Slackwater Flood Deposits Research Articles

Provenance and erosional impact of Quaternary megafloods through the Yarlung-Tsangpo Gorge from zircon U-Pb geochronology of flood deposits, eastern Himalaya

Holocene and Upper Pleistocene sediments in the eastern Himalayan syntaxis represent an extraordinary record of glacial outburst megafloods in one of the most tectonically active landscapes on Earth. Glacial damming and outburst floods in the syntaxis may have focused erosion in the steep Tsangpo Gorge and inhibited river incision into the margin of the Tibetan Plateau. However, few flood slackwater deposits have been studied, and it is unknown which of the hundreds of known glacial impoundments in Tibet may have sourced the floods. Here we report n=1438 new detrital zircon U-Pb data from individual ancient megaflood and historical outburst flood slackwater deposits to examine the provenance and erosive potential of these events. Detrital zircon U-Pb geochronology of megaflood deposits show that megaflood provenance is more consistent with impoundment of the Yarlung River drainage at or west of the Namche Barwa massif, than impoundment of eastern drainages along the Yigong and Parlung Rivers. Compared to active bedload in the Siang River, and historical floods within the same drainage area, megaflood samples overall contain a disproportionately large amount of zircons eroded from the Tsangpo Gorge. We interpret this preferential derivation to reflect preferential erosion during large discharge events—supporting the hypothesis that Quaternary megafloods are a primary contributor to rapid exhumation of the eastern Himalayan syntaxis.

Holocene palaeoflood events recorded by slackwater deposits along the Jin-shan Gorges of the middle Yellow River, China

Palaeoflood slackwater deposits (SWDs) are the natural record of overbank flooding and are often found within aeolian loess-soil profiles along the river valleys of the middle Yellow River basin. Palaeo-hydrological field investigation was carried out along the Jin-shan Gorges of the middle Yellow River after the great flood in 2012. We founded the new palaeoflood SWDs profile (YHG-B) on the basis of previous results in the YHG site, and collected the 2012 flood slackwater deposits. Analytical results, including magnetic susceptibility and particle size distribution, which is similar to the flood SWDs in 2012, indicate 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 pedo-stratigraphic correlations with the previously studied Holocene pedo-stratigraphy in the Yellow River drainage basin. According to the palaeoflood peak stages indicated by YHG-A and YHG-B, the peak discharges of these palaeoflood events vary in a range from 28 340 to 48 410 m3/s by using HEC-RAS one dimensional model. This discharges result error is less than 5% with previous published discharges by using Slop-Area method in 2012. The extraordinary flood events were documented not only on the Yellow River, but also on its tributaries such as the Weihe River, the Jinghe River, and the Qishuihe River. The flooding events are therefore considered to be a regional expression of known climatic events in the northern hemisphere and demonstrate Holocene climate was far from stable. This work will increase our understanding of the interactions between fluvial environment and climatic change in the semi-arid and sub-humid regions, and provide information that is useful for interpretation and calibration of the modern short-term gauged records.

Historical palaeohydrology and landscape resilience of a Mediterranean rambla (Castellón, NE Spain): Floods and people

This paper provides a new methodological approach to analyse secular patterns of flooding (magnitude and frequency) from sedimentary evidence (palaeofloods), taking into account changes in channel geometry, and their links to historical environmental changes and the inherent social and demographic evolution within the catchment. A case study analysis was focused in Rambla de la Viuda (drainage area of 1500 km2) whose stream flow is related to extreme rainfalls. A 500 years sedimentary archive was reconstructed from eight stratigraphic profiles comprising continuous sequences of slackwater flood deposits interbedded with episodic colluvial and edaphic horizons. Discharge estimates associated to sedimentary flood evidences were obtained from one-dimensional hydraulic modelling. The stratigraphic units were sampled to characterise their geochemical and paleobotanical (phytoliths) contents. Palaeoflood chronology was obtained from radiocarbon and luminescence (OSL) dating, supported by documentary data (written historical documents). A high frequency and high magnitude palaeoflood period took place during the 15th-middle 16th century, which seem to correlate in time with general wetter conditions. Three short-term environment stability conditions (land use and climatic) also made possible the development of three paleosols. The lowest flood magnitude and discharges in the sedimentary record was found between the mid-17th to mid-18th centuries, under prevailing drier environmental conditions. Episodic high magnitude flooding took place at late 18th century, correlating in time with palaeovegetation and geochemical evidences of important changes on land use (deforestation and grazing). Poorer developed soils were found at upper stratigraphic sequences (19th century) characterised by thick units of colluvium deposits, usually culminating sequences of short-lived continuous slackwater flood units. Despite of the potential human influence (land-use) on soil hydrology, the long-term behaviour of high magnitude floods (>1000 m3 s-1) has been stationary over the last 500 years.

Hydrological studies of the historical and palaeoflood events on the middle Yihe River, China

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, 300m3/s using the slope–area method. These are about twice the largest gauged record (7180m3/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 1761CE 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.

Late Pleistocene and Holocene extreme hydrological event records from slackwater flood deposits of theAnkang east reach in the upperHanjiangRiver valley,China

This paper reports an investigation into palaeoflooding along the upper reaches of the Hanjiang River valley, China. Based on the sedimentary evidence of the palaeohydrological regime, two bedsets of palaeoflood slackwater deposits (SWDs) were identified interbedded within the Holocene loess–soil sequence along the riverbanks of the Ankang east reach. Optical Stimulated Luminescence (OSL) dating and stratigraphical correlation with previously dated sites were used to reconstruct the chronology of the palaeoflood events. The results show that the palaeoflood events represented bySWD1 occurred between 13 000 and 12 400 a, coincident with the climatic transition from the Bølling–Allerød (BL+AL) stage to the Younger Dryas (YD) event. The palaeoflood events recorded bySWD2 were dated to 1000–800 a, corresponding to the later stages of the North Song Dynasty (AD960–1127) and the subsequent South Song Dynasty (AD1127–1279), which was a time of climatic decline according to historical documents. Palaeoflood discharges were estimated using the step‐backwater method, and the peak discharges were estimated to be in the range 35 200–47 400 m3 s−1. These results are of significance to our understanding of the regional hydrological response to global climatic change, the utilization of water resources, hydraulic engineering, flood control and disaster reduction.

A multidating approach applied to historical slackwater flood deposits of the Gardon River, SE France

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 and cave GG were excellent examples to illustrate the effects of erosion/preservation in a context of a progressively self-censoring, vertically accreting sequence. The sedimentary flood record of the terrace GE located at 10m above the channel bed is complete for years post-1958 but incomplete before. During the 78-year period 1880–1958, 25 floods of a sufficient magnitude (>1450m3/s) have covered the terrace. Since 1958, however, the frequency of inundation of the deposits has been lower: only 5 or 6 floods in 52years have been large enough to exceed the necessary threshold discharge (>1700m3/s). The progressive increase of the threshold discharge and the reduced frequency of inundation at the terrace could allow stabilization of the vegetation cover and improve protection against erosion from subsequent large magnitude flood events. The sedimentary flood record seems complete for cave GG located at 15m above the channel bed. Here, the low frequency of events would have enabled a high degree of stabilization of the sedimentary flood record, rendering the deposits less susceptible to erosion.Radiocarbon dating is used in this study and compared to the other dating techniques. Eighty percent of radiocarbon dates on charcoals were considerably older than those obtained by the other techniques in the terrace. On the other hand, radiocarbon dating on seeds provided better results. This discrepancy between radiocarbon dates on charcoal and seeds is explained by the nature of the dated material (permanent wood vs. annual production and resistance to degradation process). Finally, we showed in this study that although the most common dating technique used in paleoflood hydrology is radiocarbon dating, usually on charcoal preserved within slackwater flood sediments, this method did not permit us to define a coherent age model. Only the combined use of lead-210, caesium-137, and geochemical analysis of mining-contaminated sediments with the instrumental flood record can be applied to discriminate and date the recent slackwater deposits of the terrace GE and cave GG.

Reliability of equivalent-dose determination and age-models in the OSL dating of historical and modern palaeoflood sediments

The challenge of accurately estimating the deposition age of incompletely-bleached samples in luminescence dating has motivated developments in the analysis of single grain dose distributions, and a number of statistical approaches have been proposed over the last few years. In this study, we compare the behaviour of the arithmetic average, so-called ‘robust statistics’, the Central Age Model (CAM), the Minimum Age Model (MAM) and the Internal–External Consistency Criterion (IEU) and lowest 5% method, when applied to single-grain dose distributions from a sequence of eight recent (40–1000 years) flash-flood deposits. These sediments are expected to be incompletely bleached, but all have age control from historical records. Modifications were made to allow the use of the standard CAM and MAM models with dose distributions containing near zero and negative dose values. An assessment of minimum over-dispersion (OD) is based on dose recovery tests based on gamma-irradiated samples. We then present a detailed analysis of the effect of an additional uncertainty added to the individual dose estimates on the burial dose estimates from the MAM and the IEU approach. The results of the various models are discussed in terms of the accuracy of the resulting age, and we conclude that, overall, the IEU approach generates the most accurate ages. We also demonstrate that accurate doses can be obtained for those older samples for which uncertainties of ∼40 years are unimportant by applying the IEU model to small aliquot (∼30 grains) dose distributions. From our study we conclude that these and similar young slack-water flood deposits can be accurately dated using quartz OSL, opening up the possibility of establishing time series of flood discharge in catchments for which no instrumental or historical record exists.

Concentration of heavy metals in the modern flood slackwater deposits along the upper Hanjiang River valley, China

Modern flood slackwater deposits (SWD) were systematically collected after the great flood events which occurred on 19 July, 2010 and on 19 September, 2011 along the upper Hanjiang River valley. Concentrations of heavy metals (Co, Cr, Cu, Ni, Pb, Zn, Ba, Mn and V) in the fresh flood SWD were determined. Enrichment Factor (EF) and Index of Geoaccumulation (Igeo) were employed for heavy metal assessment. These results show that Cu, Ni, Pb, Zn and Mn are defined as only minor enrichment and unpolluted in the upper Hanjiang River basin, whereas Co, Cr, Ba and V are defined as between moderate enrichment and moderately severe enrichment, and moderately polluted in the downstream of Ankang city. Principal component analysis (PCA) and correlation analysis confirm the results of EF and Igeo. Cluster analysis (CA) identifies that Ankang city is the boundary between higher and lower heavy metals in the modern flood SWD. Therefore, we can conclude that the source of Co, Cr, Ba and V is mainly from anthropogenic inputs, but the source of Cu, Ni, Pb, Zn and Mn is mixed from natural and anthropogenic inputs. Concentration of heavy metals in the downstream of Ankang city is higher than that in the upstream. These results would help to establish non-point sources of heavy metal pollution and provide important information for water conservation strategy during the great flood periods in the upper Hanjiang River basin.

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.

Comparative study of the modern flood slackwater deposits in the upper reaches of Hanjiang and Weihe River Valleys, China

Modern flood slackwater deposits (SWD) were investigated and sampled in the upper reaches of Hanjiang River and the Weihe River valleys, China. Magnetic susceptibility and grain-size distribution were analyzed in the laboratory. The results show that the magnetic susceptibility values of the modern flood SWD vary between 20 × 10 −8 m 3 kg −1 and 60 × 10 −8 m 3 kg −1 , very close to that of the Malan Loess over the Weihe River basin . These sediments were newly deposited and uninfluenced by weathering and pedogenesis . The grain-size distribution of the modern flood SWD is dominated by silt, with sand and clay, indicating that these sediments were sourced from the suspended sediment load of floodwater. However, the grain-size distribution of the modern flood SWD has more sand-sized sediments in the upper reaches of Hanjiang River valley than that in the Weihe River valley. The modern flood SWD are defined as sandy silt and silty sand in the upper reaches of Hanjiang River valley, whereas the modern flood SWD are defined as silt, sandy silt and clayey silt in the Weihe River valley. The modern flood SWD have different sediment sources and transporting forces. The modern flood SWD are sourced mainly from the Qinling and the Bashan bedrock mountains and the related residual deposits, slopewash and mud-rock flow deposits in the upper reaches of Hanjiang River valley. During rainstorms , torrential tributary rivers can bring much coarse deposits into the mainstream of the Hanjiang River. However, the sources of the modern flood SWD are the eolian loess and the related surface soil in the Weihe River valley. Comparative study shows different sedimentary features between the modern flood SWD in the upper reaches of Hanjiang River valley and that in the Weihe River valley. These results give insight into the different sediment sources and transporting force of the modern flood SWD, which provide a basic reference to mitigate flood disasters, and to soil and water conservation.

Palaeoflood and floodplain records from Spain: Evidence for long-term climate variability and environmental changes

Palaeoflood chronologies from seven Spanish river basins and floodplain aggradation chronologies from thirteen rivers are analysed. These fluvial records were divided in to two sub-sets, namely Atlantic (10 ka record) and Mediterranean (3 ka record) river basins, which represent distinct modern hydroclimatic conditions. In Atlantic basins floods result from intense, widespread rainfalls associated with Atlantic frontal systems transported by westerly airflow. Mediterranean river flooding is related to heavy rainfall induced by mesoscale convective systems (MCSs) during autumn months. Evidence from radiocarbon dates in slackwater flood deposits shows six periods of flood clusters at 10,750–10,240; 9550–9130; 4820–4440; 2865–2350; 960–790; and 520–290 cal BP. Despite the different flood-producing weather conditions in Atlantic and Mediterranean rivers, the radiocarbon sample clusters overlap and indicate changes in large-scale atmospheric circulation and climatic conditions in the Iberian Peninsula. Comparison with proxy records of mean temperature for the Northern Hemisphere demonstrates a relationship between the period of slackwater flood deposition and cold climatic phases (e.g. the 2650 yr BP climatic event or AD 1590–1650 period of the Little Ice Age). Radiocarbon dates from aggraded floodplain sediments were clustered at 2710–2320, 2000–1830, and 910–500 cal BP. The first cluster period is in phase with the timing of slackwater deposition, whereas the third (910–500 cal BP) occurs in between two periods of increased flood frequency as indicated by the palaeoflood and documentary flood records. It is argued that the 910–500 cal BP floodplain aggradation period reflects the first post-Roman evidence of environmental change related to generalised land-use changes at the catchment scale, which produced high sediment load transported to overbank areas during high flows.

The Palaeoflood record of the Gardon River, France: A comparison with the extreme 2002 flood event

A paleoflood study in the Gardon River gorge in southern France identified extreme floods larger than any modern or historically gauged flood. During the course of our study, an extreme flood on the 8–9th of September 2002 claimed the lives of 21 people and caused millions of Euros worth of damage to the towns and villages along the river. The magnitude of this flood was larger than any known historical flood on record,. However, there is evidence of greater magnitude flood events in the form of slack-water flood deposits preserved in caves elevated 17–19 m above the normal base flow, and up to 3 m above the level reached by the 2002 floodwaters. The stratigraphy and radiocarbon dating show that at least five extreme events occurred during the past 500 years. The discharge estimation associated with these flood units, achieved using the HEC-RAS one dimensional model, indicates that at least three floods were bracketed by discharges between 6850 and 7100 m 3 s − 1 , and at least two floods reached a magnitude above 8000 m 3 s − 1 . Therefore, the extraordinary flood of 2002 was not the largest in the basin.

Late Holocene fluvial chronology of Spain: The role of climatic variability and human impact

A database of published and unpublished radiocarbon dates from Late Holocene fluvial environments in Spain was critically analysed in order to obtain a fluvial chronology for this period. The principal types of depositional environment represented in the record were slackwater flood deposits (28 dates), alluvial overbank facies (12 dates) and alluvial channel facies (7 dates). The sedimentary context of each date was classified in order to identify those samples that specifically dated change in the fluvial environment. Analysis of the different depositional environment data-sets using summed probability plots identified phases of alluviation at 2800–2350 and 800–500 cal. BP. Increased frequency of large magnitude floods occurred at 2850–2500, 1000–800 and 520–250 cal. BP. The results indicate that in Mediterranean regions slackwater flood deposits are valuable archives of climatic variability even during periods of increased human impact.

The Holocene fluvial chronology of Spain: evidence from a newly compiled radiocarbon database

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.

Ancient Floods, Modern Hazards: Principles and Applications of Paleoflood Hydrology: (Edited by P. K. House, R. H. Webb, V. R. Baker, and D. R. Levish)

Paleoflood hydrology, as it developed in the 1970s, identified slackwater flood deposits (SWD) as the primary indicator of a historically high stage of a river, i.e., a “paleostage indicator.” Such SWDs are believed to have formed in eddies at the mouths of tributaries of rivers that caused sedimentation of sands and silts in topographically elevated locales relative to the present position of such rivers. If undisturbed by even larger floods, there is a reasonable likelihood that pedogenic processes will create a soil horizon within the SWD that can be age-dated using 14C. Thus an approximate age of the paleoflood can be estimated as well as the paleostage, i.e., the maximum flood stage. Using one-dimensional hydraulic models of bedrock channels and estimates of bed roughness described by Manning's coefficient, the paleoflood hydrologist employs the same tools as used by engineering hydrologists to estimate peak flood discharges, e.g., the step-backwater method. Consequently, geomorphologists and paleoflood hydrologists have been able to contribute their insight to flood-frequency analysis, previously the sole domain of engineering hydrologists. Some years have past since Environmental & Engineering Geoscience published a research paper on low-probability floods and their estimation (Malamud et al., 1996). Paleoflood hydrology is, however, a vibrant area of scientific research that has particular significance to dam safety and floodplain protection of critical structures, such as nuclear reactors, and to engineering geology and fluvial geomorphology in general. This monograph, by the principal actors in paleoflood hydrology, is rich in detail, definitive in scope, and of moderate cost ($75 hardback). Ancient …

Palaeoflood hydrology and its role in applied hydrological sciences

This paper is a review of the methodology of palaeoflood hydrology. In particular, we focus on recent developments and the credibility of the palaeoflood data produced. The use of slackwater flood deposits as a physical record of water surface elevations reached by past floods enables the calculation of robust palaeodischarge estimates for floods that occurred during recent centuries or millennia. Over these time intervals the chronological precision from numerical age dating, such as radiocarbon, is sufficient for the structuring of the palaeoflood discharge data into different threshold levels that are exceeded by floodwaters over specific periods of time, the input data necessary for new methodologies of flood frequency analysis. The value of palaeoflood hydrology to hydrological sciences is discussed through its application in varying multidisciplinary research themes. We demonstrate the use of palaeoflood hydrology in: (1) flood risk estimation; (2) determination of the maximum limit of flood magnitude and non-exceedances as a check of the probable maximum flood (PMF) and its application in producing regional, long-term envelope curves; (3) Holocene climatic variability and (4) assessing sustainability of water resources in dryland environments where floods are an important source of water to alluvial aquifers.

A long-term flood discharge record derived from slackwater flood deposits of the Llobregat River, NE Spain

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.

Caesium-137 dating applied to slackwater flood deposits of the Llobregat River, NE Spain

During a palaeoflood study of the Llobregat River (NE Spain), analysis of 137Cs was carried out on slackwater flood deposits to obtain an improved flood chronology for the most recent flood sediments. At the Monistrol study reach, 137Cs was measured in flood deposits preserved in valley side rock alcoves, with a range of 137Cs concentrations of 2.80–10.06 mBq/g measured. The 137Cs was associated with sediment mobilised from the upstream catchment by erosion and transported to the alcoves during floods. The onset of measured caesium activity in the sedimentary profiles permitted the deposits to be divided between those from floods that occurred before the mid-1950s and those from events after this period. Combining the information derived from the slackwater flood stratigraphy, the minimum discharge estimates associated with the flood deposits, 137Cs analysis and the instrumental peak discharge series, it was possible to identify the particular flood events responsible for the majority of the slackwater flood units. At Vilomara, however, 137Cs activity was measured in palaeoflood deposits dated to the Late Bronze Age and located at an elevation not reached by the floodwaters of the largest modern events. The data from this site indicated that the 137Cs was not associated with sediments transported by floods but was rainfall-derived, the rainwater reaching the deposits through fissures in the rock. To ensure the successful application of caesium dating to slackwater flood deposits preserved within rock alcoves, or caves, some general guidelines are proposed. The technique is best applied: (1) when there is a sedimentary profile of greater than 1 m depth; (2) when the profile is sampled away from zones affected by direct percolation or springwater entering the alcove; and (3) where there are various levels of contrasting textures within the profile.

Sedimentology of high-stage flood deposits of the Tagus River, Central Spain

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 m s −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.

Sedimentary records and luminescence chronology of Late Holocene palaeofloods in the Luni River, Thar Desert, northwest India

River Luni is the only well-integrated river system in the Thar Desert of India. This river catastrophically flooded due to unusually heavy rainfall in the catchment area during July 1979. In order to establish whether floods of this magnitude have occurred in the recent geological past, sedimentary records of palaeofloods occurring in the Sindari Gorge, in the lower Luni Basin were investigated. The principal slackwater flood deposits were observed in a back-flooded tributary near Bhuka. Analysis of the slackwater flood deposits preserved in the back-flooded tributary and their luminescence dating suggests that the Luni River has experienced at least 17 extreme floods during the past millennium. Evidence from the Bhuka site also suggests that no floods comparable in magnitude to the July 1979 megaflood have occurred during this period. This observation is in conformity with the palaeoflood record of central India. Comparison of the long-term monsoon rainfall series for the Luni Basin and the Indian region reveals a clear link between the two, and indicates that the clustering of large floods in the last few decades and during the Medieval warming period is a regional phenomenon associated with wetter conditions. This correlates with a regionally extended episode of landscape stability denoted by stabilization of dunes in this region.