Using Groundwater Flow Modelling for Investigation of Land Subsidence in the Konya Closed Basin (Turkey)

Using Groundwater Flow Modelling for Investigation of Land Subsidence in the Konya Closed Basin (Turkey)

Naciye Nur Özyurt (Hacettepe University, Turkey), Pınar Avcı (Hacettepe University, Turkey) and Celal Serdar Bayarı (Hacettepe University, Turkey)
DOI: 10.4018/978-1-5225-2709-1.ch016
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Abstract

Land subsidence which is defined as gradual settling or sudden collapse of Earth's surface, is a geohazard phenomenon that occurs worldwide. Land subsidence occurs in time mainly due to excessive groundwater abstraction. This problem occurs usually in semi-arid regions where the groundwater is the sole source of water. Eliminating the adverse effects of land subsidence requires careful observations on the temporal change of elevation coupled with groundwater flow modeling. In this study, numerical groundwater flow modeling technique is applied to a confined aquifer system in the Konya Subbasin of Konya Closed Basin (KCB), central Anatolia, Turkey. Groundwater head in the KCB has been declining with a rate of about 1m/year since early 1980s. Recent GPS observations reveal subsidence rates of 22 mm/year over the southern part of KCB. MODFLOW numerical groundwater flow model coupled with subsidence (SUB) package is used to simulate the effect of long term groundwater abstraction on the spatial variation of subsidence rates.
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Introduction

The term land subsidence includes both of the processes of slow settlement and sudden collapse of a ground surface. However, in many of the cases, the subsidence is a subtle phenomenon. Both the vertical and horizontal components of the displacement can lead remarkable damages. Generally, it is the groundwater pumping that cause land subsidence in compressible aquifer systems. Such systems typically comprise of basin-fill and unconsolidated alluvial aquifer systems that include both aquifers and aquitards (Galloway and Burbey, 2011). The aquifer-system compaction and the resultant land subsidence is associated with the compaction of the aquitards which may be thick confining units within the aquifer system or may be fine-grained deposits interbedded within an aquifer.

According to an assessment by Galloway and Burbey (2011), measured subsidence rates in different locations in the world range from 6 mm/year in Kolkata, India between 1992 -1998 (Chatterjee et al., 2006) and 300 mm/year in Mexico City, Mexico between 2004-2006 (Osmanoglu et al., 2011) and among the 18 sites distributed globally the mean and median subsidence rate are 100 mm/year (+/- 1 sigma standard deviation is 99 mm/year) and 55 mm/year.

Either slow or sudden, motion of the ground due to subsidence is a life and property threatening process. Present and potential future hazards have been assessed by computer models which are based on basic relations between groundwater’s head, ground stress, compressibility of groundwater and aquifer skeleton, and the groundwater flow. These models use two different approaches: the first is based on groundwater flow theory (Jacob 1940, 1950) and secondly, the theory of linear poroelasticity (Biot 1941). The groundwater flow theory is a special case of the poroelasticity theory. As Galloway and Burbey (2011) states, both approaches are based on the Principle of Effective Stress (Terzaghi 1923, 1925) and the principal difference between these approaches is the way how the deformation of the skeletal matrix is treated. Conventional groundwater flow theory accounts only for the vertical deformation, whereas poroelasticity theory accounts for the 3-dimensional deformation. Therefore, the poroelasticity theory represents a better relationship between fluid flow and deformation, and is physically more realistic (Galloway and Burbey, 2011). However, the approach based on conventional groundwater flow theory is preferred in studies aiming the regional deformation because it requires much less data as compared to the approach based on poroelasticity theory. On the other hand, the poroelasticity approach is more suitable to address the local-scale deformations like ground ruptures and damaged engineering structures.

Key Terms in this Chapter

Effective Stress: A critical force that keeps a set of particles together. If an external force (e.g. pore water pressure) reduces the effective stress, the set of particles is fall apart.

Consolidation: A natural process by which the volume of geologic material is reduced by gradual expulsion of pore water as a results of long term static loads.

Collapse Doline: A rare form of karst landscape, forms when the roof of an underlying karst cavity collapses due to over enlargement by dissolution. Formation of obruks may be triggered the reduced buoyancy of groundwater in areas where over pumping results in appreciable head decline.

Subsidence: The slow settlement and sudden collapse of landforms to a lower level as a result of the drainage of an aquitard by means of natural and/or anthropogenic processes.

Aquitard: A hydrogeological unit comprising mainly of low-permeability interbeds that restrict the groundwater flow. A completely impermeable aquitard is called an aquiclude or aquifuge.

Compaction: A rapid and artificial process that results in the packing of soil particles more densely through the reduction of pore space.

Karst: A specific landscape formed by the dissolution of carbonate and evaporitic rocks. Karst aquifers are characterized by large underground drainage systems in which turbulent flow conditions may be achieved in contrast to the granular aquifers.

Interbed: A low-permeability geologic material within a more permeable matrix.

Modflow-Sub: A package of MODFLOW which is used to simulate the compaction of aquifers, interbeds and confining units of an aquifer system.

Obruk: A Turkish karst term used to describe collapse dolines which are commonly exist in central Anatolia.

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