a multidating approach applied to historical slackwater flood deposits of the gardon river se france article i nfo a multidating approach was carried out on slackwater flood deposits preserved in valley side rock cave and terrace of the gardon river in languedoc southeast france lead 210 caesium 137 and geochemical analysis of mining contaminated slackwater flood sediments have been used to reconstruct the history of these flood deposits these age controls were combined with the continuous record of gardon flow since 1890 and the combined records were then used to assign ages to slackwater deposits the stratigraphic records of terrace ge

effect of ocean oxidation on the chemical structure of marine kerogen abstract the burial of organic matter om in marine sediments is an important control on global climate and petroleum formation on geological timescales while the oxygen exposure time of sediment has been shown to affect the efficiency of organic carbon burial and influence om properties in recent sediments the implications for the chemical composition of kerogen have not been tested this study tests the hypothesis that oceanic redox conditions are an important control on the molecular structure and composition of kerogen samples from a number of outcropping late cretaceous mudstones of the eagle ford formation were chosen spanning a gradient of paleo redox depositional conditions oxic sub oxic to anoxic and the bulk molecular composition of the kerogen was determined using solid state 13 c nuclear magnetic resonance nmr spectroscopy the nmr spectral properties of kerogen covaried with redox sensitive trace metal abundance whereas the thermal maturity and om source biomarkers were relatively invariant anoxic environments favor the preservation of alkyl c moieties while kerogen formed in oxic depositional environments was enriched in aromatic c this study is the first to demonstrate that diagenetic consequences of bottom water redox conditions are retained in the molecular structure of the kerogen macromolecule on geological timescales

beyond the gorge palaeoflood reconstruction from slackwater deposits in a range of physiographic settings in subtropical australia the application of palaeoflood hydrology in australia has been limited since its initial introduction 30 years ago this study adopts a regional field based approach to sampling slackwater deposits in a subtropical setting in southeast queensland beyond the traditional arid setting we explore the potential and challenges of using sites outside the traditional physiographical setting of bedrock gorges over 30 flood units were identified across different physiographical settings using a range of criteria evidence of charcoal rich layers and palaeosol development assisted in the identification and separation of distinct flood units the osl dated flood units are relatively young with two thirds of the samples being 80 of them classified as extreme event this study opens up the renewed possibility of applying palaeoflood hydrology to more populated parts of australia where the need for improved estimation of flood frequency and magnitude is now urgent in light of several extreme flood events preliminary contributions to improve the understanding between high magnitude floods and regional climatic drivers are also discussed recognised regional extreme floods generally coincide with la nina and negative ipo phases while tropical cyclones appear to be a key weather system in generating such large floods

stratigraphic architecture and alluvial geoarchaeology of an ephemeral fluvial infilling climatic versus anthropogenic factors controlling the holocene fluvial evolution in southeastern spain drylands abstract two stages of depositional high discharge fluvial environments are recorded in the infilling of a rock shelter in a canyon of the segura river namely 1 first stage aggradational alluvial gravels in the lower part interpreted as braided channel bars seventh millennium to 2030 cal bc and 2 second stage slackwater flood deposits in the upper part between 2030 cal bc and after 900 ad during a stage of main channel incision reconstructed flood discharges for the second stage were a minimum of 700 m 3 s extreme palaeofloods that inundated the cave from the canyon inlet were successively higher and are correlated to quick rising flash floods the characteristics of cultural remains preserved by flood deposits reveal long periods of human cave occupation and passage through the canyon without extreme floods occurring these long periods of low discharge fluvial flow punctuated by catastrophic flash floods are linked to a torrential fluvial regime the torrential regime is the current hydrologic and climatic scenario in the streams of semi arid southeastern spain it is discusses the climatic versus anthropogenic factors affecting the fluvial development we conclude that the archaeo fluvial record of the pozo rock shelter indicates that the present day semi arid conditions of the southeastern spain drylands extend back to circa 4000 yr bp methodologically the combination of fluvial palaeohydrology and geoarchaeology appears to be a robust tool to reconstruct long term holocene climate changes

a 2000 year natural record of magnitudes and frequencies for the largest upper colorado river floods near moab utah using well established procedures for paleoflood hydrology and employing optically stimulated luminescence osl geochronology we analyzed a very well preserved natural record of 44 upper colorado river extreme floods with discharges ranging from 1800 to 9200 m3s 1 these are the largest floods occurring during the last 2140 220 years and this natural record indicates that large floods are much more frequent than can be estimated by extrapolation from the stream gaging record that extends back to 1914 most of these large floods occurred during the last 500 years and the two largest floods in the record both exceeded the probable maximum flood pmf estimated at 8500 m3s 1 300 000 cfs for nearby moab utah another four floods with discharges greater than 7000 m3s 1 occurred during the last two millennia flood frequency analyses using the fldfrq3 model yields the following values depending on the manning n roughness coefficients 100 yr flood 4670 4990 m3s 1 500 yr flood 6675 7270 m3s 1 1000 yr flood 7680 8440 m3s 1 the presumed pmf discharge 8500 m3s 1 gets assigned a recurrence interval of about 1000 years and the largest historical 1884 flood 3540 m3s 1 a recurrence interval of 100 years flood frequency analysis for the moab valley based on the gaged record 1914 2012 yield 2730 m3s 1 for the 100 yr flood and 3185 m3s 1 for the 500 yr flood this underestimation of the frequency of large floods from the gage data results from effects on that record by modern regulation of upstream river flow and associated water extraction for agriculture

assessing grain size correspondence between flow and deposits of controlled floods in the colorado river u s a abstract flood deposited sediment has been used to decipher environmental parameters such as variability in watershed sediment supply paleoflood hydrology and channel morphology it is not well known however how accurately the deposits reflect sedimentary processes within the flow and hence what sampling intensity is needed to decipher records of recent or long past conditions we examine these problems using deposits from dam regulated floods in the colorado river corridor through marble canyon grand canyon arizona u s a in which steady peaked floods represent a simple end member case for these simple floods most deposits show inverse grading that reflects coarsening suspended sediment a result of fine sediment supply limitation but there is enough eddy scale variability that some profiles show normal grading that did not reflect grain size evolution in the flow as a whole to infer systemwide grain size evolution in modern or ancient depositional systems requires sampling enough deposit profiles that the standard error of the mean of grain size change measurements becomes small relative to the magnitude of observed changes for simple steady peaked floods 5 10 profiles or fewer may suffice to characterize grain size trends robustly but many more samples may be needed from deposits with greater variability in their grain size evolution

optically stimulated luminescence dating of holocene palaeoflood deposits in the middle reach of the yongding river china abstract palaeoflood deposits record extreme climatic and environmental events while historic data show that floods occurred frequently in the yongding river in china and have caused many natural disasters the sedimentologic records of palaeoflood in the yongding river have not been well studied this study investigates the palaeoflood deposits at the tanmugou site in the middle reach of the yongding river six episodes of palaeoflood were identified in the sediment profile optically stimulated luminescence osl dating method was applied to establish the chronology of the palaeoflood deposits both the improved single aliquot regenerative dose sar protocol and standard growth curve sgc method were employed for equivalent dose d e determinations the minimum age model mam central age model cam and arithmetic mean were applied to calculate the d e values the osl dating results show that these palaeoflood events occurred in the holocene and concentrated during three stages including 8 5 7 3 ka 2 8 2 4 ka and 1 0 5 ka the timings of these palaeoflood events in the yongding river coincide with the timings of palaeoflood events in other river systems in china compared with the continuously high resolution climate record in china these palaeoflood events occurred during climate transition periods

hydrodynamics and sedimentology of concave benches in a lowland river abstract this paper describes the hydrodynamics and sedimentation associated with concave bank benches concave benches which are the fine grained sediment deposits considered geomorphically and ecologically important in meandering rivers most notions previously associated with concave bench formation ignore hydrodynamic complexities and consequently fail to recognise the importance of bankfull flows being commonly based on inference from stratigraphic profiling or casual observation of hydrodynamics concave benches are a depositional anomaly since their location in the channel is commonly associated with scour mature concave benches are formed by separation of the reverse flow upriver flow from the stem flow downriver flow in an abrupt bend whereby deposition is dominated by vertical accretion of suspended sediments for this study two dimensional hydrodynamic modelling in conjunction with the collection and analysis of contemporary sediments is used to demonstrate the persistence of reverse flow over concave benches for stages from incipient bench inundation to bankfull and greater for two mature concave benches on the ovens river southeast australia bankfull flows were depositional with shear stresses commonly

the holocene fluvial chronology of spain evidence from a newly compiled radiocarbon database abstract a critical analysis of 74 radiocarbon dates selected from a total of 102 published and unpublished dates from holocene fluvial environments in spain has identified a number of periods of increased fluvial activity in spain namely 11 170 10 230 9630 8785 7980 6860 5800 4800 3880 3085 2895 1820 1300 0 cal bp the latter period reflecting sustained fluvial response to increased human impact the radiocarbon samples were classified according to type of depositional environment a alluvial overbank b flood basin c alluvial channel gravels d fluvio torrential deposits and e slackwater flood deposits palaeofloods dates from slackwater flood deposits indicated at least five phases of increased frequency of large magnitude floods during the holocene 10 855 10 230 9530 8780 2880 2430 975 790 and 520 265 cal yr bp flood basin type deposition occurred from 7980 to 4830 cal yr bp the main clusters of dates from alluvial floodplain deposits occurred at 2750 2150 and 930 520 cal yr bp the record is discussed in relation to other palaeoenvironmental archives including pollen records of holocene vegetation change and palaeoclimatic proxies such as the north atlantic drift ice record comparison with the latter indicates that major holocene flooding coincided with cold climate phases during 9530 9280 2880 2430 and 520 265 cal yr bp a cooling phase at 10 855 10 230 and warming phases at 9030 8780 and 975 790 cal yr bp

palaeoflood record of the tagus river central spain during the late pleistocene and holocene abstract palaeoflood hydrology of the tagus river central spain was reconstructed from slackwater deposits and palaeostage indicators in two canyon reaches located at el puente del arzobispo in the central part of the catchment 35 000 km 2 in drainage area and in alcantara at the lower part of the catchment 52 000 km 2 in drainage area near the portuguese border the palaeoflood record with more than 80 flood events shows clusters of floods at specific periods from 9440 to 9210 14 c yr bp 8540 8110 bc 8500 to 8000 14 c yr bp 7500 7000 bc 6750 14 c yr bp 5000 bc 1200 to 950 14 c yr bp ad 785 1205 410 14 c yr bp ad 1450 1500 170 to 80 14 c yr bp ad 1670 1950 the largest flood s occurred during the periods 9440 to 9210 14 c yr bp 6750 14 c yr bp and 1200 to 950 14 c yr bp reaching minimum discharge estimates of between 4000 and 4100 m 3 s 1 in the el puente del arzobispo reach and 13 700 15 000 m 3 s 1 in the alcantara reach these periods with increased flood magnitude and or frequency in the tagus river are strongly related to increased moisture influx and winter precipitation in the iberian peninsula especially in the western zone proxy records sensitive to winter precipitation such as lake levels and vegetation changes indicated by pollen records are in good agreement with the clusters of floods found in the tagus river this flood variability seems to correspond to changes in the prevailing atmospheric circulation pattern affecting the iberian peninsula

analyses of the magnitude and frequency of a 400 year flood record in the fish river basin namibia abstract ephemeral rivers in dryland regions exhibit a high interannual variability of streamflow regime mainly dominated by floods in these environments floods are a water resource and a potential hazard with important socioeconomic implications the fish river 86 600 km2 is the largest ephemeral stream in namibia and recently also the focus of new development plans including construction of the largest dam in namibia the hydrological analysis to support decisions implies large uncertainties owing to spatial limited and short hydrological records since 1962 here we investigate the current and past patterns of extreme floods combining instrumental historical and palaeoflood records palaeoflood studies were performed at two reaches with preserved sedimentary evidence at the upper sector vogelkranz upstream of hardap dam 13 catchment area and in the lower part of the river in the fish river canyon national park 70 catchment area in the hardap reach the palaeoflood record identified at least eight large floods during the last 350 years with the largest flood reaching a minimum discharge of 4800 m3 s 1 150 year return period in the fish river canyon reach the sedimentary record shows at least 12 large floods over the last 400 years the largest with an estimated minimum discharge of 8700 m3 s 1 the elevation of alluvial surfaces without flood evidence provided an upper bound for flood stages associated with discharges of 6400 m3 s 1 and 16 140 m3 s 1 for the upper and mid lower fish river respectively in the upper reach the flood frequency analysis ffa combining systematic and palaeoflood data provided lower discharges 25 of the flood quantiles than the ones using only systematic data sets in the fish river canyon reach incorporation of the palaeoflood data into the ffa results in slightly higher values in the magnitude of the higher flood quantiles 4 7 the ffa analysis using an upper limited lognormal distribution function ln4 fitted with palaeoflood data shows a good performance with a slow behaviour approaching the upper limit our flood frequency results suggest that the hardap dam should increase the spillway capacity and safety check flood of the original design in order to satisfy the dam safety criteria however the projected reevaluation figures calculated from conventional hydrological methods results in an overestimation of the safety floods according to our estimations

unsteady two dimensional paleohydraulic reconstruction of extreme floods over the last 4000 yr in segura river southeast spain summary floods in the almadenes canyon segura river southeast spain have been reconstructed using unsteady two dimensional shallow water modelling slackwater deposits archaeological age dating and hydraulic modelling allowed us to reconstruct some of the oldest extreme floods reported in spain the paleoreconstruction was performed for the first time in three steps retrodicted flood discharges for the most recent flood ad 900 were quantified comparing the simulated shields and froude numbers with respect to characteristic values for the existence of ripples and upper stage plane bed at a cave that served as a shelter for humans ensuing velocity shields and froude maps were then used as a forward predictor allowing the finding of additional paleostage indicators far from the shelter finally new psis were employed to verify the reliability of the reconstruction in doing so better insight into the paleoflood landscape was derived from consideration of the association of bedforms and modelled hydraulic conditions the present study assuming a stable geometry of the bedrock canyon during the late holocene extends the estimation of flow discharge magnitudes to ca 4000 yr bp ranging from 500 m 3 s in the oldest floods 3700 yr bp up to 700 1000 m 3 s in the most recent ad 900

reconstruction palaeoflood hydrology using slackwater flow depth method in the yanhe river valley middle yellow river basin china abstract water depth above the flood deposits should be taken into account in calculations of the palaeoflood peak stages which can provide more accurately estimate of palaeoflood stage here we present a new method slackwater flow depth to assess palaeoflood peak stage and to reduce the underestimation of palaeoflood stage palaeoflood slackwater deposits swds were identified by palaeohydrological criteria in cliff riverbank on the yanhe river middle yellow river basin palaeoflood events recorded in four layers of swd were dated by optical stimulated luminescence to 9 5 8 5 ka the estimation of palaeoflood maximum stage was 778 3 m using the slackwater flow depth method and the palaeoflood peak discharge is 15 000 m 3 s using the step backwater method palaeoflood results greatly extend the current flood data series in the yanhe river basin the regional flood history including gauged flood historical and palaeoflood data was compiled and evaluated for the major tributaries of the middle yellow river the relationship between palaeoflood peak discharges and drainage areas in this region fit well with the global maximum curves the results of site specific and regional palaeoflood evaluations demonstrate that the approach estimates the true palaeoflood peak stage and discharges and improves the flood frequency analysis of extreme and rare floods for a particular basin meanwhile the advantages and uncertainties of this method need ongoing discussion in palaeoflood investigations

impact of river bed morphology on discharge and water levels simulated by a 1d saint venant hydraulic model at regional scale summary river cross sections data are required to represent channel geometry in hydrodynamic models in the absence of accurate data at regional scale simplified or parameterized cross sections are often used which might affect the performance of the hydrodynamic model in this study we assess the sensitivity of a 1d saint venant hydraulic model to different types of river morphological data the question is addressed using a 1d unsteady hydraulic model hec ras with lateral inflows provided by the hydro geo logical model eau dyssee to explore a wide spectrum of river geometry scenarios regarding river bed slopes and cross sectional shapes the target scale is the one of the seine river france as a gateway for larger and more complex regional hydro geo logical applications our case study covers a well described 89 km reach in a sub tributary of the seine river river morphology is described by high resolution cross sections and manning s roughness coefficient n is used for calibration against observed discharges and river stages in a mid reach control point with satisfactory performances over the 8 year simulation period the resulting model still simulates realistic discharge hydrographs when forced with degraded channel geometry data using either fewer cross sections or approximated ones in such cases however the hydraulic model does not always satisfactorily predict the associated water levels when compared to observations in certain geometry scenarios the rmse between simulated water levels using degraded geometry and observations may go up to 0 3 m the study confirms that the accuracy of predicted water levels and maximum water depths simulated by a saint venant model relies on an accurate representation of channel geometry and bed level slopes along the river reach from the various scenarios it appears that the longitudinal description of the bed level profiles has a larger impact on the simulation of water levels than the cross sectional shapes this offers interesting implications for flood forecast mapping applications and regional scale models that often use simplified river geometry and digital elevation models dems built by remote sensing technologies to simulate the water levels

bayesian age modelling applied to palaeoflood geochronologies and the investigation of holocene flood magnitude and frequency a new methodology based on bayesian age modelling is presented for the analysis of palaeoflood geochronologies and palaeodischarge data bayesian age models were developed using the oxcal radiocarbon calibration software for the geochronologies of three palaeoflood sites in spain namely the gaudalentin tagus and llobregat rivers in se central and ne spain respectively the age depth plots resulting from the applied sequence models enabled the construction of flood magnitude frequency plots through substitution of the original stratigraphic depth data with the associated minimum discharge quantified by hydraulic modelling the age models presented demonstrate that a bayesian approach for analyzing holocene flood magnitude and frequency prevents the loss of geomorphic and hydrologic information inherent in radiocarbon frequency methods previously used in the analysis of palaeoflood data sets frequency approaches do not allow proper consideration of flood magnitude information and only incorporate t

investigating extreme flood response to holocene palaeoclimate in the chinese monsoonal zone a palaeoflood case study from the hanjiang river abstract palaeoflood events recorded by slackwater deposits swds were investigated extensively by sedimentological criteria of palaeohydrology along the upper hanjiang river valley modern flood swds were collected for comparison with palaeoflood swd in the same reaches three typical palaeoflood swds were observed within holocene loess soil blanket on the first river terrace land the grain size distributions of palaeoflood swds are similar to modern flood swds whereas they are different from eolian loess and soil palaeoflood swd lies in three major pedo stratigraphic boundaries ts l 0 l 0 s 0 and s 0 l t in the holocene loess soil profiles the chronology of three palaeoflood episodes was established by osl dating and pedo stratigraphic correlation with the well dated holocene loess soil profiles in the upper hanjiang river basin holocene palaeoflood events were dated to 9500 8500 3200 2800 and 1800 1700 a b p respectively palaeoflood discharges were estimated by the palaeoflood model i e slope area method and step backwater method the highest discharges are 51 680 53 950 m 3 s 1 at the 11 500 time scale in the xunyang reach of the upper hanjiang river valley holocene extraordinary hydroclimatic events in the hanjiang river often result from abnormal atmospheric circulations from southwest monsoons in the chinese monsoonal zone these results provide a regional expression of extreme flood response to holocene palaeoclimate to understand the effects of global climatic variations on the river system dynamics

late neanderthals at jarama vi central iberia previous geochronological and archaeological studies on the rock shelter jarama vi suggested a late survival of neanderthals in central iberia and the presence of lithic assemblages of early upper paleolithic affinity new data on granulometry mineralogical composition geochemical fingerprints and micromorphology of the sequence corroborate the previous notion that the archaeological units jvi 2 1 to jvi 2 3 are slackwater deposits of superfloods which did not experience significant post depositional changes whereas the artifact rich units jvi 3 and jvi 1 mainly received sediment inputs by sheetwash and cave spall new ams radiocarbon measurements on three samples of cut marked bone using the ultrafiltration technique yielded ages close to or beyond the limit of radiocarbon dating at ca 50 14c ka bp and hence suggest much higher antiquity than assumed previously furthermore elevated temperature post ir irsl luminescence measurements on k feldspars yielded burial ages for subunits jvi 2 2 and jvi 2 3 between 50 and 60 ka finally our reappraisal of the stone industry strongly suggests that the whole sequence is of mousterian affinity in conclusion jarama vi most probably does not document a late survival of neanderthals nor an early upper paleolithic occupation in central iberia but rather indicates an occupation breakdown after the middle paleolithic

assessment of dam failure flood and a natural high magnitude flood in a hyperarid region using paleoflood hydrology nahal ashalim catchment dead sea israel 1 a dam failure flood in 1993 with a peak discharge of 600 700 m3 s 1 volume of 0 5 106 m3 and a natural rainfall runoff flood with a peak discharge of 420 m3s 1 volume of 2 0 106 m3 in 2004 in the nahal ashalim catchment 75 km2 in the hyperarid dead sea region were comparatively analyzed the two floods which caused serious damage to the dead sea works industries were analyzed for magnitude and frequency using regional analysis extrapolation from the short systematic record rainfall runoff modeling and paleoflood hydrology the 7600 year paleoflood record was reconstructed at the ashalim cave at the upper nahal ashalim peak discharges in this high gradient mountainous wadi are highly sensitive to the roughness coefficient and the flow regime the dam failure peak discharge is lower than the envelope curves of measured floods of the dead sea region and of paleofloods of the negev and the dead sea deserts the probability of the dam failure flood and the natural flood were estimated by various methods at 1 to 0 2 and 0 5 respectively adding the peak discharge of the october 2004 flood increased the probability from 0 2 to 0 5 0 2 the long paleoflood record encompasses different hydrological regimes related to climatic fluctuations which are not relevant to the present hydrological regime therefore the chosen period for frequency analysis was a d 1673 1996

hydrological studies of the historical and palaeoflood events on the middle yihe river china abstract palaeo hydrological and sedimentary investigations were carried out in the longmenxia gorge of the middle yihe river five bedsets of flood slackwater deposits swd were found interbedded into holocene aeolian loess soil profiles in the river bank at the longmenxia site they were identified as the deposits of the suspended sediment load during the extreme flood events of the holocene the minimum flood peak discharges were estimated to be 12 300 15 300 m 3 s using the slope area method these are about twice the largest gauged record 7180 m 3 s that has occurred since 1937 these flood events occurred at 3100 3000 a b p 1800 1700 a b p 700 550 a b p and 350 250 a b p as dated by optically stimulated luminescence osl in combination with stratigraphic correlation to which may be added the recorded events of the 223 and 1761 ce floods these were associated with holocene monsoonal shifts and abrupt climatic events this research not only provides palaeoflood discharge estimates on the middle yihe river but also provides important data for understanding interactions between regional hydro climatic systems and global climate change in humid and semi humid monsoonal regions such as china

sedimentary and hydrological studies of the holocene palaeofloods in the shanxi shaanxi gorge of the middle yellow river china holocene slackwater deposits along the river channels were used to study the magnitude and frequency of the palaeofloods that occurred prior to gauged and historical data sets all over the world palaeohydrological investigations along the shanxi shaanxi gorge of the middle yellow river china identified palaeoflood slackwater deposits swds at several sites along the cliffs bordering the river channel the swds are intercalated within holocene eolian loess soil profiles and clastic slope deposits the palaeoflood swds were differentiated from eolian loess and soil by the sedimentary criteria and analytical results including magnetic susceptibility and particle size distribution similar to the flood swds in 2012 which indicated that these well sorted palaeoflood swd beds were deposited from the suspended sediment load in floodwaters they have recorded the extraordinary palaeoflood events which occurred between 3200 and 3000 a bp as dated by the optically stimulated luminescence method in combination with pedostratigraphic correlations with the previously studied holocene pedo stratigraphy in the yellow river drainage basin manning slope area calculations estimate the peak discharged for these palaeoflood events to range from 43 290 to 49 830 m3 s the drainage area of the study site is 489 900 km2 it is 2 0 2 5 times the largest gauged flood 21 000 m3 s that has ever occurred since 1934 these events also occurred on yellow river tributaries including the weihe jinghe and qishuihe rivers these flood events are therefore considered to be a regional expression of known climatic events in the northern hemisphere and demonstrate holocene climatic instability this study provides important data in understanding the interactions between regional hydro climatic systems and global change in semiarid and subhumid regions

recirculating flow and sedimentation in the colorado river in grand canyon arizona debris fans debouching into the bottom of grand canyon create rapids and flow separation in the colorado river the patterns of flow and the behavior of recirculation zones formed by flow separation are consistent throughout the canyon s length zones of recirculating flow occur along the margin of channel expansions recirculation zones are comprised of one primary eddy secondary eddies and areas of unorganized low velocity may exist upstream from the primary eddy the longest recirculation zones are formed by channel constrictions of low width to depth ratio recirculation zones increase in length with increasing discharge sand bars form beneath recirculation zones especially near separation and reattachment points reattachment bars project upstream from the reattachment point and underlie primary eddies separation bars mantle the downstream parts of the debris fans and form beneath secondary eddies and low velocity areas sediment that forms reattachment bars is dominated by sizes characteristic o

frequency of boulders transport during large floods in hyperarid areas using paleoflood analysis an example from the negev desert israel abstract direct measurements of boulder entrainment in desert wadis are not available the 2004 flood peak discharge 470 m3 s 1 recurrence interval 120 years in the hyperarid ungauged nahal hatzera ephemeral stream 45 km2 transported and deposited 0 85 2 1 m concrete boulders and slabs detached from infrastructure upstream and natural boulders edm and drone air photographic surveys documented the geometry of the study reach and the location of boulders analyses of flood slackwater deposits established a paleoflood record of 23 floods with peak discharges of 200 760 m3 s 1 during the last 600 years 1 d hec ras hydraulic analysis provided water surface profiles discharges and hydraulics along the study reach and velocity shear stress and stream power for each boulder max program and pearson 3 distribution were used for flood frequency analysis most of the concrete boulders were deposited in the sub critical backwater of channel constrictions where velocities were 1 5 2 1 m s 1 the largest boulders were deposited in super critical flow where velocity was 8 9 2 m s 1 the alluvial channel enabled to transport these concrete boulders reflecting the unstable active sandy layer of the channel bed over which the boulders moved the maximum flood shear stress and stream power characterize medium large floods with return period of 20 120 years and not for the largest floods as expected boulders about 2 1 m and weighing about 15 t can be transported at least once in 120 years the shear stress and stream power indicate that the moderate large floods are the most geomorphically effective floods rather than the largest floods in nahal hatzera basin nevertheless the geomorphic effectiveness of the 2004 flood a typical desert flash flood with high peak and short duration was small based on the minor changes along the channel and banks indicating that their resistance thresholds were not exceeded and energy expenditure was mainly on boulders entrainment and transport

the last millennium largest floods in the hyperarid kuiseb river basin namib desert the largest kuiseb river floods initiate at the basin s semi arid headwater downstream along the hyperarid namib desert these floodwaters are feeding shallow alluvial aquifers the only available water for human activity and for the natural ecology here we characterize the largest floods and their changing frequency and magnitudes using palaeohydrological methods along 120 km of the rivers canyon 35 palaeoflood deposit sites were identified at five of these sites we conducted stratigraphic and geochronological analyses and flood discharge estimations the upper bound of the largest flood over the late holocene is 1475 m 3 s 1 over the last 1300 years more than ten floods have exceeded 1250 m 3 s 1 an additional 33 floods exceeded 400 m 3 s 1 the last millennium was characterized by one large flood every 30 40 years during the periods 1250 1335 1355 1565 and 1715 ad to the present a slight increase in flood frequency not magnitude occurred during 1565 1715 ad one large flood in 20 years and during two short episodes 1185 1205 and 1335 1355 ad seven and four floods in 20 years respectively these episodes of increased flood frequency are associated with other proxy records of higher water availability in the namib copyright 2013 john wiley sons ltd

the holocene floods and their affinity to climatic variability in the western himalaya india abstract the present study in the middle satluj valley explores the sedimentary records of past floods with an objective to understand the climatic processes responsible for their genesis based on lithostratigraphy sedimentology and grain size variability 25 flood events are identified the geochemical data indicate that the flood sediments were mostly generated and transported from the higher himalayan crystalline and the trans himalaya our study suggests that the floods were generated by landslide lake outburst floods llofs during extreme precipitation events however the existing database does not allow us to negate the contribution from glacial lake outburst floods glofs field stratigraphy supported by optical chronology indicates four major flood phases that are dated to 13 4 10 4 8 3 3 6 2 2 1 4 and

a single stage megaflood at the termination of the messinian salinity crisis geophysical and modelling evidence from the eastern mediterranean basin abstract the messinian salinity crisis was an extraordinary event that resulted in the deposition of kilometre thick evaporite sequences in the mediterranean sea after the latter became disconnected from the world s oceans the return to fully and stable marine conditions at the end of the crisis is still subject to debate three main hypotheses based on geophysical and borehole data onshore outcrops and climate simulations have been put forward these include a single stage catastrophic flood a two step reflooding scenario and an overspill of paratethyan water followed by atlantic inflow in this study two research questions are addressed i which event marked the termination of the messinian salinity crisis ii what was the sea level in the eastern mediterranean sea during this event geophysical data from the western ionian basin are integrated with numerical simulations to infer that the termination of the crisis consisted of a single stage megaflood following a sea level drawdown of 1900 m this megaflood deposited an extensive sedimentary body with a chaotic to transparent seismic signature at the base of the malta escarpment fine well sorted sediments are predicted to have been deposited within the thicker sections of the flood deposit whereas a more variable distribution of coarser sediments is expected elsewhere the north western ionian basin hosts evidence of episodic post messinian salinity crisis slope instability events in the last 1 8 ma the largest of these emplaced a 200 km3 deposit and is associated with failure of the head of noto canyon offshore se sicily apart from unravelling the final phase of the messinian salinity crisis and the ensuing stratigraphic evolution of the western ionian basin our results are also relevant to better understand megafloods which are some of the most catastrophic geological processes on earth and mars

a long term flood discharge record derived from slackwater flood deposits of the llobregat river ne spain abstract slackwater palaeoflood deposits were identified along two bedrock gorge study reaches of the llobregat river at pont de vilomara and monistrol de montserrat the compiled palaeoflood record consists of two principal flood series a a relatively complete record of low to high magnitude flood events from the last ca 100 years and b evidence of the largest palaeoflood events that have occurred over the last ca 2700 years the longer term extreme palaeoflood record indicates that the discharge of the 1971 flood the largest on record was exceeded on at least eight occasions with two periods of high magnitude flooding identified a the late bronze age 2500 2700 years ago and b the little ice age ad 1500 1700 at pont de vilomara palaeodischarge estimates of 3700 4300 m 3 s compare to a discharge of 2300 m 3 s for the 1971 event downstream at monistrol an estimate of 4680 m 3 s for flood deposits dated as ad 1516 1642 and believed to be those of the ad 1617 flood compared to 2500 m 3 s for the 1971 flood

sedimentology of high stage flood deposits of the tagus river central spain abstract this paper details the sedimentology of high stage flood deposits with the definition of sedimentary environments and their characteristic sequences along two bedrock reaches of the tagus river central spain high stage flood deposits accumulated in bedrock canyons include slackwater flood deposits swd and other types of deposits located at flow separation zones and associated with slow moving flow 1 these flood deposits are common indirect indicators of flood stages used in palaeoflood studies for estimating the discharges associated with quaternary floods depositional environments of flood deposits include 1 channel widening 2 canyon expansion 3 bedrock obstacles and 4 backflooded areas along tributary streams these flood deposits can be found associated with other non fluvial environments namely aeolian reworked and slope washflow facies channel widening due to flood stage variations comprises internal and external zones of the channel margins and their characteristic sequences contain similar facies to those of alluvial floodplains canyon expansion environments favour vertical accretion of slackwater units and the development of flood deposit benches which contain four sequences related to bench elevation and distance from the channel s main thread of flow at the lee side of bedrock obstacles characteristic sedimentary sequences are dominated by reverse flow structures e g climbing ripples migrating upstream due to eddies with a high sand concentration flood deposits located within tributary mouths contain typical sequences of reworked floodplain deposits backflooding of tributaries during flood stages produces deposition from suspension of sand silt and clay within three sequences characterised by non structure or parallel lamination and intense bioturbation a better understanding of the flood deposit sequences may contribute to the characterisation of flood magnitudes and flood hydraulics and can also be applied to some ancient depositional environments

neanderthal settlement in central iberia geo archaeological research in the abrigo del molino site mis 3 segovia iberian peninsula abstract the abrigo del molino archaeological site is located in the valley of the eresma river near segovia in central spain it consists of a shallow cave of fluvio karstic origin which has been completely filled with detrital deposits geo archaeological interpretation of the genesis of these deposits differentiates three groups a lower group with sandy loams and fine sands interbedded with pebble and gravel layers deposited respectively by palaeofloods and slope contributions a middle group with massive silt and grain supported boulders formed by alternating contributions from the overlying slope karst mudflows and rockfalls from the shelter roof and an upper group of silt cemented by carbonates final backfill alteration and degradation of the host rock the techno typological characteristics of the lithic assemblage confirm the existence of mousterian levels at the site and optically stimulated luminescence osl dating of these levels obtains an age range of 31 60 ka for the detrital deposits presenting the occupation by neanderthal groups for the first time in karstic environments in the northern iberian plateau the southern part of the duero basin

selection of appropriate topographic data for 1d hydraulic models based on impact of morphometric variables on hydrologic process abstract with the development and utilization of mountain rivers modelers of rivers using one dimensional 1d hydraulic models face the challenge of selecting appropriate cross section data for mountain rivers since previous studies have focused on alluvial rivers mountain rivers usually have larger bed slopes and smaller widths in comparison with alluvial rivers which results in differences in their hydrologic processes our case study analyzes these differences using 1d models through the utilization of prismatic reaches with different gradient and width sizes and the provision of various boundary conditions the influence of the size of these morphometric variables on the selection of optimal topographic data is related to the hydraulic problem at hand in addition mountain rivers tend to have more irregular cross sections and gradients than alluvial rivers because of their relatively erosion resistant riverbeds the effects of gradient deviation width deviation and channel storage volume deviation on hydrologic process due to insufficient topographic data are compared it is found that the influence of channel storage volume deviation which is negligible in alluvial rivers could be significant in mountain rivers and the impact of width deviation is minimal in mountain rivers and alluvial rivers thus in reservoir areas measured width could be adjusted based on published stage storage data to reduce channel storage volume deviation and improve flow simulation accuracy this method is verified on a 605 71 km long reach in the three gorges reservoir area

the expression of the oceanic anoxic event 2 oae2 in the northeast of brazil sergipe alagoas basin abstract global oceanic anoxic events occurred throughout the cretaceous and changed the sedimentation patterns in many sedimentary basins around the world there is still a lack of data and the need to identify new sections affected by these events in the southern hemisphere in order to provide insights on the extent of low oxygen oceanic conditions this study presents sedimentological petrographic and geochemical data from a continuous 439 m thick well core drilled in the sergipe alagoas basin the analytical data suggest that anoxic conditions prevailed at the end of cenomanian and the beginning of turonian being globally correlatable to the oceanic anoxic event 2 oae2 the results on the delimitation and extent of the oae2 in the sergipe alagoas basin aid to better define the paleoceanographic changes that occurred in the south atlantic ocean during oae2 and provide a new set of data that enables further studies and correlation to other sections distributed worldwide

controls on dryland mountain landscape development along the nw saharan desert margin insights from quaternary river terrace sequences dades river south central high atlas morocco abstract this study documents river terraces from upstream reaches of the dades river a major fluvial system draining the south central high atlas mountains terraces occur as straths with bedrock bases positioned at 10 m altitudinal intervals up to 40 m t1 t5 above the valley floor becoming less common between 50 and 140 m the rock strength stratigraphy and structure of the mountain belt influences terrace distribution terraces are absent in river gorges of structurally thickened limestone whilst well developed laterally continuous terraces t1 t4 form along wide valleys occupying syncline structures dominated by weaker interbedded limestone mudstone terrace staircases develop in confined canyons associated with weaker lithologies and influence from structural dip and stratigraphic configuration terraces comprise a bedrock erosion surface overlain by fluvial conglomerates rare overbank sands and colluvium this sequence with some osl irsl age control suggests terrace formation over a 100 ka climate cycle with valley floor aggradation during full glacials and incision during glacial interglacial transitions this integrates with other archives e g lakes glaciers dunes appearing typical of landscape development along the nw saharan margin south of the high atlas and similar to patterns in the western southern mediterranean the 100 ka climate cycle relationship suggests that the terrace sequence documents late middle pleistocene landscape development consistent altitudinal spacing of terraces and their distribution throughout the orogen suggests sustained base level lowering linked to uplift exhumation of the high atlas low incision rates 1 and general absence of terrace deformation suggests dominance of isostatically driven base level lowering with relief generation being early pleistocene or older

architecture analysis of a river flood dominated delta during an overall sea level rise early pliocene se spain abstract the sedimentary record of delta front to prodelta transition that developed during the early pliocene bajo segura basin peri mediterranean betic cordillera basin represents an unusual infilling of an incised valley cutting downwards into late messinian pelagic marls the complete delta succession less than 65 m thick consists of four upwardly fining and thinning sequences several metres thick each sequence consists of basal conglomeratic and coarse grained sandstone beds delta front deposits evolving upwards to alternating sandstones and silty marl beds prodelta deposits the retrogradational stratal succession pattern is interpreted as having been deposited during an overall sea level rise facies association is characterised by quasi to planar laminated sandstones and pebbly sandstones alternating with clast supported boulders to pebbles sheet like climbing cross laminated sandstones with abundant plant remains and epsilon cross stratified and channelized gravels and sandstones these are interpreted as having been deposited by highly concentrated flows interpreted as hyperpycnal turbidity flows from high cohesionless debris flows and sinuous submarine channels to moderate sheet like climbing cross laminated sandstones discharge river flood events slabs of sediment from unstable banks outsized clasts which roll onto thin beds from channel mouths slumps and intraformational breccias in delta front are other signatures including a lack of bored clasts which indicate high energy and rapid deposition at the river mouth encouraging the delta front gravitational instability failures major changes in the discharge of the river fed delta could have been caused by brief catastrophic heavy precipitation events coupled with quasi stationary convective orographic rainfall typical of the modern mediterranean

holocene flooding history of the lower tagus valley portugal the present paper aims to reconstruct the lower tagus valley flooding history for the last ca 6500 a to explore the suitability of pollen based local vegetation development in supporting the reconstruction of flooding history and to explain fluvial activity changes in terms of allogenic climate human impact and autogenic system intrinsic processes the flooding history has been determined by cored sedimentary records located 18 km apart in distal low energy backswamps on both sides of the tagus channel in these low energy backswamps fine grained sediment layers deposited from suspended load of overbank flood water reflect periods with multiple overbank floods by means of a multi proxy approach sedimentology magnetic susceptibility grain size loss on ignition carbonate content and pollen sedimentary and environmental changes were identified at both sites synchronous lithological intervals accumulated suggesting a common origin for the changes in fluvial activity since ca 6500 cal a bp based on lithological changes three phases of high fluvial activity 6500 5500 4900 3500 and 1000 0 cal a bp and two phases of low fluvial activity 5500 4900 and 3500 1000 cal a bp were identified two periods with dominant allogenic controls on fluvial activity in the lower tagus valley were identified relative sea level 6500 5500 cal a bp and human impact 1000 0 cal a bp during the intermediate period changes in fluvial activity may have been caused by climate 5500 1000 cal bp but unambiguous correlations are difficult to make this is due to the way allogenic controls are translated through the fluvial system the geomorphological differences between upstream and downstream studies and autogenic processes the comparison of local vegetation development and flooding phases as reconstructed using sedimentology shows a limited added value of using local palynology as a proxy for fluvial activity 2010 john wiley sons ltd