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Speleology in Kazakhstan

Shakalov on 04 Jul, 2018
Hello everyone!   I pleased to invite you to the official site of Central Asian Karstic-Speleological commission ("Kaspeko")   There, we regularly publish reports about our expeditions, articles and reports on speleotopics, lecture course for instructors, photos etc. ...

New publications on hypogene speleogenesis

Klimchouk on 26 Mar, 2012
Dear Colleagues, This is to draw your attention to several recent publications added to KarstBase, relevant to hypogenic karst/speleogenesis: Corrosion of limestone tablets in sulfidic ground-water: measurements and speleogenetic implications Galdenzi,

The deepest terrestrial animal

Klimchouk on 23 Feb, 2012
A recent publication of Spanish researchers describes the biology of Krubera Cave, including the deepest terrestrial animal ever found: Jordana, Rafael; Baquero, Enrique; Reboleira, Sofía and Sendra, Alberto. ...

Caves - landscapes without light

akop on 05 Feb, 2012
Exhibition dedicated to caves is taking place in the Vienna Natural History Museum   The exhibition at the Natural History Museum presents the surprising variety of caves and cave formations such as stalactites and various crystals. ...

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That transmission capacity is the property of a porous medium to conduct fluid [16].?

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Featured articles from Cave & Karst Science Journals
Chemistry and Karst, White, William B.
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Karst environment, Culver D.C.
Mushroom Speleothems: Stromatolites That Formed in the Absence of Phototrophs, Bontognali, Tomaso R.R.; D’Angeli Ilenia M.; Tisato, Nicola; Vasconcelos, Crisogono; Bernasconi, Stefano M.; Gonzales, Esteban R. G.; De Waele, Jo
Calculating flux to predict future cave radon concentrations, Rowberry, Matt; Marti, Xavi; Frontera, Carlos; Van De Wiel, Marco; Briestensky, Milos
Microbial mediation of complex subterranean mineral structures, Tirato, Nicola; Torriano, Stefano F.F;, Monteux, Sylvain; Sauro, Francesco; De Waele, Jo; Lavagna, Maria Luisa; D’Angeli, Ilenia Maria; Chailloux, Daniel; Renda, Michel; Eglinton, Timothy I.; Bontognali, Tomaso Renzo Rezio
Evidence of a plate-wide tectonic pressure pulse provided by extensometric monitoring in the Balkan Mountains (Bulgaria), Briestensky, Milos; Rowberry, Matt; Stemberk, Josef; Stefanov, Petar; Vozar, Jozef; Sebela, Stanka; Petro, Lubomir; Bella, Pavel; Gaal, Ludovit; Ormukov, Cholponbek;
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Your search for hydraulic gradient (Keyword) returned 73 results for the whole karstbase:
Showing 46 to 60 of 73
Hydraulic considerations in the development of tiankengs, 2006, Palmer Arthur N. , Palmer Margaret V.

Tiankengs are formed most commonly by the collapse of bedrock into underlying caves that contain active rivers. The collapse propagates itself by blocking and diverting the underground streams, so that hydraulic gradients become steeper and the solutional and erosional capacities are enhanced. Most of the volume of a tiankeng is produced by removal of mass by the cave streams. A large and fluctuating discharge is most favorable. As diversion passages form and enlarge, they foster further collapse and diversion. Stress release around the collapse encourages the opening of new fractures with trends that differ from regional fracture patterns. These processes account for the large scale of tiankengs in comparison to the original cave passages.


Aroca (domaine marin ctier, Pays basque, France) : un karst continental ennoy par les transgressions maritimes quaternaires, 2007, Vanara Nathalie , Perre Alain, Pernet Marc, Latapie Serge, Jaillet Stphane, Martine Olivier
AROCA (LITTORAL, BASQUE COUNTRY, FRANCE): A CONTINENTAL KARST DROWNED BY QUATERNARY MARITIME TRANSGRESSIONS. The rocky formations in shallow areas of the Atlantic coast are hardly known. Studies are rare because of the difficulties of direct observation (diving in always agitated, troubled water, depth between 20 and 40 m). Our first step was to make a detailed topography of a submarine plateau named Aroca, 4 km off Socoa harbour (bay of Saint-Jean-de-Luz). This plateau was already known for having a large variety of forms within a small surface (150 x 100 m). We gave names to most remarquable formations and defined five main characteristic zones: - in the exokarstic domain 1/ a top surface with channels, 2/ a dismantled surface with pinnacles; - in the endokarstic domain 3/ caves, galleries, arches; - at the limits 4/ three inclined plans, west, north and east, 5/ a cliff to the south. A typology of forms shows a predominance of ablation reliefs: aplanation, over-deepened channels, covered rooms and galleries, arches, residual pinnacles. Deposit accumulations regroup chaotic breakdown blocks, pebble accumulations and sand covers. Statement of explanations requires recognition of the nature and age of the outcrops and succession of erosional agents during the Pleistocene. Rocks are dated from Ypresien (limestones) to Bartonian (marls). Continental erosion during sea regressions is responsible of caracteristic landforms and deposits; for example wall banks, allochthonous pebbles The currently active marine erosion during sea transgressions is due to storms, tide, dissolution, biochemical action (lithophages) and gravity. We propose a paleogeographic reconstitution. After an essentially calcareous sedimentation in Eocene and an essentially marly sedimentation in Oligocene, the sea recedes during Miocene. From then, the platform, henceforward above the water, is subject to meteoric erosion. In Pliocene, evolution of the massif is isovolumic (under a marly cover and with a low hydraulic gradient). During the lower and middle Pleistocene, the erosion of the marly cover goes on. During the upper Pleistocene, the wurmian (18000 BP) marine regression allows entrenchment of the hydrographic system thanks to an increase of hydraulic gradients (classic functional karst). From 15000 years onwards, a general transgression of sea level happens by successive steps. During the Boreal, a break in transgression allows the formation of a paleo-shore at 20 to 30 m, inducing a peneplanation phase in the tidal or infratidal zone. From 7500 BP onwards, a a rapid transgression from 23 to 8, then a slower one from 8 to the present level stops karstification on the massif. At present, only marine abrasion is active and tends to obliterate the previously built landforms.

Alteration of fractures by precipitation and dissolution in gradient reaction environments: Computational results and stochastic analysis, 2008, Chaudhuri A. , Rajaram H. , Viswanathan H.

Precipitation and dissolution reactions within fractures alter apertures, which in turn affects their flow and transport properties. Different aperture alteration patterns occur in different flow and reaction regimes, and they are also influenced by preferential flow resulting from spatial variations in the aperture. We consider the alteration of variable-aperture fractures in gradient reaction regimes, where fluids are in chemical equilibrium with a mineral everywhere but precipitation and dissolution are driven by solubility gradients associated with temperature variations. The temperature field is defined by a geothermal gradient corresponding to a conduction-dominated heat transfer regime. Monte Carlo simulations on computer-generated aperture fields vividly illustrate pattern formation resulting from two-way feedback between fluid flow and reactive alteration. In dissolution-controlled systems, distinct dissolution channels develop along the dominant flow direction, while elongated precipitate bodies form perpendicular to the mean flow direction in precipitation-controlled systems. Aperture variability accelerates the increase and decrease of effective transmissivity by dissolution and precipitation, respectively. The dominance of precipitation versus dissolution is determined by the angle between the mean hydraulic gradient and solubility/temperature gradient. Development of pronounced anisotropy with oriented elongate features is the key feature of aperture alteration in gradient reaction regimes. A stochastic analysis is developed, which consistently predicts general trends in the aperture field during reactive alteration, including the mean, variance, and spatial covariance structure. Our results are relevant to understanding the long-term diagenetic evolution of fractures in conduction-dominated heat transfer regimes and related problems such as emplacement of ocean bed methane hydrates.


The deepest cave in the world in the Arabika Massif (Western Caucasus), 2008, Klimchouk A. B. , Samokhin G. V. & Kasjan Yu. M.

Arabika is an outstanding high-mountain karst massif in the Western Caucasus composed of Lower Cretaceous and Upper Jurassic limestones continuously dipping southwest to the Black Sea shore and plunging below the sea level. The central sector (elevations within 2000-2700 m) is characterized by pronounced glacio-karstic landscape and hosts several deep caves including the deepest cave in the world (Krubera-Voronja Cave) recently explored to the depth of -2191 m.  Dye tracing experiments conducted in 1984-1985 revealed that the Krubera Cave area is hydraulically connected with major springs at the Black Sea shore and the submarine discharge, with the flow directed across major fold structures. Krubera Cave has an extremely steep profile and reveals a huge thickness of the vadose zone. Its lower boundary is at elevation of about 110 m, which suggests a very low overall hydraulic gradient of 0.007-0.008. Reported low salinity groundwater tapped by boreholes in the shore area at depths 40-280, 500, 1750 and 2250 m, which suggests the existence of deep flow system with vigorous flow. Submarine discharge in the Arabika coast is reported at depths up to ca. 400 m bsl. Huge closed submarine depression is revealed at the sea-floor in front of Arabika with the deepest point of ca. 400 m bsl. These facts point to a possibility that the main karst system in Arabika could have originated in response to the Messinian salinity crisis (5.96 – 5.33 Myr) when the Black Sea could have almost dried up, similarly to the adjacent Mediterranean where the sea level drop up to 1600 m is well established. Further development of the huge vadose zone and a super-deep cave have been caused by subsequent uplifts during Pliocene-Pleistocene, highly differential between the shore sector (0.1-0.2 km of total uplift) and the central sector (2-2.5 km) of Arabika.


The endokarstic erosion of marble in cold climates: Corbel revisited. , 2009, Faulkner, Trevor

After the work of Jean Corbel, who compared karstification in the Scandinavian Caledonide marbles with that in sedimentary limestones in temperate and tropical regions, the understanding of underground limestone dissolution has developed considerably. Corbel concluded that “karstification proceeds much faster in a cold than in a warm climate”, based on the knowledge that the solubilities of both CO2 and CaCO3 increase with lower temperature, without realising that because cave streams in Scandinavia rarely reach saturation, this fact is not directly relevant. We now know that the dissolutional enlargement of inception channels in limestones proceeds commonly via a slow initial ‘pre-breakthrough’ laminar flow stage before conduits can enlarge chemically at maximum rates under turbulent flow conditions. Recent research has shown that the pre-breakthrough stage is speeded up at low temperatures, as occurs in cold climates now, and as occurred during the deglaciation of the Weichselian ice sheet in Scandinavia, especially under steep hydraulic gradients and, in many cases, despite the lower partial pressure of CO2. Additionally, this whole stage might be bypassed if fractures created by deglacial seismicity were wide enough and short enough. After breakthrough, although limestone dissolution is slower in cold rather than warm climates, conduit enlargement still proceeds as a significant rate, provided the water remains unsaturated, and especially if high flow rates promote mechanical erosion. The exploration of large numbers of (short) caves in central Scandinavia shows that Corbel’s conclusion is partly true for the more recent geological past, because of the special conditions that apply during the Quaternary glacial cycles.

 

 


A DARCIAN MODEL FOR THE FLOW OF BIG SPRING AND THE HYDRAULIC HEAD IN THE OZARK AQUIFER, MISSOURI, USA, 2010, Criss R. E.
The complex discharge hydrograph for Big Spring, Missouri, can be described as the sum of two terms governed by Darcys Law. The dominant, long-term component is proportional to the regional hydraulic gradient, and constitutes about 80% of the average flow of 12.6 m3/s. Superimposed on this is a transient component with a time-constant of about 1.5 days that represents the Darcian response to sharp, rainfall-driven pulses on the head of the shallow groundwater system. This transient component delivers about 20% of the average total flow, but over short intervals can exceed the long-term component. However, the long-term component is so large that the ratio of record high flows to the average flow is only about 4x for Big Spring, and 1.5 to 4.5x for most other large Ozark springs; for comparison, this ratio is 10 to 3000x for most surface streams in Missouri. The strong correlation between the discharge of the large springs and the head in the Ozark aquifer permits the extension of the Darcian rainfall-runoff model to predict groundwater levels in wells.

Effects of Karst and geological structure on groundwater flow: The case of Yarqon-Taninim Aquifer, Israel, 2010, Dafny Elad, Burg Avi, Gvirtzman Haim

This study demonstrates the significant influences of the geological structure (especially folding and lithology) and the karst system on groundwater flow regime. Folds divert groundwater flow from the general hydraulic gradient; marly layers sustain several perched sub-aquifers above the regional aquifer; and karstification increases the hydraulic conductivity by several orders of magnitude. These phenomena are quantitatively demonstrated within the Yarqon-Taninim (YT) basin, Israel, which is a complex groundwater system, combining several (extremely) opposite characteristics: humid and arid recharge zones, phreatic and confined parts, shallow and deep sub-aquifers, stratified and relatively-homogeneous sub-basins, saline and fresh water bodies, as well as stagnant and fast-flowing groundwater regions.

We have introduced a 3D geological-based grid for the basin (for the first time). It was implemented into a numerical code (FEFLOW), which was used thereafter to analyze quantitatively the flow regime, the groundwater mass balance, and the aquifer hydraulic properties. We present up to date conceptual understanding and numerical modeling of the YT flow field, especially at its mountainous parts.

Based on the calibration procedure and the sensitivity analyses, we obtained the best-fitted hydraulic conductivity values for the aquifer mesh. The general phenomenon observed is that as groundwater flow quantity increases, the hydraulic conductivity also increases. We interpret this result by the karstification mechanism (including paleo-karst). Thus, where groundwater flow-lines converge and where groundwater discharge amount increases, the karstification process intensifies and permeability increases. Consequently, at the mountainous region, along the syncline axes, where groundwater flow-lines converge, higher conductivities are found.

Modeling results also exhibit that at the lowland confined area, the geological structure does not play a major role in directing groundwater flow. Rather, the flow field is controlled by the well-developed karst system and the relatively homogenous carbonate section. It is hypothesizes that the extensive karstification took place at the Messinian Salinity Crises, 5.5 Ma, during which groundwater heads as well as sea level were lowered by several 100 m.


Karstification in unconfined limestone aquifers by mixing of phreatic water with surface water from a local input: A model, 2010, Gabrovš, Ek F. , Dreybrodt W.

When water from the surface of a limestone plain seeps down through the fractured rock to the water table of an unconfined aquifer with low hydraulic gradient containing water saturated with respect to calcite, mixing of these waters causes renewed aggressivity. A model is presented, which describes the evolution of karstification by dissolutional widening of the fractures downgradient from the local input of surface water. The model couples flow in the fractures with dissolution rates. Dissolution rates are given by F = k (1 [1] c (x)/ceq)4, where c (x) is the calcium concentration at distance x from the entrance of the fracture, ceq is the equilibrium calcium concentration of the H2O–CaCO3–CO2 solution in the fracture, and k is a rate constant. The model describes two domains of waters saturated with respect to calcite at different partial pressure of CO2. At the borders of these domains the waters mix and create dissolutional widening of the fractures by mixing corrosion. A channel evolves along the border in the downgradient direction by about 100 m in 100 ky. Below this channel a zone of fractures with aperture widths up to 1 cm has originated. The change of the hydraulic conductivity in the mixing zone shifts the border of the domains, allowing the channel to grow in the downgradient direction. Below it the zone of widened fractures is invaded by saturated phreatic water and dissolution stops. This process continues at the downgradient part of the conduit. In summary, we find cave conduits evolving close to the water table, leaving significant cavernous structures below them. A variety of modelling scenarios with different choices of parameters show that this evolution is typical and changes only in details but not in its basic behaviour.


Karstification of aquifers interspersed with non-soluble rocks: From basic principles towards case studies, 2010, Romanov D. , Kaufmann G. , Hiller T.

We have developed a numerical model able to describe the karstification of aquifers in fractured rocks containing soluble (limestone or gypsum) and insoluble layers. When water is flowing along fractures crossing the soluble layers, it is able to dissolve the material there, to increase the aperture width of the conduit, and consequently to increase the local hydraulic conductivity. Depending on the thickness and the distribution of these layers, the dissolution can be active only for limited periods, or during the whole evolution time. Fractures located in insoluble layers do not change at all. We are interested in the integral effect of these local processes and study four simplified scenarios of karstification along a prominent wide conduit crossing a fractured limestone block. We keep the initial and the boundary conditions the same for all scenarios and vary only in the amount and the distribution of the soluble material. We demonstrate that aquifers in 100% limestone, without any insoluble layers, develop along areas with high hydraulic conductivities and high hydraulic gradients, creating channel like pathways. On the other hand aquifers containing soluble layers with limited thickness develop faster and exhibit diffuse patterns determined by the chemical properties of the rock. The second part of the paper is a step towards modeling of real karst systems. We present the evolution of an aquifer located in the vicinity of a large hydraulic structure. All initial and boundary conditions, except the amount and the distribution of the soluble rock, remain the same for all scenarios. As a material example for the bedrock, we chose Gipskeuper from an aquifer along the Birs river in Switzerland. This rock consists of soluble gypsum layers and insoluble clays and marls, with typical layer thickness in the range of millimeters to centimeters. The basic processes discussed in the first part of the paper remain valid. We demonstrate that large insoluble zones can impair the karstification process and even completely block it, while areas with thin soluble layers can provide a preferential pathway and decrease the evolution times considerably. Finally we show that the evolution of the leakage rates and the head distribution within the aquifer can sometimes reveal misleading information about the stage of karstification and the safeness of the dam. Our model can be used not only to study simplified geological settings and basic processes, but also to address some of the complications arising when modeling real aquifers.


The impact of glacier ice-contact and subglacial hydrochemistry on evolution of maze caves: A modelling approach, 2010, Skoglund Rannveig Ovrevik, Lauritzen Steinerik, Gabrovsek Franci

Labyrinth and maze cave networks are a conspicuous feature in formerly glaciated stripe karst in Scandinavia. Often found in topographically “impossible” situations, their genesis is attributed to glacial ice-contact conditions. This is further supported by observing that individual networks may either be influent, effluent or through-flow; depending on the attitude of the host rock and former glacier directions. The ice-contact hypothesis is tested by using a finite difference, fracture network model where chemical and hydrological conditions can be varied. Subglacial chemistry alone (low partial pressure of CO2, low temperature) is not sufficient to favour mazes over linear caves. However, when coupled with high input saturation ratio, high and varied hydraulic gradients and glacial hydrology, the model produced cave patterns comparable in scale and complexity to our field examples.


Karstification beneath dam-sites: From conceptual models to realistic scenarios, 2011, Hiller Thomas, Kaufmann Georg, Romanov Douchko

Dam-sites and reservoirs located above soluble rock are often damaged by increased leakage through the sub-surface within the life-time of the structure. The high hydraulic gradients driving the water through the fracture and fissure system of the bedrock have a strong impact on the aquifer evolution. The increased permeability, if not prevented, leads to an imminent danger of high leakage rates (breakthrough) as well. As a result, the structural safety of the dam-site itself is at risk. Past experience has shown that this may have large environmental and economical consequences.

For a better understanding of the evolution of karst aquifer systems in the vicinity of dam-sites, a three-dimensional conceptual model is presented. We show the evolution of the karst aquifer for simple three-dimensional dam-site setups. Keeping the symmetry and simplicity of the models we can relate our results to the two- and one-dimensional scenarios presented in the past. Implementing a statistical fracture network and topographic information to this basic setup we show that these complex three-dimensional properties of the real aquifers, have a significant influence on the karstification, and cannot always be addressed by two -and one-dimensional models.

Research highlights
- Three-dimensional karst evolution modeling of dam-sites. - Relating the 3D models to former 2D and 1D models. - Implementation of statistical fracture network and topography.


Spreading of tracer plumes through confined telogenetic karst aquifers: A model, 2011, Gabrovsek Franci, Dreybrodt Wolfgang

To calculate spreading of a tracer or contaminant through an aquifer all details of the aquifer, e.g. distribution of hydraulic parameters, must be known. This is not possible in nature. To study the spreading of plumes through karst, we have used a digital model of a confined karst aquifer at different stages of early karstification. In these models all details such as fracture aperture widths, their lengths and widths, and the hydraulic boundary conditions are known. Therefore the flow velocity of water can be calculated in each fracture. Using this information a particle tracking method is employed to calculate the propagation and spreading of a plume caused by an instantaneous input pulse into selected regions of the aquifer. From this information the time dependence of the outflow of particles from any selected region is obtained. This function represents the transfer response function for an instantaneous Dirac ?-function input. Two digital karst models are designed. In the first, homogeneous one, the aperture widths of the fractures are statistically distributed but of similar width. In the second a coarse percolating net of prominent fractures with larger constant aperture width is embedded into the dense net of narrow fissures. Propagation of the plumes and the transfer-response function are presented at the onset of karstification and at different times of karst evolution. If particles are injected at the entrance of evolving karst channels propagating towards the output boundary tracer breakthrough times increase with increasing time of karst evolution until shortly before breakthrough of the karst conduit they drop to half of their maximal value. With increasing evolution of the karst aquifer the hydraulic heads are redistributed and regions of low hydraulic gradients in the upstream side of the aquifer are created. Particles injected into fractures which have stopped dissolutional widening of their aperture widths and are located in regions of low gradient are kept in these regions for long times in the order of 100 years until they have propagated towards regions of high hydraulic heads, where a “fan like” plume develops along the pathway of steepest gradient.
Highlights


Ferruginous thermal spring complexes, northwest Tasmania: Evidence that far-field stresses acting on a fracture mesh can open and maintain vertical flow in carbonate terrains, 2011, Davidson G. J. , Bavea M. , Harris K.

Far-field stress changes in the southern Australian plate since 5 Ma have produced significant areas of uplift and seismicity. In northwest Tasmania, there is evidence that this stress reorientation to maximum horizontal NW-SE stress has influenced meteoric-derived thermal (15-20°C) discharge patterns of confined karstic aquifers, by placing pre-existing NW-trending faults/fractures into a dilated state or a critically stressed state. Previous studies have shown that spring discharge has operated continuously for at least 65,000 years, and has transported large volumes of solutes to the surface to be deposited as mounds of calcite-goethite-silica up to 7 m high. The thermal spring chemistry at one site, Mella, is consistent with descent to at least 1.2-1. 5 km, although the hinterland within 50 km is less than 500 m elevation. Thermal spring chemistry is consistent with most of the deep water-rock interaction occurring in low-strontium Smithton Dolomite. While some of this water is discharged at springs, some instead intersects shallow zones of NE-fracture-controlled rock (2 ? 4 km in area) with karstic permeability where, although confined and subject to a NE-directed hydraulic gradient, it circulates and cools to ambient temperature, with only minor mixing with other groundwaters


Influence of initial heterogeneities and recharge limitations on the evolution of aperture distributions in carbonate aquifers, 2011, Hubinger B. , Birk S.

Karst aquifers evolve where the dissolution of soluble rocks causes the enlargement of discrete pathways along fractures or bedding planes, thus creating highly conductive solution conduits. To identify general interrelations between hydrogeological conditions and the properties of the evolving conduit systems the aperture-size frequency distributions resulting from generic models of conduit evolution are analysed. For this purpose, a process-based numerical model coupling flow and rock dissolution is employed. Initial protoconduits are represented by tubes with log-normally distributed aperture sizes with a mean of 0.5 mm. Apertures are spatially uncorrelated and widen up to the metre range due to dissolution by chemically aggressive waters. Several examples of conduit development are examined focussing on influences of the initial heterogeneity and the available amount of recharge. If the available recharge is sufficiently high the evolving conduits compete for flow and those with large apertures and high hydraulic gradients attract more and more water. As a consequence, the positive feedback between increasing flow and dissolution causes the breakthrough of a conduit pathway connecting the recharge and discharge sides of the modelling domain. Under these competitive flow conditions dynamically stable bimodal aperture distributions are found to evolve, i.e. a certain percentage of tubes continues to be enlarged while the remaining tubes stay small-sized. The percentage of strongly widened tubes is found to be independent of the breakthrough time and decreases with increasing heterogeneity of the initial apertures and decreasing amount of available water. If the competition for flow is suppressed because the availability of water is strongly limited breakthrough of a conduit pathway is inhibited and the conduit pathways widen very slowly. The resulting aperture distributions are found to be unimodal covering some orders of magnitudes in size. Under these suppressed flow conditions the entire range of apertures continues to be enlarged. Hence, the number of tubes reaching aperture sizes in the order of centimetres or decimetres continues to increase with time and in the long term may exceed the number of large-sized tubes evolving under competitive flow conditions. This suggests that conduit development under suppressed flow conditions may significantly enhance the permeability of the formation e.g. in deep-seated carbonate settings.


The significance of turbulent flow representation in single-continuum models, 2011, Reimann T. , Rehrl C. , Shoemaker W. B. , Geyer T. , Birk S.

Karst aquifers evolve where the dissolution of soluble rocks causes the enlargement of discrete pathways along fractures or bedding planes, thus creating highly conductive solution conduits. To identify general interrelations between hydrogeological conditions and the properties of the evolving conduit systems the aperture-size frequency distributions resulting from generic models of conduit evolution are analysed. For this purpose, a process-based numerical model coupling flow and rock dissolution is employed. Initial protoconduits are represented by tubes with log-normally distributed aperture sizes with a mean ?0 = 0.5 mm for the logarithm of the diameters. Apertures are spatially uncorrelated and widen up to the metre range due to dissolution by chemically aggressive waters. Several examples of conduit development are examined focussing on influences of the initial heterogeneity and the available amount of recharge. If the available recharge is sufficiently high the evolving conduits compete for flow and those with large apertures and high hydraulic gradients attract more and more water. As a consequence, the positive feedback between increasing flow and dissolution causes the breakthrough of a conduit pathway connecting the recharge and discharge sides of the modelling domain. Under these competitive flow conditions dynamically stable bimodal aperture distributions are found to evolve, i.e. a certain percentage of tubes continues to be enlarged while the remaining tubes stay small-sized. The percentage of strongly widened tubes is found to be independent of the breakthrough time and decreases with increasing heterogeneity of the initial apertures and decreasing amount of available water. If the competition for flow is suppressed because the availability of water is strongly limited breakthrough of a conduit pathway is inhibited and the conduit pathways widen very slowly. The resulting aperture distributions are found to be unimodal covering some orders of magnitudes in size. Under these suppressed flow conditions the entire range of apertures continues to be enlarged. Hence, the number of tubes reaching aperture sizes in the order of centimetres or decimetres continues to increase with time and in the long term may exceed the number of large-sized tubes evolving under competitive flow conditions. This suggests that conduit development under suppressed flow conditions may significantly enhance the permeability of the formation, e.g. in deep-seated carbonate settings.


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