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Three Dimensional Numerical Simulation of Fresh Water Lens in Small Islands (Case Study: Kish Island)

Rajabi, Mohammad Mehdi | 2010

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 40550 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Ataei Ashtiani, Behzad
  7. Abstract:
  8. Sensitive ecosystem and scarcity of freshwater are the main characteristics of small islands. These islands have small catchments and usually lack tangible surface water resources. As a result these islands are dependent almost entirely on groundwater. Groundwater on small islands usually occurs in the form of a freshwater lens floating on the more dense saline water. The freshwater lens is highly sensitive to various stresses such as drought, pumping, sea tide, etc. Due to the importance of freshwater lens in islands, proper and optimal use of this vital resource should be studied. Kish island`s freshwater lens was three dimensionally simulated using the numerical model SUTRA. The aim was to address the main obstacles in the path of three dimensional simulation of small island freshwater These obstacles include lack of field data, lack of knowledge about the physical parameters of the groundwater system, heterogeneities in the soil aquifer, uncertainty in the hydrological parameters such as seasonal and monthly distribution of rain and the amount of groundwater extraction and Problems regarding the nature of numerical modeling such as uniqueness or non convergence of the results. Inverse modeling was presented as a tool for overcoming some of these problems. The numerical model SUTRA was tested using the a benchmark problem called the "Infinite Horizontal Box". As a first step Kish island`s physical and hydrological data where analyzed and the conceptual model was developed using simplifying assumptions. The island`s base model was then developed. Due to the complexity of the model and lack of knowledge about the relations between different parameters, four scenarios regarding different combinations of input parameters and model outputs were used as bases for inverse modeling. The model was calibrated based on one these scenarios which best matched the available field data. Sensitivity analysis where performed for key model parameters. Finally several management scenarios for the groundwater system were numerically modeled. The results of this study show that using salinity data has far better results than using pressure data in the inverse modeling process. Sensitivity analysis show a relatively linear relationship was recognized between 10% and 50% salinity contours and the recharge rate. This was true in both homogeneous and heterogeneous conditions. If the whole system is treated as being homogenous, any increase in hydraulic conductivity of the island results in increase in the volume of the freshwater lens. In the more realistic situation where heterogeneities were applied, increase in the hydraulic conductivity of the aquitard beneath the aquifer led to increase in the freshwater lens volume. Increase in the transverse dispersivity led to increase in the thickness of the transition zone in both homogeneous and heterogeneous conditions. Dividing the island surface into two zones with different infiltration rates on the bases of how much each part is developed did not change the results significantly. Neither did the distribution of rainfall between different seasons of the year on the bases of 20 year averages. This is a case specific conclusion which must be tested
  9. Keywords:
  10. Numerical Modeling ; Sea Water Intrusion ; Inverse Modelling ; Kish Island ; Fresh Water Lens ; SUTRA Numerical Model

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