High-magnitude Flows Research Articles

The Impacts of Dams on Streamflow in Tributaries to the Lower Mekong Basin

The Lower Mekong Basin has had extensive hydropower dam development, which changes its hydrologic conditions and threatens the exceptional aquatic biodiversity. This study quantifies the degree of hydrologic change between pre-impact (1965–1968) and post-impact (2018–2021) peak hydropower development in two major tributaries of the Lower Mekong Basin—the Sekong River, with the fewest dams, and the Sesan River, with the most dams. Both rivers have historically supported migratory fishes. We used daily pre- and post-impact data and the Indicators of Hydrologic Alteration framework to evaluate streamflow changes from dam development. We found significant changes in low- and high-magnitude flows in the pre- and post-impact periods of dam development. For the Sekong River, minimum flow had large fluctuations, with increases of 290% to 412% compared to the pre-impact period, while the Sesan River’s minimum flow ranged from 120% to 160% more than pre-impact. Dry season flows increased by 200 ± 63% on average in the Sekong River, which was caused by releases from upstream dams. Meanwhile, the Sesan River’s dry season flows increased by 100 ± 55% on average. This study indicates that seasonal flow changes and extreme flow events occurred more frequently in the two basins following dam construction, which may threaten the ecosystem’s function.

Long-term geomorphic adjustments following the recoupling of a tributary to its main-stem river

River restoration and rehabilitation projects are widespread, but rarely include the data needed to fully evaluate if they are successful in achieving their goals or how long the process of readjustment takes before a new ‘recovered’ regime state is reached. Here we present a seven-year post-project dataset detailing the morpho-sedimentary responses of a river to the reconnection of a formerly diverted tributary, and relate observed changes to conditions in the river prior to the reconnection. We describe changes in the tributary and main-stem channels, including changes in channel planform, morphology, and the export of coarse and fine sediment from the tributary to the main-stem river. We use the data to develop a conceptual model of the system's response to the reconnection.Marked geomorphic changes occurred within the first two years after the reconnection. Changes during this ‘shock phase’ included dramatic erosion and subsequent deepening and widening of the tributary channel, rapid development of a confluence bar and an increase in fine sediment delivered to the main-stem. After this shock phase, and despite the continued occurrence of high magnitude flow events, the rate of geomorphic change in the tributary began to decrease, and the rate of growth of the confluence bar slowed. Fine sediment volumes in the main-stem also decreased steadily. After an adjustment phase lasting a total of approximately 4.5 yr (including the initial 2-yr shock phase), the tributary to mainstem system appeared to reach a new dynamic equilibrium that we consider the adjusted regime state. This new regime state was characterised by, among other things, an increase in geomorphic heterogeneity in the tributary and main-stem channels.Changes in both fluvial processes and forms indicate that within 4.5 yr the project was successful in achieving its goal of augmenting sediment and increasing geomorphic heterogeneity. Our conceptual model of adjustment mirrors that developed by Petts and Gurnell (2005), with the river passing through a complex and dynamic adjustment phase before reaching a new regime state. However, unlike the responses to impoundment represented by Petts and Gurnell, our model of river response to rehabilitation charts increases in dynamism and heterogeneity.

A comparison of 1D and 2D bedload transport functions under high excess shear stress conditions in laterally constrained gravel-bed rivers: a laboratory study

Abstract. Channel processes under high-magnitude flow events are of central interest to river science and management as they may produce large volumes of sediment transport and geomorphic work. However, bedload transport processes under these conditions are poorly understood due to data collection limitations and the prevalence of physical models that restrict feedbacks surrounding morphologic adjustment. The extension of mechanistic bedload transport equations to gravel-bed rivers has emphasised the importance of variance in both entraining (shear stress) and resisting (grain size) forces, especially at low excess shear stresses. Using a fixed-bank laboratory model, we tested the hypothesis that bedload transport in rivers collapses to a more simple function (i.e. with mean shear stress and median grain size) under high excess shear stress conditions. Bedload transport was well explained by the mean shear stress (1D approach) calculated using the depth–slope product. Numerically modelling shear stress to account for the variance in shear stress (2D) did not substantially improve the correlation. Critical dimensionless shear stress values were back-calculated and were higher for the 2D approach compared to the 1D. This result suggests that 2D critical values account for the relatively greater influence of high shear stresses, whereas the 1D approach assumes that the mean shear stress is sufficient to mobilise the median grain size. While the 2D approach may have a stronger conceptual basis, the 1D approach performs unreasonably well under high excess shear stress conditions. Further work is required to substantiate these findings in laterally adjustable channels.

Intraseasonal‐to‐Interannual Analysis of Discharge and Suspended Sediment Concentration Time‐Series of the Upper Changjiang (Yangtze River)

AbstractSediment transport is the main driver of the channel morphology and landscape evolution, with implications for chemical and biological river processes, and human‐related activities. Understanding the processes governing the relations between discharge and suspended sediments is essential for the management of river catchments and river networks. Here, we use the method of wavelet transformation to identify the time‐scale dependency of suspended sediment patterns concerning the temporally and spatially uneven transient processes of sediment production, accumulation, and transport. We analyze the temporal variation of concurrent discharge and suspended sediment fluxes for the Upper Changjiang (Yangtze River, China) at Pingshan station by using a long‐term database collected for over 50 years. Furthermore, we bridge the limitations of pure predictive models to learn from temporal data structures with the main purpose of identifying the mechanisms underpinning the suspended sediment patterns (e.g., climatic forces). Intraseasonal‐to‐seasonal, annual, and inter‐annual dominant time‐scales are thus identified. The short time‐scales are driven by the bi‐modal seasonal precipitation pattern specific to the climate of the region and provide a continuous supply of sediments to the river. The large time‐scales, controlled by high magnitude flow events and within‐reach sediment storage, display alternating periods of increasing and decreasing sediment fluxes; ultimately, they maintain the river channel within balance or within a moderate positive sediment accumulation process. This analysis and methodology help to understand temporal sediment dynamics, and ultimately to manage river catchments.

Sediment–water flows in mountain catchments: Insights into transport mechanisms as responses to high-magnitude hydrological events

Sediment transfer in mountain streams occurs via processes classified as debris flows, hyperconcentrated flows, debris floods, and water flows under the control of the water energy and the amount of involved sediment. Despite the extensive documentation of the channel changes caused by high-magnitude hydrological events, the analysis of the sediment–water flows occurring during such floods is currently a minimally explored issue. This study investigated how the transport mechanisms activated in a mountain stream during a high-magnitude flood differ from those triggered during ordinary floods. It also evaluated the effectiveness of three morphometric approaches in predicting high-magnitude flows expectable in a channel sub-reach. The study area is the Tegnas catchment (Dolomites, Italy), a mountain basin affected in 2018 by a severe hydrological event (Vaia Storm), whose recurrence interval is approximately 200 years. We determined the transport processes typifying the stream network in the catchment during ordinary floods through field surveys and direct monitoring and compared these flows with the high-magnitude flow types that occurred during the Vaia Storm. Additionally, we examined the flows predicted using the morphometric approaches for high-magnitude events. We observed water flow as a response to ordinary events occurring along the entire Tegnas main stem, whereas debris flow ordinarily determines sediment transfer at steep tributaries. During the Vaia Storm, water flow still dominated along the Tegnas Torrent, although debris flows and debris floods were also documented at several sub-reaches of the main stem. The morphometric approaches satisfactorily predicted debris flows but often failed to recognize the debris floods occurred during the high-magnitude 2018 flooding. The analysis of different flow types enabled us to infer relationships among transport mechanisms, hydraulic forcing, and channel dynamics and to gain new insights on the poorly explored debris flood processes. Water flows transitioned into debris floods under unit stream powers exceeding the threshold of 5000 to 5500 W m−2 or downstream of sediment-injection points. The occurrence of debris floods, which caused higher channel widening than that induced by water flows, appeared to be facilitated by the presence of tributaries prone to debris flow occurrence connected to a receiving stream, the injection of fine material into the flows, and channels characterized by high slope and narrow section. The morphometric approaches enabled adequate first-order discrimination of expectable high-magnitude flows, but a more detailed characterization that includes field observations is necessary to understand the transport mechanisms that can affect a specific channel site during high-magnitude hydrological events for a more accurate and reliable definition of flood hazard at the local scale.

Characterization of a landslide-triggered debris flow at a rainforest-covered mountain region in Brazil

Debris flows represent great hazard to humans due to their high destructive power. Understanding their hydrogeomorphic dynamics is fundamental in hazard assessment studies, especially in subtropical and tropical regions where debris flows have scarcely been studied when compared to other mass-wasting processes. Thus, this study aims at systematically analyzing the meteorological and geomorphological factors that characterize a landslide-triggered debris flow at the Pedra Branca catchment (Serra do Mar, Brazil), to quantify the debris flow’s magnitude, peak discharge and velocity. A magnitude comparison with empirical equations (Italian Alps, Taiwan, Serra do Mar) is also conducted. The meteorological analysis is based on satellite data and rain gauge measurements, while the geomorphological characterization is based on terrestrial and aerial investigations, with high spatial resolution. The results indicate that it was a large-sized stony debris flow, with a total magnitude of 120,195 m3, a peak discharge of 2146.7 m3 s−1 and a peak velocity of 26.5 m s−1. The debris flow was triggered by a 188-mm rainfall in 3 h (maximum intensity of 128 mm h−1), with an estimated return period of 15 to 20 years, which, combined with the intense accumulation of on-channel debris (ca. 37,000 m3), indicates that new high-magnitude debris flows in the catchment and the region are likely to occur within the next two decades. The knowledge of the potential frequency and magnitude (F–M) can support the creation of F–M relationships for Serra do Mar, a prerequisite for reliable hazard management and monitoring programs.

Suspended sediment dynamics during a period of consecutive explosive cyclogeneses in rural temperate catchments: A case study from small-sized catchments in NW Spain

During the winter of 2013/2014, a sequence of explosive cyclogeneses occurred in NW Spain, causing severe flooding and substantial economic damage. Peak discharges were extremely high in many areas, with return periods in excess of 2–5 years. The heavy flow registered during these episodes provides a unique opportunity to evaluate the sedimentological effects resulting from high-magnitude flow events. Such events are relatively rare and the technical difficulties in sampling with sufficiently high frequency means that few observations on sediment transport can be gathered during the same. In this study, we present detailed hydrological and sedimentological observations across a period of consecutive explosive cyclogeneses from two small-sized catchments (i.e., the Corbeira stream and headwater of the Mero river) located in Galicia (NW Spain), in order to examine the factors controlling suspended sediment (SS) dynamics during high-magnitude flow events. The study sites are representative of the biophysical conditions (e.g. rainfall pattern, land use, vegetation cover and management practice) in NW Spain. The catchments of the Corbeira stream and headwater of the Mero river are similar with respect to climate, land use and management practice. The main differences are the distribution of land use and catchment size (Corbeira: 16 km2 and Mero: 65 km2).Large amounts of sediment were transferred in both catchments in response to flow events. A high variability in SS load during these was also observed in the two catchments, which is attributed to variation in rainfall characteristics, magnitude peak discharge, antecedent conditions controlling sediment availability, and suggests that sediment supply and transport vary widely over time. Higher concentration and variability of SS was observed in the Corbeira catchment. Differences in SS dynamics were also noted between the catchments that may be attributed to differences in the spatial distribution of land use, which influence the location of the erosion source and SS availability, and directly and indirectly affects connectivity between the sediment source area and the stream. In the Corbeira catchment, the SS load was closely linked to the hydrological response and rainfall, whereas in the Mero basin, preceding conditions, i.e. the sediment depletion during the previous events, played an important role. The results demonstrated that peak SS concentrations may coincide with, or lag behind, peak discharge, and factors other than discharge (mainly the location of the sediment source and its connectivity with the drainage network) have a major influence on SS concentrations.

How much water can be captured from flood flows to store in depleted aquifers for mitigating floods and droughts? A case study from Texas, US

Extreme flooding from Hurricane Harvey (∼100 km3, ∼80 million acre feet, maf of rainfall) in Houston, Texas, US co-located with depleted aquifers raises the question of whether we can capture floodwater to reduce flooding impacts and replenish aquifers for droughts. Here we quantified how much water could be captured from high magnitude flows (HMFs) in 10 major rivers discharging to the Gulf of Mexico for potential storage in depleted aquifers along the Texas Gulf Coast. Results show that HMFs (≥95th percentile) from rivers discharging to the Gulf of Mexico total 37 km3 (30 maf) in 2015–2017, similar in capacity to US Lake Mead (32 km3, 26 maf). These flows are less than modeled unappropriated flows that consider appropriated water rights and limited analysis suggests moderate reduction from environmental flows. Similarity in high flow volumes and modeled groundwater depletion in the Gulf Coast Aquifer system (∼25 km3, 20 maf) underscores the potential to partially mitigate flooding using aquifer storage. Interim storage would be required to resolve disconnects between high flood intensities and low aquifer injection rates. Engineering approaches will become increasingly important to manage climate extremes.

Morphodynamic simulation of sediment deposition patterns on a recently stripped bedrock anastomosed channel

Abstract. Some mixed bedrock-alluvial dryland rivers are known to undergo cycles of alluvial building during low flow periods, punctuated by stripping events during rare high magnitude flows. We focus on the Olifants River, Kruger National Park, South Africa, and present 2-D morphodynamic simulations of hydraulics and sediment deposition patterns over an exposed bedrock anastomosed pavement. We examine the assumptions underlying a previous conceptual model, namely that sedimentation occurs preferentially on bedrock highs. Our modelling results and local field observations in fact show that sediment thicknesses are greater over bedrock lows, suggesting these are the key loci for deposition, barform initiation and island building. During peak flows, velocities in the topographic lows tend to be lower than in intermediate topographic areas. It is likely that intermediate topographic areas supply sediment to the topographic lows at this flow stage, which is then deposited in the lows on the falling limb of the hydrograph as velocities reduce. Subsequent vegetation establishment on deposits in the topographic lows is likely to play a key role in additional sedimentation and vegetation succession, both through increasing the cohesive strength of alluvial units and by capturing new sediments and propagules.

Availability of high-magnitude streamflow for groundwater banking in the Central Valley, California

California’s climate is characterized by the largest precipitation and streamflow variability observed within the conterminous US This, combined with chronic groundwater overdraft of 0.6–3.5 km3 yr−1, creates the need to identify additional surface water sources available for groundwater recharge using methods such as agricultural groundwater banking, aquifer storage and recovery, and spreading basins. High-magnitude streamflow, i.e. flow above the 90th percentile, that exceeds environmental flow requirements and current surface water allocations under California water rights, could be a viable source of surface water for groundwater banking. Here, we present a comprehensive analysis of the magnitude, frequency, duration and timing of high-magnitude streamflow (HMF) for 93 stream gauges covering the Sacramento, San Joaquin and Tulare basins in California. The results show that in an average year with HMF approximately 3.2 km3 of high-magnitude flow is exported from the entire Central Valley to the Sacramento-San Joaquin Delta often at times when environmental flow requirements of the Delta and major rivers are exceeded. High-magnitude flow occurs, on average, during 7 and 4.7 out of 10 years in the Sacramento River and the San Joaquin-Tulare Basins, respectively, from just a few storm events (5–7 1-day peak events) lasting for 25–30 days between November and April. The results suggest that there is sufficient unmanaged surface water physically available to mitigate long-term groundwater overdraft in the Central Valley.

The geomorphic impacts of culverts at paved forest roads: Examples from Carpathian headwater channels, Czech Republic

Road-stream crossings constructed with culverts have the potential to alter geomorphic processes in adjacent stream channels. We examined changes in channel geometry and bed sediments in longitudinal profiles related to culverts at five paved forest road crossings with perennial headwater streams and two gullies initiated by point releases of water through ditches. A dendrogeomorphic approach was used for dating of exposed tree roots and estimation of bank erosion rates in incised cross-sections downstream of road crossings. The studied forest road crossings concentrate surface runoff from the road net, act as stabilizing elements in stream longitudinal profiles and decelerate bedload transport in the case of culvert clogging, which caused notable upstream and downstream changes in fluvial processes. Notable fining of the coarsest fraction in upstream cross-sections was observed only for two culverts with the highest depositional tendencies. Much lower width-depth ratios (<4) and typically higher channel slopes were measured in channel reaches downstream of the culverts, compared with upstream cross-sections. Two road-induced gullies typically had very low width/depth ratios close to 2. At five of a total of seven studied culverts, the highest documented erosion rates occurred immediately downstream of these structures. The mean annual erosion rates reconstructed from cross-sections at the positions of individual tree root samples ranged from 0.2 to 6.5cm·year−1, and the erosion occurred over 2–5year intervals, depending on the studied location. The highest number of observed changes in cross-sectional geometry caused by channel erosion were connected with relatively high-magnitude flow events (with recurrence intervals >10years; specific discharge >1500l·s·km−2), whereas more frequent flows (with recurrence intervals of 2–10years) were not always recorded by exposed tree roots.

Two-stage opening of the Dover Strait and the origin of island Britain

Late Quaternary separation of Britain from mainland Europe is considered to be a consequence of spillover of a large proglacial lake in the Southern North Sea basin. Lake spillover is inferred to have caused breaching of a rock ridge at the Dover Strait, although this hypothesis remains untested. Here we show that opening of the Strait involved at least two major episodes of erosion. Sub-bottom records reveal a remarkable set of sediment-infilled depressions that are deeply incised into bedrock that we interpret as giant plunge pools. These support a model of initial erosion of the Dover Strait by lake overspill, plunge pool erosion by waterfalls and subsequent dam breaching. Cross-cutting of these landforms by a prominent bedrock-eroded valley that is characterized by features associated with catastrophic flooding indicates final breaching of the Strait by high-magnitude flows. These events set-up conditions for island Britain during sea-level highstands and caused large-scale re-routing of NW European drainage.

Fluvial response to Late Pleistocene and Holocene environmental change in a Thames chalkland headwater: the Lambourn of southern England

This paper describes the Late Pleistocene to Holocene stratigraphy of the River Lambourn; a minor headwater of the River Thames in the Berkshire Downs. The Quaternary valley-fill comprises around 5–8m of Late Pleistocene gravels overlain by Holocene peats and chalky clays. Quaternary deposits overlie an irregular rockhead erosion surface with deep scouring particularly evident on prominent bends in the valley. The gravels subdivide into a lower unit of chalky gravels overlain by coarse flint gravels. Ground penetrating radar suggests that gravels at depth are relatively structureless, but at the top show well-developed point-bar accretion surfaces which occur in association with peat-filled sinuous channels. These probably date from around the Pleistocene-Holocene boundary and may have formed in response to climate change and increased groundwater outflow as stream hydrology changed from the short-duration, high-magnitude flows of the Lower Dryas to the uniform, low-magnitude flows of the Holocene. Holocene peats initially infilled abandoned floodplain channels at around 10kyr BP but later encroached over much of the Lambourn floodplain. A progressive upward decrease in organic material and an increase in the proportion of chalky clays from around 4kyr BP probably occurred in response to floodplain accretion coupled with increased erosion of the chalk catchment related to agricultural clearance and a wetter climate.

Effects of flow regime on growth rate in freshwater drum, Aplodinotus grunniens

Understanding the influence of flow regime hydrology on fish life history is essential for conserva- tion and management of lotic ecosystems. The objec- tives of this study were to investigate growth patterns of Wabash River freshwater drum Aplodinotus grunniens relativetoage,sex,and longtermflowregimevariation. Growth was estimated using back-calculated lengths from otolith measurements. Flow regime was described using Indicators of Hydrologic Alteration flow metrics calculated from long-term stream flow data at USGS stations. A generalized linear mixed model was used to modelgrowthasafunctionofage,sex,andflowregime. Freshwater drum growth was explained by variation in age, sex, and flow magnitude. Freshwater drum exhib- ited indeterminate growth relative to age. Mean growth rates and mean length at age were lower in males than females. High magnitude flow events were positively correlated with increased growth rates in both males and females. However, the effect of flow magnitude variation on growth was stronger in males. The primary flow regime metrics related to growth were magnitude- based and were not related to timing offlow events. The study provides evidence that long term variation in flow regime is a catalyst for alterations in freshwater drum growth rates.

Reach-scale channel sensitivity to multiple human activities and natural events: Lower Santa Clara River, California, USA

Understanding the cumulative impact of natural and human influences on the sensitivity of channel morphodynamics, a relative measure between the drivers for change and the magnitude of channel response, requires an approach that accommodates spatial and temporal variability in the suite of primary stressors. Multiple historical data sources were assembled to provide a reach-scale analysis of the lower Santa Clara River (LSCR) in Ventura County, California, USA. Sediment supply is naturally high due to tectonic activity, earthquake-generated landslides, wildfires, and high magnitude flow events during El Niño years. Somewhat typically for the region, the catchment has been subject to four reasonably distinct land use and resource management combinations since European-American settlement. When combined with analysis of channel morphological response (quantifiable since ca. 1930), reach-scale and temporal differences in channel sensitivity become apparent. Downstream reaches have incised on average 2.4m and become narrower by almost 50% with changes focused in a period of highly sensitive response after about 1950 followed by forced insensitivity caused by structural flood embankments and a significant grade control structure. In contrast, the middle reaches have been responsive but are morphologically resilient, and the upstream reaches show a mildly sensitive aggradational trend. Superimposing the natural and human drivers for change reveals that large scale stressors (related to ranching and irrigation) have been replaced over time by a suite of stressors operating at multiple spatial scales. Lower reaches have been sensitive primarily to ‘local’ scale impacts (urban growth, flood control, and aggregate mining) whereas, upstream, catchment-scale influences still prevail (including flow regulation and climate-driven sediment supply factors). These factors illustrate the complexity inherent to cumulative impact assessment in fluvial systems, provide evidence for a distinct Anthropocene fluvial response, and underpin the enormity of the challenge faced in trying to sustainably manage and restore rivers.

Magnitude and Frequency Relations of Bankfull Flows and Bed-Material Entrainment in the Llano River Watershed, Central Texas, USA

Bankfull channel geometry and bed-material entrainment are examined in the Llano River watershed in central Texas, which is characterized by mixed alluvial-bedrock boundaries and a highly variable flow regime. Field data, including cross-section surveys and sediment samples from 17 sites, are coupled with one-dimensional hydraulic models and flood-frequency analyses to compute recurrence intervals of bankfull and entrainment conditions. Recurrence intervals of flows that entrain median-sized bed material (d 50) are consistently less than 1 year, whereas flows that define bankfull morphology range from one to four years (median of 1.75 years). Recurrence intervals of flows responsible for incipient entrainment of relatively coarse bed material (d 84) are typically less than 1.5 years, but complete mobilization of this size fraction occurs less frequently than bankfull flows. At multiple sites, morphologic surfaces defining bankfull stage are discontinuous and set within a macro-channel, which is formed by high-magnitude flows with recurrence intervals greater than five years. Finally, the frequency of bankfull flow in the lowermost, confined, sandbar-dominated channels is similar to the frequency of the mobilization of relatively coarse low-flow channel gravels; but discontinuous, sandy bank deposits indicate that sediment exchange with channel bars is the most important process for development of bankfull indicators.

Challenges of modeling current very large lahars at Nevado del Huila Volcano, Colombia

Nevado del Huila, a glacier-covered volcano in the South of Colombia’s Cordillera Central, had not experienced any historical eruptions before 2007. In 2007 and 2008, the volcano erupted with phreatic and phreatomagmatic events which produced lahars with flow volumes of up to about 300 million m3 causing severe damage to infrastructure and loss of lives. The magnitude of these lahars and the prevailing potential for similar or even larger events, poses significant hazards to local people and makes appropriate modeling a real challenge. In this study, we analyze the recent lahars to better understand the main processes and then model possible scenarios for future events. We used lahar inundation depths, travel duration, and flow deposits to constrain the dimensions of the 2007 event and applied LAHARZ and FLO-2D for lahar modeling. Measured hydrographs, geophone seismic sensor data and calculated peak discharges served as input data for the reconstruction of flow hydrographs and for calibration of the models. For model validation, results were compared with field data collected along the Páez and Simbola Rivers. Based on the results of the 2007 lahar simulation, we modeled lahar scenarios with volumes between 300 million and 1 billion m3. The approach presented here represents a feasible solution for modeling high-magnitude flows like lahars and allows an assessment of potential future events and related consequences for population centers downstream of Nevado del Huila.

Multiscale spatio-temporal variability of sedimentary deposits in the Var turbidite system (North-Western Mediterranean Sea)

The Var turbidite system is a small sandy system located in the Ligurian Basin (Mediterranean Sea). It is active during present sea-level highstand and shows four types of sediment transfer processes: (1) low-density turbidity surges generated by small-scale failures (2) low-magnitude, high-frequency (yearly) hyperpycnal turbid plumes, (3) high-magnitude, less-frequent, hyperpycnal currents and (4) high-magnitude flows generated by large slope failures. These processes have different imprints on the morphology of the system. Inversely, the topography plays a role on the behaviour of these flows. The depositional and erosional architecture of the system has been investigated in detail on the basis of SAR imagery and a set of cores using an interface corer collected repetitively from a dense group of sites. The inner terraces located in the upper part of the turbidite system are generally depositional and thus provide a detailed record of recent sediment transfer processes. But the lower the elevation, the more terraces are affected by turbulent flow erosion. Downward, the channel-floor is a complex area where flows mainly by-pass but locally erode or deposit. The levee is dominantly depositional, but only records high-magnitude events, able to spill over. Low-magnitude, high-frequency events, such as yearly hyperpycnal currents are confined in the upper part of the system and thus have little control on system architecture. Moreover, they provide only thin deposits that can be misinterpreted in terms of involved process, and will probably be erased from the geological record. High-magnitude events have strong control on the system architecture as they erode the channel-floor and participate in the construction of the Var Sedimentary Ridge.

Late Ordovician climbing-dune cross-stratification: a signature of outburst floods in proglacial outwash environments?

Climbing dune-scale cross-statification is described from Late Ordovician paraglacial successions of the Murzuq Basin (SW Libya). This depositional facies is comprised of medium-grained to coarse-grained sandstones that typically involve 0·3 to 1 m high, 3 to 5 m in wavelength, asymmetrical laminations. Most often stoss-depositional structures have been generated, with preservation of the topographies of formative bedforms. Climbing-dune cross-stratification related to the migration of lower-flow regime dune trains is thus identified. Related architecture and facies sequences are described from two case studies: (i) erosion-based sandstone sheets; and (ii) a deeply incised channel. The former characterized the distal outwash plain and the fluvial/subaqueous transition of related deltaic wedges, while the latter formed in an ice-proximal segment of the outwash plain. In erosion-based sand sheets, climbing-dune cross-stratification results from unconfined mouth-bar deposition related to expanding, sediment-laden flows entering a water body. Within incised channels, climbing-dune cross-stratification formed over eddy-related side bars reflecting deposition under recirculating flow conditions generated at channel bends. Associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys. The primary control on the formation of climbing-dune cross-stratification is a combination between high-magnitude flows and sediment supply limitations, which lead to the generation of sediment-charged stream flows characterized by a significant, relatively coarse-grained, sand-sized suspension-load concentration, with a virtual absence of very coarse to gravelly bedload. The high rate of coarse-grained sand fallout in sediment-laden flows following flow expansion throughout mouth bars or in eddy-related side bars resulted in high rates of transfer of sands from suspension to the bed, net deposition on bedform stoss-sides and generation of widespread climbing-dune cross-stratification. The later structure has no equivalent in the glacial record, either in the ancient or in the Quaternary literature, but analogues are recognized in some flood-dominated depositional systems of foreland basins.

Geomorphic evidence of ancient catastrophic flow type landslides in the mid-mountain ridges of the Western Flysch Carpathian Mountains (Czech Republic)

Ancient flow type landslides are relatively frequent fossil forms of the relief in mid-mountain conditions of the Czech Carpathian Mountains. Sixty rather distinctive displays of debris flows, rock avalanches, and debris avalanches have been mapped in the uppermost part of the territory. Unlike contemporary sporadic and low volume debris flows, ancient (Pleistocene and Lower Holocene) accumulations are a few orders of magnitude more extensive and were of considerable geomorphologic significance in forming the steep sections of mountain valleys and slopes. This geomorphic pattern does not hold for flow type slides, the source of which is material released as a consequence of numerous deep-seated landslides. Due to deep disruption of slopes, a few high-magnitude flow type landslides (e.g., rock avalanches), quite rare in flysch mid-mountain conditions, also occurred in the Late Holocene.