominate the sedimentological evidence of flooding at raise beck therefore the catchment is sensitive to high magnitude low frequency events following the 1995 flood much of the lower catchment was channelized using rip rap bank protection re establishing flow north towards thirlmere the likely success of this management strategy in containing future floods is considered based on an analysis of channel capacities it is concluded that the channelization scheme is only a short term solution which would fail to contain the discharge of an event equivalent to the january 1995 flood copyright 2002 john wiley sons ltd

critical specific stream power in gravel bed rivers experiments with marked pebbles were carried out on different sized rivers of the belgian ardenne catchment areas varying from less than 1 km2 to 2700 km2 specific stream power required to cause bedload movement was evaluated and critical values were obtained three types of relationship between critical specific stream power 0 and grain size d were established the values for 0 in the largest river the ourthe were the lowest and were close to the values obtained for mountainous rivers carrying large boulders in medium sized rivers catchment area between 40 and 500 km2 the critical unit stream power was higher it is likely that it is due to the bedform s greater resistance this resistance would use up some of the energy that can cause movement and transport of bedload the amount of resistance of the bedform can be expressed as bedform shear stress determined by the relationship between grain shear stress that determines movement and transport of the bedload and the total shear stress this ratio varies between 0 4 and 0 5 in the medium sized rivers compared to 0 7 in the ourthe in headwater streams less than 20 km2 there is greater loss of energy due to bedform resistance 0 3 critical specific stream power is higher in this third type of river than in the other two

the kinematic evolution of the nepalese himalaya interpreted from nd isotopes neodymium nd isotopes from the himalayan fold thrust belt and its associated foreland basin deposits are useful for distinguishing between himalayan tectonostratigraphic zones and revealing the erosional unroofing history as controlled by the kinematic development of the orogen neodymium isotopic data from the himalayan fold thrust belt in nepal n 35 reveal that the lesser himalayan zone consistently has a more negative nd 0 value than the greater and tibetan himalayan zones our data show the average nd 0 value in the lesser himalayan zone is 21 5 whereas the greater and tibetan himalayan zones have an average nd 0 value of 16 these consistently distinct values throughout nepal enable the use of nd isotopes as a technique for distinguishing between lesser himalayan zone and greater himalayan zone rock the less negative nd 0 values of the greater himalayan rocks support the idea that the greater himalayan zone is not indian basement but rather a terrane that accreted onto india during early paleozoic time neodymium isotopic data from eocene through pliocene foreland basin deposits n 34 show that sediment provenance has been dominated by greater and tibetan himalayan detritus across nepal the nd t values in the synorogenic rocks in western and central nepal generally show an up section shift toward more negative values and record the progressive unroofing of the different tectonostratigraphic zones at 10 ma in khutia khola and 11 ma in surai khola a shift in nd t values from 16 to 18 marks the erosional breaching of a large duplex in the northern part of the lesser himalayan zone this shift is not seen in eastern nepal where the nd t values remain close to 16 throughout miocene time because there has been less erosional unroofing in this region

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

depositional character and preservation potential of coarse grained sediments deposited by flood events in hyper arid braided channels in the rift valley arava israel abstract four coarse grained braided channels undergoing rapid deposition in the hyper arid arava rift valley have been studied to determine the dynamics of strata deposition by monitored flood events and to ascertain the sedimentologic character of these event deposits the systems span granule sand to sandy gravel bed material having longitudinal bars with simultaneously increasing grains size slope and bed relief scour and fill were monitored along cross sections using scour chains and repeat surveys while event strata fill were described with reference to sediment calibre type of support grading thickness and location unlike many previous investigations of gravel sedimentology the source of information for this study involves not only stratigraphic description but also observation of the concomitant dynamics of the channel bed as expressed by the extent of event deposition the texture of the deposited sediment generally coarsens in sympathy with the surface bed texture of the studied channels the thickness of event deposited sediments increases as the magnitude of flow events increases the stratigraphic record is characterized by 1 sub parallel stratification 2 no grading 3 clast supported gravel infilled with sand granule matrices and 4 rapid sedimentation rates this character is hypothesized to mirror the elevated bedload fluxes in this hyper arid fluvial setting lacking densely vegetated banks the proximal braidplain of this arid environment often contains no extensive fine textured floodplain successions the depth of fill as well as the extent of strata obliteration increase with flood magnitude explaining the increased preservation potential of event strata with increase in flood magnitude it also explains why most event strata including large ones are only partially preserved

recent large magnitude floods and their impact on valley floor environments of northeastern yellowstone abstract the lamar river watershed of northeastern yellowstone contains some of the most diverse and important habitat in the national park broad glacial valley floors feature grassland winter range for ungulates riparian vegetation that provides food and cover for a variety of species and alluvial channels that are requisite habitat for native fish rapid neogene uplift and quaternary climatic change have created a dynamic modern environment in which catastrophic processes exert a major influence on riverine riparian ecosystems uplift and glacial erosion have generated high local relief and extensive cliffs of friable volcaniclastic bedrock as a result steep tributary basins produce voluminous runoff and sediment during intense precipitation and rapid snowmelt recent major floods on trunk streams deposited extensive overbank gravels that replaced loamy soils on flood plains and allowed conifers to colonize valley floor meadows tree ring dating identifies major floods in 1918 ca 1873 and possibly ca 1790 in 1996 and 1997 discharge during snowmelt runoff on soda butte creek approached the 100 year flood estimated by regional techniques with substantial local bank erosion and channel widening indirect estimates show that peak discharges in 1918 were approximately three times greater than in 1996 with similar duration and much greater flood plain impact nonetheless 1918 peak discharge reconstructions fall well within the range of maximum recorded discharges in relation to basin area in the upper yellowstone region the 1873 and 1918 floods produced lasting impacts on the channel form and flood plain of soda butte creek channels may still be locally enlarged from flood erosion and net downcutting has occurred in some reaches leaving the pre 1790 flood plain abandoned as a terrace gravelly overbank deposits raise flood plain surfaces above levels of frequent inundation and are well drained therefore flood plain soils are drier noncohesive gravels also reduce bank stability and may have persistent effects on channel form overall floods are part of a suite of catastrophic geomorphic processes that exert a very strong influence on landscape patterns and valley floor ecosystems in northeastern yellowstone

estimating the sliding velocity of a pleistocene ice sheet from plowing structures in the geologic record 1 as an ice sheet slides over its sediment bed some clasts partly embedded in the glacier sole plow through the bed surface the size distribution of such clasts if it can be characterized from structures in the geologic record can be used to estimate the sliding velocity of a past ice sheet by combining a theory of glacier sliding with a geotechnical theory of cone penetration sliding velocity can be calculated in terms of clast size parameters a fluidity parameter for ice and the thermodynamic properties of ice and clasts if frictional properties of the bed are measured the effective normal stress on the bed and bed shear strength during glaciation can also be calculated we used this approach to estimate the sliding velocity of an illinoian ice sheet that left plowing structures in cemented outwash near peoria illinois fluidity parameters for normal and basal temperate ice yielded sliding velocities of 140 168 m yr 1 and 60 72 m yr 1 respectively these are overestimates if solutes impeded regelation of ice past clasts or if friction between debris laden ice and clasts retarded slip preconsolidation stresses determined in laboratory tests on silt from the bed agree with effective normal stresses calculated using clast size parameters the high shear strength of the bed 145 kpa and primary structures preserved within it indicate that additional movement due to pervasive shear of the bed was unlikely application of this method elsewhere would provide basal velocity data that are otherwise unavailable for testing and tuning of ice sheet models

dam failure and a catastrophic flood in the mekong basin bolaven plateau southern laos 2018 abstract sudden breaching of a saddle dam on july 23 2018 on the perimeter of the xe nammoy hydroelectric power reservoir recently constructed in the mekong basin southern laos caused catastrophic flooding that resulted in fatalities and displaced thousands of individuals this study aims to 1 assess the cause of the dam breach and 2 understand the hydrologic and geomorphic impact of the flood our analysis shows that the collapse of the dam occurred as the reservoir was nearly full and the water level was rising against the inner slope of the earthen dam as the water discharged through the breach we calculate that the reservoir fell 22 m and lost 350 million m3 more than a third of the total stored volume on july 23 inspection of publicly available photographs of the failed dam confirms our interpretation that the failure was structural involving both piping and rotational slumping and not due to overtopping of the dam failure to properly process the local materials before construction of the earth core of the dam is the likely cause of failure our analysis of satellite imagery bolstered by field observations reveals that the flooding inundated an area of 46 km2 of villages and rural land in the vang ngao river a tributary of the mekong river basin by using 1d and 2d hydrological modeling we calculate a peak flood discharge of 8500 m3 s about half the mean discharge of the mississippi river the dam failure and concomitant losses are consistent with common inadequacies in this region in assessing potential hazards challenges of geomorphological geotechnical engineering when using tropical materials for construction and emergency planning and warning thus this analysis has important implications for the safer design and construction of the myriad of other dams that are either operating or planned within the multi national mekong river basin

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

the barranco de aras flood of 7 august 1996 biescas central pyrenees spain abstract geomorphic effects observed in the barranco creek de aras basin are used to characterize the flood sediment features allow to qualify the flood as essentially a water flow using the critical section method the peak flood discharge is estimated to be between 400 and 600 m 3 s 1 similar results were obtained using a paleohydraulic formula based on the size of the largest mobilized clasts using the rational method with available rainfall data the discharge for a recurrence interval of 500 years is estimated to be between 150 and 200 m 3 s 1 these results agree with predictions obtained using curves of peak discharge versus basin area based on regional data several trenches dug on the fan showed that the size of boulders mobilized by the event is larger that those left by previous floods at the same place when the estimated peak flood discharge is related to the basin area values between 20 and 30 m 3 s 1 km 2 are obtained demonstrating that the barranco de aras flood was most unusual

unraveling bed slope from relative roughness in initial sediment motion understanding incipient sediment transport is crucial for predicting landscape evolution mitigating flood hazards and restoring riverine habitats observations show that the critical shields stress increases with increasing channel bed slope and proposed explanations for this counterintuitive finding include enhanced form drag from bed forms particle interlocking across the channel width and large bed sediment relative to flow depth relative roughness here we use scaled flume experiments with variable channel widths bed slopes and particle densities to separate these effects which otherwise covary in natural streams the critical shields stress increased with bed slope for both natural gravel s 2 65 g cm 3 and acrylic particles s 1 15 g cm 3 and adjusting channel width had no significant effect however the lighter acrylic particles required a threefold higher critical shields stress for mobilization relative to the natural gravel at a fixed slope which is unexpected because particle density is accounted for directly in the definition of shields stress a comparison with model predictions indicates that changes in local velocity and turbulence associated with increasing relative roughness for lighter materials are responsible for increasing the critical shields stress in our experiments these changes lead to concurrent changes in the hydraulic resistance and a nearly constant critical stream power value at initial motion increased relative roughness can explain much of the observed heightened critical shields stresses and reduced sediment transport rates in steep channels and also may bias paleohydraulic reconstructions in environments with exotic submerged densities such as iron ore pumice or ice clasts on titan

catastrophic flooding from glacial lake wisconsin abstract glacial lake wisconsin was a large proglacial lake that formed along the southern margin of the laurentide ice sheet during the wisconsin glaciation it was formed when ice of the green bay lobe came into contact with the baraboo hills in southwestern wisconsin and blocked the south flowing wisconsin river during early glacial recession the ice dam failed catastrophically and the lake drained in about a week despite early recognition of the former lake and the likelihood that it failed catastrophically outflow rates during the failure have not been previously evaluated estimates based on step backwater modeling indicate that peak discharge was between 3 6 and 5 3 10 4 m 3 s in the lower wisconsin river as an alternate method we used a previously derived empirical relationship between lake volume and peak discharge for dam break events from a digital elevation model altered to incorporate isostatic depression we estimated the lake volume to be 87 km 3 just prior to dam breach suggesting that the flooding magnitude was as high as 1 5 10 5 m 3 s at the outlet adjusting these results for downstream flood wave attenuation gives a discharge of around 4 4 10 4 m 3 s in the lower reach which closely matches the results of the step backwater modeling these estimates of discharge from the catastrophic failure of ice marginal lakes improve our understanding of the processes that have produced the morphology and behavior of present day upper midwest river systems

discussion bulk sampling of coarse clastic sediments for particle size analysis gale and hoare 1992 have provided a figure relating maximum particle diameter in a coarse clastic sediment to the size of sample required to generate a reproducible measure of the particle size distribution of that sediment however they fail adequately to justify the basis for the criterion of sample adequacy adopted in their proposal additionally there is a range of separate issues neglected by gale and hoare 1992 which makes their conclusion inapplicable in many situations a fundamental issue that they neglect is the purpose lying behind a grain size analysis this issue determines what grain size measures are appropriate rather than the empirical seemingly purpose independent approach of gale and hoare 1992

particle comminution defines megaflood and superflood energetics abstract the concept of megafloods and superfloods was introduced at the end of the last century to define exceptionally large discharge floods primarily those associated with the failure of quaternary ice dams these floods exceeded by one or two magnitudes historically recorded floods megafloods having peak discharges equalling or exceeding 1 m m3 s 1 herein the sediment associations of exceptional floods are found to be distinctive being dominated by comminuted smashed grain size distributions which contrast to the sediment deposits of more moderate floods a consideration of the energetics of moderate floods superfloods and megafloods with respect to the entrainment dis entrainment suspension and comminution of coarse gravel allows thresholds of energy expenditure to be proposed that reflect general transformations in flood behaviour that relate to distinctive flood deposits a threshold of energy expenditure 20 kj s 1 m 2 is proposed to separate moderate floods from superfloods the latter by this definition can be 0 1 m m3 s 1 this threshold separates floods competent to entrain cobbles and boulders with limited comminution of the load from superfloods that comminute the suspended cobbles and boulders to produce fine granules in suspension the granules are subsequently deposited to form distinct depositional units that are characteristic of superfloods and megafloods the energetic threshold is consistent with the lower range of power expenditures associated with the original definition of a megaflood in terms of peak fluid flux significance exceptional floods occurred in prehistoric times the largest are referred to as megafloods or superfloods to distinguish such exceptional floods from other large floods megafloods were defined as having a peak discharge exceeding 1 m m3 s 1 however many reports of megafloods are flows with a peak discharge smaller than 1 m m3 s 1 recent research has shown that megafloods and superfloods deposit distinctive layers of smashed gravel which is not the case for smaller floods the gravel is smashed by exceptionally high energy expenditures consideration of the energetics of floods to transport gravel allows a robust quantitative definition of superflood based on the energy needed to smash a range of gravel sizes

bedforms and sediment transport processes in the ephemeral streams of kobo basin northern ethiopia abstract bedforms and channel deposits architecture were studied in the ephemeral streams of the structural basin of kobo in northern ethiopia the froude numbers calculated at bankfull flow for a few of the study reaches are in agreement with the extensive occurrence of horizontal planar lamination small bedforms not previously reported in the literature are found and described as produced mainly by scouring rather than accretion processes equations to predict particle entrainment proved that large individual boulders standing scattered on the stream bed can be transported at bankfull flow for long distances direct inspection of the old alluvium and the modern channel deposits allow to identify a typical arrangement within a single bed consisting of divisions with specific characteristics this model of division association is explained in terms of vertical distribution of shear stress and compared with hyperconcentrated flow and traction carpet models in contrast with recent theories of flat asymmetrical bed waves progradation at froude numbers close to unity

evidence for the submarine weathering of silicate minerals in black sea sediments possible implications for the marine li and b cycles 1 the role of sediment diagenesis in the marine cycles of li and b is poorly understood because li and b are easily mobilized during burial and are consumed in authigenic clay mineral formation their abundance in marine pore waters varies considerably exchange with the overlying ocean through diffusive fluxes should thus be common nevertheless only a minor li sink associated with the low temperature alteration of volcanic ash has been observed we describe a low temperature diagenetic environment in the black sea dominated by the alteration of detrital plagioclase feldspars fluids expelled from the odessa mud volcano in the sorokin trough originate from shallow 100 400 m deep sediments which are poor in volcanic materials but rich in anorthite these fluids are depleted in na k li b and 18o and enriched in ca2 and sr2 indicating that anorthite is dissolving and authigenic clays are forming using a simple chemical model we calculate the ph and the partial pressure of co2 pco2 in fluids associated with this alteration process our results show that the ph of these fluids is up to 1 5 ph units lower than in most deep marine sediments and that pco2 levels are up to several hundred times higher than in the atmosphere these conditions are similar to those which favor the weathering of silicate minerals in subaerial soil environments we propose that in black sea sediments enhanced organic matter preservation favors co2 production through methanogenesis and results in a low pore water ph compared to most deep sea sediments as a result silicate mineral weathering which is a sluggish process in most marine diagenetic environments proceeds rapidly in black sea sediments there is a potential for organic matter rich continental shelf environments to host this type of diagenesis should such environments be widespread this new li and b sink could help balance the marine li and li isotope budgets but would imply an apparent imbalance in the b cycle

sedimentary record of a late saalian jokulhlaup case study in siedlątkow outcrop central poland abstract the aim of the research conducted in an outcrop of late saalian sediments in siedl tkow central poland was to reconstruct extreme meltwater flows during a jokulhlaup on the basis of the geological record a combination of field and laboratory lithological methods was used which included lithofacies analysis analyses of grain size distribution shape and roundness of gravelly clasts examination of sediments made it possible to recognise palaeohydrological sedimentation and deformation processes during and immediately after the flood in particular the simultaneous operation of water flows in four paths in the immediate foreland of the ice front was reconstructed mechanisms of transport and sedimentation of deposits were determined characteristic features of the analysed jokulhlaup deposits include erosional and deformational contact with the substrate a high content of very coarse sediments poorly sorted with predominant massive clast supported gravels and boulders the presence of rip up clasts inverse grading in the lower part of the profile normal grading in the upper part and similarities in clast shape and roundness and the ratio of local rocks to northern origin rocks when compared to the surrounding saalian till the documented features allow for the following phases of jokulhlaup to be distinguished initial rising high stage and waning stage during the reconstructed event hyperconcentrated flow was dominant whereas fluvial flow and debris flow were of lesser importance the release of meltwater in the marginal part of the ice sheet came from the depression of the kolo basin to the north of the analysed sites complex deformation processes which occurred in the flood deposits and their substratum were also interpreted the analysed jokulhlaup event resembles some minor glacial floods in the ice contact zone of present day icelandic glaciers further research in marginal zones of pleistocene ice sheets should account for their occurrence as the analysed example indicates that jokulhlaups might have had considerable influence on the course of sedimentation processes in the marginal zones of ice lobes of pleistocene ice sheets as well as in the marginal valleys and sandars

slope failure and denudation in the western himalaya slope failures glaciers and rivers constitute the three main agents of denudation in the himalaya failure of slopes is strongly controlled by bedrock geology especially at plate terrane boundaries although climatic and seismic controls of failure also occur slope failure complexes in the western himalaya studied for this report include the pakhar kaghan nanga parbat and atabad hunza areas with 23 different slope failures reported within all the complexes quantification of denudation by mass movement requires measurement of volumes and determination of timing of sediment emplacement and removal where possible to obtain better data on recurrence intervals and total geomorphic work performed in formation of the landscape the relation of mass movement to overall denudation by glaciers and rivers and its role in catastrophic floods and debris entrainment is assessed

flood channel morphology of a quiet river the lachlan downstream from cowra southeastern australia abstract detailed geomorphic mapping and sediment analyses of floodplain features were undertaken on a 30 km reach of the lachlan river in southeastern australia the reach is located in the transitional zone between the southern great dividing range and extensive alluvial plains to the west the channel resembles other fine grained low energy alluvial channels it is muddy narrow and deep and meanders irregularly it contains fine grained accreting bench deposits and in places is influenced by inherited fluvial forms both of which indicate long term channel stability by contrast the floodplain exhibits features that have been attributed elsewhere to catastrophic flood flows including elliptical scour scars swirl pits cobble and gravel deposits large chutes and chute bars zones of floodplain stripping flood levees and multiple flood channels sorting and texture of floodplain sediments generally increases with height above the bed although deposits are variable and poorly sorted indications of recent fluvial activity at several levels above the bed are confirmed by aerial photographs dating from 1954 although changes to the channel position have been small the alluvial facies model displays a compound floodplain in which a stable low energy channel is inset within a high energy flood dominated floodplain this interpretation is consistent with streamflow and documentary evidence of recurrent large floods since 1839 which have a return period of

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

hydraulics and basin morphometry of the largest flash floods in the conterminous united states abstract the maximum rainfall runoff floods measured by indirect methods in small basins 0 39 370 km2 in the conterminous united states are examined this analysis identified twelve floods that were the largest ever measured these floods all occurred in semiarid to arid areas for eleven of the twelve largest rainfall runoff floods measured in small basins by the slope area method values of hydraulic depth hydraulic radius width depth ratio n value mean velocity velocity head coefficient froude number water surface energy and channel slopes shear stress and unit stream power are tabulated estimated composite n values weighted by subsection conveyance range from 0 028 to 0 048 with a mean of 0 038 mean velocities ranged from 3 47 to 9 97 m s 1 froude numbers ranged from 0 81 to 2 49 with 9 of 12 floods having values greater than 1 00 water surface energy and channel slopes vary considerably for each flood energy slope always was less than water surface slope by values of 1 104 channel slope was greater than energy slope in eight floods shear stresses ranged between 61 and 855 n m 2 and unit stream power from 212 to 8131 w m 1 floods in these small basins produced shear stresses and unit stream powers several hundred times greater than floods in large rivers floods on other small streams with smaller unit discharges produced greater shear stresses and stream powers this indicates that the force of a flood is controlled by the depth slope product not absolute discharge in the twelve watersheds studied basin relief ranged from 165 to 1280 m elongation ratios ranged from 0 55 to 0 80 the number of first order streams basin magnitude ranged from 10 to 4297 drainage density ranged from 4 1 to 10 9 km km 2 basin slope ranged from 0 0043 to 0 2486 relief ratio ranged from 0 0097 to 0 34 ruggedness number ranged from 0 69 to 7 17 and first order channel frequency ranged from 5 1 to 38 6 km 2 elongation ratios were larger and drainage density and first order channel frequency lower than other small flash flood prone basins in the united states these twelve basins have neither the most favorable morphometric characteristics which contribute to flash flood peaks nor did the storms causing these flash floods have the greatest short duration intensities maximum flood peaks originate from an optimal combination of basin morphology and physiography and storm intensity

formation of paleovalleys in the central himalaya during valley aggradation abstract the formation of paleovalleys in mountainous regions is considered to result from extreme events such as landslides and glacial or landslide lake outburst floods according to ipcc 2012 the extreme events are rarest of the rare weather climatic events when the climate weather variable is significantly above or below the defined threshold value the present study suggests that paleovalleys can also form during years long periods of valley aggradation a series of paleovalleys thus formed runs parallel to the present river course in this study we suggest that paleovalleys in the alaknanda valley of the central himalaya have formed in two ways 1 major valley aggradation and 2 local events of landsliding and lake breaching most of the paleovalleys in the alaknanda valley formed during a major valley aggradation phase between 15 and 8 ka paleovalleys formed due to local landsliding also formed around 8 ka local landslides were triggered due to high rainfall in lower valley reaches during unstable climatic conditions therefore the formation of paleovalleys both by regional and local mode within 15 8 ka indicates that the valley was receiving excess sediment from upper catchment as well as from lower reaches during this period this phase of excess sediment supply and valley aggradation coincides well with post glacial climatic amelioration therefore the study underlines the role of climate in the time scales of 10 3 years in shaping the landscape of an active mountain like the himalaya the role of other landscape changing agencies such as tectonics and erosion is not accounted in the present study