Evolution of karst aquifers in natural and man made environments [Elektronische Ressource] : a modeling approach / von Douchko Romanov

Dushko

Informations

• Publié par: universitat_bremen
• Publié le: 01 janvier 2004
• Nombre de lectures: 34
• Langue: English
• Poids de l'ouvrage: 14 Mo

Extrait

Evolution of Karst Aquifers in Natural and Man Made
Environments: A Modeling Approach













Douchko Romanov



















Universität Bremen 2003 Evolution of Karst Aquifers in Natural and Man Made
Environments: A modeling approach



Vom Fachbereich für Physik und Elektrotechnik
der Univerität Bremen








zur Erlangung des akademischen Grades eines
Doktor der Naturwissenschaften (Dr. rer. nat.)
genehmigte Dissertation


von
Dipl. Phys. Douchko Romanov
aus Sofia, Bulgarien











1. Gutachter: Prof. Dr. W. Dreybrodt
2. Gutachter: Dr. R. Liedl

Eingereicht am: 19. 03. 2003
Tag des Promotionskolloquiums: 23. 04. 2003 Acknowledgments

To go and start to work in a foreign country is never easy. Three years ago I had the chance to
join the workgroup of Prof. Dreybrodt. It is also not easy to express feelings. Therefore I will
make it short. Thank you:
To Prof. Wolfgang Dreybrodt – he was always at the right place at the right time.
To Dr. Franci Gabrovsek. – he was the peace of BALKAN in the FAR NORTH of Bremen.
To Katrin Vosbeck – a real german friend.
To Dr. Alexander Jeschke – always ready for loooooong and interesting discussions.

I would also like to thank to the Tuebingen group. The seminars, perfectly organized by Dr.
Liedl were a place for sharing the newest ideas in the area of karst modeling.
I am really grateful to “Stiftung Constantia v. 1823, Bremen” for their financial support. Contents
Contents

Introduction 1
Goal and structure of this thesis 4

1. Basic principles of 2D modeling of karst aquifers 8
1.1. Single fracture 8
1.1.1. Hydrological part 10
1.1.2. Chemical part 10
1.1.3. Evolution of a single fracture 12
1.2. 2D networks 14
2. Influence of exchange flow on the early evolution of karst aquifers 17
2.1. Interaction of fracture and conduit flow in the early evolution of karst aquifers 18
2.1.1. Basic settings 18
2.1.2. Numerical results 21
2.1.2.1.Influence of the exchange flow on the breakthrough times 21
2.1.2.2.Evolution of the fracture aperture widths (standard scenarios A and B) 23
2.1.2.3.Numerical results for the central channel (standard scenario A) 32
2.1.2.4.Numerical results for extended scenarios 37
2.1.3. Conclusion 41
2.2. The influence of the exchange flow on the evolution of a single conduit 43
2.2.1. Basic setup 43
2.2.2. Numerical results 50
2.2.3. Discussion 57
2.2.4. Analytical approximation 58
2.2.5. Conclusion 60
3. Karstification below dam sites 61
3.1. Basic settings 63
3.1.1. Geological settings 63
3.1.2. Modeling domain 64
3.1.3. Numerical results for standard scenarios 67
3.1.3.1.Standard scenario A – uniform case 68
3.1.3.2.Standard scenario B – 73
3.1.3.3.– statistical case 75 Contents
3.1.3.4.Standard scenario B – statistical case 78
3.1.4. Evolution of the leakage rates for the standard scenarios 80
3.1.5. Standard scenario A – statistical case – gypsum 82
3.1.6. Influence on the basic hydrological and geochemical parameters on the
breakthrough time 85
3.1.7. Examples of different geological settings 88
3.1.8. Effect of mixing corrosion on the evolution of a dam site 91
3.1.9. Conclusion 92
3.2. Modeling of a catastrophic failure of the San Juan reservoir (NE Spain). 94
3.2.1. Modeling domain 94
3.2.2. Numerical results 95
3.2.3. Conclusion 101
4. The influence of the chemical boundary conditions on the evolution of karst aquifers 102
4.1. Basic settings 102
4.2. Numerical results 106
4.2.1. Evolution dominated by BT 106
4.2.2. Evolution dominated by MC 108
4.2.3. Intermediate cases (both MC and BT active) 114
4.2.4. Discussion 120
4.2.5. Conclusion 124
5. Conclusion 125
Bibliography 129 Introduction 1
Introduction

Sedimentary rocks cover approximately 75 % of the continents (Hamblin, 1992). Their
formation involves weathering of preexisting rock, transportation of the material away from the
original site, and deposition of the eroded material. Weathering is the mechanical and chemical
breakdown of rocks and minerals. Based on the way they are formed, the sedimentary rocks
can be divided into the following categories:
a) Clastic sedimentary rocks – broken rock fragments that have been lithified. They are
subdivided according to the grain size of the component materials. From the largest
grain size to the smallest, the types of the clastic rocks are: conglomerate, sandstone,
siltstone, and shale;
b) Chemical and organic sedimentary rocks – they are formed by chemical precipitation or
by biological processes. The most important are: limestone, dolostone, rock salt, and
gypsum.
This thesis will deal mainly with limestone, but in some cases also with gypsum
(CaSO •2H 0). 4 2
Limestone originates by both chemical and organic processes. It is composed principally of
calcium carbonate (CaCO ). The major types are: skeletal limestone, oolithic limestone, and 3
microcrystalline limestone. Limestones have great variety of rock textures. The limestone
deposits can be several hundreds of meters thick and extend over thousands of square
kilometers.
An important property of the limestone rock is, that it is dissolved by water containing
CO . 2
The unique landscape formed by the chemical action of water on these rocks is called
karst. The origin of the term is related to the region Kras in Slovenia. 10% to 20% of the
Earth’s land surface is covered by karst (Ford and Williams, 1989). The evolution of the karst
terrains is governed by many factors. The most important are:
a) The climate of the region - determines the amount of water entering the karst system.
It also determines the type of the vegetation and the soil cover in the region, which is
related to the amount of CO contained by the water. Swinnerton (1932) was the first 2
who stressed the importance of the soil CO for the karst evolution. 2
b) The geological settings;
c) The location and the geometry of the inflow and the outflow areas;
d) The type of the soluble rock; Introduction 2
e) The distribution of the primary fractures in the rock.
Karst has been subject of extensive research since centuries. The articles of Shaw (2000),
Lowe (2000) and White (2000) are interesting reviews about the development of the
speleogenetic studies from ancient time to the present days. People are interested in the karst
evolution not only because of the beauty of the karst landforms, but also because of their
practical importance. Karst aquifers (rock bodies sufficiently permeable to transmit
groundwater (Bear and Veruijt, 1987)) are the main source of drinking water for about 25% of
the world population (Ford and Williams, 1989).
Sinkholes, sinking streams, closed depressions, and caves characterize the topography of
the karst terrains. All these different landforms have a common element. It is the well-
developed subsurface drainage system. Initially, when the hydraulic conductivity of the rock is
low, most of the water is flowing on its surface. Only a relatively small amount is entering the
primary fractures of the soluble rock. As already discussed CO containing water is an 2
aggressive solution and is capable to dissolve a certain amount of the rock. If there is no way
for the water to leave the aquifer, it becomes saturated and is not able to change the primary
conduits further. Therefore a necessary condition for the initiating of karstification is the
existence of places where the water can leave the block and allow further inflow of aggressive
solution. The flow through the initial fractures is laminar, because their aperture widths are in
the range of several 100 mm. Some of the fissures widen faster than others. Therefore the flow
through them increases and consequently the rate of their widening is increased also. This
positive feedback loop is the reason for the development of secondary porosity and
consequently for the development of a complex, extremely heterogeneous aquifer. Flow
through some of the widened fractures finally becomes turbulent. The hydraulic conductivity
of the karst aquifer is increased by orders of magnitude. Most of the remaining initially small
fissures are also widened. Therefore the storage capacity of the aquifer is increased. At the
same time some surface karst landforms develop. The positions of the sinkholes for example
are related to the evolution of the subsurface drainage.
Because of the increased permeability, most of the water on the surface is entering the
drainage system after very short travel distances. It reappears in karst springs at the base level
of the aquifer. Con