Lagünlerde tatlı su-tuzlu su girişim yüzeyi özelliklerinin belirlenmesi

Abstract

Bu tezin konusunu, lagünlerin yeraltı suyu akımlarında, tatlı-tuzlu su girişim yüzeyi özellikleri ve bu girişim yüzeyinin farklı koşullardaki davranışı oluşturmaktadır. Lagünler denize dar bir giriş ağzı ile bağlanabildiği gibi, tamamen denize kapalı da olabilirler. Bu çalışmada, denizle irtibatı bulunmayan tamamen tecrit edilmiş bir lagün göz önüne alınmış olup mevcut kıyı akiferinde akıntının lagün ağzından dışarı ve denizden lagüne doğru olması durumu için, iki farklı hidrodinamik yapıya göre girişim yüzeyinin özellikleri belirlenmeye çalışılmıştır. Her iki durumu da ifade eden matematik ilişkiler elde edilmiş ve laboratuvar koşullarında sürdürülen bir dizi deney sonucunda, sonuçlar karşılaştırılmıştır. Ayrıca, Köyceğiz Lagünü ve Dalyan sulak alanında yapılan yerel çalışmalar deneysel çalışmalara ışık tutmuştur. Deneysel çalışmalarda, Hele-Shaw viskoz akım analojisi kullanılmış olup, izotropik, homojen ve serbest yüzeyli bir kıyı akiferindeki kararlı akım incelenmiştir. Belirgin bir girişim yüzeyi kabul edilmiş olup (dispersiyon dikkate alınmamış), Dupuit- Forcheimer yaklaşımından faydalanılmıştır. Beş alt bölüm halinde sunulan bu çalışmanın birinci bölümünde çalışmanın amacı ve kapsamı, konu ile ilgili daha önce yapılmış çalışmalar ve temel denklemler verilmiştir. İkinci bölümde ise, kıyı akiferlerinde iki sıvının hidroliğine dair bazı yaklaşımlar ve sınır koşulları sunulmuş ve girişim yüzeyinin diferansiyel denklemi elde edilmiştir. Üçüncü bölümde, girişim yüzeyinin özelliklerini belirleyen matematik ifadeler türetilmiş olup, önce lagün ağzından denize doğru daha sonra denizden lagüne doğru akım olması durumu olmak üzere konu iki başlık altında ele alınmıştır. Dördüncü bölümde, yapılan yersel çalışmalar ve seçilen pilot bölge hakkında bilgi verilmiştir. Beşinci bölümde, deney düzeneği ve yapılan deneysel çalışmalar anlatılmış ve daha önce yapılmış çalışmalarla kıyaslanmıştır. Altıncı bölümde elde edilen sonuçlar özetlenmiş ve öneriler sunulmuştur.

A lagoon is a shallow lake connected with a larger body of water comprised mostly of the sea. Lagoons interact with their environment and with the sea to which they are connected in a very complex manner within a naturally balanced framework. The main characteristic of a lagoon is its connection with the sea, which may be a long channel, a wetland, a dune formation or a narrow mouth. These types of connections have critical effects on the formation of the lagoon and on the morphological and ecological structure of the nearby seacoast. Lagoons may be completely close to the sea and have fresh water. But in this case, salinity may increase with the seepage of salt water from the barrier into the lagoon. The formation of the coastal lagoons are related with the mouth of the lagoon or the configuration of the dune seperating the lagoon and sea. The aim of this thesis is to determine the basic properties of fresh water- salt water interface and its behaviour under different groundwater flow conditions in coastal lagoons. A lagoon completely closed to sea is taken into account and the position of the interface for different hydrodynamic conditions are examined. For this purpose, steady flow in a homogeneous, isotropic, unconfined coastal aquifer is assumed. Assuming a sharp interface (no dispersion) useful relationships have been established between the interface characteristics and the shore properties. The study is modelled using Hele-Shaw Viscous Flow analogue and compared with the field measurements carried out in Köyceğiz Lagoon - Dalyan Wetland, Turkey. During field study, salinity measurements in canal leading Köyceğiz Lagoon to sea, a highly saline domain is examined near the basin, which is successfully identified with the model. In this thesis, which consists of six chapters. In the first chapter, the objectives and contents are given by GHYBEN-HERZBERG (1889), HUBBERT (1940), GLOVER (1959), COOPER (1959), Mc WHORTER and SUNADA (1981), FETTER (1972), POLO and RAMIS (1983) and ESSAID (1990). Also some previous works which have been carried out in Turkey by ERGUVANLI (1973), İRTEM (1991), KAPDAŞLI et al. (1996) and GÜNGÖR (1996) are given. In the second chapter, hydraulics of two fluids in a coastal aquifer is given and basic equations, assumptions and approximations are explained. Differential equation of the interface is inferred and boundary conditions are given. A brief summary of the approximations used in this thesis are given below: xn Sharp interface approximation: From a microscobic point of view a sharp interface seperating two different fluid does not exist due to diffusion, in a coastal aquifer. Nevertheless compared to the dimensions of the aquifer, thickness of the diffusion zone is negligible. So, non- homogeneous fluid in diffusion zone may be considered as two different fluids having constant densities, seperated with a sharp interface. So the problem is simplified to the flow of two immiscible liquids. This assumption, enables the use of piezometric head as force potential. Dupuit-Forchheimer Approximation: Difficulties encountered in the analysis of groundwater flow in coastal aquifers, have led hydrologists to use a more practical, if less rigorous, approach. Consider a sloping water table above a horizontal impermeable boundary as shown in Figure 1. The slope has been greatly exaggerated for clarity. The discharge per unit width into the plane of paper, across any vertical plane is, Q= \qx(x,z)dz (1) In Eq. 1, qx is the specific discharge in x-direction, Zf is the elevation of any point on the water table. Evaluation of the integral in Eq. 1 requires that qx(x,z) must be known. However, provided that the slope 6 of the water table is small, qx at the water table does not differ significantly from that on the boundary and qx(x,z) « qx(x,Zf). Datum Assumed Constant Head Line Actual Constant Head Line / / s s / s s / / / f *///,/// / s / / / / : / / ) ? x Figure 1. Dupuit-Forchheimer Approach xm In this case; dh Q = qx (x,zf )zf = -K-zf (2) where 'h' is the piezometric head at the water table. By definition the pressure head is zero at the water table, thus h = Zf and we have; " dh q=-kh- (3) In Eq. 3, h represents both the thickness of the flow and the piezometric head at the water table. The quantity dh/dx is the slope of water table. It is emphasized that Eq. 3 is valid for cases in which the water table slope is small. More expilicitly, dh dx 2 « 1 (4) is the condition that must be satisfied (BEAR, 1972). In the third chapter, the interface characteristics were determined for the conditions of flow from lagoon to the sea and from sea to the lagoon. In the case of fresh water level above the mean sea water level Glover approach is compared with the Ghyben - Herzberg approach. For the case of mean sea water level above the fresh water level, a relationship giving the interface position was established using Dupuit- Forchheimer approximation. In the forth chapter, site investigations carried out in Köyceğiz Lagoon, Dalyan Wetland and the canal connecting the lake to the sea are presented. By means of classified satellite photographs of the region the water balance, the current system and the temperature distribution are determined. During field measurements, a highly saline domain near the basin of the canal leading lake to the sea is examined almost 1 km to the sea. Because the canal is very long (« 8km) and its connection with the sea is very narrow («150 m), the occurrence of the salt-water plume cannot be interpreted as salt water intrusion from the opening mouth. The main reason must be the ground water flow in the coastal aquifer in different hydrodynamic conditions. In order to check the validity of this idea, a set of experiments were carried out in the hydraulics laboratory. XIV In the fifth chapter, experimental studies were given in two sections. In the first set of experiments, the condition of flow from lagoon to sea is modelled setting the fresh water level above the mean sea water level. In the second set of experiments the salty water level was set above the fresh water level in order to model the situation created by wave and wind effects. In both cases, the lagoon is assumed completely unconnected to sea. Results obtained from experiments are compared with the results of a similar study carried out by ESSAID (1990). In order to show salt water - fresh water interaction, test photographs and test conclusions were given. Viscous flow anology method (Hele-Shaw) has been selected. The Hele-Shaw instrument consists of two glass or plexiglass plates, very closely and vertically situated to each other. The distance between the plates is negligible compared to their length. Sieved and cleared sand has been placed between the plates. Salt water has been utilized to represent seawater. Salinity of the salt water has been measured with salinometer. The permeability of the domain is measured using constant head permeameter. To obtain a steady motion, salt and fresh water tanks have been fed continously. Both tanks have weirs in order to keep both the fresh and salt water levels constant. In order to follow the movement of fresh water, both in the sandy bottom and in the salty water, it was coloured with potassium permanganate. Fresh water advancements, meeting with salt water and the formation of the interface between fresh and salt water are photographed. The experiments were stopped after that a steady interface was established. Test results observed for the case of flow from sea towards the lagoon agreed with the site investigations. The interfaces observed which the flow is towards the sea are very similar to those obtained by ESSAID (1990) (Fig. 2). z TT o 0.25 0.50 0.75 1.00 ? Observed ESSAID (1990) 0 0.25 0.50 x/L 0.75 1.00 Figure 2. Interface Position Compared With ESSAID's (1990) Results xv In Figure 2, z/h is a dimensionless quantity, which is the ratio of elevation of interface to the depth of the domain, x/L is ratio of distance to the aquifer length. Also, t/T dimensionless quantity gives the ratio of time the interface is observed to the whole experience time. It is seen from the curve of t/T = 1, that the fresh water is discharged to sea agreeing with Glover's assumption. In the case of flow from sea towards the lagoon, observed interface position is compared with ESSADD's (1990) results in Figure 3. Considering the boundary and initial conditions of two different studies it is seen that the interface position is similar for t/T = 0.3 and 0.4. For small values of t/T, the initial condition of ESSAID (1990) is the steady state obtained at the end of the test considering flow from lagoon to sea. In his study, considering coastal aquifers, he did not investigate coastal lagoons. So, for last values of t/T, it is seen from Figure 3, that the interface position advances to the depths of the aquifer while in this study, salt water is blocked in the bed of lagoon. So Figure 3, must be interpreted considering these initial and boundary conditions. 0.25< z/h 0.50 - 0.75 - 1.00 1.00 Figure 3. Interface Position Compared With ESSAID (1990)'s Results For the Case of Flow From Sea Towards Lagoon. In the sixth chapter, the test results were interpreted and conclusions were discussed.