Franci Gabrovšek

Dam sites in soluble rocks: a model of increasing leakage by dissolutional widening of fractures beneath a dam

Abstract Water flowing through narrow fissures and fractures in soluble rock, e.g. limestone and gypsum, widens these by chemical dissolution. This process, called karstification, sculptures subterranean river systems which drain most of their catchment. Close to dam sites, unnaturally high hydraulic gradients are present to drive the water impounded in the reservoir downstream through fractures reaching below the dam. Under such conditions, the natural process of karstification is accelerated to such an extent that high leakage rates may arise, which endanger the operation of the hydraulic structure. Model simulations of karstification below dams by coupling equations of dissolutional widening to hydrodynamic flow are presented. The model scenario is a dam 100 m wide in limestone or gypsum. The modelling domain is a two-dimensional slice 1 m wide directed perpendicular to the dam. It extends 375 m vertically and 750 m horizontally. The dam is located in its center. This domain is divided by fractures and fissures into blocks of 7.5×7.5×1 m. The average aperture width of the fractures is 0.02 cm. We performed model runs on standard scenarios for a dam site in limestone with the height H of impounded water 150 m, a horizontal impermeable apron of width W =262 m and a grouting curtain reaching down to a depth of G =97 m. In a second scenario, we changed these construction features to G =187 m and W =82 m. To calculate widening of the fractures, well-established experimental data on the dissolution of limestone and gypsum have been used as they occur in such geochemical settings. All model runs show similar characteristic behaviour. Shortly after filling, the reservoir exhibits a small leakage of about 0.01 m 3 s −1 , which increases steadily until a breakthrough event occurs after several decades with an abrupt increase of leakage to about 1 m 3 s −1 within the short time of a few years. Then, flow in the fractures becomes turbulent and the leakage increases to 10 m 3 s −1 in a further time span of about 10 years. The widths of the fractures are visualized in various time steps. Small channels propagate downstream and leakage rises slowly until the first channel reaches the surface downstream. Then breakthrough occurs, the laminar flow changes to turbulent and a dense net of fractures which carry flow is established. We performed a sensitivity analysis on the dependence of breakthrough times on various parameters, determining breakthrough. These are the height of impounded water H , the depth G of grouting, the average aperture width a 0 of the fractures and the chemical parameters, which are c eq the equilibrium concentration of Ca with respect to calcite and the Ca-concentration c in of the inflowing water. The results show that the most critical parameter is a 0 . At fracture aperture widths of 0.01 cm, breakthrough times are above 500 years. For values of a 0 >0.02 cm, however, breakthrough times are within the lifetime of the structure. We have also modelled dam sites in gypsum, which exhibit similar breakthrough times. However, after breakthrough, owing to the much larger dissolution rates of gypsum, the time until unbearable leakage is obtained, is only a few years. The modelling can be applied to complex geological settings, as phreatic cave conduits below the dam, or a complex stratigraphy with varying properties of the rock with respect to hydraulic conductivity and solubility. A few examples are given. In conclusion, our results support the assumption that increasing leakage of dam sites may be caused by dissolutional widening of fractures.

The impact of hydrochemical boundary conditions on the evolution of limestone karst aquifers

Abstract The early evolution of karst aquifers depends on a manifold of initial and boundary conditions such as geological setting, hydrologic properties of the initial aquifer, and petrologic properties of the rock. When all water entering at various inputs into the aquifer has equal chemical composition with respect to the system H2O–CO2–CaCO3 early evolution under conditions of constant head exhibits breakthrough (BT) behaviour. If the chemical compositions of the input waters are different, deep in the aquifer where the saturated solutions mix renewed aggressiveness occurs, and additional dissolutional widening of fractures by mixing corrosion (MC) changes the hydrologic properties of the aquifer. To study the impact of MC on the evolution of karst we have modelled a simple karst aquifer consisting of a confined limestone bed, with two symmetrically located inputs at constant head and open flow conditions along the entire width at base level. To calculate dissolutional widening of the fractures the well-known dissolution kinetics of limestone was used, which is linear up to 90% of saturation with respect to calcite and then switches to a nonlinear fourth order rate law. First, two extremes are modelled: (a) Both inputs receive aggressive water of equal chemical composition with [Ca2+]=0.75[Ca2+]eq. In this case two channels migrate downstream with that from one input more competitive and reaching base level first, causing BT. (b) Water at both inputs is saturated with respect to calcite, but in equilibrium with different partial pressures of CO2. Therefore, dissolution widening can occur only where these waters mix. A central channel starts to grow extending down-head until base level is reached. Flow rates through the aquifer first rise and become constant after the channel has reached base level. In the following runs these two extreme modes of karstification are combined. The waters entering have different chemical compositions and therefore different equilibrium concentrations [Ca2+]eq. This allows MC to be active. They are also undersaturated with the inflowing solutions at concentration [Ca2+]in=f[Ca2+]eq where f is the ratio of saturation. In comparison to the extreme limit (a) the action of MC now creates permeability where the solutions mix and diverts the evolution of conduits into this region. Finally one conduit reaches base level and causes BT. This behaviour is found for f=0.7, 0.9, and 0.96. For solutions more close to equilibrium with respect to calcite (f=0.99, 0.9925, and 0.995) BT behaviour is replaced by a steady increase in flow rates. In the early state as in the case of MC controlled evolution (case b) a central channel not connected to the input is created by MC and reaches base level. After this event, further increase in flow rates is caused by slow dissolutional widening by the slightly undersaturated input solutions flowing towards the central channel. Comparison of the various model aquifers at termination of the computer runs reveals significant differences in their properties caused solely by changes of the hydrochemical boundary conditions.

Hydrodynamic aspect of caves

INTRODUCTION Karst aquifers are dominated by a network of connected conduits developed within a fractured and/or porous rock. Flow of groundwater in karst is complex, and extreme variation of conductivities enables practically all natural flow regimes. Interaction between conduits and the adjacent fracture-matrix system plays an important role in karst hydrology. Nevertheless in conduit dominated aquifers with low bedrock porosity and conductivity, such exchange can be neglected when discussing short term flood response of the system (Peterson and Wicks, 2005; Jeannin, 2001). Our focus is flow in large

Deep conduit flow in karst aquifers revisited

Caves formed in soluble rocks such as limestone, anhydrite, or gypsum are efficient drainage paths for water moving through the aquifer from the surface of the host rock toward a resurgence. The formation of caves is controlled by the physical solution through dissociation of the host rock by water or by the chemical solution through reactions of the host rock with water enriched with carbon dioxide. Caves as large underground voids are simply the end-member of secondary porosity and conductivity characterizing the aquifer. Caves and their relation to a present or past base level are found both close to a past or present water table (water table caves) and extending far below a past or present water table (bathy-phreatic caves). One explanation for this different speleogenetic evolution is the structural control: fractures and bedding partings are preferentially enlarged around more prominent faults, thus the fracture density in the host rock controls the speleogenetic evolution. This widely accepted explanation can be extended by adding other controls, e.g., a hydraulic control: as temperature generally increases with depth, density and viscosity of water change, and particularly the reduction of viscosity due to the increase in temperature enhances flow. This hypothesis was proposed by Worthington (2001, 2004) as a major controlling factor for the evolution of deep bathy-phreatic caves. We compare the efficiency of structural and hydraulic control on the evolution of a cave passage by numerical means, adding a third control, the chemical control to address the change in solubility of the circulating water with depth. Our results show that the increase in flow through deep bathy-phreatic passages due to the decrease in viscosity is by far outweighted by effects such as the decrease in fracture width with depth due to lithostatic stress and the decrease in solubility with depth. Hence, the existence of deep bathy-phreatic cave passages is more likely to be controlled by the structural effect of prominent faults.

Impact of Motorways on Karst Waters

Impervious surfaces of road networks accumulate contaminants and pollutants, which are washed off during rain and snowstorm events into nearby waters and lands. Especially in areas with a developed traffic network, runoff from motorways is a significant source of pollution. Various protection and remediation measures were developed and implemented for preventing or at least diminishing its negative influences. Three major sources of pollution are associated with the road network: vehicles (emissions of motor vehicles, spilled and released oil, tires’ particles, de-icing agents), road characteristics and paint markers, and atmospheric depositions influenced by the adjacent land use. Besides pollutants directly associated with traffic, the impermeable surfaces can collect and drain a negligible quantity of organic waste, nitrogen and phosphorous, herbicides, pesticides and faecal pathogens. Soil infiltration treatment, building of collection basins and constructed wetlands are used as protection and remediation measures. In karst areas, motorway runoff has an amplified impact on ground water, compared to other types of landscapes. The soil layer is commonly thin or non-existent and thus soil infiltration treatment is practically inexistent. Stormwater runoff may immediately flow into the aquifer through subsurface conduit networks, fractures, sinkholes and sinking streams, which makes karst groundwater especially vulnerable to pollution. Accidents that cause greater quantities of hazardous substances to flow into the karst, for whatever reason, endanger our environment, karst waters, and even in smaller quantities also the quality of the karst springs that have been captured for drinking water supply. Especially dangerous are spills of oil derivatives, since we know too little about the flow of such substances that do not dissolve in water and are lighter than it. Based on observations of Globocec spring following the accident near Ortnek and of the Rižana Spring following the spillage near Obrov and other similar cases, we anticipate a longer retention time and washing away of oil derivatives due to the possibility of adsorption on sediments and retention in siphons.

Unroofed Caves Provide Important Clues to the Karst Development

Unroofed caves are old caves that were revealed on account of the lowering of the karst surface. They are preserved by their infill—mostly alluvium and flowstone. It became clear during the motorway construction undertaking in Slovenia that unroofed caves constitute a relatively common karst landform. In fact, more common than karstologists had imagined before the karst surface was uncovered through earthworks. The various types of notches occurring on the surface have long been interpreted as types of dolines or as the result of the lithological properties of rock and its fracturing. 75 km long and, on average, 25 m wide stretch of the motorway across the classical Karst gave up 350 caves, of which 90 are unroofed caves. Some of them make up the same cave system. New findings prompted karstologists to become more aware of these unique surface karst forms. In the process, numerous unroofed caves filled with all types of alluvium were discovered. There were several attempts at typification of the characteristic shapes of unroofed caves and to design partial models to explain their typical formation processes. The surface and subcutaneous dissolution of carbonate rock and its disintegration from back in the Ice Age, brought about the lowering of the karst surface. Old caves, which were formed by erstwhile water flows and are partly intersected by shafts which drain water from the permeable karst surface, pop up as either empty or filled with alluvium. The caves were formed as a part of a system of cavities in a period when impermeable rocks had enclosed the aquifer higher up, causing the underground water in the aquifer to be at a higher level. The hypothesis was that the karst topography and its remarkable systems of valleys can be traced back to former surface water throughflow. However, revealing the surface did not provide the evidence to support this hypothesis, instead obvious signs of former water throughflow in carbonate rock were identified—manifested as open and cut through old caves.

Unroofed Caves Near Kozina and Their Identification

Unroofed caves are an important karst form that makes up a part of the karst surface and epikarst, and provides us with valuable evidence of the karst development. They are old caves that became exposed by the lowering of the karst surface. In fact they are preserved by their fill—mostly fine-grained alluvium. It is also often that they feature preserved flowstone and an intact rock rim. During earthworks preceding the motorway construction, this important karst feature, also characterizing the surface, attracted special attention. Earthworks revealed that the karst surface is scattered with several distinct types of unroofed caves, which in itself are not an uncommon phenomena. The typical shapes of unroofed caves found on karst terrain could be singled out, i.e. individual doline-like forms that occur in strings, and oblong notches. Because the surface of the Karst region has lowered so dramatically, there are old caves and shafts opening up all the time in the course of the construction of the motorways. Old caves are either void or filled with alluvium. The caves were formed as a part of a system of cavities in a period when impermeable rocks had enclosed the aquifer higher up, causing the ground water in the aquifer to be at a higher level. But karstification gave rise to a drop in the water table in the aquifer—today it is 200 m and more below ground, and the karst surface is still lowering. Unroofed caves are therefore regarded as distinct surface karst forms which were in part reshaped by surface processes that make up an important part of epikarst. In the course of the earthworks for the construction of the Kozina motorway, the typical shapes of unroofed caves, passages and large cave systems carved in horizontal or inclined karst surfaces were discovered.

Palaeomagnetic Research of an Unroofed Cave Near Kozina

The unroofed cave from which samples for the palaeomagnetic analysis were taken, is located to the northeast of Kozina, near the existing Ljubljana–Koper main road, in the cutting which was formed during the construction of the Divaca–Klanec motorway. The sampled profile consists of more than 5 m of alluvium in two main sequences. The lower sequence comprised ochre-coloured sandy to clayey sediments that were app. 3 m thick. These sediments were sampled to undergo the palaeomagnetic method. The lower sequence was covered with collapsed breccia and limestone blocks, and no samples were taken from it. The profile’s lithology clearly manifests a two-phase depositing in the past. The lower sequence underwent erosion after having been deposited. Subsequently, during the collapse, the empty space in the cave filled with rubble ranging in size from rocks to blocks mixed with brown karst soil. The ochre-coloured intercalations in the upper part of the upper sequence may indicate the presence of eroded sediment comparable with the lower sequence. The thinning of cave ceilings through erosion and karst denudation triggered collapsing. The sediment originates most probably from the weathered Eocene flysch. The Kozina profile is older than the Brunhes/Matuyama boundary (0.78 million years). The distribution of individual magnetozones supports the claim that the sediment is older than the end of the Olduvai epoch (1.77 million years) as the magnetostratigraphic profile concludes with the inverse polarity magnetozone and contains two normal polarity zones.

The Karst in the Breccia of Rebrnice in the Vipava Valley

The road runs in the NW–SE direction across two landscape units: across the bottom of the Vipava Valley and the southwestern slopes of Mount Nanos, Breg and Rebrnice. The geological, geomorphological, speleological, and hydrological diversity of the Slovene karst has been demonstrated also by the study of karstification of breccia that have been forming beneath the western slope of Mount Nanos. Water, in most cases percolating diffusely through the permeable surface of rubble or breccia and into the more or less impermeable flysch bedrock, is forming young karst phenomena. The percolating water accumulates where breccia is most consolidated. Earthworks have revealed the early stages in the formation of unique dolines. Since the motorway runs diagonally to the slopes, at the same time it cuts many and various relief forms in the upper and lower sections of the slopes. Road construction has also indirectly affected many, particularly fluvial relief forms. Due to the changes in the outflow from the slopes, changes in the old and the formation of the new relief forms are likely. Characteristic types of cavities developed in the young and very porous breccia which is consolidated only in places that lie on the more or less slanting flysch, i.e. on an impermeable bedrock. The true karst cavities are small and their development was influenced by the alluvium that has been depositing on their bottom and mainly fills them. They formed in a locally and periodically flooded zone or they were paragenetically enlarged. The largest cavities formed above the contact with the impermeable flysch bedrock where the big streams joined together. Their shape reflects the varying degrees of consolidation of breccia. In areas where breccia is less solid and along fissures they rise into domes. Along fissures which emerged due to the sliding of breccia and rubble down the slanting bedrock of frequently soggy flysch, fissure caves formed diagonally to the dip direction of the slope; some of them are very long and wide enough in places to make them accessible. Their walls are mainly covered with flowstone.

The Section Between Divača and Kozina Revealed Many Characteristics of Karst Development

The motorway section from Divaca to Kozina, stretching 7.5 km, revealed 50 old caves, most of them filled with alluvium, many of which unroofed, and six of them qualifying as shafts. The old caves were once, in a time when the underground water table was still high enough, characterized by through-flowing water. At that time, the aquifer was encased in flysch high up and partly covered with it. The runoff from the flysch introduced loam and sand into the caves. After they had been dry for a while and featured flowstone deposits, flood waters filled the caves with fine-grained sediments in one of the last cave development phases. The alluvium sealed the cracks and withstood even the long periods of the aquifer development during which its surface lowered for several tens of metres. It thus appears that old unroofed caves are an important component of the karst surface, providing clues to the development of the aquifer. Seeing as today the underground water level is 200 m and more below the surface, the traces of ancient water flow through the aquifer are now preserved only in caves and their respective alluvium. There was no indication of surface water flows that could have reshaped the aquifer’s surface. Old caves therefore contain the oldest geological traces dating back to the very early periods of the karst aquifer development. The more significant caves were able to be preserved. Today, these are either hidden below the road or accessible via man-made entrances.