Slackwater Deposits Research Articles

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.

A multi-index analysis of the extraordinary paleoflood events recorded by slackwater deposits in the Yunxi Reach of the upper Hanjiang River, China

Paleoflood investigations were carried out on the first river terrace along the upper reaches of the Hanjiang River, a major tributary of the Yangtze River in China. Several bedsets of paleoflood slackwater deposit (SWD) were found within the eolian loess-soil profiles in the Yunxi Reach of the upper Hanjiang River. The pedo-stratigraphic sequences were studied using a multi-index approach, including magnetic susceptibility, particle-size distribution, loess-on-ignition, chemical elements and micro-morphology. Both the sedimentary criteria and the analytical results show that the paleoflood SWDs were clearly differentiated from eolian loess and paleosol in the same profile by their abrupt vertical change in the pedo-stratigraphy. The studies indicate that these paleoflood SWDs were deposited from the suspended sediment load in floodwaters and hardly affected by the subsequent weathering and pedogenesis. We have identified four episodes of extraordinary paleoflood events occurred on the Hanjiang River during the last 18,000years. The paleoflood events were dated to 12,500–12,000aB.P., 7500–7000aB.P., 3100–3000aB.P., and 1000–900aB.P. respectively, by using the OSL method in combination with archeological dating of anthropogenic remnants, and with pedo-stratigraphic correlation with well dated profiles in the upper Hanjiang River. The flooding events are considered to respond to climate variability in relation with the monsoonal shift and coincide with the global climatic events recorded by other climatic proxies in different regions of the world. This study is of great significance for understanding the interactions between hydro-climatic systems and global change in sub-humid regions.

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.

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.

Extreme paleoflood events 3200–3000 a BP in the Jingyuan–Jingtai reaches of the upper Yellow River, China

A set of paleoflood slack water deposit (SWD) beds was identified within slope clasts at the Jinpingcun (JPC) site in the Jingyuan–Jingtai reaches of the upper Yellow River gorges based on sedimentary criteria and analytical results. Paleoflood hydrology methods were applied to reconstruct paleoflood events in the Jingyuan–Jingtai reaches. The paleoflood peak stages were calculated based on the highest end-point elevations of the SWDs. The HEC-RAS model and optically stimulated luminescence dating enabled robust paleo-discharge estimates to be calculated between 12,750 and 16,310 m3/s for paleoflood events that occurred during 3200–3000 a BP. The peak discharges, including gauged and historical flood data at the Lanzhou gauge station and the reconstructed paleoflood peak discharges at the JPC site, were input for flood frequency analysis. These paleo-hydrological approaches can also be applied to other regions in the Yellow River gorges. Furthermore, the ages of the paleoflood events correspond well with the known climatic events at 3100 a BP during Holocene climatic variability. These findings are significant for flood risk estimation, in the assessment of water sources in dryland environments, and in understanding the interactions between hydrological systems and climatic change in arid and semi-arid regions of the upper Yellow River in China.

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.

Soils of Canaan Valley and Adjacent Mountains

The genesis of all soils is a function of the interactions of climate, organisms, relief, parent material, and time, which are often called the five factors of soil formation. Because temperatures are cooler and precipitation is greater in Canaan Valley (hereafter, the Valley) than in most other parts of West Virginia, the Valley's soils tend to be wetter during most of the year than the soils on similar landscape positions in other parts of the state. Although the Valley's various soils differ in age, have developed on different parent materials, and are found in diverse landscape positions, most are acidic and/or wet. The Valley's soils vary from organic soils in depressions to sandy soils on the surrounding ridges. In the Valley, the common bedrock under the organic soils is Greenbrier Limestone. Mineral soils also formed on the Valley's floodplains and terraces. Many of these soils, especially those on the wet terraces, have slowly permeable, clayey subsoils that formed in alluvium or slack-water deposits. It is consistent that these soils are somewhat poorly, poorly, or very poorly drained. In some areas, residuum or colluvium occur below the water-deposited material; in other areas the soils formed completely within residuum or colluvium. The residual materials weathered in place from limestone, shale, and/or sandstone. The colluvium weathered from the same parent materials, but has moved downslope. The soils on the sideslopes surrounding the Valley formed in shales and sandstone and are normally dryer than the soils of the Valley's floor. However, some of these low-lying soils are moderately well to somewhat poorly drained. Soils on the ridgetops formed in Pottsville Sandstone. Unique soil horizons developed where Picea rubens (Red Spruce) and Tsuga canadensis (Eastern Hemlock) occur. These spodosols are acidic, sandy, and have very low water-holding capacities.

Investigating extreme flood response to Holocene palaeoclimate in the Chinese monsoonal zone: A palaeoflood case study from the Hanjiang River

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/L0, L0/S0, and S0/Lt) 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 51,680–53,950m3s−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.

颍河上游褚河段古洪水事件及其沉积响应

通过对颍河上游禇河镇段一处典型的古洪水滞流沉积层剖面(SWD)的粒度、磁化率等指标进行综合研究，并结合野外特征描述，分析了该剖面理化特征，探讨了该地区发生洪水的次数。研究结果表明：1) 在YH-CH剖面粒度组成中以粉砂为优势粒径，含量介于47%~85%之间，平均含量为64.3%；粘粒含量较少，含量波动于3.2%~9.5%之间，平均为5.3%；砂级颗粒平均含量为17%；在累计频率曲线图上，B、C、D、E等层位偏左，SWD1和SWD2偏右；SWD1和SWD2分选较好且沉积颗粒较粗。2) 该剖面低频磁化率(X-lf)和高频磁化率(X-hf)具有相似的变化趋势，都呈现出B、C、D、E磁化率值相对较高，而SWD1与SWD2磁化率值相对较低，说明了SWD1和SWD2为典型的古洪水滞流沉积物。综合研究表明，YH-CH剖面记录两期古洪水事件，对应的剖面深度分别为60~80 cm、180~200 cm，其中第一期洪水较第二期洪水能量较高，携带的沉积物颗粒较粗。同时这些特征也为颍河上游的古洪水事件的识别标志提供参考。 Based on the test of grain size and magnetic ability from typical flood slack water deposits (SWD) in the Chuhe site, in upstream reaches of the Ying River, coupled with filed description, we analyze its physicochemistry features and discuss the sedimentary characteristics when floods happen. The results show that: 1) in the grain size of YH-CH profile, silt that varies from 47% - 85% is the main constituent, and the average level is 64.3%; clay content is less, between 3.2% and 9.5%, and its average level is 5.3%. Average sand-size particles’ content is 17%; on the cumulative frequency cure, B, C, D, and E incline to the left, and however, SWD1 and SWD2 incline to the right. It is obvious that SWD1 and SWD2 are special ancient flood slack water sediments; 2) the section of low-frequency magnetic susceptibility (X-lf) is in line with high-frequency magnetic susceptibility (X-hf) broadly in a changing trend; the magnetic susceptibility value of B, C, D, and E is relatively high, and the magnetic susceptibility value of SWD1 and SWD2 is relatively low. According to the experimental analysis, we conclude that the profile recorded two kinds of flood. And the depth of the corresponding profiles respectively is from 60 to 80 cm and 180 to 200 cm; the energy of the first ancient flood that carried a lot of coarse matters is higher than that of the second ancient flood. In addition, the study of sedimentary characteristics provides the reference for the identification of ancient floods in upstream reaches of the Ying River.

Holocene flooding and climate change in the Mediterranean

Mediterranean fluvial hydrology is characterised by decadal-to-multi-centennial length wet and dry episodes with abrupt transitions related to changes in atmospheric circulation. Since the mid-1990s site-based flood chronologies from slackwater deposits in bedrock rivers and regionally aggregated flood histories from alluvial deposits have developed increasingly higher resolution chronological frameworks, although regional coverage is still uneven. This paper analyses the spatial and temporal distribution of extreme Holocene hydrological events recorded in fluvial stratigraphy in the Iberian Peninsula (Spain and Portugal), southern France, southern Italy, Northern Africa (Morocco and Tunisia) and eastern Mediterranean (Greece, Crete, Turkey, Cyprus and Israel). This study constitutes the most comprehensive investigation of Holocene river flooding ever undertaken in the Mediterranean and is based on the analysis of 515 14C and 53 OSL dates. It reveals that flood periods in different regions cluster into distinct time intervals, although region-wide flooding episodes can be identified at 7400–7150, 4800–4600, 4100–3700, 3300–3200, 2850–2750, 2300–2100, 1700–1600, 1500–1400, 950–800, ca. 300, 200–100cal. BP. Periods with more frequent floods in the western Iberian region coincide with transitions to cool and wetter conditions and persistent negative NAO mode. In Northern Africa increased flood frequency coincides with periods of generally drier climate, while in the eastern Mediterranean there is a higher incidence of extreme flood events under wetter conditions. Our meta-data analysis identifies an out-of-phase pattern of extreme events across the Mediterranean over multi-centennial timescales, which is particularly evident between the western Iberian and eastern Mediterranean regions. This centennial-to-multi-centennial see-saw pattern in flooding indicates that bipolar hydroclimatic conditions existed in the Mediterranean during the Holocene.

Late Pleistocene and Holocene palaeoflood events recorded by slackwater deposits in the upper Hanjiang River valley, China

Summary Slackwater palaeoflood deposits (SWDs) were identified in a bedrock gorge in the upper reaches of the Hanjiang River of central China. The Hanjiang River is the longest tributary of the Yangtze River, one of the most flood-prone rivers in China, and the main source of water for the South-to-North Water Transfer Project (SNWTP). Three main loess-soil profiles with late Pleistocene and Holocene palaeoflood SWD bedsets were found. Palaeoflood SWDs identified interbedded in the loess–soil sequence of late Pleistocene and Holocene age within the cliff riverbanks were studied by field observations and laboratory analysis, including particle−size distribution and Optically Stimulated Luminescence (OSL) dating. At least eight extreme flood events documented by palaeoflood SWDs occurred in the Wufeng reaches of the upper Hanjiang River. The discharge estimation associated with palaeoflood SWDs, indicates that the minimum flood peak discharges of these flood episodes range from 42,220 to 63,400 m3/s. The SWDs were OSL dated to between 12,600–12,400, 4200–4000, 3200–2800 and 1900–1700 a BP and these dates were corroborated with pottery remains retrieved from the profiles and dated by archaeological methods. These periods of increased flood magnitude coincide with contemporaneous global climatic events dated to 12,500, 4200, 3100 and 1900 a BP worldwide. These findings are of great significance in understanding the interactions between hydrological systems and climatic change in monsoonal zones.

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.

Paleofloods recorded by slackwater deposits in the upper reaches of the Hanjiang River valley, middle Yangtze River basin, China

A paleo-hydrological study was carried out along the upper-reach gorges of the Hanjiang River that drains from the Qinling and Dabashan Mountains. Two bedsets of paleoflood slackwater deposits were found embedded in Holocene loess soil sequences within riverbank cliffs. All of these were studied by field observations and laboratory analyses, including magnetic susceptibility, particle size distribution and optically stimulated luminescence (OSL) dating. These analyses provided a complete catalog of Holocene flood events on a watershed scale that is important for understanding the regional hydrological responses to monsoonal climatic change. The results show that these hydro-climatic events were dated to 9000–8500 and 3200–2800 a B.P. The paleoflood peak discharges were estimated to be 44,350–49,860 m3/s by using the HEC-RAS model and were verified by different approaches. These results have extended the flood data sequence of the Hanjiang River to a 10,000-year time-scale. The additional data allowed us to refine the flood frequency-peak discharge curve and potentially increase its accuracy. These results will play an important role in mitigating flood hazards and will be useful for hydraulic engineering and the development of water resources in the Hanjiang River basin.

Holocene palaeoflood events recorded by slackwater deposits along the middle Beiluohe River valley, middle Yellow River basin, China

Palaeohydrological investigations were carried out in the middle Beiluohe River valley in the middle Yellow River basin. Palaeoflood slackwater deposits (SWDs) are natural records of overbank flooding and are often identified in the Holocene aeolian loess–soil profiles along the river valley. Three bedsets of palaeoflood SWDs were found within the mid‐Holocene Climatic Optimum palaeosol (S0) on the right riverbank. Both the sedimentary and the analytical results, including magnetic susceptibility, grain‐size distribution and concentrations of chemical elements, indicate that these palaeoflood SWDs were deposited from the suspended sediment load of overbank flooding. We identified three episodes of extraordinary overbank flooding events. These hydro‐climatic events were dated to 7600–7400, 5800–5000 and 4200–4000 a BP, from optically stimulated luminescence (OSL) dating and pedostratigraphical correlation. The minimum flood peak discharges were estimated to have been between 12 600 and 14 100 m3 s−1 using the slope–area method. These hydro‐climatic events are considered to be a regional expression of known climatic events and demonstrate that the mid‐Holocene climate was far from stable. These results show that the magnitudes of floods are highly sensitive to climate change in semi‐arid and sub‐humid regions with a monsoonal climate.

Reconstructing the palaeoflood events from slackwater deposits in the upper reaches of Hanjiang River, China

Through investigation in the upper reaches of Hanjiang River, China, an aeolian loess-soil profile with four palaeoflood slackwater deposit (SWD) interbeds was found on the cliffed riverbanks in Yunxian county reach of Hubei province. Based on field observation, and laboratory analysis including magnetic susceptibility and particle-size distribution, the four palaeoflood SWDs were differentiated from aeolian loess and soil by sedimentary criteria and analytical results, and recorded four episodes of palaeoflood events in the upper reaches of Hanjiang River. By using stratigraphic correlation and OSL dating, the four palaeofloods were dated to 12,600–12,400 a B.P., 11,600–11,400 a B.P., 5500–5000 a B.P. and 1000–900 a B.P., respectively. According to the calculated maximum palaeoflood stage, the maximum peak discharges of the palaeofloods events were reconstructed by using the HEC-RAS one-dimensional model. The same hydrological method was used to calculate the peak discharge of the 2010 flood in the same reach. The error between the calculated and gauged discharges was 4.9%, showing that the reconstructed four palaeoflood discharges were reliable. The flood peak discharge–frequency relationship at 10,000-year timescale was established by a combination of the gauged flood, historical flood and palaeoflood hydrological data in the upper reaches of the Hanjiang River, assessing reliability of peak discharge data in flood design for the water conservancy construction and flood mitigation.

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.

Sedimentary records of palaeofloods in the Wubu Reach along the Jin-Shaan gorges of the middle Yellow River, China

The Yellow River is known for the heavy suspended sediment load resulting from intensive soil erosion and human disturbance in the drainage basin. Palaeohydrological investigations were carried out along the Jin-Shaan gorges in the middle reaches of the Yellow River. A set of palaeoflood slackwater deposits (SWDs) were identified interbedded in the Holocene loess-soil sequence within the cliff riverbanks. Analytical results of magnetic susceptibility and particle-size distribution indicate that these well-sorted palaeoflood SWD beds were deposited by the suspended sediment load of the floodwater sourced from the loess tablelands in the middle reaches of the Yellow River. Four episodes of extraordinary palaeoflood events have been recorded in the Lianghekou (LHK) site. These hydro-climatic events were dated to 3200–2900 a by using the optically stimulated luminescence (OSL) method in combination with pedo-stratigraphic correlations with the previously studied Holocene pedo-stratigraphy (YHG site) in the Yellow River drainage basin. The peak discharges were estimated to be 41,268–47,415 m3 s−1 by using the slope-area method of streamflow measurement. They are about twice the largest gauged flood (24,000 m3 s−1) that has ever been measured in the Wubu gauge station. These extraordinary palaeoflood events are correlated with Holocene climatic variability characterized by cooling and drying during the period 3200–2900 a. This study provides sdata for hydraulic engineering, development of water resources and mitigating flood hazard in the Yellow River drainage basin, and also gives insight into understanding the sensitivity of regional hydro-climatic systems and the effects of climate change.

Reconstructing the extraordinary palaeoflood events during 3200–2800 a BP in the upper reaches of Hanjiang River Valley, China

Palaeoflood hydrology study is a leading subject in global change study. Through field investigation in the upper reaches of Hanjiang River, palaeoflood slackwater deposits (SWD) were found in the bedrock gorges of the Yunxi reach. The results of field observation, laboratory analysis including particle-size distribution and magnetic susceptibility, and comparison with modern flood deposits, the SWD were identified as the most typical Holocene palaeoflood deposits of the Hanjiang River. By using stratigraphic correlation and OSL dating method, the palaeoflood event was dated to be 3200–2800 a BP at the turn from the middle to late Holocene. According to the palaeoflood peak stage and hydraulic parameters, the peak discharges of the palaeoflood were reconstructed as 48,830–51,710 m3/s by using the slope-area method. At same time, the flood peak discharges of 1983, 2005 and 2010 severe floods were reconstructed with the same method and hydraulic parameters in the same cross section. The error between the reconstructed and gauged discharges was 1.99%–4.21%. This showed that the reconstructed palaeoflood peak discharges were reliable. The flood peak discharge-frequency relationship at 10,000-year time scale was established by a combination of the gauged flood, historical flood and palaeoflood hydrological data. These results are very important for hydraulic engineering and flood mitigation on the Hanjiang River.

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.

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.