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Louisiana State University
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LSU Historical Dissertations and Theses Graduate School
1989
Land Subsidence and Saltwater Intrusion in Coastal Areas.
Song-kai Yan Louisiana State University and Agricultural & Mechanical College
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Recommended Citation Yan, Song-kai, "Land Subsidence and Saltwater Intrusion in Coastal Areas." (1989).https://digitalcommons.lsu.edu/gradschool_disstheses/4826 LSU Historical Dissertations and Theses. 4826.
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LAND SUBSIDENCE AND SALTWATER INTRUSION
IN COASTAL AREAS
A Dissertation Submitted to the Graduate Faculty ofLouisiana State University and Agricultural and Mechanical Collegein partial fulfillm ent of the requirements for the degree ofDoctor of Philosophy in The Department of Civil Engineering
by Song-kai Yan B.S., Beijing Institute of Hydraulic Engineering, 1964 M.S., Colorado State University, 1983 August 1989
ACKNOWLEDGEMENTS
I wish to thank Professor Dean Adrian, my advisor, who first addressed to me the practical significance of land subsidence and saltwater intrusion problems in coastal Louisiana areas. This motivated me to conduct this dissertation research. Without his persistent guidance, instruction, and above all, his encouragement and versatile support, the completion of this dissertation would not have been possible. I also wish to thank Flora Wang, James Cruise, Helmut Schneider, Stephen Field, Dipak Roy and Mark Walthall for serving as members of my Ph.D committee and for their review of this dissertation. Professors Adrian and Wang, in particular, reviewed this dissertation with great effort on a word by word basis. Professor Wang not only encouraged me spiritually, but also helped organise and finalize a paper based on my dissertation and submitted to an ASCE journal. Her enthusiasm and conscientiousness at work will always be remembered. Thanks to Dr. Cruise, who taught me three courses from which I greatly benefitted. His comments on my dissertation in terms of time schedule and other aspects were most practical and helpful. My thanks also go to the Department of Civil Engineering, who supported me financially for the first two years of my study and to Woodward-Clyde Consultants, without whose financial support this dissertation may not be have been accomplished successfully. Partial financial support for the study of saltwater intrusion in New Orleans area, Louisiana, was provided by the Louisiana Water Resources Research Institute and the Department of Marine Science, Louisiana State University. Thanks
TABLE OF CONTENTS page Acknowledgements ** List of Tables List of Figures ** Abstract
Chapter 1 Introduction and Purpose 1
Chapter 2 Derivation of Governing Equations forCompressible and Sloping Aquifer ^ 2.1 Derivation of Flow Equation for Compressible Aquifer 12 2.1.1 Integration Along the Thickness of A Confined Aquifer 31
2.1.2 Integration Along the Thickness of An Unconfined Aquifer 44 2.2 Derivation of Land Subsidence Equation 46 2.2.1 Confined Aquifer ^ 2.2.2 Unconfined Aquifer 2.3 Derivation of Ground Water Flow Equation for Sloping 54 Aquifer 2.3.1 Unconfined Aquifer 54 2.3.2 Confined Aquifer 61
TABLE OF CONTENTS (continued) Page
- Chapter 3 Computer(STRALAN - Solute Transport and Land Subsidence) Model Development and Modification - 3.1 The Major Governing Equations - 3.2 Numerical Methods - 3.3 Computer Program Description - 3.4 Simulation Examples - 3.4.1 Analytical Solution for Land Subsidence Problem - 3.4.2 Example at Rocky Mountain Arsenal
- Chapter 4 A Case Study: Land Subsidence and Saltwater Intrusion - 4.1 Introduction in New Orleans Area. - 4.2 The Study Area - 4.3 Study Objectives - 4.4 Model Calibration and Verification - 4.4.1 Construction of the Finite Element Mesh - 4.4.2 Initial Condition and Boundary Condition - 4.4.3 Vertical Leakage to the Gonzales-New Orleans Aquifer
LIST OF TABLES
Page Table 3-1 Calculated subsidence for the analytical solution 92 Table 4-1 Total pumpage in Jefferson and Orleans Parishes 106 Table 4-2 Drawdowns in well level in the past 15 years * Table 4-3 Chloride concentration in some observation wells 124 Table 4-4 Results of sensitivity tests of hydraulic conductivityon piezometric drawdown, chloride concentration and subsidence in 1981-1982 135 Table 4-5 Results of sensitivity tests of aquifer compressibility on piezometric drawdown, chloride concentration and subsidence in 1981-1982 137 Table 4-6 Results of sensitivity tests of dispersivity and molecular diffusivity on piezometric drawdown, chloride concentration and subsidence for calibration run (1961-1981) 144 Table 4-7 Predicted results of piezometric drawdown, chlorideconcentration and subsidence in 1995 under projected pumping rates 151 Table 4-8 Predicted results of piezometric drawdown, chloride concentration and subsidence in 2010 under projected pumping rates 162 Table 4-9 Water withdrawal in Jefferson and Orleans Parishes in 1985 159 Table 4-10 Responses of piezometric drawdown, chloride concentration and subsidence to additional pumpage to supplement surface water supply shortage (20 years of simulation) 161 Table 4-11 Piezometric drawdown, chloride concentration and subsidence with different pumping rates at node # 210 for 30 days, 2 years, 10 years and 20 years 163
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LIST OF TABLES fcontinuectt
Table 4-12 Predicted results of piezometric drawdown, chloride concentration and subsidence under recharging rate of 10 Mgal/d
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LIST OF FIGURES (continued! Page 1964, New Orleans (from U.S. Coast and Geodetic Survey, after Kazmann and Heath, 1968) 109 Land surface subsidence compared with water level in wells along line A-A’ (after Kazmann and Heath, 1968) U 0 Location map of the study area and its vicinity 115 Geologic cross sections along A-A" and B-B" in Figure 4- (after Dial and Sumner, 1988) 116 Thickness and coefficient of permeability of the Gonzales- New Orleans aquifer (The '700-foot" sand), New Orleans Area, Louisiana (after RoIIo, 1968) 118 Contours of water level (in meters below mean sea level) in the Gonzales-New Orleans aquifer in 1961- (after Rollo, 1968) U Contours of piezometric level (in meters, MSL) in the Gonzales- New Orleans aquifer, Novenber 1985, New Orleans area (based on data from the U.S. Geological Survey, Louisiana office) 1^ Contours of chloride content (in ppm) obtained from wells in the Gonzales-New Orleans aquifer in 1961-1962 (after Rollo, 1966) 122 Subsidence in the New Orleans area, 1964- (from Zilkoski and Reese, 1986) 126 Land subsidence compared with piezometric drawdown in the New Orleans area along line A-A’^127
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LIST OF FIGURES (continued! Page Average monthly pumpage from the Gonzales-New Orleans aquifer in 1963 in New Orleans (after Rollo, 1963) 129 Model calibration. Piezometric drawdown (in meters) below mean sea level in 1981-1982. (a) Observed data; (b) Simulated result 138 Model calibration. Approximate positions of freshwater and saltwater interface (250 ppm isochlor) in the Gonzales- 139 New Orleans aquifer in the vicinity of New Orleans, Louisiana Model calibration. Comparison of drawdown (1981) in the Gonzales-New Orleans aquifer along A-A’ cross section 140 Model calibration. Comparison of observed and simulated chloride concentration (1961-1981) in the Gonzales- New Orleans aquifer along A-A’ cross section 141 Model calibration. Comparison of land subsidence (1961-1981) in the New Orleans area along A-A’cross section 142 Simulated land subsidence (in centimeters) in the New Orleans area between 1961 and 1981 145 Model verification. Piezometric drawdown (in meters) below mean sea level in 1985-1986: (a) Observed data; (b) Simulated result 147 Model verification. Approximate positions of freshwater and saltwater interface (250 ppm isochlor) in the Gonzales- New Orleans aquifer in the vicinity of New Orleans, Louisiana 148 xi
Page Figure 4-29 Predicted piezometric drawdown (in meters) in the New Orleans area in 1995 with annual average pumping rate of 230 Mgal/d 166 Figure 4-30 Contours of predicted land subsidence (in centimeters) in the New Orleans area in 2005 with annual pumping rate of 140 Mgal/d if surface water supply is cut off by 4 % (subsidence is relative to 1961) 167 Figure 4-31 Contours of predicted land subsidence (in centimeters) in the New Orleans area in 2005 with annual pumping rate of 230 Mgal/d if surface water supply is cut off by 9% (subsidence is relative to 1961) 168 Figure 4-32 Model prediction with recharging wells. Approximate positions of freshwater and saltwater ( ppm isochlor) in the Gonzales-New Orleans aquifer in the vicinity of New Orleans, Louisiana (recharging rate is 10 Mgal/d) 1^
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ABSTRACT
Land subsidence and saltwater intrusion problems are encountered concurrently or individually in many coastal areas, especially where heavy pumping is exerted. In order to evaluate the extent and progress of these two phenomena in quantity and quality, a two-dimensonal model, the STRALAN (Solute Transport and Land Subsidence) model, is developed to simulate fluid movement, solute transport and land subsidence in a non-homogenous, anisotropic, compressible and sloping aquifer system. (^) i The governing equations of the model, i.e., the fluid mass balance equation, solute mass balance equation and land subsidence equation for confined or unconfined as well as for sloping aquifers are derived in detail. Hybridization of finite element and finite difference methods is employed for the numerical solution of the governing equations which describe the interdependent processes of fluid density-dependent saturated ground water flow, solute transport and land subsidence caused by changes of pressure due to pumping. STRALAN model provides, as primary calculated results, piezometric drawdown (or pressure), solute concentration and subsidence spatially and temporally in the simulated subsurface system. It has a wide variety of options from which one can select to simulate either a confined aquifer or unconfined aquifer; either a horizontal or sloping aquifer; land subsidence only; saltwater intrusion only; or both of the above; steady or transient state condition for ground water flow, jriv
CHAPTER 1
INTRODUCTION AND PURPOSE
Coastal ground water management is subject to the constraints of saltwater intrusion and land subsidence associated with a lowering of the inland piezometric head due to extensive pumping. The moving interface between fresh water and salt water, whether it is sharp or dispersed, can be regarded as a dynamic boundary of the aquifer varying in time and space. Saltwater encroachment may reduce the potable ground water storage and influence the extraction of ground water. Within the past two decades, analysis of an aquifer system containing both fresh water and salt water was based on a number of different conceptual models. Many computer models using these conceptual models have been developed for the analysis, prediction and control of saltwater intrusion in coastal aquifer systems (Bear and Verruijt, 1987; Willis and Finney, 1988). Based on the dynamic behavior of fresh and salt water, these models can be classified into: (1) sharp interface models in which the fresh water and salt water are considered as two immiscible fluids, and the Dupuit assumption is valid so that the flows in the aquifer are essentially horizontal (Shamir and Dagan, 1971; Kashef 1975; Wilson, Townley and Coasta, 1979; Bear and Kapuler, 1981); and (2) transition zone models in which the fluids are miscible, and both horizontal and vertical movement of fluids occur in the flow region caused by advection and hydrodynamic dispersion (Pinder and Cooper, 1970; Lee and Cheng, 1974; Huyakom and Taylor, 1976; Volker and Rushton, 1982). These model require the solution of the advective-dispersion-diffusion equation. According to the dimensionality, saltwater intrusion models can also be grouped into three categories (Reilly and Goodman, 1985): (1) two-dimensional cross-sectional models with both sharp or dispersed interface (Wilson, Townley and
