Extraordinary Flood Events Research Articles

Sedimentary characteristics of mixed source fine-grained gravity-flow and its significance for shale oil exploration in a lacustrine depression basin: A case study of the Chang 73 Sub-member of the Triassic Yanchang Formation in Ordos Basin, NW China

This study investigates the transport and evolution of fine-grained gravity-flow deposits in lacustrine basins, focusing on the relationship between fine-grained deposits and their respective source–reservoir combinations. Such knowledge is vital for the exploration and development of unconventional oil and gas resources within the source-rock series of continental lake basins in China. We investigated the fine-grained gravity-flow deposits of the Late Triassic Chang73 Sub-member within the Yanchang Formation in the Ordos Basin. Our approach combined core observations, field outcrop evaluations, thin section analysis, scanning electron microscopy, geochemical data, and laboratory measurements. The sedimentary facies, characteristics, controlling factors, and sedimentary model of the fine-grained gravity-flow deposits are investigated. The primary sources of fine-grained sediments are terrestrial clastic and volcanic–hydrothermal fine-grained sediments. These sediments are mainly developed in warm, humid environments with abundant precipitation, favoring a highly reducible hypoxic terrestrial brackish–freshwater environment. Ten lithofacies and eight bed types of fine-grained gravity-flow deposits are categorized. Bed types represent deposits of muddy slide deposits, muddy slump deposits, muddy debris flow deposits, muddy flow deposits, hybrid event beds, fine-grained transitional flow deposits, surge-like turbidity flow deposits, and fine-grained hyperpycnal flow deposits. As a result of paleo-seismicity, volcanic events, and paleogeomorphology, instability and subsequent collapse of the delta front sediments led to the formation of gravity-flow deposits. These deposits formed the “channel–lobe complex” and “sublacustrine fan” sedimentary systems in both the southwestern and northeastern parts of the basin, respectively. Meanwhile, extraordinary flood events formed during the Carnian Pluvial Episode (CPE) played a significant role. It prompted increased fluvial drainage that drove fine-grained hyperpycnal flows through pre-existing valley systems, resulting in the establishment of the “channel–lobe complex” sedimentary system. The Chang 73 Sub-member contains high-quality hydrocarbon source rocks, laying the foundation for shale oil. The introduction of volcanic ash and the influence of hydrothermal activities amplify the enrichment of organic matter within the lacustrine basin. Fine-grained gravity-flow deposits yield siltstone-rich sweet intervals, primarily seen in surge-like turbidity and hyperpycnal flows. The interbedding of sandy and muddy sediments, influenced by hybrid event beds and fine-grained transitional flows, establishes a stratigraphic arrangement in which the upper layer nourishes the underlying reservoir. This structure is particularly beneficial for the development of shale oil and gas sweet intervals.

The frequency, intensity, and origin of floods in Poland in the 11th–15th centuries based on documentary evidence

Documentary evidence is increasingly being recognized as a precious source for assessing flood records. We have used this type of proxy data to identify the occurrence of floods in Poland from the 11th to the 15th centuries. In addition, we estimated the intensity of each flood event using the best-known classifications for Europe (Barriendos and Coeur, 2004; Brázdil et al., 2006) and assessed their origin based on modified Lambor’s (1954) criteria. The database of floods in Poland contains 166 occurrences in the study period. Most occurred in the 15th century (61.4%). Of the studied regions, Silesia and the Baltic Coast and Pomerania regions were the two most affected by flood events, each accounting for 33–34% of instances. Based on the Brázdil et al. (2006) classification, 77 of the recorded floods are above-average or supra-regional. Also, the indexation of floods based on Barriendos and Coeur (2004) demonstrated that 99 were extraordinary flood events. Rain and its subtypes were the leading causes of floods, with 79 records (47.6%). Flood occurrence in Poland exhibited good spatial coherency with neighboring countries. The updated and most complete inventory of floods in medieval Poland that we present here with a detailed analysis of their frequency, intensity and origin, improves the existing knowledge about this phenomenon in Central Europe. The results of this study, similarly to many other previous studies, also confirm the great capacity for documentary evidence to provide valuable and reliable information about flood records for the pre-instrumental period.

Geochemistry of a paleo-oxbow lake sediments and its implications for the late Holocene extreme overbank flooding history of the Yellow River within the Zoige Basin, NE Tibetan Plateau

Paleo-oxbow lake sediments can provide archives to reconstruct paleo-channel evolution and flooding history of the river. Multi-proxy approaches including detailed sediment stratigraphy, sedimentology and geochemistry have been implemented in a high-resolution sedimentary section of paleo-oxbow lake of the Yellow River within the Zoige Basin on the NE Tibetan Plateau, to reconstruct regional environmental changes and extreme overbank flooding history. Our results suggest that not only traditional sedimentological proxies, but also chemical elements can be applied in defining sequences with different genetic types, especially the paleoflood deposits in the paleo-oxbow profile. Two units of late-Holocence extreme overbank flooding deposits (OFDs) are identified in terms of the significantly higher proportions of sand, high contents of SiO2, Na2O, Ba, low contents of Al2O3, Fe2O3, MgO, K2O, Ti, Rb and high values of Zr/Fe, Zr/Rb ratios. These extraordinary flood events within the Zoige Basin were dated back to 2,960 ± 240–2,870 ± 270 a and 1840 ± 200–1700 ± 160 a, in response to the mid-Holocene climatic optimum to the late Holocene and the Dark Age Cold Period (DACP). And the strong rainfall caused by the abnormal atmospheric circulation during the period of climate transition and abrupt change may led to the frequent occurrence of extreme flood events in the source region of the Yellow River. These findings are important for understanding the response of regional fluvial system to high climatic instability and provide a new perspective for us to analyze the risk of flood disasters on the Tibetan Plateau under the background of climate change.

A comprehensive analysis of Holocene extraordinary flood events in the Langxian gorge of the Yarlung Tsangpo River valley

Increasing extreme temperature, precipitation and rapid meltwater events have added stress to the Himalaya's hydrological sensitivity and major flood risks, however, current extreme hydrological dataset and their genesis are insufficient to assess future flood discharge extremes in High Asian’ rivers. Here, Holocene extreme floods in the Yarlung Tsangpo River valley were reconstructed by using physic-chemical analysis, optically stimulated luminescence dating and palaeohydraulic techniques. Palaeoflood slackwater deposits (SWDs) were identified by means of palaeohydrological criteria and comparison with SWDs from large flood that occurred in 2018. Palaeoflood SWDs consist of well-sorted silt and sand with a consistent geochemical composition, implying a similar sedimentary source. Such results suggest that these SWDs were transported in suspension over long distances during flood events. The chronological analysis indicates that there are three palaeoflood events, dated to 5.7, 3.9 and 2.9–1.2 ka, during the mid-late Holocene. Palaeoflood peak discharges in the bedrock reach and meandering channel were estimated to be 27,600–35,000 m3/s using one-dimensional and two-dimensional hydrodynamic modelling. The simulation results clearly show the potential palaeoflood depositional range in the lower–velocity and eddy backwater environments between Jiacha and Langxian gorge. The palaeoflood magnitudes redefine the regional largest flood discharge, and fit well with global maximum flood curves. And mid-late Holocene extreme flood magnitudes were generally 2.5–3.5 times larger than the current maximum gauged flood, but lower than the Jiedexiu glacial lake outburst floods. Comprehensive analysis highlights the three extraordinary floods were possibly induced by monsoon rains and glacial meltwater. Site-specific palaeoflood information advances our knowledge of rare and extraordinary floods in the highest and largest river in the southern Tibetan Plateau.

A giant lacustrine flood‐related turbidite system in the Triassic Ordos Basin, China: Sedimentary processes and depositional architecture

AbstractTurbidites have been regarded as an important sedimentary infilling component in both oceans and lakes, but limited studies have been performed on the mechanisms governing the initiation and development of lacustrine turbidite systems. The present study offers unique insight into the controls and potential extent of ancient lacustrine turbidite systems by an investigation of the Triassic Ordos Lake, where a large turbidite system had been traced across >25 653 km2. This article shows by comparison that the Triassic Ordos Lake turbidite system is larger than all known modern and ancient lacustrine counterparts. The exceptionally large intracontinental sag basin provided a relatively unconfined environment for the development of the turbidite system, explaining its vast extent. Extraordinary flood events formed during the Carnian Pluvial Episode facilitated continuous sediment supply into the turbidite system, supporting its accumulation. Lacustrine flood‐related turbidity currents travelled as sediment‐laden turbulent flows, showing an increase in the proportion of suspended‐load deposits and a decrease in the proportion of bed‐load deposits downstream from the river mouth. Five architectural elements have been revealed, reflecting a distinctive assemblage of erosional bedforms and depositional bedforms in channel‐lobe systems, and their recognition criteria were established. This study changes the traditional understanding of lacustrine turbidite systems, generally interpreted as having smaller sizes, and demonstrates likewise in the lacustrine realm, that extreme flood events can generate a world‐class deep‐water turbidite system, which can even be comparable with its submarine counterparts. This study also confirms that the combination of low‐gradient slopes and a long‐lived, mixed‐load, prograding fluvial feeder system can produce exceptionally large‐scale deep‐lake flood‐related turbidites. Furthermore, it has implications for the prediction of facies and reservoir quality in ancient lacustrine turbidite systems.

Stationary vs non-stationary modelling of flood frequency distribution across northwest England

ABSTRACT Extraordinary flood events occurred recently in northwest England, with several severe floods in Cumbria, Lancashire and the Manchester area in 2004, 2009 and 2015. These clustered extraordinary events have raised the question of whether any changes in the magnitude and frequency of river flows in the region can be detected. For this purpose, the annual maximum series of 39 river gauging stations in the study area are analysed. In particular, non-stationary models that include time, annual rainfall and annual temperature as predictors are investigated. Most records demonstrate a marked non-stationary behaviour and an increase of up to 75% in flood quantile estimates during the study period. Annual rainfall explains the largest proportion of variability in the peak flow series relative to other predictors considered in our study, providing practitioners with a useful framework for updating flood quantile estimates based on the dynamics of this highly accessible and informative climate indicator.

Calculation of nonstationary flood return period considering historical extraordinary flood events

With climate change and intensification of human activities, the flood generation mechanism has been influenced which results in the nonstationarity of flood time series. In this study, in order to use historical flood data for flood frequency analysis, a nonlinear AutoRegressive exogenous (ARX) model was employed to complement missing flood data between the historical and recorded data. The key statistics of the complemented time series agreed well with those of the documented record. Using EWT (the expected waiting time until an exceedance) and ENE (the expected number of events in m year is one), nonstationary flood return periods were estimated by the complemented and recorded annual maximum flood peak series, respectively. The results showed that the return levels under nonstationarity are smaller than those under stationarity due to the construction of small hydraulic structures and many check dams after 1980s. This study implies that it is necessary to refine the flood return levels and the corresponding flood control measures for nonstationary flood series due to environmental change.

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.

Extraordinary flood events and the response to monsoonal climatic change during the last 3000 years along the middle Yangtze River valley, China

Well-dated long-term flood data series is very important in studying of hydrological response to past climatic change along the mainstream of the Yangtze River. Five palaeoflood events recorded by slackwater deposits were identified by the sedimentological criteria and analytical results. A detailed chronological framework of palaeoflood events during the late Holocene was established by optically stimulated luminescence dating in combination with anthropogenic remains, and with pedo-stratigraphic correlations with well dated Holocene loess-soil profiles within the middle Yangtze River basin. The five palaeoflood events were dated to 2.43±0.18ka, 1.82±0.17ka, 0.94±0.07ka, 0.72±0.06ka and 0.59±0.05ka, respectively. These results are consistent with geological records at other palaeoflood sites and historical flood records along the middle Yangtze River valley. A regional synthesis of the palaeoflood chronology over the last 3000years was compiled along the middle Yangtze River. Extraordinary flood events generally seem to be correlated with late Holocene climatic variability (i.e. Neoglacial cooling, Roman Warm Period, Dark Ages Cold Period, Medieval Climate Anomaly, and Little Ice Age). The high-resolution climatic proxies from stalagmites of the Dongge cave and Heshang cave, ice-cores of the GRIP, the total solar irradiance variations and sunspot number, and ENSO activities suggest that these hydroclimatic events are possibly related to the weaker Asia summer monsoon and the cooling climatic events, as well as the stronger ENSO activities during the late Holocene in the Yangtze River valley. These results provide insights into the response of hydroclimatic system to global change in the large rivers of Asia.

Holocene climatic events recorded in palaeoflood slackwater deposits along the middle Yiluohe River valley, middle Yellow River basin, China

Palaeohydrological investigations were carried out in the middle reaches of the Yiluohe River, a major tributary in the lower–middle Yellow River basin. Typical palaeoflood slackwater deposits (SWDs) were identified in the Holocene pedostratigraphy on the cliffy river banks. Analytical results, including magnetic susceptibility and grain-size distribution data, indicated that these SWDs were deposited from the suspended sediment load in flood water. These SWDs are different from eolian loess, soils and aeolian sands in the riverbank profile. They recorded several episodes of extraordinary palaeoflood events. In the Longmenxia reaches of the Yihe River valley, these flood events were dated at 3100–3000a, 1800–1700a, 770–610a, and 420–340a using the optically stimulated luminescence method in combination with the pedostratigraphic correlations. In the Longhutan reaches of the Luohe River valley, the palaeoflood events were dated at 1975–1466a, i.e., from the Han to Wei dynasties (AD 25–534), during which the capital city on the river banks was flooded many times, as recorded in the literature. These extraordinary flood events are well correlated chronologically with the known Holocene climatic events that occurred in the Northern Hemisphere. Thus, the monsoonal climate was highly variable with both floods and droughts occurring frequently during these episodes. These results are important for understanding the response of river systems in eastern Asia to global changes.

Historical floods in Tabasco and Chiapas during sixteenth–twentieth centuries

It is presented a chronology of historical floods during sixteenth–twentieth centuries in the Mexican states of Tabasco and Chiapas. There were 41 historical flooding events during 1528–1948, 16 of them were catastrophic flooding and 25 were extraordinary ones. There were periods of historical floods between 1651–1652, 1676–1677, 1679–1680, 1888–1889, 1927–1929, 1931–1933 and 1940–1944. During the instrumental period (1949–1999) there were only four extraordinary flood events in the Usumacinta River. Most of flood periods coincided with the warm phase of the Atlantic Multidecadal Oscillation (AMO). The flood period of 1940–1944 was as long as the most recent one (2007–2011). Wavelet analysis found flood periodicities of 2.5, 52 and 83 years, but only the last one was statistical significant and their occurrence was in phase with the AMO. Logistic regression showed that AMO index was the most correlated index with flood events. In fact, the odds ratio showed that floods were 1.90 times more likely to occur when AMO index was positive. This regression model predicted correctly 64.70 % of flood occurrences during twentieth century using its flood information only as validation data.

Major elements in the Holocene loess-paleosol sequence in the upper reaches of the Weihe River valley, China

Palaeohydrological investigations were carried out in the Guchuan Basin in the upper reaches of the Weihe River valley, China. A set of palaeoflood slackwater deposits (SWDs) was found interbedded in the Holocene loess-paleosol sequence at the Guchuanzhen site (GCZ). These palaeoflood SWDs were studied by field observations and laboratory analyses including concentrations of chemical elements and optically stimulated luminescence (OSL) dating. The results showed that the palaeoflood SWDs were the result of the secondary separations of the surface soil and weathered soil layers during the process of water transport and deposition, and without obvious weathering during soil development. These extraordinary flood events were dated back to 3,200−3,000 a B.P. with the OSL method and checked by archaeological dating of the human remains retrieved from the profile. These extraordinary flood events were therefore considered as regional expression of known climatic events and demonstrated the climatic instability in the Holocene. This result is important for understanding the effects of global climate change on the dynamics of river systems.

The propagation of complex flood-induced head wavefronts through a heterogeneous alluvial aquifer and its applicability in groundwater flood risk management

Summary The extraordinary rise of piezometric surface in aquifers may induce an unexpected interaction of groundwater with anthropogenic elements known as groundwater inundation phenomena. This could result in several damage processes, including building foundation destabilization, groundwater infiltration and pollutant remobilization, which are responsible for considerable economic losses worldwide. To improve our knowledge concerning flood-induced head wavefront propagation and groundwater inundation phenomena, the reconstructed kinematics of the head wavefronts obtained from a calibrated groundwater model simulating 12 ordinary flood events have been statistically analyzed. The correlation between kinematic variables of a flood-induced head wavefront, flood event characteristics and aquifer properties was the basis for the study of the aquifer response to river flood events along four different trajectories, i.e. the identification of river level rise rate, absolute–relative height of the maximum rise stage, hydraulic parameter variability, river–aquifer exchange rates, pre-event state of the aquifer and distance to the aquifer boundary as the key-influencing factors in groundwater inundation. Finally, model and scenario results were used to develop two synthetic models for groundwater inundation as novel decision-support tools for assessing both ordinary and extraordinary flood events in groundwater flood risk management and in urban development planning. These synthetic models gives the technical criteria to predict extraordinary rise of piezometric surface and therefore allows increasing the confidence of stakeholders in the risk assessment and improving the subsurface infrastructure design to mitigate damage processes derived from groundwater interaction.

Extraordinary hydro-climatic events during 1800–1600 yr BP in the Jin–Shaan Gorges along the middle Yellow River, China

Paleoflood slackwater deposits (SWDs) of the Holocene were found at many sites along the Jin–Shaan Gorges in the middle Yellow River basin. A set of four paleoflood SWD beds was identified at the Pingduguan (PDG) sites and studied by field observations and laboratory analysis including particle-size distribution and Optically Stimulated Luminescence (OSL) dating. Hydrological reconstruction using the HEC-RAS model shows that the paleoflood peak discharges were between 39,000 and 50,220m3s−1. They are about double the largest gauged flood (24,000m3s−1) that has occurred since 1976 along the Jin–Shaan Gorges. These extraordinary flood events were OSL dated to between 1800 and 1600yr BP during which climatic deterioration and disasters were documented over the Yellow River basin. Various evidences show that severe flood and drought disasters during the episode resulted in frequent harvest failures, famines, social upheavals, and population reduction, invasions by nomads, and even the fall and replacement of the dynasties. Climate decline documented by proxy records such as tree-rings, stalagmites, ice-cores and lake sediments from over the world is in agreement with the paleoflood events identified along the Jin–Shaan Gorges. These mean that the extraordinary paleoflood events are closely related to increased climatic variability and instability. This result provides solid evidence for understanding the response of hydroclimatic system to global climate change in the semi-arid and sub-humid regions of the world.

Regional flood frequency analyses involving extraordinary flood events at ungauged sites: further developments and validations

Flood frequency analyses are often based on recorded series at gauging stations. However, the length of the available data sets is usually too short to provide reliable estimates of extreme design floods. Hence, hydrologists have tried to make use of alternative sources of information to enrich the datasets used for the statistical inferences. Two main approaches were therefore proposed. The first consists in extending the information in time, making use of historical and paleoflood data. The second, spatial extension, consists in merging statistically homogeneous data to build large regional data samples. Recently, a combination of the two techniques aiming at including estimated extreme discharges at ungauged sites of a region in the regional flood frequency analyses has been proposed. This paper presents a consolidation of this approach and its comparison with the standard regional flood frequency approach proposed by Hosking & Wallis. A modification of the likelihood function is introduced to enable the simultaneous calibration of a regional index flood relation and of the parameters of the regional growth curve. Moreover, the efficiency of the proposed method is evaluated based on a large number of Monte Carlo simulated data sets. This work confirms that extreme peak discharges estimated at ungauged sites may be of great value for the evaluation of large return period (typically over 100 years) flood quantiles. They should therefore not be neglected despite the uncertainties associated to these estimates.

Extraordinary hydro-climatic events during the period AD 200−300 recorded by slackwater deposits in the upper Hanjiang River valley, China

The Hanjiang River, a major tributary of the Yangtze River in China, is noted for the current national South-to-North Water Diversion project. Palaeo-hydrological investigations were carried out along the upper reach gorges of the Hanjiang River that drains the Qinling and the Dabashan Mountains. A set of palaeoflood slackwater deposit beds (SWDs) was identified in Holocene pedo-stratigraphy of the riverbanks along the valley. These SWDs are interbedded in the eolian loess-soil profiles in the cliffy riverbanks and they thin out toward the upper slopes. The palaeoflood SWDs were differentiated from eolian loess and soil by the sedimentary criteria and analytical results. The minimum flood peak discharges were estimated to be 65,400−65,830m3s−1 by using palaeo-hydrological methods. They are about twice of the largest gauged flood (34,300m3s−1) that has ever been measured. They represent the largest flood events in the upper reaches of the Hanjiang River over the Holocene. These extraordinary flood events were dated to 1810−1710 a (AD 200−300) with the optically stimulated luminescence method and checked by archaeological dating of the human remains retrieved from the profiles. This indicates that the extraordinary floods occurred during the dynasties of Eastern Han to Western Jin (AD 25−316) in Chinese history, during which severe droughts and floods were recorded in documents. In the reconstructed 2000-year temperature time series based on high-resolution climatic proxies from tree-rings, stalagmites, ice-cores and lake sediments from over the world, these extraordinary palaeoflood events are correlated with an increased climatic variability characterized by cooling and drying during the period AD 150−350. This result is important for understanding the effects of global change on river system dynamics.

Sedimentary and hydrological studies of the Holocene palaeofloods in the middle reaches of the Jinghe River

Palaeo-hydrological field investigation was carried out in the middle reaches of the Jinghe River. A set of palaeoflood slackwater deposit beds was identified in the Holocene loess-soil sequence in the riverbanks. The sediment samples were collected from the profile, and the particle-size distribution, magnetic susceptibility, loss-on-ignition were analyzed in laboratory. The analytical results showed that the palaeoflood slackwater deposits have recorded extraordinary flood events in the Jinghe River valley. According to stratigraphic correlation and OSL dating, the palaeoflood events were dated to 4100–4000 a BP. The palaeoflood peak discharges were estimated to be 19,410–22,280 m3/s by using the hydrological model and checked by different approaches. These results have the flood data sequence of the Jinghe River extended to 10,000-year time-scale. It provided significant data for hydraulic engineering and for mitigation of flood hazards in the Jinghe River drainage basin.

Sedimentary records of extraordinary floods at the ending of the mid-Holocene climatic optimum along the Upper Weihe River, China

Sedimentary records of the Holocene extraordinary floods were investigated in the upper reaches of the Weihe River, a major tributary in the middle Yellow River basin. Palaeoflood slackwater deposits (SWD) were identified at several sites on the riverbanks. These clayey silt beds are inserted into the Holocene aeolian loess-soil profiles and slope clastic deposits. They have recorded the extraordinary palaeoflood events which occurred between 3200 and 3000 yr BP as dated by OSL method and checked by the archaeological remains of the Neolithic and Bronze Age retrieved from the profile. The minimum flood peak discharges were estimated at between 22 560 and 25 960 m3/s by using palaeohydrological methods. It is 4.5–5.0 times the largest gauged flood (5030 m3/s) that has ever occurred since 1934. The palaeoflood slackwater deposits were found inserted into the pedostratigraphic boundary between the late-Holocene loess (L0) and the mid-Holocene Luvisol (S0) in the riverbank profile. This indicates that the extraordinary flood events were synchronous with the pedogenic regression at the end of the mid-Holocene climatic optimum. The climatic proxies from the studied profile and the correlative profiles in the river valley and the Loess Plateau show that the pedogenic regression was forced by climatic aridity and intensified dust storms and dust falls in connection with monsoonal shift over the Yellow River basin at about 3100 yr BP. The extraordinary flood events were documented not only on the Weihe River, but also on the mainstream and other tributaries of the Yellow River. These suggest that both extraordinary floods and droughts were parts of the climatic variability during the monsoonal shift. These findings are of great importance in understanding the interactions between hydrological system and climatic change in the semi-arid and subhumid regions of the world.

Is it possible to study palaeoenvironmental changes in Alpine spring habitats? A few examples from the south-eastern Alps (NE Italy)

Palaeolimnological techniques were applied to spring habitats in the Trentino Region (NE Italy) as a test in order to reconstruct past environmental changes in these crucial and fragile milieus. Three different sites were selected from a database of more than one hundred on the basis of morphological, geological, and biological factors, including human impact evaluation: Nambrone, Paul and Madonnina Val Lomasona. Sampling was performed by adapting standard lake-sediment coring methods, and the retrieved sediment was described and subsampled following standard "lake" procedures. Field work and sedimentological analyses revealed clear influence from nearby fluvial environment for the Nambrone site, whereas Paul and Madonnina Val Lomasona were selected for further analyses (e.g., dating, diatoms, chironomids). Lead-210 and Caesium-137 dating methods were used to obtain a chronological framework, in addition to historical information on land use and other events in the area. The upper part of the Paul sequence presented organic rich sediment compatible with spring environments, but the lower and essentially detrital part appeared to record alluvial deposition, probably linked to extraordinary flood events (1882, 1966). This interpretation is indirectly confirmed by the records of the fallout nuclides Caesium-137 and Lead-210, that were both unfortunately concentrated in the topmost 1 cm, making it impossible to construct a normal age-depth profile. Madonnina Val Lomasona sedimentological and biological indicators point to a marked change around ca 5 cm, dividing the sequence in two units, corresponding to different environmental conditions. Although the interpretation of the dating results is not completely straightforward, the assembled data suggest that the upper 5 cm of the record represents ca the last 50 years. During this period we found typical limnocrenic, clear-water spring conditions, while before 1960s the coexistence of lotic and lentic chironomid species including several opportunistic ones and the low richness, emphasized the presence of a disturbance (e.g., highly variable flow and/or trophic conditions), as confirmed also by diatom assemblages and sedimentological features. The recorded disturbance is probably linked to a water-level change due to the local land use, as also testified by historical data. The results of this study support the idea that carefully selected spring sites might be approached with adapted palaeolimnological techniques to extract valuable palaeoenvironmental information. However, it also highlighted the low percentage of "good sites" (1%) and considerable difficulties in coring and sampling without disturbing the sediment.

Historical floods and dendrochronological dating of a wooden deck in the Old Mint of Segovia, Spain

AbstractDuring recent archaeological work carried out in the 16th‐century Segovia Mint, Spain, a wooden deck of Pinus sylvestris was revealed covering the hydraulic canals. Dendrochronological analyses allowed us to estimate the dates that trees were felled in order to effect repairs to canals damaged after extraordinary flood events. There are at least three age clusters: (1) 1677–1678; (2) 1690–1698–1700; and (3) 1713–1720–1721–1742. By comparing dendrochronological data with both documentary records of repair work and the inventory of historical floods, the dates of the main floods affecting the building between 1583 and 1771 can be identified. Our work is innovative in that it uses tree‐ring dating in historical buildings to reconstruct past flood events, which are then linked to documentary records. It has three main applications: validating documented flood dates, estimating their relative magnitude, and locating new non‐documented flood events or previously unknown historical repair work at this UNESCO World Heritage Site. © 2011 Wiley Periodicals, Inc.