Displays

          The Indus River originating from the Manasarovar Lake runs along the Indus Tsangpo Suture Zone at Ladakh separating the Tethyan Himalaya in the south from the Karakoram Zone in the north. Due to the barrier created by the Pir Panjal Ranges and the Higher Himalaya, Ladakh falls in the rain shadow zone of ISM (Indian Summer Monsoon) with an average annual temperature of ~7.3°C. Random catastrophic hydrological events are known to endanger lives and properties of people residing here. So, determination of frequency, recurrence and forcing mechanism of past extreme floods are crucial in this highly vulnerable area.

          Here we studied Holocene mega flood history of the Upper Indus River at Ladakh using slack water deposits (SWDs). SWDs are composed of stacks of sand-silt couplets deposited during high flooding events. They are deposited instantly from suspension associated with sharp reduction of flow velocity due to local obstructions. Each couplet represent a flooding event. These events are dated employing Optically Stimulated Luminescence (OSL) using sand and AMS ¹⁴C using charcoal specks and hearth layers. The frequency of these events suggest higher occurrence of mega floods during pronounced northward penetration of ISM. Recurrence Interval (RI) analysis of these events suggest spatial variation in forcing mechanism between the trunk and the main tributary channel (Zanskar). Sedimentary provenance of these events are also analyzed using detrital zircon geochronology. The provenance analysis indicate more efficient sediment transportation along the Zanskar River as compared to the main Indus channel. Post LGM (Last Glacial Maximum) human migration along the channel is revealed from hearths found within these SWDs which generally occurs during post flooding episodes. Materials found within the hearths, chronology and the fashion of occurrence imply migration and cultural connectivity between the Indian sub-continent and the Central Asia along the ancient Silk Road at Ladakh as old as ~14 ka.

How to cite: Pankaj Sharma, C., Chahal, P., Kumar, A., Srivastava, P., Singhal, S., Agnihotri, R., Wasson, R. J., Ziegler, A. D., and Shukla, U. K.: Holocene megaflood history and provenance of the upper Indus-River. Implication for human migration along the ancient Silk Road at Ladakh., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-384, https://doi.org/10.5194/egusphere-egu2020-384, 2020.

In recent times global climatic changes are gaining due importance. One of the ways in which climate change affects humankind is the frequent occurrence of extreme climatic disruptions, such as high magnitude climate-flooding events. In order to understand the present and future trend and pattern of the changing climate, it is important to identify high magnitude palaeoflood events and reconstruct the palaeohydrology. Instrumental/historic records have helped to understand the extreme flood-climate relationship in the modern environment. However, to understand their long-term relation (10²-10³ years) studying sedimentological archives of large magnitude floods (eg. slack water deposits) is important, also, leading us to understand the future climatic disruptions even more effectively. Robust estimation of palaeoflood discharges and frequencies will also lead to the formulation of better flood-related policies.

Current study is undertaken in the upper reaches of Kaveri basin, Southern India and shows noteworthy link between the major climatic transitions (from fluvial dormancy to sudden outburst of monsoons around 2 ka, onset of Little Ice Age (LIA) in the 14^th century, end of LIA in the 19^th century and then the 20^th century) and increased frequency of large magnitude floods. Detailed flood chronology was established using the optically stimulated luminescence (OSL) dating technique. OSL dates the last daylight exposure of the sediment. In addition, palaeoflood discharge estimations were made based on Manning’s equation. Together with numerical dating, it allowed the reconstruction of flood magnitude and frequency over an extended period of time. The study suggests that the magnitudes of recent flood events are higher than the palaeoflood magnitudes in the study area. We also observe that the two major flood events of the 20^th century reported from the upper Kaveri were produced by high-intensity short-duration storm events. Rainfall precipitation analyses of the last 10 years (2010-2019) demonstrate the increase in erraticity of rainfall also causing extreme floods. Analyses of other hydrological variables such as soil moisture, basin shape, and size in producing floods in the study area suggest that rainfall alone may not always be the ultimate proxy for subsequent flooding.

How to cite: Goswami, K., Jaiswal, M. K., and Kale, V.: Paleohydrology of high magnitude floods from upper Kaveri basin, Southern India: Implication to late Holocene climate variations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-645, https://doi.org/10.5194/egusphere-egu2020-645, 2020.

The Judea Desert constitutes a distinctive hydrological region characterized by short and steep ephemeral streams draining eastward to the Dead Sea Valley. The aridity of the Judea desert is caused by the rain-shadow effect of the north-south, mountainous back bone (MBB) of Israel, as well as by the low elevations within the Dead Sea valley. The hydrological data for these streams is scarce, which leads to poor estimation of the magnitude and frequency of floods. The lack of data is particularly significant when planning infrastructure such as roads, bridges, reservoirs, dams etc. Flood frequency analysis for risk assessment is therefore, based on various models such as rainfall-runoff, empirical, regional models etc.

The current study is based on Palaeoflood Hydrology  which uses geomorphological evidence for real floods that accumulate in typical natural traps, along the course of the streams for hundreds and thousands of years. The collection of these data enables us to reconstruct the history of the floods in the streams including/at least the largest event that occurred in the stream in the last hundreds to thousands years. By combining these data with measured and historical data (if any), a long, solid database can be reconstructed. The applicability of the system in Israel has been proven in the larger streams in the Negev.  However, the Negev Desert is a significantly different hydrological environment. The  largest flood that occurred in the stream is important for regional envelope curves. Long palaeoflood records can indicate on changes in the hydrological regime, which testify for climatic fluctuations.

The method is based on field evidence in the form of slackwater deposits and other high water marks, which accumulate in typical sites and indicate on the minimum water elevation enabling discharge calculations using HECRAS hydraulic engineering software. The ages of the floods are determined by dating the flood deposits using radiocarbon and OSL.

In the Upper Nahal Rahaf stream (50 km²), three sites were located with 2-4 flood deposits at each site, including  a rock shelter within which  2 flood remnants with reconstructed peak discharges of 1,200-1,300 m³/s. These flood sediments are overlying  an Upper Paleolithic site dated to about 30 ka.

In Nahal Ze'elim stream (245 km²) 5 sites were located - 4 of which close to the outlet. Each site recorded between 2-8 sedimentary units with reconstructed peak discharges of 200-900 m³/sec.

The integration of the floods from all sites with their age revealed a vast information regarding major events. In further study this will also allow a renewed frequency analysis on the basis of wider knowledge.

How to cite: Zituni, R., Greenbaum, N., and Zilberman, E.: Magnitude and Frequency of the Largest Palaeofloods during the Holocene in Nahal Ze'elim and Nahal Rahaf, Judea Desert, Israel, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-842, https://doi.org/10.5194/egusphere-egu2020-842, 2020.

The Moksha River valley was studied in its lower part between the Tsna River confluence and the mouth of the Moksha River. Wide floodplain and two levels of terraces are presented on the studied part of the valley. The height of the floodplain is from 1 to 6 m, of the first terrace – about 9-11 m, of the second terrace – 18-22 m. The width of the valley in this area is about 14-16 km, but sometimes it can reach 20-22 km and more. The width of the floodplain is about 12-14 km.

The Moksha River is a meandering channel. Large and small (modern-size) meandering palaeochannels spread widely on the floodplain surface. These palaeochannels were the main objects of our study. Small palaeochannels have the same parameters as the modern river channel: their width is about 100-150 m, wavelength is between 300-400 and 600-700 m. For the large palaeochannels (macromeanders) the mean parameters are the following: width is about 250-300 m, wavelength is about 1500-2000 m. These large palaeochannels are the signs of high flood activity epoch(s).

In our study we used a number of field and laboratory methods. Twelve boreholes in large and small palaeochannels were made during fieldwork in August-September 2019. Organic material from studied palaeochennels was sampled to make radiocarbon (AMS) dating to find the time of palaeochannels’ formation and infilling. Also we made the reconstructions of paleo-discharges of the Moksha River based on paleochannels’ parameters.

We studied both large and small palaeochannels to reconstruct palaeohydrology and history of the Moksha River valley development in Late Pleistocene. Large palaeochannels correspond to the time of high river runoff. The oldest ones of small palaeochannels were studied to know the time of lowering of the river runoff. Presumably, large palaeochannels were formed at the end of Late Glacial (after LGM) when river runoff was much higher than the modern one. This period of extremely high runoff was previously distinguished in many river valleys of East European Plain, where formation of large paleochannels is usually associated with Late Glacial (the end of MIS 2). Lowering of runoff on the central part of the East European Plain is usually associated with the beginning of the Holocene.

This study is supported by Russian Science Foundation (Project № 19-17-00215).

How to cite: Matlakhova, E., Panin, A., and Ukraintsev, V.: Late Pleistocene Palaeohydrology of the Moksha River (the Volga Basin), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5640, https://doi.org/10.5194/egusphere-egu2020-5640, 2020.

A dimensionality reduction-feature selection approach to streamflow reconstruction using dendrochronological data

Marco Obertelli¹, Alessandro Amaranto¹, Andrea Castelletti¹

¹Department of Electronics, Information and Bioengineering, Politecnico di Milano, Milano, Italy

Instrumental gauge records are the basis for understanding streamflow variability and for informing water management plans accordingly. Yet, these records seldom exceed 100 years and might thus be insufficient for capturing the whole range of streamflow variability for a given river section. Tree ring width from climatically sensitive trees provides a means for developing long-duration chronologies that extend beyond instrumental recording. Considering their dependence on temperature and surface water availability in a specific year, dendrodata have been recognized in the literature as representative climate proxies; and have been therefore widely used in recent years for reconstructing chronologies of hydrological variables as precipitation frequency, drought severity and streamflow variability. Paleoclimate reconstructions are usually carried by combining a dimensionality reduction-feature selection technique with linear regression methods.  The goal of this study is to better understand long-term hydrological variability in the Rhine and the Po river basins by reconstructing for the first time their streamflow trajectories from paleoclimatic data. We apply Principal Component Analysis (PCA) for dimensionality reduction and Multiple Linear Regression (MLR) as a reconstruction model.  Palmer Drought Severity Index (PDSI) trajectories, inferred from tree-rings chronologies are employed as paleoclimate proxy. Numerical results show a good accuracy in the reconstruction approach, especially in the Rhine basin (average R² = 0.60). The accuracy decreases in the Po basin, probably due to the Alpine hydrologic regimes which includes complex nonlinear phenomena (e.g. solid precipitation and snowmelt) not fully described by the PDSI drought index. Historical evidence of the reconstructed 1817 drought in the Po river basin has been found as a proof of the reliability of this approach. The variety of the morphological and hydrological characteristics reflected in the two river basins considered in this study, allows to explore how some of their peculiarities reflect on the streamflow reconstruction, increasing the possibility to replicate the approach in other areas of the world.

How to cite: Obertelli, M., Amaranto, A., and Castelletti, A.: A dimensionality reduction-feature selection approach to streamflow reconstruction using dendrochronological data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10326, https://doi.org/10.5194/egusphere-egu2020-10326, 2020.

Anthropogenic activities such as mining are responsible for acid drainage and metal-enriched waters that in turn contaminate river ecosystem downstream due to the weathering of exposed minerals or tailing dam failures. The release of heavy metals is especially disturbing because of their high toxicity and long permanence. Detecting highly polluted areas and their links with high (low) water flow stages can contribute to a better land management of affected areas. Here, we test if trees growing in different geomorphic positions along a river record heavy metal uptake during past floods. To this end, we applied dendrochemical analysis to twenty-five Pinus pinaster Ait. growing on the banks of Odiel River flowing into the Atlantic Ocean located at in south-western Spain. In addition, five trees disconnected from the river channel were sampled as references values. For each tree, we extracted 1 cm-sized increment cores. After dating dendrochronologically, we isolated tree-ring sequences into 5-year blocks matching with the dates of major floods in the catchments. Samples were then analyzed using an Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Our results suggest coherence between tree locations and the amount of heavy metal accumulated in the tree over the last decades. Thus, we clearly show a control of river morphological units on the  heavy metal concentrations in trees, being higher in those trees located on meander cut banks than in trees on point-bar sedimentary structures. We conclude that trees could be a natural proxy to trace chemical dispersion and pollution related to flood events in highly anthropogenic catchments.

How to cite: Delapierre, A., Ballesteros Canovas, J. A., Buzzi, J., Stoffel, M., and Slaveykova, V. I.: Trees as sensors of metallic pollution dissemination during past flood events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10887, https://doi.org/10.5194/egusphere-egu2020-10887, 2020.

According to the present knowledge, the second half of the 19^th century meant the end of the Little Ice Age and gradual warming.  This is, however, undoubtedly a fairly simplified statement.  Our contribution presents the period of 1858–1878: (1) from the point of view of drought but also (2) regarding frequency of floods. The aggregation for this period of weather-driven risks such as droughts, floods, strong winds and high tides, is worth attention.  The length of the drought period of 1858–1878, the absolute value of rainfall deficits and the length of seasonal droughts, as well as their impacts, are a certain warning in terms of our present.

Surprisingly, in such a dry period we witness an accumulation of important and extreme flood episodes as well. The regional catastrophic floods of 1858, and winter extensive floods of 1862 and 1876, may serve as excellent examples.  Furthermore, the Elbe catchment recorded floods with return periods of 10–20 years in 1860, 1865 and 1872. For this period, an occurrence of intensive mesoscale flash flood events with extreme hydrological parameters, high number of fatalities and large damages are of the utmost importance (e.g. 1868-Switzerland, 1872-Czechlands, 1874- Catalonia, 1875-South France). Our contribution builds on earlier analysed flood events of 1872, 1875 and drought period presented at EGU earlier. The contribution stresses the analogies and differences with present situation in 2014–2019.  We mainly address the situation in Czech lands, Central Europe interpreted in wider European context.

How to cite: Elleder, L., Kašpárek, L., Krejčí, J., Šírová, J., and Racko, S.: The unusual floods and flood frequency in 1858–1878 dry period in Central and Western Europe, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17459, https://doi.org/10.5194/egusphere-egu2020-17459, 2020.

The Lake Moo plain has a surface area about 0.15Km². It is located near the boundary between Emilia-Romagna and Liguria regions, at an altitude of 1130m a.s.l. (northern Apennines, Italy). This site is strategic to the dominant atmospheric currents, very prone to high intensity precipitation events (HIP) and related high-density flood. Indeed, Lake Moo area has been partially covered by a flood deposit released by a record-breaking rainfall intensity in September 2015. The intensity and wide spatial scale of those phenomena has leads us to investigate their frequency in the past, beyond the instrumental time. The lacustrine succession (ca. 13 m-thick) was studied through the extraction of one core and framed within sedimentary facies analysis approach. The paleoenvironmental interpretation of the succession was achieved combining sedimentological, pollen and pedological data and radiocarbon dating. Thirteen different facies types have been identified and the core succession is was subdivided into five informal units. The different coarse-grained layers interbedded with organic-rich silty clays and peaty layers have been interpreted as the extreme flood deposits triggered by high-intensity convective rainfall events in the catchment area that flow into the Lake Moo plain.

These coarse-grained deposits were grouped according to the genetic approach and therefore based on facies tract concept. The goal of this study is how the facies tract approach may represent a novel method that can be used to improve our understanding of flood reconstruction dynamics and may be applied to other similar deposits. We interpret the local lacustrine succession is like to the infill of a structural depression produced by gravitational block sliding that was induced by post-glacial fluvial incision.

Finally, the observed depositional cycles were put in relation with other specific paleoclimatic proxies available in literature for the area.

How to cite: Segadelli, S., Grazzini, F., Aguzzi, M., Chelli, A., Rossi, V., De Nardo, M. T., Francese, R., Marvelli, S., Marchesini, M., and Nanni, S.: Multidisciplinary analysis at Lake Moo: Changes in high intensity precipitation on the Northern Apennines (Italy) over the last 9000 years, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11258, https://doi.org/10.5194/egusphere-egu2020-11258, 2020.

The Glomma river is the largest in Norway and repeated destructive floods continue to represent a major climate hazard. Area planning and dam safety assessment in Norway, including the large catchment that feeds Glomma, are based on estimates of design flood sizes from 200 to 1000 years return periods despite the fact that most streamflow time series are ≤50 years. Consequently, design flood estimates are subject to sample uncertainty. Other data than streamflow measurements such as historical data and lake sediment cores can be employed not only to increase knowledge about floods, but also to reduce uncertainty in design flood estimates. By merging different data sources, it is possible to reduce the uncertainty associated with flood frequency analysis. The primary objective of this study is to combine systematic- historical and paleo-information in a methodological effort to improve flood frequency analysis.

We approach this ambition by (i) compiling historical flood data from the existing literature, (ii) presenting  high resolution XRF, MS and CT scanning data from a sediment core covering the last 10 000 years, and (iii) combining flood data from systematic streamflow measurements, historical sources and lacustrine sediment cores for estimating design floods and assessing non-stationarities in flood frequency.

Based on the lake sediments from Lake Flyginsjøen, which faithfully records flood events in Glomma, we can estimate flood frequency in a moving window of 50 years. Whenever the discharge is sufficient the floodwater crosses a local threshold and suspended sediments are deposited in the lake, providing information about how flood frequency has changed over the last 10 00 years. 

The lake sediment data shows that past flood frequency is non-stationarity on different time scales. Periods with increased flood activity corresponds broadly to similar timeseries from eastern Norway and also in the Alps on centennial time scales. The flood frequency shows significant non-stationarities within periods with increased flood activity as was the case for the 18th century. The lake data indicates that the major historical flood in 1789 is the largest on record for the last 10 000 years at this site.

The results show that estimation of flood quantiles can benefit from the inclusion of historical and paleodata. The paleodata were in particular useful for evaluating how the flood information in historical data represent flood frequency on longer time scales. Using the frequency of floods obtained from the paleo-flood record resulted in minor changes in design flood estimates.   

This study has shown that the potential advantage of including paleoflood data and we suggest that paleodata has a high potential for detecting links between climate and flood frequency. The data presented here can be used alone, or in combination with paleoflood data from other locations in Norway and Europe, to assess and better understand the potential links between changes in climate and the corresponding changes in flood frequency.

How to cite: Engeland, K., Støren, E., Aano, A., and Paasche, Ø.: New flood frequency estimates for the largest river in Norway based on a novel combination of streamflow-, historical- and paleo-data , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18345, https://doi.org/10.5194/egusphere-egu2020-18345, 2020.

The number and strength of weather and climate extremes, such as severe floods and droughts, have increased significantly during recent decades. Part of such increase is attributed to human-induced climate change. To better assess the role of natural and anthropogenic forcing we should put the recent climate extreme variability into a long-term perspective. We analyze the variability of extreme precipitation and temperature over Europe in connection with observed daily discharge data of the River Ammer (southern Germany) and the 5500-year flood layer record from varved sediments of the downstream Ammersee. We show that daily River Ammer floods are related to upper level  Rossby waves breaking over Europe. This process, which is related to European scale extreme precipitation and temperature anomalies, is consistent with extreme precipitation and temperature patterns associated with River Ammer floods. From a synoptic scale perspective, the observed out-of-phase relationship between solar irradiance forcing and river Ammer floods, as presented in previous studies, is related to enhanced blocking activity over Eastern Europe-western Russia during low solar forcing which favors upper level positive potential vorticity anomalies over western Europe, a more unstable atmosphere and more floods. A singular spectrum analysis of a flood layer record from lake Ammer and a total solar irradiance reconstruction, going back in time to the mid-Holocene, reveals coherent variability at ~900 years and ~2300 years. We argue that similar cycles should dominate the millennial scale variations of blocking activity in the Eastern Europe-western Russia as well as the frequency of extreme temperatures, precipitation and floods over Europe. 

How to cite: Rimbu, N., Ionita, M., and Lohmann, G.: Interannual to millennial scale variability of the River Ammer floods and its relationship with extreme climate and solar forcing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19881, https://doi.org/10.5194/egusphere-egu2020-19881, 2020.

Floods are among the most destructive natural disasters, and robust knowledge about their past, present and future trends is therefore crucial for the sustainable development of societies worldwide. Paleoflood deposits in lacustrine sedimentary sequences enable the continuous reconstruction of flood variability at centennial to millennial scale beyond instrumental hydrological datasets. FLOODARC MSCA project (2019-2021) aims to provide a more comprehensive understanding of the long-term flood variability in the Iberian Peninsula by investigating flood archives in several Spanish lake records. Preliminary results show a see-saw pattern in flood frequency variability during the Medieval Climate Anomaly and the Little Ice Age transition with more (less) flood events occurring during cold (warm) phases in Atlantic and Mediterranean areas respectively. These dissimilarities seem to be controlled by the hydroclimate variability at the regional scale as well as by historical land use changes in different areas of northern Spain.

How to cite: Corella, J. P., Wilhelm, B., Benito, G., Favre, A.-C., and Valero-Garcés, B. L.: Flood variability in northern Spain during the last millennium recorded in lacustrine sedimentary archives, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19406, https://doi.org/10.5194/egusphere-egu2020-19406, 2020.