Slackwater Deposits Research Articles

Loess and floods: High-resolution multi-proxy data of Last Glacial Maximum (LGM) slackwater deposition in the Flinders Ranges, semi-arid South Australia

Terrace remnants of late Pleistocene fine-grained valley-fill deposits (Silts) deeply entrenched by ephemeral traction load streams in arid areas remain a puzzle. They have been attributed to a variety of origins ranging from lacustrine to alluvial floodplains. We here report a centimetre-scale multi-proxy study of a 7 m section of Silts in the semi-arid Flinders Ranges of South Australia, which span the lead-up to and peak of the Last Glacial Maximum (LGM). The results of detailed lithostratigraphic mapping, high-resolution parametric particle-size analysis, Automated Mineralogy, induced magnetic susceptibility, carbon stable isotope geochemistry, and a chronostratigraphy based on 27 AMS radiocarbon and 6 luminescence ages are discussed in terms of sediment provenance, depositional environment and weathering history with the aim of reconstructing the regional palaeo-environment. The data are consistent with a fluctuating aeolian-fluvial interplay dominating the extended LGM environment with a greater impact on the landscape than all combined geomorphic processes since then. Accordingly, weathered slope mantles and loess accessions were eroded and entrained by numerous small and at least a dozen large-scale flood events, and trapped in an intra-montane floodplain extending into Brachina Gorge. Upstream of this narrow constriction, recurrent backflooding is discussed resulting in a thick sequence of layered to laminated slackwater couplets. Aggradation and degradation of valley-fills appear to be largely controlled by fine-sediment supply from the valley slopes, replenished by wind-blown dust from upwind playa lakes and source-bordering dunefields. In conclusion, this study demonstrates how dust storms and flooding rains can account for ‘pluvial’ features previously explained by the opposing effects of reduced precipitation and evaporation in the colder more arid glacial landscape of southern Australia.

Holocene flood histories in south‐western USA

AbstractRiver basins in south‐western USA are some of the most extensively studied arid land fluvial systems in the world. Since the early 1960s their hydro‐climatic histories have been reconstructed from the analysis of alluvial cut‐and‐fill cycles, while from the late 1970s there have been investigations of slackwater deposits and palaeostage indicators for large floods in stable‐boundary bedrock reaches. However, no studies have regionally integrated Holocene fluvial histories from these two different types of fluvial environments. The current study combines the alluvial archive with flood records from bedrock reaches to generate a probability‐based 12,000 year record of flooding in south‐western USA. Using more than 70014C‐dated fluvial units, the analysis produces a high resolution (centennial) flood record. Seven episodes of increased flooding occurred at 11,250–10,400, 8800–8350, 8230–7600, 6700–5700, 5600–4820, 4550–3320 and 2000–0 cal. BP. Bedrock reaches are found to record more frequent floods during the middle to late Holocene, while in alluvial rivers more flood units are dated to the early and middle Holocene. These differences are primarily the result of selective preservation with alluvial reaches tending to erode during periods characterised by very large floods. Episodes of major Holocene flooding recorded in slackwater deposits within bedrock systems correspond with periods of increased precipitation in the region and lower temperatures. In contrast, within alluvial rivers above‐average flooding probabilities, as well as regionally extensive channel entrenchment episodes, match with reduced annual precipitation and lower temperatures. The results of this study clearly demonstrate the value of the Holocene fluvial archive for reconstructing regional, short‐term hydro‐climatic change in south‐western USA. Copyright © 2010 John Wiley & Sons, Ltd.

Identification Marks of Paleo-flood Events and the Preliminary Application of Micro-morphology in It

Paleo-flood hydrology study is a hot field in global change study.It is the key for studying paleo-flood events how to look for and identify the paleo-flood slack-water deposits effectively and accurately.In this paper,the effects of paleontology,archeology,sedimentology and geochemistry in studying paleo-flood events are summarized at first,and then the micro-morphological features of paleo-flood deposits and other strata are compared and analyzed based on the theory of micro-morphology.The results show that the grain characteristics,mineral composition,pores and micro-configuration in paleo-flood deposits are obviously different from those in other strata:(1) The grains in the paleo-flood slack-water deposits are fine,well-sorted and subrounded or rounded;(2) Mineral composition in the paleo-flood deposits is similar to that in other strata,but the contents of unstable minerals are relatively low in paleo-flood deposits;(3) Soil in paleo-flood deposits is dominated by clay,and the clay content is second only to that in paleosol;(4) The pores are irregular,and the pore walls are coarse.These reveal that the micro-morphology can be used to research and identify the paleo-flood events.

Extraordinary Floods of 4100−4000 a BP recorded at the Late Neolithic Ruins in the Jinghe River Gorges, Middle Reach of the Yellow River, China

Holocene palaeohydrological investigations were carried out in the middle reach of the Jinghe River that drains the central part of the Loess Plateau. A set of palaeoflood slackwater deposits was found interbedded in the Holocene loess-soil sequence within the cliff riverbanks. Both the sedimentary criteria and the analytical results show that this sediment was sourced from a suspended sediment load of floodwater. The major cultural layer of a late Neolithic settlement (4300−4000 a BP) occurring at the same site is blanketed by this flood deposit directly. Slopewash with pottery shards, charcoal and burnt earth occurs in between the palaeoflood slackwater deposit beds. It indicates that each of the slackwater deposit beds has recorded one individual flood event, and that these extraordinary floods occurred when the late Neolithic people occupied the riverbank terrace land. This pre-historical settlement was abandoned after the floods eventually due presumably to repeated inundation by overbank floodwater. The flood events were OSL dated to between 4100 and 4000 a BP and checked by archaeological dating of the anthropogenic remains retrieved from the sequences. Peak discharges of the floods were estimated to between 19,500 and 22,000 m 3 s − 1 , which is several times gauged maximum floods. During the 4200–4000 a BP climatic event, severe droughts were documented in China's monsoonal regions and great floods were known from an ancient legend. This study provides a well-dated sediment record of pre-historical floods which occurred at the transition from the Neolithic to the Bronze Age in the Yellow River basin. It shows that extraordinary floods were part of the abrupt climatic variability during the 4200−4000 a BP climatic event. The highly variable, unstable and catastrophic climate typified by both droughts and floods resulted in settlement abandonment, and possibly the decline of the highly developed late Neolithic civilizations in China's monsoonal regions.

Statistical Characterization of Grain-Size Distributions in Sandy Fluvial Systems

Measured particle-size distributions are commonly reduced to one characteristic value (e.g., median grain diameter) that is used in sediment transport modeling and other analyses. These values are often interpolated from empirical distributions or from fitted distributions, usually assuming that observed grain-size populations are adequately represented by Gaussian or Normal distributions. In order to investigate the implications of this approach, we (1) statistically characterize grain-size distributions in samples of bed-material load, suspended load, and slackwater deposits from the sand-bedded Calamus, North Loup, and Niobrara rivers (Nebraska, USA), and (2) explore the potential impact of misfitting distributions on estimating percentile grain diameters. Although log-normal distributions are commonly used to characterize complete grain-size distributions in sedimentary systems, in this study, samples of transported sediment are best modeled with log-hyperbolic distributions, and slackwater deposits are best fitted by mixtures of distributions. Despite large overlaps in the grain sizes of bed-material-load and suspended-load samples, estimated parameters of fitted log-hyperbolic distributions show consistent differences between these samples across all rivers. Samples of bed-material load have higher modes and positive (coarse-grained) asymmetry, whereas suspended-load samples have lower modes and weaker asymmetry. Because it is has a general form, the log-hyperbolic distribution should adequately characterize unimodal grain-size samples because its parameters can yield both normal-shaped distributions as well as asymmetric distributions. In all three rivers, slackwater deposits contain the entire range of grain sizes present in suspension as well as a significant component of very fine-grained (< 0.02 mm) material that is not present in suspended-sediment samples. This suggests some degree of fractionated deposition of suspended sediment in areas of near-zero flow velocities. Ultimately, fitting parametric grain-size distributions to grain-size data can be a useful way to find effective particle-size values for use in sediment transport modeling and other studies. However, particularly with asymmetric grain-size distributions, fitting log-normal distributions to data may result in errors of estimated percentile grain sizes, which should be considered in studies relying on characteristic grain-diameter values.

Palaeofloods recorded by slackwater deposits on the Qishuihe River in the Middle Yellow River

Palaeoflood hydrological study is a frontal subject of global change study. Using sedimentology, geomorphology and OSL dating methods, the typical palaeoflood slackwater deposits were studied in the Qishuihe River valley. The results showed that five flooding episodes with 21 palaeoflood events occurred during 4300–4250 a B.P., 4250–4190 a B.P., 4190–4100 a B.P., 4100–4000 a B.P. and 3100–3010 a B.P., respectively, during the Holocene period. The palaeoflood peak discharges were calculated with hydrological models. With a combination of the gauged flood, historical flood and palaeoflood hydrological data, the archives of flood events were extended to over 10,000 years in the Qishuihe River valley, and the flood frequency-peak discharge relationship curve was established accurately. These research results played important roles in mitigating flood hazard, hydraulic engineering and also the development of water resources in the semiarid Weihe River basin.

Sediment characteristics at river confluences: a case study of the Mula-Kas confluence, Maharashtra, India

River bed and floodplain sediments are a direct reflection of river channel processes. This paper examines sediment transfer processes at river junctions, using as a case study the River Kas, a tributary of the River Mula, Godavari basin, Maharashtra, India. The study uses four cross-sections, two from upstream tributaries, one in the main confluence zone, and one downstream. Sediment samples are collected from each cross-section and adjacent banks. To understand the tributary impact on the main channel, variations in sand, silt and clay percentages and variations in the shape of sediment particles were recorded. Data suggest that the percentage of silt and clay increased away from the active channel towards banks along a cross-section, except for the mid-channel bar and the downstream segment of the confluence. The `a axis' and `c axis' lengths of particles increased for the samples on the confluence compared to upstream and downstream samples. Sediment characteristics between tributary and the main stream reflect both downstream distance from sediment source and the characteristics of the respective transport processes. The distribution of fine material at the tributary mouth suggests that there have been instances in the past where the mainstream flow has dominated the confluence and has led to slack water deposits on the tributary mouth. Construction of a weir for local flow regulation also affects the pattern and character of sediments. In this large, seasonal river, confluence sedimentology is a joint product of flow variation, confluence morphology and the additional effects of human activity. The study thus provides insights into confluence dynamics and characteristics which may not be revealed in the more intensively researched temperate confluences of smaller scale.

Residual Luminescence in Quartz from Slack Water Deposits in Kaveri Basin, South India: A Single Aliquot Approach

Residual Luminescence in Quartz from Slack Water Deposits in Kaveri Basin, South India: A Single Aliquot Approach Optical bleaching of quartz may be incomplete due to decreased daylight intensity and narrower wavelength spectrum in water column, leading to age overestimation in young fluvial sediments. This hypothesis was tested on a low energy slack water deposits from Kaveri Basin in south India using the SAR protocol. Various tests suggest successful application of SAR on quartz from the study area despite low sensitivity and few numbers of bright grains. Partial bleaching can be ruled out if using SAR on small aliquots. The results suggest a uniform residual luminescence and a common sedimentary condition in this area. Due to partial bleaching, the lowest values of palaeodoses should be considered in preference to the average values.

Breakout flood from an ignimbrite-dammed valley after the 5 ka Numazawako eruption, northeast Japan

Numazawa volcano in northeast Japan erupted most recently at ca. 5 ka, forming a 2-km-diameter caldera and emplacing at least 4 km 3 of valley-confined ignimbrite. The ignimbrite dammed the Tadami River to a depth of >100 m, temporarily impounding >1.6 km 3 of water. Overtopping of the barrier triggered breaching and catastrophic release of the dam lake. Pyroclastic material redeposited by the ensuing flood is widely distributed along the Tadami and Agano Rivers in deposits tens of meters thick as far as the coastal Niigata Plain >150 km downstream of the volcano. Evidence for damming and flooding found along the rivers includes (1) fine-grained, thinly laminated lacustrine deposits upstream of the pyroclastic blockage; (2) 30-m-thick, pumiceous debris-flow and hyperconcentrated-flow deposits that show continuous sedimentation with no major hiatus downstream of the volcano; (3) fine-grained slackwater deposits at tributary river mouths; and (4) large flood boulders that form lags on stripped ignimbrite surfaces and younger terrace surfaces, or cropping out within hyperconcentrated-flow deposits. Paleohydraulic estimation techniques indicate a peak discharge of 30,000–50,000 m 3 /s at the breach point. Burial of Neolithic Jomon settlements by flood and volcaniclastic aggradational deposits at distal locations indicates that such indirect volcanogenic hazards are significant, even where direct primary consequences of volcanic eruptions are minor or absent.

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

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

Glacial activity and catchment dynamics in northwest Greece: Long-term river behaviour and the slackwater sediment record for the last glacial to interglacial transition

Pleistocene glacial activity transformed the runoff regimes and sediment loads of many river systems in the Mediterranean region. New data from northwest Greece show marked contrasts in ice volume and extent between cold stages of the Middle and Late Pleistocene. These altered the long-term meltwater routes and sediment supply conditions in glaciated river catchments. The Voidomatis River basin (384 km 2) of northwest Greece now contains one of the best dated records of Middle and Late Pleistocene glacial and fluvial activity that is constrained by over 40 radiometric dates. This paper evaluates the glacial and fluvial geomorphological records to establish a series of correlations and to produce a fuller picture of Middle and Late Pleistocene landscape dynamics in a glaciated Mediterranean catchment. We propose a model of long-term river response to headwater glaciation for the Voidomatis River basin that may have some broader significance for mountain catchments in the Mediterranean and elsewhere. The transfer and storage of coarse-grained river sediment during the Late Pleistocene was strongly conditioned by the major Middle Pleistocene glaciations of MIS 12 and MIS 6. Glaciers were much smaller during the last cold stage (MIS 5d to 2), but meltwater floods still dominated suspended sediment fluxes throughout this period. Uniquely, the Voidomatis River basin contains fine-grained slackwater sediments that record the transition from glacial to interglacial conditions in the Mediterranean region at the end of the last cold stage. Sediment source data from the slackwater deposits record a relatively rapid reorganisation of the hydrology and sediment supply conditions in the catchment between about 20 and 17 ka. These deposits have allowed us to explore flood history and the dynamics of suspended sediment sources during this period for the first time. The use of quantitative sediment fingerprinting in palaeoflood studies has yielded a new understanding of long-term catchment dynamics that has not hitherto been attainable using conventional techniques.

Paleoflood hydrology: Origin, progress, prospects

From an origin in diverse studies of flood geomorphology and Quaternary geology, paleoflood hydrology emerged as a geophysical and an applied hydrological science during the 1970s and 1980s. Since acquiring its formal name in 1982, the most productive approach in paleoflood hydrology has become energy-based inverse hydraulic modeling of discrete paleoflood events, recorded in appropriate settings as slackwater deposits and other paleostage indicators (SWD-PSI), or as various threshold indicators of non-exceedence. Technological advances, particularly in hydraulic modeling and geochronology, were instrumental in moving the discipline to its present status. The most recent advances include (1) new techniques for the accurate geochronology of flood sediments, notably TAMS radiocarbon analyses and OSL dating, and (2) the phenomenal increase in computer power that allows complex hydraulic calculations to become feasible for routine studies. From its initial demonstration in the southwestern United States, SWD-PSI paleoflood hydrology proved its widespread applicability to various landscape environments. Particularly important studies have been accomplished in Australia, China, India, Israel, South Africa, Spain, and Thailand. Paleoflood hydrology has also generated its share of controversy, in part because of the differing viewpoints and attitudes of the two scientific traditions from which it emerged: Quaternary geology/geomorphology versus applied hydrologic/hydraulic engineering. Nevertheless, the future growth of the discipline is assured, given the rapid pace of discoveries that it engenders. Indeed, so many international studies exist that it is appropriate to pursue global syntheses to address interesting and timely questions of extreme flood phenomena in relation to global climatic change.

Impact of monsoonal climatic change on Holocene overbank flooding along Sushui River, middle reach of the Yellow River, China

In the semiarid loess regions, slackwater deposition of overbank flooding over the piedmont alluvial plains was episodic and alternated with dust accumulation and soil formation throughout the Holocene. The records of past hydrological events are therefore preserved within the architecture of loess and soils and are protected from subsequent erosion and destruction. Several Holocene loess–soil sequences with the deposits of overbank flooding over the semiarid piedmont alluvial plains in the southeast part of the middle reaches of the Yellow River drainage basin were investigated by field observation, OSL and C 14 dating, measurement of magnetic susceptibility, particle-size distribution and chemical elements. This enables the reconstruction of a complete catalog of Holocene overbank flooding events at a watershed scale and an investigation of hydrological response to monsoonal climatic change as well. During the Holocene, there are six episodes of overbank flooding recorded over the alluvial plain. The first occurred at 11,500–11,000 a BP, i.e. the onset of the Holocene. The second took place at 9500–8500 a BP, immediately before the mid-Holocene Climatic Optimum. After an extended geomorphic stability and soil formation, the third overbank flooding episode came at about 3620–3520 a BP, i.e. the late stage of the mid-Holocene Climatic Optimum, and the floodwater inundated and devastated a Bronze-age town of the Xia Culture built on the alluvial plain, and therefore the town was abandoned for a period of ca 100 years. During the late Holocene, the alluvial plain experienced three episodes of overbank flooding at 2420–2170, 1860–1700 and 680–100 a BP, respectively. The occurrence of these overbank flooding episodes corresponds to the anomalous change in monsoonal climate in the middle reaches of the Yellow River drainage basin when rapid climate change or climatic decline occurs. During at least the last four episodes, both extreme floods and droughts occurred and climate departed from its normal condition, which was defined as a balanced change between the northwestern continental monsoon and southeastern maritime monsoon over time. Great floods occurred as a result of extreme rainstorms in summers caused by rare intensive meridianal airflows involving northwestward moving tropical cyclone systems from the Pacific. These results could be applied to improve our understanding of high-resolution climatic change, and of hydrological response to climatic change in the semiarid zones.

Fluvial evolution of the lower Mississippi River valley during the last 100 k.y. glacial cycle: Response to glaciation and sea-level change

The lower Mississippi valley contains multiple large braid belts for which age control has been limited. Application of the optically stimulated luminescence technique has produced a new chronology of lower Mississippi valley channel-belt formation and insight into the valley9s evolution during the last glacial cycle. Fluvial deposits range from last interglacial meander belts (85 ± 7 to 83 ± 7 ka) to multiple braid belts (64 ± 5 to 11 ± 1 ka) and record large-amplitude responses of the Mississippi River to glacially induced changes in discharge and sediment supply during the last glacial cycle. Slackwater deposits in buried tributary valleys from the middle Mississippi valley and northern lower Mississippi valley suggest that the river was flowing at a position 8–21 m below the present flood plain during the last interglacial, then rapidly aggraded and switched to a braided regime to form the highest and oldest braid belt by 64 ± 5 to 50 ± 4 ka, coincident with initial glaciation of the upper drainage basin. The Mississippi River remained braided until final meltwater withdrawal from its headwaters in the earliest Holocene. Braid-belt formation and incision was controlled by fluctuations in meltwater and sediment discharge, while glacio-eustatic sea level controlled the elevation to which the river was graded, causing late glacial braid belts to dip below the Holocene flood plain in the southern lower Mississippi valley. Moreover, avulsions in the middle Mississippi valley and northern lower Mississippi valley during the last glaciation have pinned the river over regions of shallow bedrock, preventing the modern river from incising to its last interglacial profile. The new chronology and longitudinal profiles presented here provide insight into the response of this continental-scale river system to climatic (glacial) and base-level forcing during the last 100 k.y. glacial cycle.

Modelling coupled hydraulics and sediment transport of a high-magnitude flood and associated landscape change

AbstractProcesses and mechanisms of erosion, transport and deposition within high-magnitude outburst floods such as jökulhlaups and lahars are poorly understood and remain largely unquantified. This study therefore applies a two-dimensional or depth-averaged hydrodynamic model, with fully integrated sediment transport, to reconstruct a Holocene jökulhlaup to have occurred from Kverkfjöll volcano, Iceland. Results indicate simultaneous inundation of multiple channels, flow around islands, hydraulic jumps and multi-directional flow including backwater areas and hydraulic ponding. These flow characteristics are typical of outburst floods that are volcanically triggered, flow through steep volcanic terrain and contain high concentrations of volcaniclastic sediment. Kverkfjöll jökulhlaups had low frontal flow velocities but as stage increased, velocities reached 5–15ms–1. Peak stage was prolonged in zones of hydraulic ponding, but generally attenuated in magnitude and duration downstream. Suspended load transport persisted over the entire hydrograph but bed load transport was spatially discontinuous and comprised distinct pulses. A hierarchy of landforms is proposed, ranging from highest energy zones (erosional gorges, scoured bedrock, cataracts and spillways) to lowest (valley-fills, bars and slackwater deposits). Bedrock erosion was generally where flow exceeded ∽3m flow depth, ∽7ms–1 flow velocity, ∽1×102Nm–2 shear stress and 3 ×102Wm–2 stream power. Deposition occurred below ∽8m flow depth, 11 ms–1 flow velocity, 5 ×102Nm–2 shear stress and 3 ×103Wm–2 stream power. Hydraulic ranges associated with erosion and deposition have considerable overlap due to transitional flow phenomena, transitions in sediment concentration and the influence of upstream effects, such as hydraulic ponding behind topographic constrictions. These results are the first of coupled hydraulic and sediment transport phenomena in high-magnitude outburst floods with fluid rheology and high sediment content, such as jökulhlaups and dilute lahars. Modelled changes in sediment mass closely resembled field-mapped zones of erosion and deposition. This paper therefore introduces a capability to simulate rapid landscape change due to high-magnitude outburst flood.

A mid–late Holocene flood record from the alluvial reach of the Mahi River, Western India

Flooding of rivers in India is linked with the peak monsoons. Investigating the linkage between monsoonal patterns and flood history of various rivers is therefore of fundamental importance in the Indian context. In the present study, the slackwater deposits in the alluvial reaches of the Mahi river basin, western India have been documented. These occur in the ravines incised during the early Holocene on an alluvial surface comprising sediments of Late Pleistocene age. The slackwater deposits occur at elevations up to 20 m from the present river level and extend to about 500 m inland. The carbonate rich sediments forming the ravine cliffs have provided bank stability and the dissections in the ravines have helped in the accumulation of slackwater deposits due to backflooding of the floodwater from the main channel. Recent gullies have incised the sediments and exposed deposits related to major flood events. The best exposures of slackwater deposits have been observed at Dodka. The sediment succession of the slackwater deposits is dominated by bedsets and laminasets of silt and sand separated by colluvial sediments. Four events of flood deposition occurred during the mid to late Holocene. Two units of slackwater deposits, SWD 2 and 4 have been dated by IRSL at 4.6 ± 1 ka and 1.7 ± 0.5 ka. The stratigraphy of these deposits indicates that the first two slackwater units (SWD1 and SWD2) have resulted due to flooding in a regime of intense monsoon. The other two units, however, represent extreme high magnitude floods in a period of low average precipitation.

Hydrodynamics and geomorphic work of jökulhlaups (glacial outburst floods) from Kverkfjöll volcano, Iceland

AbstractJökulhlaups (glacial outburst floods) occur frequently within most glaciated regions of the world and cause rapid landscape change, infrastructure damage, and human disturbance. The largest jökulhlaups known to have occurred during the Holocene within Iceland drained from the northern margin of Vatnajökull and along the Jökulsá á Fjöllum. Some of these jökulhlaups originated from Kverkfjöll volcano and were routed through anastomosing, high gradient and hydraulically rough channels. Landforms and sediments preserved within these channels permit palaeoflow reconstructions.Kverkfjöll jökulhlaups were reconstructed using palaeocompetence (point measurements), slope–area (one‐dimensional), and depth‐averaged two‐dimensional (2D) hydrodynamic modelling techniques. The increasing complexity of 2D modelling required a range of assumptions, but produced information on both spatial and temporal variations in jökulhlaup characteristics. The jökulhlaups were volcanically triggered, had a linear‐rise hydrograph and a peak discharge of 50 000–100 000 m3 s−1, which attenuated by 50–75% within 25 km. Frontal flow velocities were ∼2 m s−1; but, as stage increased, velocities reached 5–15m s−1. Peak instantaneous shear stress and stream power reached 1 × 104 N m−2 and 1 × 105 W m−2 respectively.Hydraulic parameters can be related to landform groups. A hierarchy of landforms is proposed, ranging from the highest energy zones (erosional gorges, scoured bedrock, cataracts, and spillways) to the lowest energy zones (of valley fills, bars, and slackwater deposits). Fluvial erosion of bedrock occurred in Kverkfjallarani above ∼3 m flow depth, ∼7m s−1 flow velocity, ∼1 × 102 N m−2 shear stress, and 3 × 102 W m−2 stream power. Fluvial deposition occurred in Kverkfjallarani below ∼8 m flow depth, 11 m s−1 flow velocity, 5 × 102 N m−2 shear stress, and 3 × 103 W m−2 stream power. Hence, erosional and depositional ‘envelopes’ have considerable overlap, probably due to transitional flow phenomena and the influence of upstream effects, such as hydraulic ponding and topographic constrictions, for example.Holocene Kverkfjöll jökulhlaups achieved geomorphic work comparable to that of other late Pleistocene ‘megafloods’. This work was a result of steep channel gradients, topographic channel constrictions, and high hydraulic roughness, rather than to extreme peak discharges. The Kverkfjöll jökulhlaups have implications for landscape evolution in north‐central Iceland, for water‐sediment inputs into the North Atlantic, and for recognizing jökulhlaups in the rock record. 2D hydrodynamic modelling is likely to be important for hazard mitigation in similar landscapes and upon other glaciated volcanoes, because it only requires an input hydrograph and a digital elevation model to run a model, rather than suites of geomorphological evidence and field‐surveyed valley cross‐sections, for example. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Depositional environment and OSL chronology of the Homeb silt deposits, Kuiseb River, Namibia

Previous studies suggest that the Homeb silts of the Kuiseb valley, Namibia (i) accumulated in a dune-dammed lake, (ii) are end-point deposits, (iii) represent an aggrading river bed, and (iv) are slackwater deposits. Thus, they have been used alternatively as evidence of past drier conditions or past wetter conditions. Lithostratigraphic analysis of two sediment sequences at Homeb indicates sedimentation by aggradation of the Kuiseb River triggered by a transition from an arid to humid climate. OSL ages for the sequences were obtained by the SAR protocol on aliquots of 9.6-mm and 4.0-mm diameter and on single grains. Four-millimeter aliquot minimum ages closely approximate the single-grain minimum ages and are younger than 9.6-mm aliquot minimum and central ages. Based on these results, the small-aliquot (4-mm) approach appears to provide ages comparable to those obtained by the more laborious and time-consuming single-grain method. Minimum ages indicate rapid deposition of the Homeb Silts in at least two episodes centered at ∼15 ka and ∼6 ka during climate transitions from arid to humid. Flash floods eroded the valley fills during slightly more arid conditions.

Large floods during late Oxygen Isotope Stage 3, southern Negev desert, Israel

Slackwater deposits were found in a cave in the Nahal Netafim catchment (35 km 2), near the head of the Gulf of Aqaba in the southern Negev, Israel. The sedimentological record includes 27 large paleofloods, dated by infrared stimulated luminescence to 33,000–29,000 years ago. The scatter of the ages and their large uncertainties prevented an assessment of the exact duration of the record and the specific timing of each flood. Bayesian analysis was used to adjust the dense dating results so that the time interval between the first and last flood deposit preserved in the cave could be estimated. Minimum peak discharges were reconstructed based on the estimated elevation of the Late Pleistocene channel bed as indicated by fluvio-pedogenic layers found near the cave. The average frequency of these large floods (200–600 m 3 s −1) for the period between 33,000 and 29,000 years ago is about 1 flood per 150 years, while for the mid–late Holocene it is only 1 large flood per 1000 years. Eight floods out of the 27 recorded deviate from the envelope curves of mid–late Holocene paleofloods and measured floods in the hyperarid Negev desert, indicating a different hydroloclimatological regime. The anomalous large floods are hypothesized to have resulted from an increase in regional rainfall intensity and/or duration, attributed to increased frequency of the Red Sea Trough low-pressure system that affects the region. Available records indicate that the northern Negev and areas farther north in Israel controlled by Mediterranean pressure systems were wetter 40–20 ka BP. At the same time, the southern Negev, probably in response to the Red Sea Trough system, also experienced short episodes of more and/or larger rainstorms. The timing of these episodes in both the northern and the southern Negev towards the Last Glacial Maximum points to a potential synchronous strengthening of both the Mediterranean and Red Sea systems, currently acting at different seasons. These episodes of increased storminess in the area were brief and were not able to alter the general hyperarid conditions in this area.

The historical and palaeoflood record of Katherine river, northern Australia: evaluating the likelihood of extreme discharge events in the context of the 1998 flood

AbstractThe discharge regimes of the large rivers of northern Australia are characterized by the occurrence of extreme flood events with far‐reaching environmental and societal impacts. In January 1998 the largest flood ever recorded on the Katherine River, northern Australia, resulted in widespread inundation and resultant damage to the town of Katherine. The occurrence of the flood emphasized the unreliability of the then available flood probability estimates and prompted a palaeoflood approach to estimate the recurrence interval of the event. The location of Katherine is ideal for such a study, as the town is located immediately downstream from Katherine Gorge, which provides the necessary bedrock‐confined channel required for such an approach. In addition, previous work in Katherine Gorge had demonstrated that the gorge sections hold suitable deposits for palaeoflood stage reconstruction. The results of the present study show that at least two flow events with discharges similar to the 1998 flood have occurred within the last 600 years, and that high‐magnitude floods are a general feature of the discharge record of the Katherine River over the last c. 2000 years. Furthermore, because the study was undertaken within a few months of the occurrence of the 1998 flood, it provided the opportunity to evaluate the previously obtained flood discharge estimates and draw attention to the general uncertainties associated with palaeoflood studies. Our results emphasize that palaeoflood stage estimates based on slackwater deposits need to be treated as conservative estimates only. More specifically, with respect to the 1998 event, our study demonstrates that the controls of flood peak were more complex than simply flood routing through the gorge sections. It is clear that the areas downstream from Katherine Gorge made an important contribution to the flood peak of the 1998 event. Copyright © 2005 John Wiley &amp; Sons, Ltd.