cross-sectional stability of a two-inlet bay system

Cross-sectional Stability of a Two Inlet Bay System Master Thesis C.S. Borsje March 2003 Faculty of Civil Engineering and Geosciences Section of Hydraulic Engineering Cross-sectional Stability of a Two Inlet Bay System Master Thesis Chris Borsje Supervisors: Prof.dr.ir. M.J.F. Stive (TU Delft) Dr.Ir. A. van Mazijk (TU Delft) Ir. H.J. Verhagen (TU Delft) Prof.dr.ir. J. van de Kreeke (University of Miami) TU Delft, Faculty of Civil Engineering and Geosciences, Section of Hydraulic Engineering Delft, March 2003 III Preface This master thesis deals with a study on cross-sectional stability of estuaries with two inlets. This has been carried out within the framework of the Faculty of Civil Engineering at Delft University of Technology. Special thanks go to the examination committee, for their supervision during my work: prof. Stive, ir. Verhagen, dr. van Mazijk and prof. van de Kreeke for their devotion and much valuable information and assistance. Also, I would like to thank ir. Tghiem Tien Lam for helping me out with his report. Chris Borsje IV V Abstract Tidal inlets are connections between small, shallow bays and the sea. In a tide-dominated environment, the status of this inlet is a balance between local given conditions such as channel geometry, and the hydraulic environment. According to Escoffier, when the equilibrium is disrupted, it should generally return to its original state. In tidal lagoon systems with more inlets, it is more uncertain to predict a stable condition for each inlet. There are cases where some channels will close and others stay open, while other systems remain overall stable, i.e. each inlet has an equilibrium state. An analytical method to determine this stability is yet no available. Therefore, the focus attends to research a model bay system with two ocean inlets, where there could be an unconditionally stable environment. For this, the following important assumptions and starting angles are set up:  The water level in the bay area moves uniformly;  Variations in ocean tide and phase levels may give new insights;  A third inlet, imaginary or real, with resulting small water level differences in the bay, is added. For this case, a double inlet lagoon system is modelled: this consists of a bay area with no significant shape, two channels with relevant friction and inertia parameters and an idealized ocean tide, with a tidal range and phase at each inlet throat. Further, a variation of this is done by (virtually) splitting up the bay. On each model, a series of tests has been done with calculation and graphic software. Every inlet has an equilibrium flow curve, i.e. a cross-sectional area where the maximum currents are just enough to flush sediment and keeping the channel open. If this is the case for both cross-sections, the whole lagoon system is referred to as stable (E-type flow condition). In the graphic images, this is shown by the intersections of the equilibrium flow curves. The normal model bay system showed that there is a stable condition when the tidal range at the seaside is different at each inlet, provided that this is not too large. In addition, the channels should be sufficiently short, with friction not too high. This is also valid for higher tidal ranges, where the differences are even smaller. The bay system with the partition had the same stability conditions. Higher tide levels at both inlets could also ensure stability, as the third inlet becomes relatively small. Next, the E-type flow condition is more likely if the third inlet is smaller or longer. Further, the ratio in bay area on either side of the partition channel has some negative influence on this, but in combination with tide difference, a balanced system can remain. The models are further implemented on a present-day situation, a lagoon inlet system located in the Hue province in Vietnam. Here, there are two inlets, located far apart, letting the system be relatively stable. In this particular case, repeated flooding and breaching of the sand barrier resulted into another inlet next to the main inlet in the northern part of the lagoon. The appearance of this third channel has raised the question whether the two close inlets could be stable, although the authorities would rather have this channel closed. The partition model indicated that this can be the case. Although the lagoon is also largely influenced by wave action, frequent storm surges and river runoff, the tidal inlet model is a useful tool in order to understand the basic behaviour of this lagoon better. It should be understood that the models that were used could never fully represent real hydraulic environments, as there are always other influencing factors present. For tide-dominated estuaries, they can however be a valuable starting point. Nevertheless, the results shown by both models are satisfactory. A deeper investigation could be made on parts of this subject. V I V II Table of Contents PREFACE III ABSTRACT V TABLE OF CONTENTS VII LIST OF FIGURES XI LIST OF TABLES XIII CHAPTER 1 INTRODUCTION 1 CHAPTER 2 PROBLEM ANALYSIS 3 2.1 Introduction 3 2.2 One - inlet bay system 3 2.3 Two inlet bay system 4 2.4 Goal of this study 8 CHAPTER 3 MODEL FOR CROSS–SECTIONAL STABILITY 9 3.1 Principles of stability analysis 9 3.2 Classification of equilibrium flow curves. 11 3.2.1 Type A 11 3.2.2 Type B 12 3.2.3 Type C 12 3.2.4 Type D 13 3.2.5 Type E 15 3.3 Inlet-bay schematization 17 3.3.1 Two inlet bay system 17 3.3.2 Two inlet bay system with partition 18 V III CHAPTER 4 INLET HYDRODYNAMICS 19 4.1 Introduction 19 4.2 One-inlet bay system 19 4.3 Two-inlet bay system 22 4.4 Two-inlet bay system with partition 23 4.5 Verification of the solutions 24 C HAPTER 5 CALCULATIONS 27 5.1 Introduction 27 5.2 Reference situations 27 5.2.1 Two inlet bay system 27 5.2.2 Two inlet bay system with partition 28 5.2.3 Discussion 29 5.3 Symmetrical ocean tide conditions for both inlets 30 5.3.1 Variations with one parameter 30 5.3.2 Variations with two parameters 31 5.3.3 Discussion 31 5.4 Different ocean tide conditions for both inlets 32 5.4.1 Variations with one parameter 32 5.4.2 Variations with two parameters 39 5.4.3 Discussion 39 5.5 Phase differences inside the bay 40 5.5.1 Variations with one parameter 40 5.5.2 Variations with two parameters 48 5.5.3 Discussion 49 CHAPTER 6 APPLICATION TO THE TAM GIANG – CAU HAI LAGOON, VIETNAM 51 6.1 Introduction 51 6.2 Description of the system 52 6.3 Objectives 53 6.3.1 Simulation excluding the Hoa Duan inlet 54 6.3.2 Simulation excluding the Thuan An inlet 56 6.3.3 Simulations excluding the Tu Hien inlet 57 CHAPTER 7 CONCLUSIONS AND RECOMMENDATIONS 59 REFERENCES 61 APPENDICES 1 [...]... cross-sectional values for different bay area ratios 52 Average inlet characteristics of the Tam Giang-Cau Hai lagoon system (Lam, 2002) 55 Inlet parameters of the Tam Giang-Cau Hai lagoon system 56 XIII XIV Cross sectional stability of a two inlet bay system Chapter 1 Introduction Over the years, a lot of study has been done on coastal inlets and tidal basins Estuaries or lagoons, connected... important equations A brief outline is given of the hydrodynamics of a one – inlet bay system, followed by the schematization and equations for the two-inlet bay system and the two-inlet bay system with the partition The model for the two-inlet bay system is validated using information on Matagorda Bay in Texas, U.S .A 4.2 One-inlet bay system Assume that the water level inside the bay fluctuates uniformly,... bay area ratios 42 Equilibrium cross-sectional values for different bay area ratios 45 Equilibrium cross-sectional values for different bay area ratios 47 Equilibrium cross-sectional values for different bay area ratios 49 Equilibrium cross-sectional values for different bay area ratios 50 Equilibrium cross-sectional values for different bay area ratios 51 Equilibrium cross-sectional. .. Water level amplitude differences inside the bay at a bay ratio of 1 (a) , 2 (b) and 10 (c) 46 Phase level differences inside the bay at a bay ratio of 1 (a) , 2 (b) and 10 (c) 47 Equilibrium flow curve at ĥ01 = 0.56 m 48 Map of the Tam Giang – Cau Hai lagoon system 51 Overview of lagoon areas and inlets with cross-sectional profiles 52 Model of the lagoon system. .. thick part of the curve is the stable section: this runs from (A1 , min; A2 , max), a 4 Cross sectional stability of a two inlet bay system maximum value of A2 where A1 is just stable, to (A1 , max; 0), where A1 reaches its stable value at point SP2 in absence of Inlet 2 Figure 2.2 Closure surface of Inlet 1 plotted against Inlet 2 A2 A2 , max A1 , max SP1 A1 , min SP2 A1 Figure 2.3 Equilibrium flow curve of. .. basin models are used to find an explanation on the stability of a lagoon system in Vietnam that is frequently exposed to typhoons and floods It thus might serve as a basis to understand this coastal area better Finally, some conclusions are drawn, followed by a few recommendations for further study 1 Cross sectional stability of a two inlet bay system 2 Cross sectional stability of a two inlet bay. .. Sensitivity of partition inlet 36 D.4 Variations with bay ratio 41 D.4.1 Bay Ratio 41 D.4.2 Bay Ratio with small partition inlets 42 D.4.3 Bay Ratio with tidal amplitude difference 49 APPENDIX E VIETNAM INLET AND BAY PARAMETERS OF THE TAM GIANG - CAU HAI LAGOON, 54 IX APPENDIX F SIMULATIONS ON THE TAM GIANG - CAU HAI LAGOON, VIETNAM... Inlet -bay schematization To attain the first three goals of this study, two schematizations of the two-inlet bay system are used: one model with and the other without a partition in the bay The partition has an opening that allows water to flow between the two bay compartments The purpose of the partition is to remove the condition of a uniformly fluctuating bay level 3.3.1 Two inlet bay system The bay Referring... R and cf) In the present application the inlets are assumed to be triangular After a change, the crosssections remain geometrically similar The ocean tide The ocean tide forces water in and out of the bay A semi-diurnal tide is assumed, but one can also use one tidal period a day Amplitudes and phases of the ocean tide can be different for the two inlets Suppose that the tidal wave arrives first at... can be determined when the value of c is known form observations O’Brien (1931, 1969) originally determined a similar relationship between minimum throat crosssectional area of an inlet below mean tide level and the tidal prism The above relation implies that a larger tidal prism means a larger cross-sectional area As an example, for inlets in the U.S .A the relationship between cross-sectional area . different bay area ratios. 51 Table D.10 Equilibrium cross-sectional values for different bay area ratios. 52 Table E.1. Average inlet characteristics of the Tam Giang-Cau Hai lagoon system (Lam,. consists of a bay area with no significant shape, two channels with relevant friction and inertia parameters and an idealized ocean tide, with a tidal range and phase at each inlet throat. Further,. D.4.3. Bay Ratio with tidal amplitude difference 49 APPENDIX E INLET AND BAY PARAMETERS OF THE TAM GIANG - CAU HAI LAGOON, VIETNAM 54 X APPENDIX F SIMULATIONS ON THE TAM GIANG - CAU HAI LAGOON,