Sediment entrainment and transport in fluviokarst systems (original) (raw)
Related papers
he complex drainage systems within karst settings can result in atypical longitudinal profiles. Features, such as cave entrances, can be expressed as anomalous ‘bumps’ in the longitudinal profile of a stream if downcutting has continued upstream of the area in which the water is pirated to the subsurface. Horn Hollow, a fluviokarst valley located in Carter Caves State Park Resort in northeastern Kentucky, was examined for these types of features. The objectives of this study were to determine if sediment mobility can be used as a proxy for anomalous areas along the profile of the valley and if detailed cross-sections can reveal and/or differentiate areas of cave collapse from natural down-cutting of the system. To accomplish these objectives, the longitudinal profile of Horn Hollow and numerous cross-sections through the valley were surveyed. Armor point counts were performed at cross-sections unless the section was predominantly bedrock. Although Horn Hollow’s waters have been predominantly pirated to the subsurface, the longitudinal profile of the system is graded to that of a stream near equilibrium, but anomalous areas are present. The progression of sediment size along the length of the profile does not follow a typical fining-downstream pattern. Some of the largest sediments can be found within the lower segment of the profile. Taken together, the anomalous bumps and the sediment size suggest that the shape of the longitudinal stream profile is strongly influenced by karst processes such as stream piracy and cave collapse.
The activation of ephemeral streams in karst catchments of semi-arid regions
Catena, 2012
Ephemeral streams are morphological elements of karstic low-relief areas, characterized by relatively large and flat transects, which occasionally drain runoff, in particular when generated by extraordinary or extreme rainfall events. The main problem related to this streams and affecting the analysis and prediction of flood events in karst semi-arid regions is the almost complete absence of discharge time series. Floods are relatively rare events for semi-arid karst regions; however, they can be really severe and disruptive, causing serious damages to people and infrastructures. This work presents an analysis of the response to rainfall of ephemeral streams in a karst semi-arid regions of southeast of Italy by an innovative approach. The analysis is based on a 2D simulation model of the behavior of a network of ephemeral streams. The used approach integrates the hydrological and hydraulic models in order to account first for the dynamic of catchment response to rainfall and activation of the streams, and then for the hydraulic behavior of the streams. Results show the slow response of ephemeral streams to the rainfall, whereas more extreme rainfall events generate quick and high discharge responses of the ephemeral streams. In addition, modeling results emphasize the non-linearity of runoff generation, in particular for some flow paths, which are biased by the local morphology as well as by the intensity of the rainfall event.
Sedimentology, 2013
Associate Editor -Karl F€ ollmi ABSTRACT Modern mixed alluvial-bedrock channels in mountainous areas provide natural laboratories for understanding the time scales at which coarse-grained material has been entrained and transported from their sources to the adjacent sedimentary sink, where these deposits are preserved as conglomerates. This article assesses the shear stress conditions needed for the entrainment of the coarse-bed particles in the Glogn River that drains the 400 km 2 Val Lumnezia basin, eastern Swiss Alps. In addition, quantitative data are presented on sediment transport patterns in this stream. The longitudinal stream profile of this river is characterized by three ca 500 m long knickzones where channel gradients range from 0Á02 to 0Á2 m m À1 , and where the valley bottom confined into a <10 m wide gorge. Downstream of these knickzones, the stream is flat with gradients <0Á01 m m À1 and widths ! 30 m. Measurements of the grainsize distribution along the trunk stream yield a mean D 84 value of ca 270 mm, whereas the mean D 50 is ca 100 mm. The consequences of the channel morphology and the grain-size distribution for the time scales of sediment transport were explored by using a one-dimensional step-backwater hydraulic model (Hydrologic Engineering Centre -River Analysis System). The results reveal that, along the entire trunk stream, a two to 10 year return period flood event is capable of mobilizing both the D 50 and D 84 fractions where the Shields stress exceeds the critical Shields stress for the initiation of particle motion. These return periods, however, varied substantially depending on the channel geometry and the pebble/boulder size distribution of the supplied material. Accordingly, the stream exhibits a highly dynamic boulder cover behaviour. It is likely that these time scales might also have been at work when coarsegrained conglomerates were constructed in the geological past.
2020
Karst development influences the hydrological response of catchments. However, such an impact is poorly documented and even less quantified, especially over short scales of space and time. The aim of this article is thus to define karst influence on the different hydrological processes driving runoff generation, including interbasin groundwater flow (IGF) for elementary catchments at the storm-event timescale. IGFs are estimated at the scale of the river reach, by comparing inlet and outlet flows as well as the effective rainfall from the topographic elementary catchment. Three types of storm-event descriptors (characterizing water balance, hydrograph shape and lateral exchanges) were calculated for the 20 most important storm events of 108 stations in three French regions (Cévennes Mountains, Jura Mountains and Normandy), representative of different karst settings. These descriptors were compared and analysed according to catchment geology (karst, non-karst or mixed) and seasonality in order to explore the specific impact of karst areas on water balance, hydrograph shape, lateral exchanges and hydrogeological basin area. A statistical approach showed that, despite the variations with study areas, karst promotes (i) higher water infiltration from rivers during storm events, (ii) increased characteristic flood times and peak-flow attenuation, and (iii) lateral outflow. These influences are interpreted as mainly due to IGF loss that can be significant at the storm-event scale, representing around 50 % of discharge and 20 % of rainfall in the intermediate catchment. The spatial variability of such effects is also linked to contrasting lithology and karst occurrence. Our work thus provides a generic framework for assessing karst impact on the hydrological response of catchments to storm events; moreover, it can analyse flood-event characteristics in various hydro-climatic settings and can help with testing the influence of other physiographic parameters on runoff generation.
Effects of an extreme flood event on an alpine karst system
Journal of Hydrology, 2020
The effects of an extreme storm (501 mm of rainfall in less than five days) were monitored in an alpine show cave to assess the characteristics of its aquifer unsaturated and saturated zones. The selected cave, Bossea (SW Piedmont, Italy), is an excellent test site for hydrological investigation of karst systems, because it hosts an underground karst laboratory since the late 70 s. The investigation was carried out by means of an integrated approach that combined measurements of flow discharge, physico-chemical parameters, major chemical components, and trace elements (metals, Rb, Ba, Sr and REE). The hydrology and hydrochemistry of the main underground river and two of its secondary tributaries (a drip site and a small secondary inflow) draining the unsaturated zone were monitored during the November 2016 flood, an exceptional hydrological event (estimated recurrence time of 200 years) that caused severe damages in the whole southwestern Piedmont region. The results of the 2016 monitoring were compared with those of another less extreme flood occurred in 2011 (recurrence time of 20 years). The karst system showed an impulsive response to flooding at the catchment and at the individual inflow scales, but each site exhibited a characteristic response due to the complex geological and structural setting of the cave system. The development of this karst system is, in fact, related to the lateral and vertical juxtaposition of rocks with different lithologies and mechanical properties, which form different hydrogeologic compartments. Coupling hydrograph, chemograph analysis and REE normalized patterns permitted to assess which compartments were activated during each phase of the floods. In particular, it was possible to recognize a progressive increasing contribution of the non-carbonate lithologies to flow during the peak of the flood for both the two unsaturated inflows and the main underground river, suggesting the activation of larger portions of the aquifer. However, the response of each individual unsaturated inflow is more influenced by its recharge system architecture rather than the magnitude of the meteorological event. Similar complex geological and lithological karst systems are typical of many areas characterized by orogenetic processes (Alps, Rocky Mountains, Caledonides, etc.), but despite them being relatively widespread, the hydrodynamics of these aquifers is still poorly understood. This study affirms that only long-term and well-integrated monitoring and sampling can help unravel the behavior of such complex karst systems.
1992
Abstract,The dynamics,of a catastrophic flood/multiple debris flow on 30 June 1990 was,reconstructed along a 2 km stretch of the River Schmiedlaine. The upper bedrock reach was mainly subject to sediment transport and erosion, giving way to an erosionally and depositionally interactive boulder-bed reach with confined bedrock meanders,and finally to a well-sorted purely,aggradational,braided,reach. Sediment was delivered from several major slumps and broken check dams, combined with two major debris flows. Resulting deposition consisted of logjams, levees and small end lobes. During the flood's rising limb, flows were highly viscous and hyperconcentrated, with flood-water reworking following near the end. Reconstruction of the flood maximum,allowed determination of varying velocities and flood mechanics. Up to 1 m of vertical water level difference occurred in areas of high stream curvature. Flood power, bed resistance and bed friction varied markedly according to gradient and s...
Geomorphology, 2020
Bedload transport measurements in the formerly glaciated Chochołowski catchment located in the Western Tatras in Poland were performed in the period 1975 to 2018. Measurement of bedload displacement allowed to determine the role of bedload transport disturbances in both system connectivity and sediment transfer. Bedload transport occurred as many as triggered up to several times a year. The longest distances were observed during rain-on-snow events: between 12 m in headwaters and over 100 m in the fluvial valley. Bedload became activated along the entire length of the channel system (10.5 km) every 2 to 5 years when the stream discharge exceeded 10 m 3 s −1 (with an average of 1.25 m 3 s −1). In such situations, bedload dynamics increased downstream, and stream power was sufficient to overcome local barriers (i.e. boulder and log steps). Downstream increases in bedload dynamics may become disturbed and inverted by sudden snowmelt and locally heavy rainfall. These types of events caused the dynamics of bedload transport to be 190% to 320% greater in the upper part of the studied catchment and to decline in the downstream direction over a distance of 7 km to yield an attenuation effect. The same pattern was observed in local tributaries where the dynamics of bedload transport were 200% higher in the upper part and attenuation was observed along a distance of 1.5 km in the downstream direction. All events of this type cause seasonal disturbances in bedload transport. However, the greatest effect on bedload dynamics was produced by natural deforestation. A 16% decrease in forest cover causes a fluvial system disequilibrium manifested in an intensification of hydro-geomorphologic processes and formation of new landforms.
Sediment transport in the proglacial Fagge River (Kaunertal/Austria)
2012
The fluvial system in proglacial areas is more-or-less continuously fed with sediment by glacial melt water and infrequently supplied with sediment by landslides, debris flows, rock fall or fluvial transport from the coupled slopes. A part of the sediment input is temporarily stored in intermittent sinks (river bed, bars, braid plains). These stores can be reworked and then become sources for fluvial sediment transport during floods. Sediment transporting processes are highly variable in both the temporal and spatial scale. In consequence of this high variability, field-data based detailed knowledge of sediment fluxes and the interrelated geomorphological processes in proglacial areas is lacking. The present work is part of the research project "High-resolution Measurements of Morphodynamics in Rapidly Changing Proglacial Systems of the Alps", that is set up in the Kaunertal, Austrian Alps. The project is focused on the quantification of fluvial sediment transport. Suspended sediment load and bed load are measured at different locations in the proglacial Fagge River. Surface changes of sediment sources are quantified by a comparison of multi-temporal terrestrial and airborne laser scanning data.
Sediment transport and erosion in mountain streams
IAHS-AISH publication, 1998
The paper presents first results of laboratory experiments with bed load transport over steep sloped streams with staircase-like structures, the so-called step-pool systems. The initial state of the experiments consisted in the development of armour layers of maximum bed stability at critical discharges. Coloured sediment was added at the beginning of each run to analyse the transport rates of the original bed material and of the feeding material separately. Velocity measurements were carried out by the salt dilution method before, during, and after the procedure of sediment feeding.
Sediment-transporting flows in headwater streams
1999
The equilibrium alluvial stream channel has a geometry that allows it to pass the water and sediment supplied from the watershed. At the same time, the equilibrium alluvial channel is built and maintained by the flows and sediment delivered to it. A prerequisite for understanding the creation of the equilibrium channel is an understanding of the sediment conveyance and competence of the flows the channel receives. This study describes the bed-load transport regime as it is linked to hydrology and geomorphology in 23 headwater gravel-bed streams in snowmelt-dominated parts of central and northern Idaho. At sites, drainage areas range from 1.29 to 381 km 2 , stream gradients range from 0.0042 to 0.0747, and median bed surface particle sizes range from 4 to 207 mm. Stream architecture includes riffle-pool, planar, and step-pool beds. The bed load is much finer than the surface and subsurface material, suggesting selective transport of the finer sizes. Nonetheless, the majority of the load is sand at all flow discharges. Progressively coarser sediment was collected as flow discharge increased, and painted rock experiments documented the transport of coarser particles at higher discharges. The supply of sediment to the streams appears limited, as indicated by observed clockwise hysteresis in bed-load transport rates during each spring snowmelt and by the coarse surface armor observed at sites. Flows above bankfull discharge move 37% of the bed load, whereas flows between mean annual discharge and bankfull move 57% of the bed load. The bed-load effective discharge has a recurrence interval that averages 1.4 yr and the magnitude of effective discharge averages 80% of bankfull discharge. The recurrence interval of bankfull discharge averages 2.0 yr. The ratio of effective discharge to bankfull discharge is independent of basin size, grain size, and gradient, although the ratio increases with the relative magnitude of large infrequent events.