Modeling estuarine morphodynamics under combined river and tidal forcing
|
Leicheng GUO
This research is dedicated to studying long-term estuarine morphodynamic behavior under combined river and tidal forcing. Analysis of river tides in the Yangtze River estuary (YRE) in China, schematized morphodynamic modeling in 1D and 2D mode and
morphodynamic modeling of the YRE based on a process-based numerical model (Delft3D) are conducted. Morphodynamic sensitivities to river discharge magnitude and time variations, tidal strength and tidal constituents are then systematically explored.
Analysis of river tides in the YRE reveals strong river-tide interactions and non-linear modulation of tides by river discharge. River discharge alters tidal asymmetries and resultant tidal residual sediment transport.
Analysis of morphodynamic modeling results exposes significant mechanisms inducing tidal residual sediment transport and controlling long-term morphodynamic development. Morphodynamic equilibria in 1D and 2D simulations can be defined by vanishing gradients of tidal residual sediment transports and meeting empirical morphodynamic relationships.
This research indicates the value of numerical modeling in examining long-term morphodynamic development in millennia time scale. Understanding of the controls on morphodynamic behavior in estuaries under river and tidal forcing is to the benefit of managing estuaries’ functions in a long-term point of view.
Modeling estuarine
morphodynamics
under combined river
and tidal forcing
Modeling estuarine morphodynamics under combined river and tidal forcing
by
Leicheng Guo
Delft, 8
December 2014
1. Non-stationary river flow modulates tidal dynamics non-linearly through friction.
(this thesis)
2. River-tide current interactions are of profound morphodynamic importance by
enhancing seaward sediment export. (this thesis)
3. A medium river discharge leads to deepest equilibrium estuarine depth. (this thesis)
4. Combination of different tidal constituents has a diversified impact on residual
sediment transport and associated tidal basin morphodynamics over large space and
long time scales. (this thesis)
5. Schematized models unveil the governing mechanisms of morphodynamic processes
in a more transparent way than more complex models. Thus schematized models are
more useful.
6. Nature is awesome in complexity and at the same time beautiful in simplicity. The
ultimate objective of science is to formulate simple and applicable laws from the
complex reality.
7. To pursue a doctoral degree in the Netherlands is like biking in a rainy, windy and
dark winter night. You need a light to show the way ahead.
8. Negative research results showing that certain methodologies do not work contribute
to science in the same way as positive results. Negative results deserve publication
and citation.
9. The long-term value of government mass sponsoring of doctoral candidates studying
abroad needs to be reflected by a reducing need to do that in the next generation.
10. Follow your heart in the Netherlands and follow your duty in China.
These propositions are regarded as opposable and defendable, and have been
approved as such by the supervisors Prof. dr. ir Dano (J.A.) Roelvink and Prof. dr.
Qing He.
behorende bij het proefschrift
Modeling estuarine morphodynamics under combined river and tidal forcing
Van
Leicheng Guo
Delft, 8 Dec 2014
1. Niet-stationaire rivier afvoer verandert getij dynamica op een niet-lineaire wijze door
toedoen van wrijving. (deze thesis)
2. Interacties tussen getij en rivier afvoer hebben een belangrijke invloed op de
morfodynamica doordat ze het zeewaarts transport vergroten. (deze thesis)
3. Een medium rivier afvoer leidt tot de grootste estuarine evenwichtsdiepte. (deze
thesis)
4. Combinatie van verschillende getij-constituenten heeft een brede invloed op
residuele sediment transporten en gerelateerde getij bekken morfodynamica over een
groot gebied en over een lange periode. (deze thesis)
5. Geschematiseerde modellen onthullen belangrijke morfodynamische processen op
een meer transparante manier dan complexe modellen. Daarom zijn
geschematiseerde modellen nuttiger.
6. De natuur heeft een overweldigende complexiteit, maar laat tegelijkertijd ook
schoonheid in eenvoud zien. Het uiteindelijke doel van wetenschap is om
eenvoudige en toepasbare wetten te formuleren uit de complexe realiteit.
7. Het volgen van een promotie traject in Nederland is als fietsen in een regenachtige,
winderige, donkere winter nacht. Je hebt licht nodig om de weg voorwaarts te zien.
8. Negatieve onderzoeksresultaten die laten zien dat bepaalde methodes niet werken
dragen net als positieve resulaten bij aan de wetenschap. Negatieve resultaten
verdienen daarom publicatie en citatie.
9. De lange termijn waarde van door de overheid gefinancierde buitenlandse doctoraal
studies moet worden gereflecteerd in een afname van het nut van die financiering in
volgende generaties.
10. Volg je hart in Nederland en doe je plicht in China.
Deze stellingen worden opponeerbaar en verdedigbaar geacht en zijn als zodanig
goedgekeurd door de promotoren Prof. dr. Ir. J.A. Roelvink en Prof. dr. Q. He.
MODELING ESTUARINE MORPHODYNAMICS
UNDER COMBINED RIVER AND TIDAL FORCING
under combined river and tidal forcing
DISSERTATION
Submitted in fulfillment of the requirements of the Board for Doctorate of Delft University of Technology
and of
the Academic Board of the UNESCO-IHE Institute for Water Education for the Degree of DOCTOR
to be defended in public
on Monday, 8 December 2014 at 15:00 hours in Delft, the Netherlands
by
Leicheng GUO
born in Ganzhou, Jiangxi, ChinaProf. dr. Q. He
Composition of the Doctoral Committee:
Chairman Rector Magnificus, TU Delft Vice-Chairman Rector of UNESCO-IHE
Prof. dr. ir. J.A. Roelvink UNESCO-IHE/ TU Delft, promotor
Prof. dr. Q. He East China Normal University, China, promotor Dr. ir. M. van der Wegen UNESCO-IHE
Prof. dr. ir. Z.B. Wang Delft University of Technology Prof. dr. D.A. Jay Portland State University, USA
Prof. dr. ir. A.E. Mynett UNESCO-IHE/ Delft University of Technology Prof. dr. ir. M.J.F. Stive Delft University of Technology, reserve member
CRC Press/Balkema is an imprint of the Taylor & Francis Group, an informa business © 2014, Leicheng Guo
All rights reserved. No part of this publication or the information contained herein maybe reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, by photocopying, recording or otherwise, without written prior permission from the publishers. Although all care is taken to ensure the integrity and quality of this publication and the information herein, no responsibility is assumed by the publishers nor the author for any damage to the property or persons as a result of operation or use of this publication and/or the information contained herein.
Published by: CRC Press/Balkema
PO Box 447, 2300 AK Leiden, the Netherlands e-mail: Pub.NL@taylorandfrancis.com
www.crcpress.com - www.taylorandfrancis.co.uk - www.ba.balkema.nl Cover image: modeled fluvio-deltaic morphodynamics after 3000 years
This work is financially supported by the China Scholarship Council
(No.2009101208), the National Natural Science Foundation of China
(No.41276080), and the ReSeDUE project (No.60038881).
It is always advisable to have in mind a basically correct
mental picture of a physical phenomenon in order to guide one’s
intuition.
By P.H. LeBlond, 1978
In On tidal propagation in shallow rivers,
Journal of Geophysical Research, 83, C9, 4717-4721.
Abstract
Estuarine morphodynamics are of broad importance to estuaries’ functions related to navigation, human settlement and ecosystems. Inspired by the Yangtze River estuary (YRE), this study aims to explore the impact of river discharge, tides and their interaction on long-term estuarine morphodynamics. Use is made of 1D and 2D process-based models.
In first instance we focus on purely hydrodynamic characteristics in a 560 km long basin. We analyze the non-stationary river tides in the YRE by harmonic analysis and continuous wavelet transformation which reveals a wide range of subtidal variations and non-linear modulation by varying river discharges. An intermediate river discharge could be defined at which the amplitudes of the internally generated overtides and compound tides reach maxima.
Based on these hydrodynamic insights we conduct long-term (millennia time scale) morphodynamic simulations in schematized long basins. Vanishing spatial gradients in tidal residual sediment transport indicate an approach towards morphodynamic equilibrium. Morphodynamic equilibrium is also reached in case of a seasonally varying river discharge, which is reflected by a balance between erosion and accretion during low and high river discharge periods, respectively.
River flow supplies sediment, accelerates ebb currents, and alters tidal asymmetries. Each of these processes has its own effects on tidal residual transport and morphodynamics. For example, the interaction between a mean flow (i.e., Stokes return flow or river flow) and tidal currents induces significant tidal residual sediment transport which explains net ebb transport dominance in the presence of a flood tidal asymmetry. A larger river discharge does not necessarily lead to deeper equilibrium bed profiles. An intermediate river discharge is found which induces largest residual sediment transport gradients along the estuary leading to deepest equilibrium bed profile. Quantification of this medium river discharge is case dependent because of the non-linearities involved.
The 2D model approach applied in a large scale fluvio-deltaic system reveal river, estuarine and deltaic types of morphodynamic features, such as alternating sand bars, meandering channels inside the estuary and more elongated sand bars and distributary channels in the mouth zone and delta. The cross-sectionally averaged depth of the 2D model responds in a similar way to increased river discharge as a 1D model. Furthermore, a high river discharge induces ebb transport dominance, restricts development of flood channels and prolongs meander wavelength.
In summary, this research unveils the fundamental effects of tidal asymmetries, river discharge, and river-tide interaction in governing residual sediment transport and associated long-term estuarine morphodynamics under combined river and tidal forcing.
Samenvatting
* (Abstract in Dutch)Estuarine morfodynamiek speelt een belangrijke rol in estuarine functies gerelateerd aan navigatie, menselijke nederzettingen en ecosystemen. Geïnspireerd op het Yangtze Rivier Estuarium (YRE) is het doel van deze studie om de invloed te onderzoeken van rivier afvoer, getijbeweging en hun interactie op de lange termijn morfodynamiek van estuaria. Daarbij is gebruik gemaakt van 1D en 2D proces gebaseerde modellen.
In eerste instantie hebben we ons gericht op de hydrodynamische karakterisering in een 560 km lang bekken. We hebben de niet-stationaire rivier getijden van het YRE geanalyseerd door harmonische analyse en continue wavelet transformatie. Deze analyse toonde subgetij variaties aan die niet-lineair beïnvloed worden door rivier stroming. We konden een intermediate rivier afvoer definiëren die zorgt voor een maximalisatie van de amplitudes van intern opgewekte hogere harmonische getij componenten en gebonden getijden.
Op basis van dit hydrodynamische inzicht hebben we lange termijn (op een tijdschaal van millennia) morfodynamische simulaties uitgevoerd in lange, geschematiseerde bekkens. Verdwijnende ruimtelijke gradiënten in getij-residueel sediment transport duidden op een ontwikkeling naar morfodynamisch evenwicht. Deze ontwikkeling vond ook plaats in geval van seizoensvariërende rivier afvoer, waarin het evenwicht weerspiegeld wordt door een balans tussen erosie en depositie gedurende periodes van, respectievelijk, lage en hoge rivier afvoer.
Rivier afvoer voert sediment aan, versnelt ebb stroming en verandert getij asymmetrie. Elk van deze processen heeft een eigen effect op getij-residuele transporten en morfodynamiek. Bijvoorbeeld, de interactie tussen gemiddelde stroming (dat wil zeggen Stokes retour stroming of rivier afvoer) en getij stroming genereert een significant getij-residueel sediment transport wat een netto ebb transport verklaart in de aanwezigheid van een vloed getij asymmetrie. Een hogere rivier afvoer leidt niet noodzakelijkerwijs tot een dieper evenwichtsprofiel. We konden een intermediate rivier afvoer definiëren die leidde tot maximale getij-residuele transporten en een evenwichtprofiel met maximale diepte. De waarde van deze rivier afvoer is afhankelijk van lokale omstandigheden, vanwege de aanwezige niet-lineairiteiten.
Toepassing van een 2D model benadering in een groot fluviaal-delta systeem liet verschillende typen morfodynamische vormen zien welke karakteristiek zijn voor rivieren, estuaria en delta's. Voorbeelden zijn alternerende zandbanken en meanderende geulen binnen in het estuarium en meer uitgestrekte zandbanken en nevengeulen in de monding en delta. De doorsnede gemiddelde diepte van het 2D model liet hetzelfde gedrag zien onder verhoogde rivierafvoer als het 1D model. Bovendien leidde een hogere rivier afvoer tot een grotere ebb dominantie, een beperkte ontwikkeling van vloedgeulen en een verlenging van de meander lengteschaal.
Samenvattend heeft dit onderzoek het fundamentele effect onderzocht van getij assymmetrie, rivier afvoer en rivier-getij interactie op de belangrijkste residuele
sediment transporten en bijbehorende lange termijn estuarine morfodynamiek als gevolg van een gecombineerde rivier en getij forcering.
* This abstract is translated from English to Dutch by Dr. Mick van der Wegen, but
中文摘要
(An extended abstract in Chinese) 河口海岸地貌对于人类在海岸带的生产生活及生态环境具有重要的社会经济价值。 以往海岸地貌研究以现场观测和物理模型为主，数学模型研究仅限于相对短时间尺度 （潮周期至年）的地貌变化。人们逐渐认识到地貌过程的长期性，如河口海岸地貌对人 类工程及海平面上升等的响应，均在十年、百年尺度以上。传统研究方法也难于揭示水 流运动-泥沙输运-地貌演变之间动力地貌相互作用的过程和机制，由此提出了中长时间 尺度的动力地貌数学模型的需求。中长尺度的河口海岸动力地貌模型通过地貌加速方 法，实现了有限水沙动力计算时间下的长期地貌演变过程，使得模拟或者反演十年乃至 百年和千年的动力地貌发展和演变过程成为可能。本研究受长江河口地貌研究需要的启 发，采用Delft3D 模型，系统研究了径流和潮汐相互作用下的河口动力地貌演变和发育 特征，重点探讨径流大小和径流季节变化、潮汐组分和潮汐强度对河口地貌形态的控制 作用。 本研究首先分析了长江河口受径流影响的潮汐变形规律和特征。长江径流巨大，东 海潮波入侵，长江口在此动力下实现了径流和潮汐的相互作用。一方面，上溯潮波受到 摩擦和径流影响，潮差向上游逐渐减小；另一方面，潮波也因此加剧变形，表现为涨潮 历时逐渐缩短。数据分析表明，长江口潮波变形显著，潮差表现出半月、月、半年和年 际的变化，由此导致洪季潮平均水位大于枯季、洪季-大潮潮差会大于枯季-大潮、小潮 期间潮平均水位低于大潮(江阴以上)。这些低频信号反应了季节性径流、外海潮汐和潮 汐变形的综合作用。利用连续小波变换的方法，分析了牛皮礁、徐六泾、南京和大通四 站的实测长系列水位过程，结果表明：从下游往上，半日潮和全日潮能量衰减；四分之 一高频潮振幅先增长再减小；半月及月潮等低频潮振幅向上游逐渐增强；表明潮汐能量 从天文分潮向浅水分潮转换，由此导致AM4/AM2潮汐振幅比(A 表示振幅)向上游先增大 而后减小，反映出半日潮潮汐不对称的空间变化特征。同时也发现证实了潮汐相对相位 之间关系，即2ΦM2-ΦM4=ΦM2+ΦS2-ΦMS4 (Φ 表示相位)，由此表明潮波变形的单向性。 一维潮汐数学模型结果显示：天文潮受径流增大逐渐衰减，而高频浅水倍潮和混合潮则 受到径流影响，先增大而后减小，上下游之间表现出非线性特征；低频混合分潮则表现 出上游振幅极大的特征，尤其是半月 MSf 潮。由此可以解释长江口潮波衰减和变形现 象。对摩擦项非线性作用的敏感性分析表明，非线性的摩擦作用是控制潮波变形及径流 和潮汐相互作用的主要因素。 本研究建立了简化的一维河口动力地貌模型，模型在恒定径流和潮汐的驱动下，模 拟了千年尺度的地貌演变。发现: 随着径流逐渐增大，河口纵向平衡剖面先变深，然后 变浅，反映了河流供沙(源)和河口输沙(汇)之间的平衡关系；其结果也表明中等径流最 有利于河口泥沙的净向海输运。基于径流和潮流相互作用的机制分解，发现三个控制泥 沙净输运的重要机制，即: 径流、潮汐不对称、径潮流相互作用；这三者之间的平衡控 制了河口泥沙净输运的方向和大小，以及动力地貌发展的方向。 径流的显著作用表现为：一方面向河口供沙，加大落潮流和落潮输沙能力；另一方 面作用于周期性的潮流，进一步加强了向海的泥沙净输运。长时间尺度下，河口能够达 到一个动力地貌平衡状态，表征是泥沙余输运趋于均匀，即梯度趋于零。进一步考虑径 流的季节性变化，表明长期的地貌平衡仍可以实现，指征是季节性交替的侵蚀和淤积， 而年均的泥沙余输运则趋于均匀。一维动力地貌模型测试也表明偶发的大洪水引起的地 形变化、在长时间尺度下可以复原，由此表明中等径流及相应的年均泥沙余输运对河口 长尺度动力地貌的控制作用。二维模型结果表明，径流大小对滩槽格局形态具有重要影 响，表现为中小径流驱动下河槽趋于加深，滩槽格局趋于多变；大径流导致河势趋直，蜿蜒河段加长。从陆到海的地貌演变显示了单向蜿蜒河道 (河流)-双向蜿蜒河道(河口)-分汊河道体系(河口/三角洲)一体的地貌形态，反应了径潮流作用下、自然控制的一般河 口/三角洲地貌发育特征。 潮汐的显著作用表现为：各主要潮汐组分均和径流产生相互作用，由此加强向海的 泥沙净输运；同时潮汐组分之间的相互作用，产生浅水分潮；天文潮和浅水潮之间的相 互作用，如M2-M4、M2-S2-MS4、O1-K1-M2等，导致潮汐不对称，由此产生重要的泥沙 净输运效果。敏感性分析表明，在类似于长江河口潮汐组分的情况下，S2潮和径流的相 互作用、O1-K1-M2潮汐之间的相互作用是两个重要机制，前者有利于向海的泥沙净输运， 后者则引起向陆的泥沙净输运，由此将引起不同深度的地貌平衡剖面。二维模型结果也 表明，M2-S2作用引起的大小潮变化，可以引起增深的河槽和淤高的潮滩。综合来看， 多重潮汐不对称和非线性的径潮流作用对长时间尺度的河口地貌过程具有重要影响。 注意到径潮流相互作用反映的是潮平均余流和周期性潮汐水流之间的相互作用。在 没有径流的情况下，非驻波情况下的Stokes 作用也能产生一个向海的余流(若干厘米每 秒)，该余流和周期性潮流相互作用，仍能产生显著的向海的泥沙净输运，由此导致涨 潮不对称的情况下、净向海的泥沙余输运。径潮流相互作用对泥沙净输运的重要效果也 表明：强潮有利于径流影响下的河口向海的泥沙净输运，而受到抑制的潮汐作用(如潮 差减小、潮流减弱、潮量减小)可能减弱泥沙的净输运。由此综合表明：中等径流和强 潮作用对维持较强的河口泥沙净输运及较深的河道水深具有重要意义。 以上模型测试结果在简化了的长江河口动力地貌模拟上得到初步验证。长江河口口内 (南支及以上)地貌模拟验证较好，表明其受径潮流控制的泥沙运动(sand)及岸线束缚的作 用。而开敞口门的分汊河道特征尚需进一步模型测试和验证。该研究简化了层化环流、风浪、 沿岸水流和泥沙粘性(mud)等因素，这些因素对大空间和长时间尺度的河口动力地貌发育和 演变的影响亟待进一步系统研究。
Abstract
……….…....….…IXSamenvatting
……….……….….…....……XI中文摘要
……….………....…...… XIIITable of Contents
……….……….………....……..……XV1. Introduction
...11.1. Definition and classification of estuaries ...2
1.2. Driving forcing and morphodynamics...3
1.2.1. River flow and tides ...3
1.2.2. Estuarine morphodynamics ...4
1.3. Morphodynamic modeling...6
1.4. Objectives and research questions ...9
1.5. Thesis organization ... 11
2. Inspiration from the Yangtze River estuary
... 132.1. Introduction...14 2.2. Driving forces ... 15 2.2.1 River discharge... 15 2.2.2. Sediment discharge ... 17 2.2.3. Tides...18 2.2.4. Estuarine circulations... 20
2.3. Sediment transport dynamics ...21
2.4. Morphodynamics...21
2.4.1. Millennial geomorphological evolution ...21
2.4.2. Centennial morphodynamic evolution ... 23
2.5. Concluding remarks... 26
3. River tidal dynamics
... 273.1. Introduction... 28
3.2. Setting, data and methods ... 31
3.2.1. Introduction to the Yangtze River estuary ... 31
3.2.2. Data source ... 33
3.2.3. Tidal analysis methods ... 33
3.2.4. Numerical model setup ...35
3.3. Data analysis results... 38
3.3.1. Subtidal variations ... 38
3.3.2. Time-frequency spectra of tidal species ... 40
3.3.3. Time-frequency spectra of tidal constituents... 43
3.3.4. Time-frequency structure of tidal currents ... 46
3.3.5. Numerical model results ... 48
3.4. Discussion ... 51
3.4.1. Non-linear tidal interactions... 51
3.4.3. How important is friction to tidal dynamics? ...53
3.4.4. Implications of river tidal dynamics...54
3.4.5. Thoughts about river tide analysis ...56
3.5. Conclusions...56
Appendix A. Harmonic analysis results ... 59
4. Role of tides
...63 4.1. Introduction... 64 4.2. Model setup... 66 4.3. Model results... 68 4.3.1. Morphodynamic development... 68 4.3.2. Tidal hydrodynamics... 70 4.3.3. Modeled TRST ...75 4.3.4. Analytical TRST ...76 4.4. Discussion ... 804.4.1. TRST by multiple tidal asymmetries ... 80
4.4.2. Impact of river flow on TRST...81
4.4.3. Rethinking the concept of representative tides... 83
4.4.4. Feedback to reality... 84
4.5. Conclusions... 85
5. Role of river discharge magnitude
... 875.1. Introduction... 88
5.1.1. Tidal hydrodynamics and sediment transport... 88
5.1.2. Modeling efforts on estuarine morphodynamics ... 89
5.1.3. Aim and methodology...91
5.2. Model setup...91
5.3. Model results ... 93
5.3.1. Hydrodynamics of the schematized model... 93
5.3.2. Morphodynamics of the schematized model... 94
5.3.3. Tidal residual sediment transport... 96
5.3.4. Mechanism analysis...97
5.4. Discussion ... 101
5.4.1. Impact of basin geometry ... 101
5.4.2. Role of river discharge...102
5.4.3. Morphodynamic equilibrium ...104
5.4.4. Shape of equilibrium profiles...106
5.5. Conclusions...107
6. Impact of river discharge seasonality
...1096.1. Introduction... 110
6.2. Model setup... 112
6.2.1. Model schematization ... 112
6.3. Model results... 116
6.3.1. Sediment transport and fluxes ... 116
6.3.2. Morphodynamic sensitivity of hydrograph schematization .... 118
6.3.3. Morphodynamic sensitivity of hydrographs ... 119
6.4. Discussion ... 121
6.4.1. River-tide interactions ... 121
6.4.2. Hydrograph schematization ...122
6.4.3. Morphodynamics by seasonally varying river discharges...123
6.4.4. Impact of extreme floods ...126
6.4.5. Thought about MF approach... 127
6.5. Conclusions...128
7. Fluvio-deltaic morphodynamics
...131 7.1. Introduction ...132 7.2. Model descriptions...134 7.3. Model results ...136 7.3.1. Morphodynamics ...1367.3.1.1. Morphodynamic sensitivity to river discharge ...136
7.3.1.2. Morphodynamic sensitivity to tides ... 141
7.3.1.3. Hypsometry...142
7.3.1.4. Flat areas and channel volumes ...145
7.3.2. Tidal hydrodynamics ...147
7.3.3. Residual currents and residual sediment transport...148
7.3.4. Sediment budget ...152
7.4. Discussion...153
7.4.1. Channel-shoal patterns ...153
7.4.2. Impact of river discharge 0n 2D morphodynamics ...158
7.4.3. Impact of tides on 2D morphodynamics ...159
7.4.4. Morphodynamic equilibrium in a fluvio-deltaic system ...160
7.4.5. Transition between river and tide dominance ...163
7.5. Conclusions ...164
8. Modeling the Yangtze River estuary
... 1678.1. Introduction...168
8.2. Model setup... 171
8.2.1. One-dimensional model setup ... 171
8.2.2. Two-dimensional model setup ... 172
8.2.3. Model calibration ...174
8.3. Model results... 175
8.3.1. One-dimensional modeling of the South Branch... 175
8.3.2. Two-dimensional modeling of the South Branch...176
8.3.3. Two-dimensional modeling of the entire estuary ... 180
8.4. Discussion ... 181
8.4.2. Channel patterns in river-influenced estuaries...182
8.4.3. Morphodynamic time scales and equilibrium ...183
8.5. Concluding remarks...184
9. Conclusions and reccomendations
... 1879.1. Concluding remarks... 188
9.1.1. Introduction ... 188
9.1.2. Answering the research questions... 188
9.1.3. Overall conclusions...190
9.1.4. Implications for the YRE ... 191
9.2. Recommendations...192
References
………...………….….195Acknowledgements
………..………..…….……….211Exposure
……….………..….……..……...213Chapter 1
Introduction
This chapter briefly introduces definition and classification of estuaries and the physical driving forces, mainly river flow and tides, followed by descriptions of estuarine morphodynamics with emphasis on the channel-shoal patterns. Morphodynamics modeling techniques are then discussed with details on the time scale gap between hydrodynamics and morphodynamics, the morphological updating approach, and other relevant issues. Before the end, research questions and objectives addressed in this thesis are proposed and also the structure of this thesis.
1.1. Definition and classification of estuaries
Estuaries are the regions where rivers meet the sea and where freshwater and saltwater interact (Perillo, 1995). Located at the interface of land drainage basins and open seas/oceans, estuaries form a dynamic dispersal point of river-derived sediments. Estuaries are of widely recognized social, economical and ecological importance. Estuaries and associated deltas are the habitats for human beings, a variety of wildlife and species, for human settlement and liquid waster disposal. Environmentally, estuaries are vulnerable to alteration of river flow, water pollution, habitat degradation and destruction, and sea level rise.
Definition of estuary is widely reported. Dionne (1963) defined an estuary as an inlet of the sea, reaching into the river valley as far as the upper limit of the tidal wave.
Cameron and Pritchard (1963) defined an estuary as a semi-enclosed coastal body of
water with free communication to the ocean, within which ocean water is measurably diluted by freshwater derived from land drainage. Dalrymple et al. (1992) proposed that an estuary is the seaward portion of a drowned valley system which receives sediment from both fluvial and marine sources, whereas sedimentary facies are influenced by tides, waves, and fluvial processes. Overall, estuaries are the transitional regions between rivers and seas, where both fluvial and marine processes are active. By that estuaries are found worldwide (Figure 1-1).
Estuaries are classified into different categories by their physical nature, e.g., general physical behavior, formation, or geomorphology (Dyer, 1997). By tidal strength, an estuary is classified into micro-tidal (a mean tidal range of 0~2 m), meso-tidal (2~4 m) and macro-tidal (>4 m) types (Davis, 1964). Strong tides leads to formation of tidal morphological features such as funnel-shaped plan form, meandering tidal channels and tidal flats, linear sand bars or sand ridges, tidal creeks, (ebb- or flood-) tidal deltas.
Figure 1-1. Worldwide distribution of rivers, estuaries, and deltas. The image is directly
A triangular classification of estuaries/deltas into wave-, tide-, and river-dominated types is widely accepted (Galloway, 1975, Figure 1-2a). River dominated estuaries exhibit more distributaries with marshes, bays, or tidal flats in the inter-distributary regions, such as the Mississippi River mouth. Wave dominated systems have more regular shorelines, beach ridges, like the Sao Francisco estuary. Tide-dominated estuaries are characterized by large tidal ranges and strong tidal currents, which leads to an estuarine bay filled with many stretched islands parallel to the tidal flow and perpendicular to the shoreline, e.g., the Brahmaputra delta (Seybold et al., 2007). A fourth dimension is added by considering the time during which progradation or transgression happens (Dalrymple et al., 1992; Figure 1-2b). In that sense, an estuary is an unfilled pre-delta system.
More than one primary forcing can be present in the real world. Estuaries influenced by comparable strength of river and tides are the focus of this study. In such circumstances, an estuary can be highly stratified (a salt-wedge), partially stratified or partially mixed, weakly stratified or well-mixed (Pritchard, 1955;
Cameron and Pritchard, 1963). In this work, we assume a well-mixed situation
throughout this thesis thus salinity intrusion, stratification dynamics and associated density currents are excluded.
Wave Tide River Delta Estuary Estuary Delta Strand plain/ Tidal flats Delta Tidal flat Strand plains Wave
dominated Tide dominated
Time
(progradation
) Wave Tide River Delta Estuary Estuary Delta Strand plain/ Tidal flats Delta Tidal flat Strand plains Wave
dominated Tide dominated
Time
(progradation
)
Figure 1-2. The ternary classification of estuaries and deltas: (a) from Seybold et al. (2007),
and (b) modified from Dalrymple et al. (1992). The red dots indicate the position of the Yangtze River estuary/delta.
Estuaries are also classified by their geomorphological features, as coastal plain estuaries, rias, and fjords (Davidson and Buck, 1997). Coastal plain estuaries are formed by infilling of pre-history drowned river valleys and they are dominantly funnel-shaped in plan form (Dyer, 1997), Coastal plain estuaries are studied extensively because of their wide distribution and significance for human settlement and port development. The Yangtze River estuary (YRE) in China is a typical funnel-shaped coastal plain estuary influenced by a large river flow and strong tides, thus providing an ideal system for this study (see chapter 2).
1.2. Driving forcing and morphodynamics 1.2.1. River flow and tides
River flow and tides are two primary forces driving water motion and sediment
transport in estuaries. River flow enhances tidal currents and induces seaward residual currents and likely associated seaward residual sediment transport. River supplied sediment can be the major material source in estuaries and deltas. River flow is one prominent process flushing sediment to the sea and shaping river deltas (Wright and Coleman, 1973). Specifically, an episodic high river discharge may play a profound role in governing morphodynamic evolution both in fluvial environments (Leopold et al., 1964) and in estuaries (Cooper, 1993, 2002; Yun, 2004; Gould et al., 2009). Field experiences have suggested the potentially crucial importance of river flow and river floods in controlling estuarine morphodynamics case by case, but it is insufficiently understood how river discharge (its magnitude and variations) influences estuarine morphodynamics.
Tides induce periodically reversed currents and water level variations in estuaries. Tides are responsible for a greater fraction of sediment-transporting energy in causing bidirectional sediment transport compared to that in rivers. Tidal currents redistribute river-supplied sediments, creating a funnel-shaped geometry and stimulating formation of coexisting ebb and flood channels and associated channel-shoal patterns. Tidal asymmetry is one controlling mechanism driving tide-averaged sediment transport and resultant morphodynamic change. A tidal estuary in the absence of intertidal flat and with a minor river discharge is prone to have a flood tidal asymmetry due to landward shallowness (Speer and Aubrey, 1985;
Lanzoni and Seminara, 2002). A flood tidal asymmetry favors landward net
sediment transport while an ebb tidal asymmetry benefits seaward net sediment transport. Note that tidal asymmetry can be generated by a variety of mechanisms, such as tidal wave distortion because of shallow waters (Friedrichs and Aubrey, 1988), linear tidal interactions (i.e., O1-K1-M2 interaction; Hoitink et al., 2003), tidal
flat storage (Friedrichs and Aubrey, 1988), and also river flow (Horrevoets et al., 2004). But it is still poorly known how combination of these tidal asymmetries affects estuarine morphodynamics.
1.2.2. Estuarine morphodynamics
When using the term ‘morphodynamics’ (instead of ‘morphology’), we emphasize (1) the dynamic evolution of morphology, and (2) the dynamic interaction between hydrodynamics and morphology. The latter operates through a series of feedback loops linked by sediment transport, which in turn results in morphological changes (Cater and Woodroffe, 1994). Thus morphodynamics are defined as the mutual adjustment of topography/bathymetry and fluid dynamics involving sediment transport (Wright and Thom, 1977).
Estuarine morphodynamics are the product of geological and climatic conditions, marine and fluvial forcing, and human interventions. Regional geological background, sea level variations, and sediment availability constrain the preliminary formation of estuaries (Inman and Nordstrom, 1971; Davis, 1994). Fluvial and marine forcing controls the long-term estuarine morphodynamic changes while human activities impose external disturbances (Lane, 2004; Thomas et al., 2002; Guillen and
changes provide the initial conditions upon which future evolutionary processes build (Cowell and Thom, 1994).
Estuarine morphodynamics are featured by individual morphological elements, such as tidal channels or inlets, tidal flats or sand bars or shoals, and ebb deltas. Distinct channel-shoal patterns are widely observed in reality, like meandering channels, braided channels, distributary channels, and elongate tidal sandbars (Dalrymple et al., 1992; ABPmer, 2008; Schuurman et al., 2014). Controls on the channel patterns include the planform of an estuary, and the combination of primary forcing with respect to river, tides, and waves.
F F F F F E E E E F F E E E F F E F F (a) (b) (d) (c) (f)
Ebb channel (E) Flood channel (F) (a) sea side E F E (e)
Figure 1-3. Sketches of channel patterns of (a) a long tidal basin, (b) circulating sediment
transport cells, (c) a meandering channel system, (d) an estuary-delta system, (e) a continuous river-estuary morphological pattern, and (f) distributaries in Wax delta. The capital letter E indicates ebb channels and F indicates flood channels. The curved arrows show the residual sediment transport patterns. Panel (a), (b), and (d) are redrew from van Veen (1950); panel (c) is from Ahnet (1960); panel (e) is from Dalrymple et al. (1992).
A number of channel patterns in tide-influenced systems are proposed based on extensive observation and field studies of real-world estuaries (van Veen, 1950;
Ahnert, 1960; Dalrymple et al., 1992). One typical example is a meandering channel
pattern featuring separated flood and ebb channels as that in the Western Scheldt estuary (Figures 1-3a, 1-3b, and 1-3c). River-dominated estuaries or deltas may consist of a number of distributary channels and sub-delta lobes (Figures 1-3d, 1-3e, and 1-3f). Tide-dominated estuaries/deltas have morphological elements such as ebb deltas located seaward of the estuary mouth or tidal inlet and flood deltas located
inside the estuary mouth (ABPmer, 2008). Meandering tidal channel patterns are reproduced by long-term morphodynamic simulations (Hibma et al., 2004; van der
Wegen and Roelvink, 2008 etc.) but not distributary channels in the open mouth
zones where estuarine-deltaic morphodynamics coexist.
Morphodynamic equilibrium is defined by stable morphodynamic behavior.
Wright et al. (1973) stated that ’the simultaneous, co-adjustment of both the process
and form had yielded an equilibrium situation’. A number of empirical relationships can be found in literature to quantify the approaching to equilibrium, such as that between tidal prism and cross-sectional area (of tidal linnets or tidal channels) (O’Brien, 1969; Jarret, 1976). Reduced energy dissipation, or decreasing entropy level are other indicators when exploring equilibrium conditions (van der Wegen et
al., 2008; Townend, 1999).
1.3. Morphodynamic modeling
Field measurements, physical modeling, and numerical modeling are prime measures in studying estuarine and coastal morphodynamics. Bathymetric measurements can date back to a century ago in some regions, such as in the Western Scheldt estuary (Dam et al., 2013), in the Mersey Estuary in UK (Thomas et al., 2002); but long-term bathymetric data are rare in many other estuaries. Physical modeling of morphodynamic development of river deltas is broadly reported (Martin
et al., 2009; Clarke et al., 2010) but less on estuaries, maybe due to the scaling
problem. Numerical morphodynamic modeling has its advantages in its feasibility, low cost, and easy management, thus it has undergone development in the past couple of decades.
Numerical morphodynamic models are classified into simple or complex, top-down or bottom-up, and behavior based or process-based categories (de Vriend, 1996; Huthnance et al., 2007). Long-term process-based morphodynamic models account for the feedback mechanisms between hydrodynamics and morphodynamics. The non-linear effects, notably the advection and friction effect, are nicely integrated thus no assumptions are needed. This is important for studies of the non-linear tidal dynamics because tidal asymmetry is strongly related to the non-linear processes.
One primary issue raised in long-term morphodynamic modeling is the time scale gap between hydrodynamics and morphodynamics (Figure 1-4). Davis (1994) stated that ‘any single coast is the result of processes at all three time scales: the slow geological processes of mountain formation and erosion that requires millions of years; the gradual sea level changes requiring thousands of years; and superimposed over these the day-to-day and year-to-year combination of long-term and short-term action of waves, currents, and tides’. In this study, we are concerned about large scale, long-term morphodynamic processes related to river flow and tides, whereas the geological condition and sea level variations are excluded. Note that simply summing up short-term erosion and deposition will not lead to long-term morphodynamics due to the dynamic interactions and non-linearity. Long-term morphodynamics are generally seen as a trend whereas shorter-term processes are seen as fluctuations superimposed on the present trend (Stive et al., 2002). To bridge the time scale gap,
one method is to decouple hydrodynamics and morphology so that the morphology remains unchanged during hydrodynamic calculation and the current field is assumed to be static when the morphology is updated (Wang et al., 1995). By frequent feedback between hydrodynamics and morphological changes, it is able to simulate nearly realistic long-term morphodynamic development.
0.1 mm 1 m 100 m 10 km 100 km 0.01 s 1 s 100 s Day Year Decade Century turbulence sea and swell infragravity waves tides sea level rise suspension events megaripples ripples cusps bars meanders delta basin river flow 0.1 mm 1 m 100 m 10 km 100 km 0.01 s 1 s 100 s Day Year Decade Century turbulence sea and swell infragravity waves tides sea level rise suspension events megaripples ripples cusps bars meanders delta basin river flow
Figure 1-4. The space and time scales of coastal dynamics and morphology, modified from
de Vriend (1991).
In this study we use the Delft3D software as a process-based morphodynamic modeling toolkit. The model system includes modules to calculate the hydrodynamics by fluvial process, tides and waves, and sediment transport and bed level changes (Lesser et al., 2004). In 2DH mode, the continuity and momentum conservation equations read as:
' ) ( ) ( q y hv x hu t ∂ = ∂ + ∂ ∂ + ∂ ∂ξ (1-1) x f M y u x u h v u u c x g y v v x u u t u = ∂ ∂ + ∂ ∂ − + + ∂ ∂ + ∂ ∂ + ∂ ∂ + ∂ ∂ ) ( ) ( ) ( 2 2 2 2 2 2 υ ξ (1-2) y f M y v x v h v u v c y g y v v x v u t v = ∂ ∂ + ∂ ∂ − + + ∂ ∂ + ∂ ∂ + ∂ ∂ + ∂ ∂ ) ( ) ( ) ( 2 2 2 2 2 2 υ ξ (1-3) 0 ) 1 ( = ∂ ∂ + ∂ ∂ + ∂ ∂ − y S x S t z x y ε (1-4) where
ξ water level with respect to the reference datum (mean sea level) (m) h water depth below mean sea level (m)
u depth averaged velocity in x direction (m/s) v depth averaged velocity in y direction (m/s) q′ river discharge per unit area (m3/s/m2)
g gravitational acceleration (m/s2)
cf dimensionless friction coefficient defined by cf=gn2h1/3
n Manning’s coefficient (s/m3)
υ eddy viscosity (m2/s)
ε bed porosity, default 0.4 (dimensionless) z bed level (m)
Mx external momentum source in x-direction (m/s2)
My external momentum source in y-direction (m/s2)
Sx sediment transport in x-direction (m3/m/s)
Sy sediment transport in y-direction (m3/m/s)
More details regarding the numerical scheme are referred to Delft3D-FLOW users’ manual (Deltares, 2011) and Lesser et al. (2004). A number of sediment transport formula are available in Delft3D from which we apply the Engelund and
Hansen (1967) formula because it is simple, it works and it is comparable with
analytical method. The formula reads as,
50 2 3 5 05 . 0 D C g U S S S s b Δ = + = (1-5) where: Ss and Sb are suspended load and bed load transport transports (m3/m/s),
respectively, U is current velocity (m/s), C is the Chézy friction parameter (m1/2/s),
Δ
is the relative density defined by (ρs-ρw)/ρw, and D50 is the median diameter ofbed material (m).
A robust bed level updating scheme, namely morphological acceleration factor approach, is imbedded in Delft3D to speed up morphodynamic evolution (Roelvink, 2006; Roelvink and Reniers, 2011). The MF approach works simply by multiplying sediment erosional/depositional fluxes in each hydrodynamic time step by a user-defined factor (the morphological factor, MF), thereby effectively extending the morphological time scales. It indicates that the bathymetry is updated frequently thus the feedback mechanism between currents and morphology is considered dynamically. The essence of the MF approach is as follows:
y x T x S y S MF Sbed bu bx hydro Δ Δ Δ Δ + Δ × = Δ ( , , ) (1-6) hydro mor MF T T = ×Δ Δ (1-7) where ΔSbed is sediment fluxes of bed load transport, Sb,u and Sb,v are bed load
transport vectors in the x and y direction, respectively, Δx and Δy are cell size of in the x and y direction, respectively, ΔThydro is hydrodynamic time scale, and ΔTmor is
morphodynamic time scale.
One assumption of the MF approach is that the bed level change in one tidal cycle is small compared to the water depth so that bed level changes can be extrapolated linearly (Roelvink, 2006). The advantages of the MF approach are: (1) it helps to achieve decadal to millennial estuarine and coastal morphodynamic modeling by bridging the time scale gap, and (2) it considers the feedback mechanism between currents and bathymetry. Note that the selection of a MF is sensitive to process and input reductions, thus it requires careful selection (Lesser et
al., 2004). A too large MF may lead to disastrous, unrealistic model results. Studies
have shown that the MF can be as large as 500 in some cases. Sensitivity simulations regarding MF selection are recommended. Up to now, the MF approach has been widely validated and deployed (van der Wegen and Roelvink, 2008; Ranasinghe et
al., 2011; Dastgheib et al., 2008; Walstra et al., 2013), indicating its effectiveness in
enabling medium- to long-term morphodynamic simulations.
Input and processes reduction techniques are introduced for long-term morphodynamic modeling, aiming to reduce model complexity. For instance, the full spectrum of tidal constituents is simplified to a limited number of representative tidal constituents (Latteux, 1995; Lesser, 2009), and the highly varying wave climate is schematized into representative wave groups (Dastgheib, 2012; Walstra et al., 2013).
Flooding and drying processes are addressed by removing grid cells that become dry when the bed level falls below a threshold and reactivating the cells that become wet. Dry cell erosion is achieved by introducing a portion of erosion in the nearby wet cell to the closest adjacent dry cells (Lesser et al., 2004; Deltares, 2011). Bed slope effects are considered by Bagnold-Ikeda approaches in the longitudinal and lateral directions (Lesser et al., 2004; van der Wegen et al., 2008). Neumann boundary conditions are prescribed on the cross-shore boundaries on the seaside (Roelvink
and Walstra, 2004; Roelvink and Reniers, 2011).
Morphodynamic modeling can be in 1D, 2D, or 3D mode to answer specific research questions. One-dimensional (1D) models are simple and can be used to study cross-sectionally averaged bed profiles in estuaries, 1D tidal networks, coastal profiles, and coastlines (Roelvink and Broker, 1993). Two-dimensional models (2D) capture morphological features such as channel patterns (Nicholson et al., 1997). Quasi-3D model takes into account the secondary flow, and fully 3D model can represent the effect of breaking waves on the currents profile and cope with density-driven flow etc. De Vriend (1997) proposed that there is a need for modeling-oriented research on 3D morphodynamic modeling. However, 3D morphodynamic modeling is still less documented because of its high computation cost and lack of knowledge on the degree to which 3D flow is important for long-term morphodynamic behavior. In this study, we use both 1D and 2D models because they meet our research demand.
The effectiveness of medium- to long-term morphodynamic modeling is widely accepted nowadays. The hindcast power of medium-term morphodynamic models is improved due to increasing understanding of estuarine physics (Dam et al., 2013;
external force changes in terms of sea-level rise and intensified human activities etc.
1.4. Objectives and research questions
Morphology is the arena to many estuarine functions. Understanding of estuarine morphodynamic development and the mechanisms governing morphodynamic changes is of vital importance for management initiatives. Numerical modeling has proven to be a powerful approach in studying morphodynamic development. Applications of long-term morphodynamic modeling, both in idealized and realistic cases, are increasingly reported. The effect of overtides and geometry (e.g., Schramkowski et al., 2002, 2004), the initial formation and development of 2D channel and shoal patterns (e.g., Hibma et al., 2003; van der
Wegen and Roelvink, 2008), the concept of morphodynamic equilibrium
(Schuttelaars and de Swart, 1996, 2000; Lanzoni and Seminara, 2002; van der
Wegen et al., 2008), the impact of sea level rise (Dissanayake et al., 2012; van der Wegen, 2013), and the effect of human interferences on large scale morphological
features in tidal basins (Dastgheib, 2012, Dam et al., 2013) etc. are widely reported. These extensive modeling efforts, however, are confined to tide- and/or wave-dominated, relatively short (<100 km) basins/estuaries. In this work, we aim to investigate the effects of river discharge and tides on long-term, large scale estuarine morphodynamics. The formulated research questions are as follows.
(1) How do the river and tides interact with each other in tidal rivers?
River tides are still rarely examined due to their strong non-stationary, non-linear behavior in response to highly variable river discharge as well as a lack of analyzing methods. Though it is known that river discharge has significant effect on damping, distorting and retarding incoming oceanic tides (Godin, 1985, 1991; Jay
and Flinchem, 1997), we have limited understanding about how tidal waves are
modulated by river discharge, how do the river and tides interact. To address these questions, we apply harmonic analysis and continuous wavelet transformation to long time series of water levels in the YRE and analyze river tidal dynamics. We also employ a numerical tidal model to explore the impacts of river discharge magnitude on tidal wave propagations.
(2) What is the impact of tides on estuarine morphodynamic behavior?
Oceanic tides consist of a number of tidal constituents in a wide frequency spectrum. In mixed tidal regimes, diurnal tides (e.g., O1 and K1) and semi-diurnal
tides (e.g., M2 and S2) are the most significant constituents. The effect of M2-M4,
M2-O1-K1 tidal interactions in generating tide-averaged sediment transport is known
(Hoitink et al., 2003) but it is poorly understood how these tidal constituents can influence estuarine morphodynamics. Recent work by Canestrelli et al. (2013) suggested that consideration of more tidal constituents can lead to deepened estuaries but without explaining the mechanisms and without providing an explanation. To link these studies and provide a synthesis for estuarine morphodynamics, we apply a 1D model driven by different tidal constituents to
explore the effects of multiple tidal asymmetries on morphodynamic behavior.
(3) What is the role of river discharge magnitude on long-term estuarine morphodynamics?
Morphodynamics in tide-dominated inlets, basins and estuaries are widely studied when river discharge is zero or minor. It is increasingly apparent that river discharge, as one of the primary forces shaping estuarine and deltaic morphodynamics, has significant effect in controlling morphodynamic development but it is unknown how and to what degree. Herein we aim at answering a question: how does river discharge control estuarine morphodynamics? Use is also made of 1D schematized long-term morphodynamic modeling.
(4) What kind of effect do seasonal river discharge variations and river floods have on estuarine morphodynamics?
River discharge may vary over a large range. It is unknown what impact the seasonal river discharge variations have on the long-term estuarine morphodynamics. In particular, exploring the role of high river discharge remains a challenge. As a prerequisite to address these questions, we first need to find out a way to schematize river discharge hydrograph when the MF approach (a MF>1) is used. We use a 1D model to evaluate the three methods to schematize hydrograph and based on that explore the impacts of river discharge variations and river floods on long-term estuarine morphodynamic behavior.
(5) What is the effect of river flow and tides on 2D morphodynamics in a fluvio-deltaic system?
Two-dimensional estuarine channel-shoal patterns are of particular importance for estuary’s management. The impact of river and tides on long-term estuarine morphodynamic pattern development needs a 2D model approach. In addition, development of channel-shoal patterns has a feedback on tidal wave propagation and tidal asymmetry, eventually influencing morphodynamic development. Inherently linked estuarine-deltaic morphodynamics are rarely examined. Thus, we build up a 2DH model consisting of a large river feeding an outer basin to examine fluvio-deltaic morphodynamics.
1.5. Thesis organization
This thesis in all has nine chapters (Figure 1-5):
(1) Chapter one, this introduction, presents an overview of general estuarine physics, process-based morphodynamic modeling techniques, and research questions.
(2) Chapter two is a review of processes in the YRE in terms of fluvial and marine forcing, sediment dynamics, and morphodynamic evolution. It provides a realistic ‘model’ for this study.
(3) Chapter three is about examination of river tidal dynamics in the YRE and that by using a numerical tidal model (research question 1).
(4) Chapter four studies the effects of multiple tidal asymmetries on tidal residual sediment transport and estuarine morphodynamics (research question 2).
(5) Chapter five deals with 1D simulations forced by different (constant) river discharges (research question 3).
(6) In chapter six, we evaluate three methods to schematize a river discharge hydrograph in long-term modeling when using the MF approach. Then we examine the effects of seasonal river discharge variations on short- and long-term estuarine morphodynamics (research question 4).
(7) Chapter seven introduces 2DH morphodynamic modeling results on an idealized fluvio-delta system (research question 5). The sensitivity to river discharges and tides on channel patterns and 2D morphodynamic changes are investigated.
(8) In chapter eight, we validate the role of river flow and tides on YRE’s morphodynamics by using simplified 1D and 2D models.
(9) Chapter nine summarizes the whole work and makes suggestions for future study.
River tidal dynamics
Tidal asymmetries River-Tide interaction River flow 1). Tidal asymmetries: 2M2-M4; O1-K1-M2; M2-S2-MS4… 2). Tidal strength: Strong tides vs. Weak tides
1). River flow magnitude: Low vs. Medium vs. High 2). Seasonal variations: intra-annual balance 3). River floods 1). Stoke’s flow interact with tidal currents; 2). River flow interacts with tidal currents Conclusions
(data analysis, chapter 3)
The Yangtze
River estuary (theme, chapter 1)
(Inspiration, chapter 2)
(chapter 4) (chapter 5) (chapter 5 and 6) 1D morphodynamic modeling
2D morphodynamic modeling (chapter 7)
fee d back to rea lity (chapter 8 ) River-Tide controlled estuarine morphodynamics (chapter 9) Residua l se dim ent tra n sp ort
Chapter 2
Inspirations from
the Yangtze River estuary
The Yangtze River estuary (YRE) is a large scale, river- and tide-controlled, coastal plain estuary. River discharge and tides form the primary forcing, while wind and waves play a role as well but waves are poorly studied. River discharge supplies huge amount of sediments to the river mouth and flushes sediment downward. Nearly half of the river-supplied sediment deposits around the estuary, building up a mega-delta, and remainders are transported offshore. Tidal waves and currents propagate landward and interact with river flow, creating stratification and gravitational circulations. Strong tidal currents redistribute sediment and stimulate formation of tidal channels, flats and shoals. Meandering channel patterns took shape inside the YRE, whereas the open mouth zone is characterized by limitedly bifurcated channels with neighbored broad tidal flats. The mechanisms shaping morphodynamics in a river-influenced estuary like the YRE are still insufficiently understood.
2.1. Introduction
The Yangtze River, also called the Changjiang River (means a long river in Chinese), is the longest river in China and also among the longest rivers on earth. Located in the middle China and springs from the Tibetan high land in the west and flows eastward, the Yangtze River has a mainstream length of ~6,300 km, a basin area of 1.8 million km2 (20% of the China’s territory area). The Yangtze River basin is
home to 0.45 billion people (one third of China’s population). Economically, about 41% national GDP is produced in the Yangtze River basin (in 2007). The huge river flow (904.5 km3 per year) accounts for 37% of national river runoffs. These figures
apparently indicate the importance of the Yangtze River basin to China.
The Yangtze River meets the marginal East China Sea in the west Pacific. The Yangtze’s silt-laden water brings huge amount of sediment to the river mouth and builds up a mega delta. The Yangtze River delta starts to develop beyond Nanjing, from an initially incised deep and broad valley 7,500-6,000 years ago (Chen et al., 1985; Li et al., 2002; Song et al., 2013a). Nowadays there forms both mega sub-aerial (eastward of Nanjing) and sub-aqueous deltas covering an area of approximately 23,000 km2 and 29,000 km2, respectively (Li, 1986). The Yangtze River delta is the
place where a cluster of big cities, including the metropolis Shanghai, locate at. About 6% of Chinese population live here and they contribute about 20% of annual national GDP, also suggesting the importance of the Yangtze River delta.
Figure 2-1. Topography of the YRE with a bathymetry in 1997. The bathymetry is
measured below local datum, i.e., lowest low water level. The mouth zone includes the region seaward of the SB and South Channel and until the 10 m isobath.
The Yangtze River estuary (YRE) rests upon the delta and stretches downstream of Datong and seaward until the region where the Yangtze plume becomes invisible. By that the YRE has a length more than 800 km in a broad mean so that more than one tidal wavelength can be accommodated. Datong is the tidal wave limit where
river discharge is regularly monitored; and Jiangyin is the tidal current limit (about 405 km downstream of Datong) upstream of which the currents never reverse in direction under medium discharge and tides.
Large river flow and strong tides are dominant forcing in the YRE while monsoon driven wind, waves and storm events have effect as well. Nearshore, the Yangtze plume, coastal currents, and offshore shelf currents interact near the YRE. The present YRE is characterized by multiple bifurcations into four major channels (Figure 2-1). It first divides into the North Branch (NB) and the South Branch (SB) by the Chongming Island. Nowadays the SB receives the major portion of ebb discharge whereas the NB is influenced by limited fluvial forces and it is a tide dominant environment. The SB splits into the North Channel and South Channel by the Changxing and Hengsha Islands. The South Channel is further divided into the North Passage and South Passage by the Jiuduan shoal (Figure 2-1). In the mouth zone, broad tidal flats and shoals develop between the channels, such as the eastern Chongming flat, the eastern Hengsha flat, the Jiuduan shoal and the Nanhui marginal flat (Figure 2-1). The higher parts of these flats are colonized by vegetation, e.g., reeds and Spartina (Yun, 2004). The channels in the mouth zone are overall shallower, usually less than 9 m below reference datum, than the landward SB and than the seaward sea, thus behaving as a transverse mega bar blocking the navigation channel connecting the sea and the inland river.
Studies of hydrodynamics and morphodynamics of the YRE are of wide interest and concern in broad subjects because its social, economical and ecological value and functions. However, ‘dominated by huge river flow and macro-tides and moderate wave, the sheer size of the YRE, plus variations in river flow, tidal stage and phase, as well as the bifurcations of flow through various channels, combine to create a complex hydrographic and sedimentologic environment, which are only being understood in a semi-quantitative sense’ (Milliman et al., 1985). Though increasing knowledge is obtained in the past three decades, there are still big gaps between what we know and what we do not know, for instance, regarding its long-term morphodynamic behavior. In that sense, herein we provide a brief introduction to demonstrate its sophisticated complexity in terms of driving forcing, sediment transport and morphology.
2.2. Driving forces 2.2.1 River discharge
River flow is one of the prime forcing controlling saltwater intrusion, water residence time, and morphological changes in the YRE. The yearly averaged river flow is 903.4 km3 or a mean river discharge of 28,600 m3/s at Datong (1950-2005)
(CWRC, 2010). Maximum daily discharge is recorded by 92,600 m3/s (on 1 Aug 1954)
and the minimum is 4,620 m3/s (on 31 Jan 1979). Maximum monthly discharge is
84,200 m3/s (in Aug 1954) and minimum monthly discharge is 6,730 m3/s (in Feb
1963) (Zhang et al., 2003). The big variation range of river discharge suggests the occurrence of extreme floods or droughts potentially. Long time series data suggest that the yearly river flow varies between 667.1 and 1359.0 km3 between 1950 and
2012 at Datong. Extremely big floods occurred in 1954 and 1998, and droughts in 1978 and 2006 (Figure 2-2). No evident decreasing or increasing trend is detected during this period (Shen et al., 2001a; Yang et al., 2005a; CWRC, 2010) even in the century time scale based on regressed data since 1865 (Yang et al., 2005a).
0 200 400 600 800 1000 1200 1400 1600 1950 1960 1970 1980 1990 2000 2010 Year R iv er fl ow (k m 3 ) 0 100 200 300 400 500 600 700 800 S ed im ent lo ad ( M tons ) -River flow Sediment load
Figure 2-2. Inter-annual variations of yearly river flow and sediment load at Datong
between 1950 and 2012. Data are from CWRC (2012) and Yun (2010).
Precipitation and river flows in the Yangtze River basin are featured by a wet season in summer (during May and October) and dry season in winter (during November and April) because of the influence of Asian monsoon. About 71% of the annual water is discharged in the wet season at Datong (Chen et al., 2007; Figure 2-3a). Mean discharges in the wet and dry seasons are typically 40,000-60,000 m3/s
and 10,000-20,000 m3/s, respectively. Based on the daily discharges between 1980
and 2012, we estimate the possibility of a discharge between 10,000 and 50,000 m3/s
is 83%, less than 1% for <10,000 m3/s, and 17% for > 50,000 m3/s (Figure 2-3b).
0 20 40 60 80 100 0 20 40 60 80 100 Discharge (103 m3/s) C um ul at ed f requen cy ( % ) (b) 01/01 03/01 05/01 07/01 09/01 11/01 01/010 20 40 60 80 100 Date (mm/dd) Di sc ha rg e ( 10 3 m 3 /s) (a)
Figure 2-3. (a) Intra-annual variations of daily mean discharges, and (b) cumulative
frequency distribution of daily mean discharges at Datong based on the data between 1980 and 2012.
2003 onwards modulates river flow processes in the Yangtze River. It reduces flood peak discharges by up to 20,000 m3/s whereas dry season discharges is less
influenced at Datong though the Three Gorges Dam averagely discharges 500-3,000 m3/s more water in the dry season (Guo et al., 2014a). Time series of yearly flood
peak river discharge at Datong suggest a sharp decreasing in the early 2000s, because of increasing dam regulations over the Yangtze River basin (Guo et al., 2014a). Also note that water withdrawal along the river downstream of Datong may reduce river flow reaching the lower estuary (<10%) (Zhang et al., 2003).
2.2.2. Sediment discharge
The Yangtze River delivers large amount of sediment into the estuary and further to the outer sea. River-supplied sediment is the major sediment source that fills the pre-delta valley and builds up the delta. Annual sediment load is measured by 414 million tons with a mean suspended sediment concentration of 0.581 km/m3 at
Datong (1950-2005) (Yun, 2010; CWRC, 2010). Maximum and minimum yearly sediment load is 678 million tons in 1964 and 84.8 million tons in 2006. A dramatic decreasing of sediment discharge is detected since 1980s, which is mainly caused by soil conservation measures and dam constructions (Yang et al., 2006, 2011; Guo et
al., 2014a; see Figure 2-2). The yearly averaged sediment discharge is only about 143
million tons between 2003 and 2011, less than half of the mean value between 1950 and 2005. The sharp sediment load decreasing tends to slow down the accretion rate or even induce erosion in the YRE (Yang et al., 2011). But the long-term effect on the estuarine morphodynamics is not well studied to the author’s knowledge.
0 10 20 30 40 50 60 70 1 2 3 4 5 6 7 8 9 10 11 12 Month Se di m ent di sc ha rg e (t on/ s) --0 10 20 30 40 50 60 70 1 2 3 4 5 6 7 8 9 10 11 12 Month (a) (b)
Figure 2-4. Variability of monthly sediment discharges at Datong (a) between 1950 and
2005 (Data are from Zhao, 2006), and (b) between 2003 and 2012. Data are from CWRC (2010, 2012).
Sediment transport also concentrates in the wet season, with more than 87% is delivered between May and September (Chen et al., 2007; Figure 2-4a). Figure 2-4b also reveals significant sediment discharge reduction in the wet season since 2003 because of the impact of TGD operation. Sediment rating curves indicate that sediment discharge increases exponentially with increased river discharge (Figure 2-5). Xu et al. (2005) simulated that the sediment fluxes during the 2.5-month flood season in 1998 was 450 million tons at Datong, which is about 3.8 times of that
