Hydrodynamics and morphodynamics of a seasonally forced tidal inlet system

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(1)Hydrodynamics and Morphodynamics of a Seasonally Forced Tidal Inlet System.

(2) Font and back covers: The Tu Hien inlet, Thua Thien-Hue, Vietnam (photo courtesy M.B. de Vries, 2004).

(3) Hydrodynamics and Morphodynamics of a Seasonally Forced Tidal Inlet System. Proefschrift. ter verkrijging van de graad van doctor aan de Technische Universiteit Delft op gezag van de Rector Magnificus Prof. dr. ir. J.T. Fokkema voorzitter van het College voor Promoties, in het openbaar te verdedigen op maandag 22 juni 2009 om 14:00 uur. door. Tien-Lam NGHIEM Master of Science in Hydraulic Engineering, IHE Delft geboren te Bac Ninh, Vietnam.

(4) Dit proefschrift is goedgekeurd door de promotor: Prof. dr. ir. M.J.F. Stive Copromotor: Dr. ir. Z.B. Wang Samenstelling promotiecommissie: Rector Magnificus. voorzitter. Prof.dr.ir. M.J.F. Stive. Technische Universiteit Delft, promotor. Dr.ir. Z.B. Wang. WL | Delft Hydraulics / Technische Universiteit Delft, copromotor. Prof.dr.ir. J.A. Roelvink. UNESCO–IHE / Technische Universiteit Delft. Prof.dr.ir. J. van de Kreeke. University of Miami, USA. Prof.dr.ir. A.W. Heemink. Technische Universiteit Delft. Dr.ir. W.S.J. Uijttewaal. Technische Universiteit Delft. Dr.ir. A.J.H.M. Reniers. University of Miami, USA / Technische Universiteit Delft. Ir. H.J. Verhagen heeft als begeleider in belangrijke mate aan de totstandkoming van het proefschrift bijgedragen (Ir. H.J. Verhagen has provided substantial guidance and support in the preparation of this thesis).. This thesis should be referred to as: Lam, N. T. (2009). Hydrodynamics and morphodynamics of a seasonally forced tidal inlet system. Ph.D. Thesis, Delft University of Technology. Keywords: tidal inlet; hydrodynamics; morphodynamics; numerical modeling; microtidal wave dominated coast; river flow dominant. ISBN 978-90-9024356-6 Copyright © 2009 by Nghiem Tien Lam. All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means including photocopy, without prior permission from the author. Printed by PrintPartners Ipskamp B.V., the Netherlands..

(5) To my family. “You raise me up, so I can stand on mountains; You raise me up, to walk on stormy seas; I am strong, when I am on your shoulders; You raise me up: To more than I can be”. (Brendan Graham and Rolf Løvland).

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(7) SUMMARY Tidal inlets are among the most variable and complex geomorphological features of the Earth’s surface. The morphology of tidal inlets is controlled by the interaction of oceanographic, hydrologic, meteorological, topographic and geologic factors. The majority of the inlets is associated with mesotidal, moderate wave energy coasts where freshwater flow is small or non-existent. A small number of the inlets is located in tropical monsoon regions where fresh water flow plays a significant role in the inlet hydrodynamics. Tidal inlets in such conditions are highly dynamic and variable under the influence of wave climate and river flow, which are seasonally varying according to the monsoon regime. Strong episodic influences of river flow discharge and wave action have been mentioned as the cause of the seasonal closure of these tidal inlets. They are found on microtidal coasts of India, Sri Lanka, Japan, Australia, South Africa, USA, and Brazil. Many river mouths and tidal inlets along the central coast of Vietnam also have characteristics of seasonal closure. During the flood season, river floods have a strong influence on inlet morphology by scouring their channels and cutting through coastal barriers. In the dry season, wave action dominates over the tide and river flows. Wave-induced sediment transport eventually closes some inlets or river mouths. The importance of fluvial processes for inlet flushing and opening, therefore, has been recognized recently. However, the mechanisms behind the behavior of tidal inlets which are strongly influenced by severe episodic river floods like the tidal inlets of the Thua ThienHue province in the central coast of Vietnam are still poorly understood. The main objective of this study is to get further insight into the physical processes underlying the morphodynamics and the behaviour of Thuan An and Tu Hien tidal inlets, located in the Thua Thien-Hue province, under a tropical monsoon climatic regime on a microtidal wave-dominated coast. The main processes of tides, river flow, wave action and sediment transport which control the tidal inlet morphology are investigated and analyzed using observed data and numerical model simulation to address their role and their influence on the morphology and the behaviour of the inlets. The stability condition of the tidal inlets is initially examined using Van de Kreeke’s (1990a, b) approach and a new analytical solution derived for tidal inlet hydraulics including tributary flow and inertia. The Thuan An inlet is found to be in a stable equilibrium state while the Tu Hien inlet is always in the unstable equilibrium state. However, the seasonal variation of wave climate and river flow and the alternation of their dominance suggest that inlet stability should be considered on a longer timescale than a tidal cycle, e.g., over a year, which requires a more advanced approach such as numerical modeling to evaluate the equilibrium condition of the inlets. The ocean influence of tides and wave action on inlet morphodynamics is simulated and investigated using a physical based numerical model Delft3D. The fluvial processes are simulated using a numerical hydraulic model SOBEK coupling 1D and 2D modules for freshwater flow in a channel network of the rivers, lagoons and inlets and on the floodplain. The morphological changes at the inlets due to river flow are then simulated by Delft3D. During the flood season, the inlets are river dominated. Episodic events of river floods and typhoons have a strong impact on the hydrodynamics and morphology of the inlets and these are the main factor to maintain the existence of the inlets. Strong river flood currents erode i.

(8) ii. Summary. inlet channels and deltas and enlarge inlet gorge cross sections. The expansion of inlet cross sectional areas also weakens the tidal currents in the inlet channels and stops them transporting sediment to build up the flood tidal deltas in the Tam Giang lagoon. Tidal flushing on this microtidal coast is, therefore, subordinate to the river flow and wave action. The Thuan An inner inlet channel can be re-orientated by the distribution of river flood waters from different rivers. High flood water level is the reason of sand barrier breaching by overflow through the barrier. The difference in river flow magnitudes is the main cause for the distinction in the stability conditions of the two inlets. River flows transport the total sediment amounts of 1.43 Mm³/year and 0.16 Mm³/year in an average year through the Thuan An and Tu Hien inlets, respectively. The seasonal variation of wave climate has a strong influence on the sediment transport pattern as well as on the morphology of the inlets and adjacent coasts. Wave induced longshore sediment transport during the winter monsoon season is dominant over the sediment transport from rivers through the inlets. When the river flows diminish, the inlets become wave dominated. The scouring by river floods makes the inlet channels becoming sediment sinks for wave induced sediment transport that causes the shoreline erosion of the adjacent coasts at both inlets. Channel sedimentation and recovery of the ebb tidal deltas and adjacent coasts are gradually promoted by summer monsoon wave reworking. The growth of Thuan An southern sand spit is due to the net longshore sediment transport of 0.25 – 0.36 Mm³/year to NW direction. At the Tu Hien inlet, the net longshore sediment transport on its north side is 0.4 Mm³/year to SE direction while this value on its south coast is 0.05 Mm³/year to NW direction. This is due to the inlet being sheltered from waves partly by the SE headlands. Weak river flow and tidal forcing in the inlet make it very unstable. The long-term behaviour of the Thuan An inlet after an extreme river flood is investigated by the aggregated model ASMITA with a timescale of decades. Each element of the inlet system responds to a different degree to the river and wave influences. The river influence is reflected by the seasonal variation of element volumes in the system which is strongest in the inlet channel then reduces at the ebb tidal delta and it is weakest along the coasts. The wave influences on the system elements are reflected by the time required to recover the elements by coastal erosion/accretion. Contrary to the river influence, the wave influence is strongest at the coast and the ebb tidal delta and it is weaker in the inlet channel. The influences of river flow and wave climate on the morphology and behaviour of the inlets is then summarized in a conceptual model. In conclusion, further insights into the morphodynamics and behaviour of an inlet system on a microtidal wave dominated coast in central Vietnam under tropical monsoon climatic regime have been gained in the present study. The key processes which control the morphodynamics and their influence on the morphology and behaviour of Hue tidal inlets have been indicated. Although the study is focused on the Thuan An and Tu Hien inlet system, the principles, approaches and some results of the study may be applied to, or at least, give a hint for other inlets in the region. Finally, several recommendations following from this study have been suggested for future research including data observation, linearization of friction term of inlet hydraulic equation with strong tributary flow, typhoon influence and the challenge of numerical long-term simulation using process based models taking into account the intermittent river floods..

(9) SAMENVATTING Zeegaten behoren tot de meest variabele en complexe geomorfologische verschijnselen op aarde. De morfologie van zeegaten wordt bepaald door de interactie van oceanografische, topografische en geologische factoren. De meeste zeegaten worden geassocieerd met mesogetij en gematigde golf energie kusten waar de zoetwater instroming klein of afwezig is. Een klein aantal van de zeegaten is gelegen in de tropische moesson regio’s waar zoetwater instroming een belangrijke rol speelt in de zeegat hydrodynamica. Zeegaten onder zulke condities zijn zeer dynamisch en variabel onder invloed van golfklimaat en rivier afvoer, wat seizoensafhankelijk varieert volgens het moesson regime. Sterke episodische invloeden van rivier afvoeren en golf werking worden aangemerkt als de oorzaak van de seizoensgerelateerde sluiting van deze zeegaten. Deze komen voor langs micro-getij kusten van India, Sri Lanka, Japan, Australië, Zuid Afrika, de Verenigde Staten van Amerika, en Brazilië. Veel rivier mondingen en zeegaten langs de centrale kust van Vietnam hebben ook karakteristieken van seizoensgerelateerde sluiting. Gedurende het overstromingsseizoen, hebben rivier overstromingen een sterke invloed op de zeegat morfologie door uitschuring van de geulen en afsnijding door de kust barrières. In het droge seizoen domineren golf invloeden over getijde- en rivier-invloeden. Golfgedreven sedimenttransport kan eventueel enkele zeegaten of riviermondingen afsluiten. Het belang van fluviale processen voor het doorspoelen en openen van zeegaten is daarom recent onderkend. Echter, over de mechanismen achter het gedrag van zeegaten die sterk beïnvloed worden door grote episodische rivierafvoeren, zoals de zeegaten in de Thua Thien-Hue provincie langs de centrale kust van Vietnam, is slechts weinig bekend. Het hoofddoel van dit onderzoek is om meer inzicht te verkrijgen in de fysische processen die de morfodynamica en het gedrag van de Thuan An en Tu Hien zeegaten, in de Thua ThienHue provincie, onder een tropisch moesson regime langs een micro-getij golfgedomineerde kust bepalen. De belangrijkste processen van getij, rivier afvoer, golf werking en sediment transport welke de morfologie van het zeegat sturen, zijn onderzocht en geanalyseerd met behulp van geobserveerde data en numerieke model simulaties om hun rol en invloed op de morfologie en het gedrag van het zeegat te adresseren. De stabiliteitsconditie van de zeegaten is initieel onderzocht met gebruik van Van de Kreeke’s (1990a, b) aanpak en met een nieuwe analytische oplossing afgeleid voor zeegat hydraulica inclusief zij-instroom en inertie. Het Thuan An zeegat is bevonden in een stabiel evenwicht te zijn terwijl het Tu Hien zeegat altijd in een onstabiel evenwicht verkeert. Echter, de seizoensgebonden variatie van golfklimaat en rivier afvoer en hun afwisselende overheersing suggereert dat zeegat stabiliteit moet worden beschouwd op een langere tijdschaal dan een getijde cyclus, bijvoorbeeld over een jaar, hetgeen een meer gecompliceerde aanpak zoals numerieke modellering vergt om de evenwichtscondities van het zeegat te bepalen. De oceaan invloed van het getij en de golfwerking op de zeegat morfodynamica is gesimuleerd en onderzocht met behulp van een fysisch gebaseerd numeriek model Delft3D. De fluviale processen zijn gesimuleerd met behulp van een numeriek hydraulisch model SOBEK dat 1D en 2D modules koppelt voor zoetwater instroom in een geulen netwerk van de rivieren, lagunes en zeegaten en op de uiterwaarden. De morfologische veranderingen bij de zeegaten ten gevolge van rivier afvoer zijn gesimuleerd met Delft3D. Tijdens het overstromingsseizoen zijn de zeegaten riviergedomineerd. Episodische gebeurtenissen van rivieroverstroming en tyfoons hebben een grote impact op de iii.

(10) iv. Samenvatting. hydrodynamica en morfologie van de zeegaten en zijn de belangrijkste oorzaak van het behoud van de zeegaten. Grote rivier afvoerstromen eroderen zeegatgeulen en delta’s en vergroten de dwarsdoorsneden van het zeegat. De expansie van zeegat doorsnede oppervlakten verzwakt ook de getijdestroming in de geulen en verhinderd sediment transport om vloeddelta’s op te bouwen in de Tam Giang lagune. Getijdoorspoeling langs deze mcirogetij kust is daarom ondergeschikt aan rivier afvoer en golfwerking. De Thyan An binnenzeegat geul kan hergeoriënteerd worden door de distributie van rivier afvoer van verschillende rivieren. Hoog water niveau is de oorzaak voor zand barrière boorbraak door overstroming door de barrière. Het verschil in rivier afvoer is de belangrijkste oorzaak van onderscheid in de stabiliteitscondities van de twee zeegaten. Rivier afvoer transporteert een totale sedimenthoeveelheid van 1.43 Mm³/jaar en 0.16 Mm³/jaar in een gemiddeld jaar door de Thuan An en Tu Hien zeegaten, respectievelijk. De seizoensvariatie van het golfklimaat heeft grote invloed op het sediment transport patroon alsmede op de morfologie van de zeegaten en de aangrenzende kusten. Golfgedreven kustlangs sediment transport tijdens het winter moesson seizoen is dominant over het sediment transport van rivieren door de zeegaten. Als de rivier afvoeren afnemen worden de zeegaten golfgedomineerd. De uitschuring door rivier overstromingen zorgt ervoor dat de zeegat geulen sediment put worden voor golfgedreven sediment transport dat kustlijn erosie van de naastgelegen kusten van beide zeegaten veroorzaakt. Geulsedimentatie en herstel van de eb delta’s en naastgelegen kusten worden geleidelijk bevorderd door zomer moesson golfwerking. De groei van de zuidelijke Thuan An zandtong komt door het netto kustlangse sediment transport van 0.25 – 0.36 Mm³/jaar in NW richting. Bij het Tu Hien zeegat is het netto kustlangse sediment transport aan de noordzijde 0.4 Mm³/jaar in ZO richting terwijl deze waarde aan de zuid kust 0.05 Mm³/jaar in NW richting is. Dit komt doordat het zeegat deels afgeschermd is van golven door de SO kaap. Matige rivier afvoer en getij forcering in het zeegat maken het erg onstabiel. Het lange termijn gedrag van het Thuan An zeegat na een extreme rivier overstroming is onderzocht met het geaggregeerde model ASMITA op een tijdschaal van decades. Elk element van het zeegat systeem reageert in verschillende mate op de rivier en golf invloeden. De rivier invloed wordt gereflecteerd door de seizoensvariatie van element volumes in het systeem welke het sterkst is aan de kust en de eb delta en zwakker in de zeegat geul. De invloeden van rivier afvoer en golfklimaat op de morfologie en gedrag van de zeegaten zijn samengevat in een conceptueel model. In conclusie, meer inzicht in de morfodynamica en het gedrag van een zeegat systeem langs een micro-getij golfgedomineerde kust in centraal Vietnam onder tropisch moesson klimatologisch regime is verkregen in deze studie. De sleutelprocessen die de morfodynamica sturen en hun invloed op de morfologie en gedrag van Hue zeegaten zijn geïdentificeerd. Hoewel dit onderzoek gefocust is op de Thuan An en Tu Hien zeegaten systemen de principes, benaderingen en een deel van de resultaten kunnen ook toegepast, of tenminste, helpen voor andere zeegaten in de regio. Tenslotte zijn verschillende aanbevelingen naar aanleiding van dit onderzoek gedaan voor toekomstig onderzoek inclusief data observatie, linearisering van de wrijvingsterm van de hydraulische zeegat vergelijking met sterke zijinstroming, tyfoon invloed en de uitdaging van numerieke lange termijn simulatie met procesgebaseerde modellen met inbegrip van intermitterende rivieroverstromingen..

(11) CONTENTS Summary ..................................................................................................................................... i Samenvatting............................................................................................................................. iii Chapter 1 1.1 1.2 1.3 1.4 1.4.1 1.4.2 1.4.3 1.5 1.6. Study area..................................................................................................................... 1 Coastal inlets ................................................................................................................ 1 Hydrodynamic classification........................................................................................ 3 Tidal inlet stability ....................................................................................................... 4 Empirical equilibrium relationships ........................................................................ 4 Cross sectional stability........................................................................................... 5 Sediment bypassing criteria..................................................................................... 5 Study objective............................................................................................................. 6 Thesis structure ............................................................................................................ 8. Chapter 2 2.1 2.2 2.3 2.3.1 2.3.2 2.4 2.4.1 2.4.2 2.4.3 2.5 2.5.1 2.5.2 2.6 2.6.1 2.6.2 2.6.3 2.6.4 2.7 2.8 2.8.1 2.8.2 2.8.3 2.9. Physical Settings .................................................................................................. 11. Introduction ................................................................................................................ 11 The coastal lagoon ..................................................................................................... 11 Tidal inlets and associated features............................................................................ 13 Thuan An inlet....................................................................................................... 13 Tu Hien inlet.......................................................................................................... 18 Climatic conditions .................................................................................................... 21 Climatic regime ..................................................................................................... 21 Wind ...................................................................................................................... 21 Rainfall .................................................................................................................. 23 River basin and hydrologic conditions....................................................................... 24 Topography ........................................................................................................... 24 River flow.............................................................................................................. 26 Coastal and marine characteristics ............................................................................. 28 Coastal and continental shelf topography ............................................................. 28 Tides ...................................................................................................................... 31 Wind waves ........................................................................................................... 33 Typhoons and storm surges ................................................................................... 35 Salinity ....................................................................................................................... 36 Sediment..................................................................................................................... 36 Sediment characteristics ........................................................................................ 36 River sediment transport ....................................................................................... 37 Longshore sediment transport ............................................................................... 38 Summary and discussion............................................................................................ 39. Chapter 3 3.1 3.2. Introduction ............................................................................................................ 1. Tidal Inlet Hydraulic Analysis ............................................................................. 41. Introduction ................................................................................................................ 41 Basic equations .......................................................................................................... 41. v.

(12) vi. 3.3 3.4 3.5 3.5.1 3.5.2 3.5.3 3.5.4 3.5.5 3.5.6 3.6 3.6.1 3.6.2 3.7 3.8 3.8.1 3.8.2. Contents. Assumptions............................................................................................................... 42 Linearization of the friction term ............................................................................... 43 Single bay-inlet system with inertia and river flow ................................................... 46 River flow velocity in the inlet.............................................................................. 47 Inlet horizontal tidal amplitude ............................................................................. 47 Bay vertical tidal amplitude .................................................................................. 48 Phase lag of the tide in the inlet and bay............................................................... 48 Analytical solution in case of no river discharge .................................................. 49 Conclusions ........................................................................................................... 52 Application to the tidal inlets of Hue ......................................................................... 52 Actual shear stress ................................................................................................. 52 Equilibrium shear stress ........................................................................................ 53 Summary and discussion............................................................................................ 58 Appendix 3.A – Deriving basic equations of tidal inlet hydraulics ........................... 60 Equation for the bay section.................................................................................. 60 Equation for the channel section ........................................................................... 60. Chapter 4 4.1 4.2 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 4.2.6 4.3 4.3.1 4.3.2 4.3.3 4.4 4.4.1 4.4.2 4.4.3 4.5 4.5.1 4.5.2 4.6. Ocean Influences .................................................................................................. 63. Introduction ................................................................................................................ 63 Tidal hydrodynamics.................................................................................................. 63 Basic equations...................................................................................................... 63 Numerical schemes................................................................................................ 65 Model domain........................................................................................................ 66 Boundary conditions.............................................................................................. 66 Model calibration and verification ........................................................................ 67 Results and discussion........................................................................................... 70 Wave dynamics .......................................................................................................... 73 Basic equations...................................................................................................... 73 Model domain and boundary conditions ............................................................... 74 Results and discussion........................................................................................... 76 Sediment transport ..................................................................................................... 78 Basic equations...................................................................................................... 78 Model domain and boundary conditions ............................................................... 79 Computational results and discussion ................................................................... 81 Conclusions................................................................................................................ 84 Tidal hydrodynamics ............................................................................................. 84 Wave dynamics and sediment transport ................................................................ 84 Appendix – Bijker’s (1971) sediment transport formula ........................................... 86. Chapter 5. River Flow Influences .......................................................................................... 89. 5.1 Introduction ................................................................................................................ 89 5.2 River flow hydrodynamics......................................................................................... 89 5.2.1 Basic equations and numerical scheme ................................................................. 90 5.2.2 Model domain........................................................................................................ 91 5.2.3 Boundary conditions.............................................................................................. 92 5.2.4 Model calibration and validation........................................................................... 93 5.2.5 Results and discussion........................................................................................... 98 5.3 River flood influence on tidal inlet morphology...................................................... 101.

(13) Contents. vii. 5.3.1 Model domain and boundary conditions ............................................................. 102 5.3.2 Results and discussion......................................................................................... 102 5.4 Conclusions.............................................................................................................. 106 Chapter 6 6.1 6.2 6.2.1 6.2.2 6.2.3 6.2.4 6.3 6.3.1 6.3.2 6.4. Morphodynamics of Thuan An Inlet .................................................................. 107. Introduction .............................................................................................................. 107 Long-term behavior.................................................................................................. 107 ASMITA model................................................................................................... 108 Model of the Thuan An inlet ............................................................................... 110 Model calibration ................................................................................................ 111 Model results and discussions ............................................................................. 114 Conceptual model .................................................................................................... 116 Fast processes ...................................................................................................... 116 Slow processes .................................................................................................... 118 Summary and conclusions ....................................................................................... 121. Chapter 7. Conclusions and Recommendations................................................................... 123. 7.1 Conclusions.............................................................................................................. 123 7.1.1 Characteristics of main processes governing Hue inlet hydrodynamics ............. 123 7.1.2 Role and influence of river flow.......................................................................... 124 7.1.3 Sediment transport pattern................................................................................... 124 7.1.4 Long-term behaviour of Thuan An inlet ............................................................. 125 7.2 Recommendations .................................................................................................... 125 7.2.1 Linearization of friction term .............................................................................. 125 7.2.2 Field observations................................................................................................ 126 7.2.3 Long-term simulation .......................................................................................... 126 7.2.4 Typhoon influence............................................................................................... 126 References .............................................................................................................................. 127 Abbreviations ......................................................................................................................... 135 List of Symbols ...................................................................................................................... 137 List of Tables.......................................................................................................................... 141 List of Figures ........................................................................................................................ 143 Acknowledgements ................................................................................................................ 145 Curriculum Vitae.................................................................................................................... 147.

(14) viii. Contents.

(15) Chapter 1. INTRODUCTION 1.1. Study area. The Tam Giang-Cau Hai lagoon is located in the Thua Thien-Hue province in the central coastal area of Vietnam. This is a system of connected lagoons and two tidal inlets connecting to the South China Sea. The lagoon has a surface area of 216 km² and elongates 68 km in NW-SE direction along the coastline. The lagoon water body is separated from the sea by a system of sandy spits and island barriers. It receives water from the Huong River Basin which has a catchment area of about 4400 km² and discharges to the sea through two tidal inlets: Thuan An in the north and Tu Hien in the south (Figure 1.1). The area is located in a tropical monsoon region and is characterized as a microtidal, wave-dominated coastal environment. Under the tropical monsoon climatic conditions, the morphology of the inlets is highly dynamic and variable. The tropical monsoon regime exerts its influence on the tidal inlet morphology through the seasonal variation of river flow and wave climate.. Figure 1.1: Tam Giang-Cau Hai lagoon and tidal inlet system. 1.2. Coastal inlets. A coastal inlet is a relatively small-scale waterway that connects an inland body of water such as a lagoon, bay, or estuary with a larger tidal body as an ocean and it serves as a conduit for the exchange of water between these bodies. Tidal basins, including tidal lagoons and 1.

(16) 2. Chapter 1. estuaries with their associated coastal inlets, interrupt a significant part of the world’s shorelines (Stive and Wang, 2003). Coastal inlets are also a common feature along the central coast of Vietnam. Nearly 60 inlets and estuaries are found along 2000 km of its shoreline. Coastal inlets provide the connection between the tidal basins and the open oceans. They play an important role in nearshore processes and the exchange of water, sediments, nutrients, planktonic organisms, larvae and pollutants between the basins and the oceans. Due to the fact that coastal inlets interrupt the continuity of shorelines and nearshore processes, they also play a central role in sediment budgets and coastal erosion (Aubrey and Weishar, 1988). 1) Coastal barrier or spit headland; 2) The tidal gorge; 3) The main ebb channel and ebb ramp; 4) Swash platforms; 5) Marginal flood channels; 6) Marginal shoals; 7) Ebb tidal levee; 8) Ebb delta terminal lobe; 9) The flood ramp; 10) The ebb shield; 11) Main ebb dominated inner channel; 12) Ebb spit; 13) Spill over channels. Figure 1.2: Morphological features of a tidal inlet system (Smith, 1984). The morphology of tidal inlets is controlled by the interaction of oceanographic, hydrologic, meteorologic, topographic and geologic factors. The diversity in hydraulic signature, sediment transport patterns and morphology of tidal inlets indicates the complexity of their processes. The variability in external forcing, such as tidal range, wave energy, freshwater inflow, sediment supply, storm intensity and frequency, together with topographic and geologic controls, and the interactions of these factors, are responsible for this wide range in tidal inlet settings (FitzGerald, 1988, 2005; Stive, 2006). As described by Smith (1984), morphological features of a typical tidal inlet system are shown in Figure 1.2. According to Bruun (1978), tidal inlets are normally divided into three main sections: a) the ocean section, which is the entrance that may include outer shoals (ebb tidal delta) and bars and one or more channels, and whose development is significantly influenced by wave action; b) the gorge, which is the channel section having the minimum cross-sectional area and usually with less exposure to waves; and c) the bay section, which may include the inner shoals (flood tidal delta) and channels, with dominant current influence and relatively little wave action..

(17) 1.3 Hydrodynamic classification. 1.3. 3. Hydrodynamic classification. Hayes (1979) introduced a hydrodynamic classification for coasts and tidal inlets based on the main driving hydrodynamic forces of waves and tides; both of these are independent of the inlet system configuration. The wave conditions are generated seaward of the inlet and thus are independent from it. The tidal range outside an inlet mainly depends on the ocean shelf tides and their interaction with the ocean shelf bottom topography (Stive, 2006). Hayes’ classifications for wave and tide environments are summarized as in Tables 1.1 and 1.2. Table 1.1: Classification of wave climate Mean significant wave height HS (m). Wave energy class Low wave energy. < 0.6. Medium wave energy High wave energy. 0.6 – 1.5 >1.5. Table 1.2: Hydrographical classification of coast and tidal inlets Class. Tidal range (m). Microtidal. <1. Low-mesotidal. 1–2. High-mesotidal. 2 – 3.5. Low-macrotidal Macrotidal. 3.5 – 5 >5. Low. Medium. 6. Mean tidal range (m) . 5 de Ti. -do. ( ted a n mi. 4 T. High wave energy High macrotidal. h) hig. m -do e d i. in. 3 n de xe i M. w) (lo d ate. y erg. (tid. e. Low macrotidal. nt ) na i m do. ant) min. e do wav ( y nerg ed e x i M ant Wave-domin. 2 1. High mesotidal. Low mesotidal. Thua Thien-Hue coast. Microtidal. 0 0. 0.5. 1. 1.5. 2. 2.5. Mean wave height (m). Figure 1.3: Hydrodynamics classification tidal inlets (Hayes, 1979).

(18) 4. Chapter 1. Using the morphology and hydrographic regime of 21 coastal plain shorelines, Hayes (1979) proposed a morphological classification based on the ratio of tidal range and mean wave height as in Figure 1.3 distinguishing five regions of wave dominated, mixed energy – wave dominated, mixed energy – tide dominated, tide dominated – low, and tide dominated – high. He also found that tidal inlets occur primarily in mesotidal coasts with moderate wave energy. According to this classification diagram, the coast of the Thua Thien-Hue province with a tidal range of about 0.5 m and a mean wave height of about 1 m, is classified as a microtidal, wave-dominated coast (Figure 1.3). 1.4 1.4.1. Tidal inlet stability Empirical equilibrium relationships. 1.4.1.1 Inlet channel cross section The study on the stability of tidal inlets has been paved by the early works of O’Brien (1931), Escoffier (1940), Bruun and Gerritsen (1960). O’Brien (1931), through his purely empirical work from field studies found that inlet cross-sectional area could be related to the tidal prism, and also to the average throat velocity over a tidal cycle. Later it was found that for various tidal inlets, the general form of the relationship is presented by Ac = a ⋅ P n. (1.1). where Ac is the cross sectional area of inlet gorge (m²), P is the spring tidal prism (m³), a and n are empirical coefficients. For many coasts the value of n is found to be of the order 1 such as by (Jarrett, 1976; Van de Kreeke, 1990b). Stive and Rakhorst (2008) noticed that the coefficient a is not dimensionless but its dimension depends on the power n. They reviewed that the value of a is close to 1·10-4 and suggested a can be determined using the following expression. a=. 2 1 C ⋅ T θ s ( s − 1) D50. (1.2). where T is the tidal period, C is the Chezy roughness coefficient, D50 is the sediment median diameter, s is the sediment specific density (ρs/ρ) and θs is the Shields mobility parameter. By varying the value of θs from 0.1 for protected inlets or small alongshore drift situations to 0.3 for unprotected inlets with large alongshore drifts, the coefficient a (and therefore, the inlet cross-sectional area Ac) varies about 30% which agrees with Kraus’ (1998) discussion. 1.4.1.2 Inlet channel volume. The channel volume of a tidal basin or estuary relates to the tidal prism as (Eysink, 1991) Vc = α c ⋅ P1.5. (1.3). where Vc is the volume of a tidal basin or estuary below MLW (m³), αc is an dimensional empirical coefficient..

(19) 5. 1.4 Tidal inlet stability. 1.4.1.3 Ebb tidal delta volume. A relationship for the ebb tidal delta sand volume and the tidal prism is presented by (Walton and Adams, 1976; Biegel, 1993) Vd = α d ⋅ P1.23. (1.4). where Vd is the sand volume of an ebb tidal delta (m³), αd is an empirical coefficient. 1.4.2. Cross sectional stability. Velocity, V or shear stress, τ. Escoffier (1940) introduced a hydraulic stability curve, referred to as the Escoffier diagram, in which maximum flow velocity is plotted against cross sectional flow area. The Escoffier diagram illustrates that a change in flow area induces a change in flow velocity that will perpetuate the induced change. According to this diagram, an inlet having a cross sectional area larger than a critical flow area is termed hydraulically stable. Any change in the inlet that brings its cross-sectional area out of equilibrium size will result in a change in inlet velocity that forces to return the inlet to its equilibrium value by associated deposition or scour. An inlet is hydraulically unstable if its cross sectional area is smaller than this critical flow area value. Since any initial change in flow area is accentuated, the hydraulically unstable inlet will either continuously scour until the equilibrium flow area is attained, or continuously shoal until inlet closure.. Stable root A. Critical value, Vcr or τcr B Unstable root. Inlet cross-sectional area, Ac. Figure 1.4: Escoffier diagram or inlet closure curve. Based on Escoffier’s (1940) approach, Van de Kreeke (1985) developed an inlet closure curve by replacing maximum flow velocity with maximum bottom shear stress in a stability analysis (Figure 1.4) realizing that the transport capacity of the inlet currents can be best characterized by the maximum bottom shear stress during a tidal cycle, τˆ . Later this approach was extended to multiple inlet systems (Van de Kreeke, 1990a, b). However, this is still only a limited approach concentrating solely on the gorge channel cross section and its tidal forcing but not accounting for wave action, river flow, nor the hydrodynamic interaction between flows and sediment transport explicitly. 1.4.3. Sediment bypassing criteria. The stability of a tidal inlet can also be considered in the light of its capacity for sediment bypassing which is the transport of sediment from the updrift to the downdrift margin of the.

(20) Chapter 1. 6. tidal inlet. Bruun and Gerritsen (1959) first described the natural mechanisms of inlet sediment bypassing and proposed a parameter r’ to indicate the type of this process. M mean Qmax. r′ =. (1.5). where Mmean is the mean value of long-shore sediment transport rate to the inlet and Qmax is the maximum discharge through the inlet during spring tidal conditions. A value of r’ < 10 – 20 indicates predominant tidal flow by-passing (with little or no bar formation), the system bypasses sand through transport of sand by tidal currents in channels and by the migration and accretion of sandbars and tidal channels. A value of r’ > 200 – 300 indicates predominant bar by-passing with typical bar or shoal formation, the system bypasses sand by wave-induced sand transport along the outer margin of the ebb-delta (terminal lobe). Later (Bruun and Gerritsen, 1960; Bruun, 1968, 1978, 1990) this ratio was converted to P/Mtot to describe the overall inlet stability. r=. P M tot. (1.6). where Mtot is the total annual littoral drift (m³/year) and P is the tidal prism (m³/tidal cycle). According to the value of P/Mtot, the stability of an inlet is rated as good, fair, or poor as further detailed in Table 1.1. Table 1.1. The overall criteria for inlet stability in terms of by-passing capacity r=. P Mtot. > 150. Good – the inlet is predominant tidal flow by-passers; entrance with little or no ocean bar outside gorge and good flushing. 100 – 150. Fair – mixed of bar-by-passing and flow-by-passing the entrance has low ocean bars, navigation problems usually minor. 50 – 100. Fair to poor – the inlet is typical bar-by-passing and unstable; entrance with wider and higher ocean bars, increasing navigation problems. < 50. 1.5. Inlet stability situation. Poor – inlet becomes unstable with non-permanent overflow channels; entrance with wide and shallow ocean bars, navigation difficult. Study objective. Since coastal inlets are a primary navigation pathway for ships and boats to travel between the open oceans and sheltered waters behind barrier islands where safe harbors are located (FitzGerald, 2005), they have been scrutinized since the early twentieth century (O'Brien, 1931; Escoffier, 1940). As tidal inlets occur primarily on coasts with moderate wave height (Hayes, 1979), the major amount of studies on tidal inlets is associated with coastal systems having small or non-existent freshwater flow. For coastal inlets in the tropical monsoon areas, fresh water discharge plays a dominant, or at least significant, role in the inlet hydrodynamics. Strong episodic influences of river flow discharge and wave action have been mentioned as the cause of seasonal closure of tidal inlets as found on microtidal coasts of India (Bruun, 1986), Sri Lanka (Wikramanayake and Pattiarachchi, 1999), Japan (Tanaka et al., 1996), Australia (Gordon, 1990; Ranasinghe and Pattiaratchi, 1997, 1998, 1999, 2003), South Africa.

(21) 1.5 Study objective. 7. (Cooper, 1990, 1994; Largier et al., 1992), USA (Elwany et al., 1998), and Brazil (Moller et al., 1996). Most of the river mouths and tidal inlets along the central coast of Vietnam also have characteristics of seasonal closure. During the flood season, river floods have a strong influence on inlet morphology by scouring their channels and cutting through coastal barriers. In the dry season, wave action dominates over the tides and river flows. Wave-induced sediment transport eventually closes some inlets or river mouths. Nevertheless, studies of the physical processes governing the morphodynamics of tidal inlets including waves, tides, river flow and our understanding of the behavior of tidal inlets in the tropical monsoon areas are very limited. The role and the significance of each forcing factor and the associated morphodynamic response of a tidal inlet on a microtidal wave dominated coast in a tropical monsoon, like the inlets of Hue, are still poorly understood. Figure 1.5 shows the topography of the Thuan An inlet and the historical changes of its channel. The inlet topography well shows the gorge and the ocean section with an ebb tidal delta, but the bay section of Thuan An inlet comprises only one channel without an inner shoal, or in other words, a flood tidal delta is not present. The channel reorientation in the Thuan An inlet since 1960 to 2002 (Figure 1.5) has been recorded by the Thuan An Port Authority. But the cause of this reorientation, which is important for inlet stabilization, until now has not been understood. On account of the importance of inlets in navigation and military defense, the change of Hue tidal inlets has been recorded since the 1400s and the attempts of stabilizing them is demonstrated by the large number of human interventions to either close or open the inlets (Tien et al., 2001). Before 1950s, these interventions were mainly empirical based. Although some tidal inlet processes have been described, there is still a lack of good scientific descriptions for the morphodynamic processes of Hue tidal inlets.. Figure 1.5: Thuan An inlet and its channel historical changes.

(22) Chapter 1. 8. The main objective of this thesis is to increase the fundamental understanding of the physical processes underlying the hydrodynamic and morphodynamic behaviour of Hue tidal inlets under a tropical monsoon climatic regime on a microtidal wave-dominated coast. Specifically, the study is aimed to get further insight into the processes tides, river flow, wave action and sediment transport which control the morphologic behaviour of the inlets. The following questions express the research objectives and the answers to these questions are presented in the indicated thesis chapters: 1-. What are the characteristics of the tide, river flow and wave processes and how do they govern the hydrodynamics at the inlets in the microtidal wave dominated environment under the tropical monsoon climatic regime? These are the key forcing parameters that need to be understood to get further insight in the tidal inlet morphodynamics and behaviour. Part of the information of the forcing factors is available from the analysis of data collected in Chapter 2. The hydraulic characteristics of the tidal inlet system are analyzed by the analytical approach in Chapter 3. Other important information of the forcing processes is ‘interpolated’ from available data using a physical based numerical modeling approach described in Chapter 4 and Chapter 5.. 2-. What are the role and the influences of river flows on the morphology of the tidal inlet system? This question is answered by using a numerical modeling approach for the hydraulic and morphologic processes in the system of a tidal inlet and river network described in Chapter 5.. 3-. How do the sediment budget and sediment transport patterns evolve in the tidal inlet morphodynamics? The sediment balance of the river system is simulated in Chapter 5 while the tide- and wave-induced sediment transport patterns in the coastal waters and inlet areas are simulated using a 2D morphodynamic model in Chapter 4.. 4-. How will the system behave on the long-term? The long-term behaviour of the Thuan An inlet is investigated in Chapter 6 using an aggregated model, assuming its equilibrium condition can be defined.. 5-. What is the conceptual model of the inlet under the influences of the monsoon climate regime? From analysis of basic data, inlet hydraulics and numerical simulation, a conceptual model is developed for the Thuan An inlet and presented in Chapter 6.. 1.6. Thesis structure. The chapter structure of this thesis is presented in Figure 1.6. After the introduction in this chapter, more details on the physical settings of the tidal inlet system are presented in Chapter 2. Chapter 3 provides a hydraulic analysis for single inlet systems. The ocean forcing on the tidal inlets of Hue including tidal and wave influences is investigated using numerical models in Chapter 4. Chapter 5 focuses on the influences of river flow on the inlet system. Chapter 6 presents a long-term analysis on the behaviour of the inlet system and the conceptual model of the Thuan An inlet. The thesis ends with the study conclusions and recommendations in Chapter 7..

(23) 1.6 Thesis structure. 9. Introduction (Chapter 1). Tidal inlet hydraulics (Chapter 3). Physical settings (Chapter 2). Tidal and wave influences (Chapter 4). Conceptual model (Chapter 6). TIDAL INLETS OF HUE. Longterm behaviour (Chapter 6). Figure 1.6: Structure of the thesis. River flow influence (Chapter 5). Conclusions and recommendations (Chapter 7).

(24) 10. Chapter 1.

(25) Chapter 2. PHYSICAL SETTINGS 2.1. Introduction. The system of Tam Giang – Cau Hai lagoon and associated tidal inlets is the largest and most important system of its kind in Vietnam. It receives water from rivers and discharges to the sea through tidal inlets in a tropical monsoon climate region. This system of lagoons and inlets is strongly influenced by both inland and oceanic conditions. In the rainy season, the dominant influence is due to the floods from the rivers, whereas during the dry period, the main influences come from the ocean. The tidal inlets are located on a micro-tidal wavedominated coast so its morphology is highly dynamic and frequently changing in terms of both cross-section and plan location. To obtain a general view on the natural condition and development of the tidal inlets, this chapter will provide some basic information on the physical characteristics of the system. The relevant data for the study are also briefly described and analyzed.. 2.2. The coastal lagoon. The Tam Giang – Cau Hai lagoon is located in the Thua Thien-Hue province in the central coastal area of Vietnam. It is a system of connected lagoons namely from north to south Tam Giang, Thanh Lam, Thuy Tu, and Cau Hai. The lagoons receive water from the Huong river basin and discharge to the South China Sea through two tidal inlets: Thuan An in the north and Tu Hien in the south (Figure 2.1). The Tam Giang – Cau Hai lagoon has a total surface area of 216 km² and a length of 68 km. Bathymetric data surveyed in 2000 (An, 2000) show that the typical water depth in the lagoons is 1 – 5 m in the northern lagoon of Tam Giang and 1 – 3 m in the southern lagoon at Cau Hai. The deepest areas (5 – 10 m deep) are found in the narrow channels near the Thuan An inlet (Table 2.1). Based on lagoon bathymetries surveyed in 1939 and 2000, water depths of the lagoon have changed significantly due to sedimentation processes. Surface areas of the lagoons are also reduced due to human activities like the development of shrimp farms and/or the construction of reclamation dikes fringing the lagoon. Table 2.1: Characteristics of the lagoons Lagoon. *. Name. Area (km²). Length (km). Average width (km). Average depth (m). Tidal inlet. 1. Tam Giang. 52. 27. 2. 2. Thuan An. 2. Thanh Lam. 25. 5. 5. 0.5 – 1.5. Hoa Duan*. 3. Thuy Tu. 35. 25. 1.5. 2. no inlet. 4. Cau Hai. 104. 15. 7. 1 - 1.5. Tu Hien. Hoa Duan inlet is currently closed. 11.

(26) Chapter 2. 12. Figure 2.1: The system of lagoons, tidal inlets and rivers of Thua Thien-Hue province. Figure 2.2: Topography of the lagoon system.

(27) 2.3 Tidal inlets and associated features. 2.3 2.3.1. 13. Tidal inlets and associated features Thuan An inlet. Historically, the lagoon has discharged to the sea through various arrangements of inlets and breaches. The main inlet nowadays is located at Thuan An. Its creation dates back to 1404 as a natural breach of the sand barrier. Before that time, the discharge from the lagoon was solely via the Tu Hien inlet in the south. As soon as the Thuan An inlet opened, the Tu Hien inlet started to decline towards a small inlet under pressure by natural closure processes, and the Thuan An inlet naturally evolved as the stable main inlet for discharging flood water from the lagoon. Natural seasonal closure and migration due to longshore sediment transport, subsequent storms and flooding, and human interventions made the inlet very dynamic and variable. From the 15th to 19th century, various attempts have been undertaken to close the breach at this location with structures to maintain the Tu Hien inlet but all failed. Natural or artificial closure of the inlet restricted its ability to discharge the flood waters so this location remained vulnerable to subsequent storms and flooding. The breach was located at different places in different periods. But mostly it was located at the two weakest places of the sand barrier at Thuan An and Hoa Duan. A historical record of the development of the inlet is shown in Table 2.2. Under the prevailing regime of river flows and waves, the ebb tidal delta is less developed and a flood tidal delta is almost not present. The ebb channels develop deeply inside the lagoons. This is mainly due to the high flow velocity of river floods. The average width of the inlet is 350m and the depth of the channels reaches 12m. These dimensions of the channels are too large to be sustained by the flood tides of a small tidal range so flood tidal currents become too small to carry sediment to build up flood shoals and to create flood channels. On the ebb delta, terminal lobes are visible but flood channels on the ebb delta are not clearly distinguished. This can be observed from the latest topography of the area surveyed in March 2002 (Figure 2.3).. Figure 2.3: Thuan An inlet topography based on soundings surveyed in March 2002 (TEDI, 2002).

(28) Chapter 2. 14. Table 2.2: Historical development of the main inlet Inlet \ Year: 1. Thuan An 2. Hoa Duan. 1404. LEGEND. 1467. 1498. 1504. 1880. 1883. open. 1889. partly open. 1897. 1903. 1904. close. 1909. 1999. 2001. no-data. 1) For the first time, breaches opened new inlets at Thuan An in 1404, and at Hoa Duan around 1500. This caused Tu Hien to decline to a small inlet with intermittent closure by natural migration of the sand bar across its entrance.. 2) To maintain the main waterway, the inlet at Tu Hien was dredged and the breaches were attempted to be closed with structures. Various attempts in 1404, 1467 and 1823 have failed but the ability to discharge flood waters was restricted.. 3) From 1868 to 1883, the Hoa Duan inlet and the Huong River were closed to prevent the landing of French battle ships.. 4) In August 1883, the closure dams were removed for navigation.. 5) In October 1897, a breach occurred at Thuan 6) The breaches and sand barriers at Thuan An An. Since then, the Hoa Duan inlet declined and and Hoa Duan remained vulnerable to subsequent storms and flooding. Thuan An has become the primary inlet..

(29) 2.3 Tidal inlets and associated features. 15. 7) The breach at Thuan An was closed in 1903 and then opened again in 1904.. 8) In September 1904, the Hoa Duan inlet was practically closed in a severe typhoon. It closed completely in 1909.. 9) From the Thuan An inlet, salt intrusion penetrated extensively at lower Huong river delta and affected agriculture.. 10) In 1928, French engineers started building a closure dam to prevent salt intrusion. A dyke system surrounding the lagoon was also built.. 11) From 1928 to 1953, the closure dam was damaged several times by floods and subsequently rehabilitated.. 12) Tides were almost blocked by the dam so the southern sand bar migrated further northwestward..

(30) Chapter 2. 16. 13) In 1960s, Thuan An port was built by US Army. The inlet was dredged and a large part of the closure dam was removed to get access to the port.. 14) In 1969, the Thuan An inlet was stabilized by a 200 m steel jetty constructed 150m south of the inlet. This jetty lasted until the end of 1970s.. 15) Without the jetty, the inlet became more unstable. 16). 17) The channel path changed frequently.. 18) Idem, see 17. 19) Idem, see 17. 20) Idem, see 17.

(31) 2.3 Tidal inlets and associated features. 17. 21) Idem, see 17. 22) The situation before the flood of November 1999. 23) In November 1999, an extreme flood caused several breaches in the sand barriers. This included an inlet at Hoa Duan.. 24) Sudden changes in the inlets were accompanied by serious coastal erosion. The flood also changed the direction of the inlet channel.. 25) In July 2000, the gap at Hoa Duan was closed with sandbags and concrete blocks but it opened again due to a flood.. 26) The gap was closed completely in September 2000..

(32) Chapter 2. 18. 27) Thuan An inlet in 2002. 28) Thuan An inlet in 2003. During the dry season, the inlet cross-sections become smaller, i.e. both narrower and shallower. The Thuan An channel tends to migrate northward because the dominant longshore sediment transport in the inlet area is in the southeast - northwest direction. The southeast sand spit can grow north-westward at a speed of about 15m/year (Hoi et al., 2001). In the period from 1928 to 1953 when a closure dam was present, the tides could not enter the lagoon and the sand spit extended 4km further northwest (Table 2.2). River floods are likely to be the main reason for the cutting of the sand bars and deepening of the inlet channels. The interruption of sand bypassing in the ebb delta due to floods may also be the cause for coastal erosion which propagates along the shore for several kilometers at both sides of the inlet. During a severe flood in 1999, the inlet became very wide and deep. After that it recovered due to longshore transported sediment entering the channel from both sides. This led to a disturbed pattern of erosion and accretion that caused the northern dune to continue to retreat at a rate of about 10m per year in 2000-2002 leading to the loss of a lighthouse and other properties, after which the retreat has slowed down again.. 2.3.2. Tu Hien inlet. For centuries, the system had only one inlet located at Tu Hien. In 1404, as described, a new inlet was opened at Thuan An. After the opening of the Thuan An inlet in 1404, the Tu Hien inlet carried far less flow discharge and gradually declined. The northern coast of the inlet is a sandy beach while the southern coast is a rocky shore, constrained and protected by a headland. Therefore the longshore sediment transport due to waves is mainly from the northwest direction. Under the action of waves, littoral drift and river floods, the Tu Hien inlet is frequently changing and continues to migrate between Vinh Hien and Loc Thuy in a morphological cycle of about 9 years during which it is closed for more than half of the time. The cycle starts with a breakthrough of the sand barrier at Vinh Hien creating a new inlet during an extreme river flood. Due to the dominant wave induced littoral drift south-eastward, the northern sand spit of the inlet is accreting and extending in the southeast direction. When the inlet reaches the rocky headland at Loc Thuy near the cape of Chan May Tay, it declines and then closes being the last stage of its cycle (Table 2.3). At the first stage of the morphological cycle, normally the inlet has dimensions of 200 m wide and 3 m deep. However, at the last stage of its cycle, it becomes a narrow and shallow channel which is about 4 km long, 50 m wide and 1 m deep..

(33) 2.3 Tidal inlets and associated features. 19. Table 2.3: Historical development of the southern inlet Inlet \ Year: 1. Tu Hien 2. Loc Thuy. 1404. 1811. LEGEND. 1823. 1844. 1953. open. 1959. 1979. 1984. 1990. 1994. close. 1999. no-data. 1) Tu Hien inlet in 1938. 2) The inlet at Loc Thuy in 1952. 3) Tu Hien inlet in 1992. 4) Tu Hien inlet before the closure in 1994. 5) Tu Hien inlet in 1997. 6) Tu Hien inlet opened again in the severe flood of November 1999. 7) Tu Hien inlet in 2001. 8) Tu Hien inlet in 2002.

(34) 20. Chapter 2. Figure 2.4: Present situation of Tu Hien inlet from satellite imagery (Google Earth, 2005). Figure 2.5: Tu Hien inlet and its morphological features (photo courtesy M.B. de Vries, 2004).

(35) 2.4 Climatic conditions. 21. Unlike the Thuan An inlet, the flood tidal delta of the Tu Hien inlet is well-developed. Flood shoals and marginal ebb channels can be distinguished. Channels on the ebb delta are not clearly recognized except for the main channel extending from the gorge. On the seaward edge of the ebb tidal delta, a system of bypassing bars is also visible (Figures 2.4 and 2.5).. 2.4. Climatic conditions. Climate influences the inlet morphology through its impact on marine and fluvial processes such as wind waves and river flows. The variation of the climate regime results in the variation of these processes and therefore, of the morphological development of the tidal inlets. The central region of Vietnam is characterized by a tropical monsoon climate, dominated by the northeast monsoon from September through March and the southwest monsoon from May to September.. 2.4.1. Climatic regime. 2.4.1.1 Northeast monsoon The northeast or winter monsoon (September-March) which is cool and dry has traveled down from Siberia and has crossed a few hundred kilometers of the warm and humid South China Sea. This monsoon is characterized by stable weather conditions and cooler northeasterly winds. The persistent winds are N and NW with a speed of 1.6 – 3 m/s. Strong winds during cold fronts and cold surges in winter can reach 17 – 18 m/s. The circulation brings abundant precipitation to the coastal mountains of Vietnam. The wind picks up moisture over the South China Sea and releases it in the form of torrential rain along the central coast and the eastern edge of the Annam (Truong Son) Cordillera. The period of the first few months of the monsoon season is also the most active period of tropical cyclones in the area. Tropical cyclones and northeast monsoon winds bring abundant rainfall to the Huong river basin. Most of the rainfall is concentrated in the period of 4 months from September to December which is called the “rainy” or “flood” season accounting for more than 70% of the annual rainfall of 3100 mm in the Huong river basin.. 2.4.1.2 Southwest monsoon In summer, southwest wind blows from the Bay of Bengal and has the “Föhn” effect when passing the Annam Cordillera. The rains associated with the southwest monsoon are often topographically driven. Rainfall is concentrated on southwest-facing slopes at elevations below 2000 m of Laos. The climate becomes dry and hot in the coastal area. The dominant wind is SW offshore. Prevailing wind near the coast changes to the east direction. Because rainfall and river flow diminish significantly, this period is called the “dry” or “low flow” season.. 2.4.2. Wind. The characteristics of the monsoon wind regime are very important in defining the wave climate in a semi-enclosed marginal sea like the South China Sea. The wave climate in turn, has a strong influence on tidal inlet morphology, especially for micro-tidal coasts..

(36) Chapter 2. 22. a) January. b) February V (m/s). N. NW. W 30%. 15. 15. 14. 14. NE. NW. E 25%. 20%. 15%. 10%. c) March 13. 12. 12. 11. 11. 11. 10. 10. 9. 9. 8. W 30%. E 25%. 20%. 15%. 10%. 3. 20%. 3. 20%. SW. SE. 2. 25%. S. V (m/s). N. 15. 15. 14. 14. NE. NW. NE. 13. 12. 12. 11. 11. 11. 10. 10. 9. 9. 8. W 30%. E 25%. 20%. 15%. 10%. 8. 5%. 7. 10 9 W 30%. E 25%. 20%. 15%. 10%. 6. 10%. 10%. 5. 5. 15%. 4. 15%. 4. 15%. 20%. 3. 20%. 3. 20%. SW. SE. 1 30%. S. S. h) August. i) September. V (m/s). V (m/s). N. 15 14. 15. 14 NW. 14. NE. NW. NE. 13. 13. 13. 12. 12. 12. 11. 11. 11. 10. 10. 9. 9. 8. 5%. W 30%. E 25%. 20%. 15%. 10%. 8. 5%. 7. 10 9 W 30%. E 25%. 20%. 15%. 10%. 6. 10%. 10%. 5. 5. 5. 15%. 4. 15%. 4. 15%. 20%. 3. 20%. 3. 20%. SE. SW. 2. 25%. SE. 1 30%. S. S. k) November. l) December V (m/s). N. V (m/s). NW. NE. NW. 12 11. W 30%. E 25%. 7. 20%. 15%. 10%. S. 9 E 25%. 20%. 15%. 10%. 8. 5%. 7. 5%. 6 10%. 5. 15%. 4. 15%. 20%. 3. 20%. SW. 1 30%. 10. W 30%. 5. 3. 25%. 8. 10%. 4. 2. 11. 7. 5. SE. 12. 11. 6. 6. 20%. 13. 12. 5% 5%. 10%. SW. 13. 9. 9. 15%. NE. 10. 10. 5%. 14. NE. 13. 8. 15. 14. 14. E. V (m/s). N. 15. 15. 5%. 2. 25%. 30% S. NW. 3 SE. 1. 30%. N. 4. SW. 2. 25%. 1. j) October. 7. 5%. 6. 10%. 8. 5%. 7. 5%. 6. SW. V (m/s). N. 15. NE. 5%. 2. 25%. 30% S. E. 3 SE. 1. 30%. NW. 4. SW. 2. 25%. 1. g) July. 7. 5%. 6. 2. 8. 5%. 7. 5%. 5. 10%. 14 NW. 12. 10%. 15%. 15. NE. 13. 5%. N. V (m/s). N. 13. 6. 20%. 1 30%. f) June. V (m/s). 25%. 2. 25%. S. SE. 3 SE. 1. e) May. SW. 4. SW. 30%. E. 25%. 5. 20%. 5%. W 30%. 6 10% 15%. NW. 7. 5%. 4. 2. 8. 5%. 15%. N. 10%. 10%. 4. d) April. 15%. 15%. 5. S. 20%. 20%. 6. 30%. 25%. 25%. 7. 5%. 1. W 30%. 9 E. 15%. 25%. 10%. 10. W 30%. 10%. SE. 15%. 8. 5%. 5. 20%. NE. 12. 10%. 25%. 14 NW. 13. 6. W 30%. 15. NE. 7. SW. V (m/s). N. 13. 5% 5%. V (m/s). N. SE 25%. 2. 4 3. SW. SE 25%. 1 30%. 2 1. 30% S. S. Figure 2.6: Wind roses at Con Co from 1992-2001 observations. Long-term observations of wind speed and direction are available at meteorological stations of Hue, Nam Dong, A Luoi, Con Co and Da Nang, where Con Co is located offshore (Figure 2.7). Wind data are also available at some locations along the coast like Thuan An, Cua Tung for some periods. Figure 2.6 shows the wind statistics at Co Con for the period of 1975 – 2005. The monsoon regime is clearly observed. The northeast monsoon starts in late September and flourishes in November and December with N and NW winds dominantly at a wind speed of 6 – 7 m/s. NW wind still intermittently prevails until March or April when SE.

(37) 2.4 Climatic conditions. 23. winds take over. Average wind speeds of N and NW in January and February are 4.5 – 5.5 m/s. SE wind initiates in March – May with a speed of 3.5 m/s. The Southwest monsoon is dominant from June to August with an average speed of 4.5 m/s. The strongest winds may be over 20 m/s during severe typhoons.. 2.4.3. Rainfall. The hydrologic regime of the rivers has a strong influence on the morphology of the tidal inlets along the central coast of Vietnam. It is defined by the characteristic rainfall pattern which depends on basin topography and monsoon regime. The monsoon regime defines the temporal variation of rainfall in a year. The topography defines largely the spatial variation of rainfall over the basin.. Figure 2.7: River basin topographic map and measurement stations. The annual rainfall in the Huong river basin is about 3300 mm but it is mostly concentrated in 4 months of the flood season from September to December which accounts for about 70% of the total (Table 2.4) when the northeast monsoon encounters the Intertropical Convergence Zone and tropical cyclones operate most actively. Rainfall over the basin is topographically.

(38) Chapter 2. 24. driven so its magnitude increases from the Hue delta to the higher elevations in mountainous areas. In the delta the annual rainfall is 2600 – 2800 mm such as at Kim Long, Hue and Phu Oc stations. This value increases to about 3600 mm in Ta Luong and Nam Dong. The rainfall over the southwest part of the basin reaches its highest value in Vietnam. The annual rainfall at Truoi is 4661 mm, and on an even higher elevation at Bach Ma, its value is nearly 8000 mm (Tuan et al., 2001). Table 2.4: Annual rainfall and flood season contribution Station. Subbasin. Geographic coordinates. Period of record. Latitude Longitude. Annual rainfall (mm). Flood season Rainfall (mm). Percentage (%). Co Bi. Bo. N16°32’. E107°21’. 1978-1988. 2972. 2216. 74.6. Phu Oc. Bo. N16°37’. E107°26’. 1977-2005. 2815. 2073. 73.6. Ta Luong Binh Dien. Bo Huong. N16°18’ N16°17’. E107°19’ E107°29’. 1978-2005 1979-2005. 3595 3557. 2398 2346. 66.7 66.0. A Luoi. —. N16°13’. E107°15’. 1973-2005. 3459. 2407. —. Nam Dong. Huong. N16°10’. E107°44’. 1973-2005. 3608. 2397. 66.4. Thuong Nhat Huong Duong Hoa Huong. N16°06’ N16°17’. E107°36’ E107°38’. 1979-2005 1986-1987. 3292 2767. 2126 1740. 64.6 62.9. Kim Long. Huong. N16°29’. E107°35’. 1977-2005. 2616. 1891. 72.3. Hue. Huong. N16°25’. E107°43’. 1901-2005. 2820. 2139. 75.8. Bach Ma Truoi. Truoi Truoi. N16°10’ N16°21’. E107°50’ E107°47’. 1932-1935 1992-1996. 7977 4661. N/A 3809. N/A 81.7. Loc Tri. Cau Hai. N16°19’. E107°52’. 1978-1991. 2817. 2255. 80.1. Lang Co. —. N16°14’. E108°05’. 1978-1989. 2245. 1754. —. 3306. —. 71.3. Basin averaged. (Note: “N/A” means no data available, “—“ means uncounted values because these locations are outside the Huong River Basin). 2.5 2.5.1. River basin and hydrologic conditions Topography. The lagoon and inlet system receives water from a drainage basin of about 4400 km² and is influenced by the flow regime of the basin. The major rivers discharging into the lagoon are Huong River (with three tributaries of Ta Trach, Huu Trach, and Bo River), O Lau River, Dai Giang River, Nong River, and Truoi River (Figure 2.7). The areas and boundaries of the catchments of the rivers and gauging stations are shown in Table 2.5 and Figure 2.7. Almost all river flow originates from the territory of the Thua Thien-Hue province. Tables 2.5 and 2.6 are derived from SRTM 90 m resolution DEM (Farr et al., 2007) of the basin showing the sub-basin catchment areas and the distribution of ground level in the basin. There are other topographic maps of the province at scales of 1:50,000 (50K), 1:10,000.

(39) 2.5 River basin and hydrologic conditions. 25. (10K), and 1:5000 (5K). Only the 50K map covers the entire river basin. The 10K map covers the lowland plain only partly. The 5K map is available for a small area of Hue city. Table 2.5: Catchment area of the rivers Location. River. Latitude. Longitude. Catchment area (km2). Annual flow (m³/s). Peak flow (m³/s). Lowest flow (m³/s). Thao Long Sinh. Huong Huong. N16°33’ N16°32’. E107°36’ E107°34’. 2619 2560. — —. 12500 —. 5.80 —. Kim Long. Huong. N16°27’. E107°34’. 1557. —. —. —. Tuan. Huong. N16°23’. E107°35’. 1457. —. —. 5.80. Duong Hoa Thuong Nhat. Ta Trach Ta Trach. N16°21’ N16°07’. E107°38’ E107°41’. 688 198. 58.8 15.8. 1860 1470. 4.50 1.15. Binh Dien. Huu Trach. N16°21’. E107°30’. 585. 42.1. 4020. 2.16. Phu Oc. Bo. N16°31’. E107°28’. 872. —. —. —. Co Bi Cong Quan. Bo Dai Giang. N16°30’ N16°22’. E107°26’ E107°46’. 735 286. 61.2. 2810. 4.26. —. —. —. Cua Lac. O Lau. N16°39’. E107°26’. 745. 78.6. 3310. —. Truoi. Truoi. N16°21’. E107°47’. 125. 18.6. 1271. 0.41. Cau Hai. Cau Hai. N16°17’. E107°54’. 38. 2.70. —. —. 4363. —. —. —. The whole system — means no data available. Table 2.6: Distribution of elevation in the river basin Elevation range (m). Percentage of basin area (%). Area of level below higher limit (%). <2. 3.32. 3.32. 2–5. 9.82. 13.14. 5 – 10. 14.32. 27.46. 10 - 20. 7.05. 34.51. 20 - 50. 8.79. 43.30. 50 - 100 100 - 200. 7.72 12.06. 51.02 63.08. 200 - 500. 22.57. 85.65. 500 - 1000. 12.80. 98.45. 1000 - 1500. 1.52. 99.97. 1700 - 1800. 0.03. 100.00. A large part of the river basin area is mountainous and hilly accounting for about 70% of its total area. The remaining part is a narrow lowland plain. The basin is surrounded by high mountains of the Annam Cordillera on the southwest, south and southeast sides. The mountains on the southwest side near the border with Laos stretch over a length of 60 km in.