This dissertation has been approved by the promotor: Prof. dr. ir. J.A. Roelvink

This study was supported by Shanghai Estuarine and Coastal Science Research Center and UNESCO-IHE Institute for Water Education. Funding was provided by UNESCO-IHE Partnership Research Fund (UPARF, No. 60038881), Shanghai Municipal Natural Science Fund of China under grant No. 11ZR1415800 and National Key Technology R&D Program of China under grant No. 2013BAB12B00.

Summary : Estuaries are natural highly dynamic and rapidly changing systems, comprising a complex
combination of physical processes on many different time- and space- scales. Fine sediment
physical processes are attracting increasing attention by coastal engineers. One reason is that we
need more coastal reclamations for sustainable society development and more and more harbors,
ports and navigational channels. These are increasingly constructed in those fine sediment
surroundings, where used to be considered as unfavorable place for waterway development, due
to the high possibility of confronting with sedimentation issue. Another reason is because we are
interested in the pursuit of the fascinating nature of fine sediment dynamics.
The Yangtze Estuary is an excellent example of a fine sediment estuarine system with a
moderate tidal range (~1-5 m) as well as a highly seasonally-varying (~10000-50000 m³/s)
freshwater inflow. The sedimentation ranks as a key issue in the Yangtze Estuary recently.
Before conceiving a measure to mitigate channel siltation, the reasons and mechanisms related
to the characteristics of sedimentation should be investigated as the first step. Thus,
understanding the underlying mechanisms associated with fine sediment transport, ETM
(estuarine turbidity maxima) dynamics and sediment trapping in the Yangtze Estuary is
considered as a major challenge to maintain the "golden waterway".
According to the systematic study on the topic of multiscale physical processes of fine
sediment in a meso-tidal convergent alluvial estuary, the main contents and conclusions are
summarized as follows. In Chapter 1, the complexity of fine sediment transport in estuaries is briefly identified
Apart from the interactions among riverine inflow, oceanic tide, wind wave, the Coriolis force,
saline water intrusion and bed resistance in an estuarine system, the micro-scale effects from
flocculation and hindered settling, baroclinic forcing, turbulence damping and drag reduction
show the obvious influences on internal structures of current, salinity and suspended sediment
concentration (SSC) which in turn have an impact on the macro-scale current and sediment
regimes and morphological evolution. After highlighting the unique features and the challenge
of the Yangtze Estuary, the main objective and organization of this study are introduced.
Chapter 2 focuses on the recent decadal hydrodynamic evolutions in the whole Yangtze
Estuary from a series of hydrological data. Over the past few years, the Yangtze Estuary has
witnessed an unprecedented scale of human intervention through extensive resource utilization.
We found that, (i) the water level along the main outlet of the Yangtze Estuary increased from
1998 to 2009; this increase was induced by the variation in the whole river regime (including
natural morphodynamic processes and local topography feedbacks from extreme meteorological
events and human activities); (ii) the decrease of the flow portion ratio at the 3rd bifurcation is
directly induced by the Deepwater Navigational Channel (DNC) project and the corresponding
morphological changes at the North Passage; and (iii) the estuarine environmental gradients
(salinity and suspended sediment concentrations) were compressed, and the fresh-salt gradient
became steeper. This has the indirect effect of back-silting on the waterway, i.e.,
strengthening the stratification effect near the area of estuarine turbidity maximum and
enhancing the tendency of up-estuary sediment transport. In Chapter 3, observations of storm-induced fluid mud dynamics have been conducted at the DNC of the Yangtze Estuary from October to December 2010, during the occurrence of a cold-air front. The observed data reveal that just after the critical wind wave event, a large
amount of fine sediment was trapped in a state of fluid mud along the channel. The observed
thickness of the fluid mud was up to about 1-5 m, which caused some significant economic and
safety problems for shipping traffic in the Yangtze Delta area. The mechanisms and transport
processes of the storm-induced fluid mud are analyzed and presented from the angles of both
process-oriented and engineering-oriented methods. With the help of tidal hydrodynamics and emodeling, cold be infeed the easier of the find event mainly depends on the overall hydralynamic regimes and the exchanges of sement which is released by serve agitation from jacent tidal flats. These sediments are Bad, and cyclate and persist at that with wegsies in the river and tie-dominated airy. In addition, the domlope son of feed is also thought to have stated and won the fluid evvel in this stay. Our modding real and observations deem that the transport of fadad is an advective phenodermining the lotion of fluid mayerang the channel, and it's also a tidal energy influenced phenomenon convinge andation of fluid ; and (5) both suspended periculate mater and local residual flow regiorical importance in dermining the ping of sediment and the cum offi blit In Chapter 4 spatial and empol med describing spring-visions of veloci, salinity and SSC in the DNC of the Yangtze Estuary were obtained in the wet of 2012. These wollected in the middle of the INC for the fin time, and document the foon of a rather stable demity sification interface and saltw especially during neptides and slack water. The convergent zone of residual currents, salinity and sediment during neap and ringde ees in the middle and lower each of the DN In the foon of big-55C low, which vessation in the drog channel. Both the bed grade dober and the lager Richar ically from around to to shaded from spring to neapsides. Sication and traffter the TM are indeve the upper half we of the water body to be dominant and the lower pan bebe-Theed reveal that the side putem of us, all fand sisted the saltyd back gradnj controlling the variation of vertical velocity stredob thered on gend by y pasa dominole in mingly the In Chapter 5, by means of and app, ingel (V) of the Yang was dry. The experimental dataSSC and all at Stone) the SV of urine fists and its controlling factors are highly dependent on speci vions; (4) the dependencies of various determinants Clay and temperatur SV free, and (v) for the Yang the SV pks when the SSC is in the range, and their flocculation sending apply and 10 pea in dry and w Charms to explore the backs of the - plical processes the findmy wits the plane of the Yingy. Though americal styly donate-dimension (10) dos mod, the effects of me physical poses read to focus and ficce are tied (Seating velocity is a separate determining the overall entrained and suspended the water. The effect of folation on sending velocity coals ETM ()sion in an ostry firstly creates logikadinal demity gradient which introducing a hardinc cffct. The direction of bric pressure genting i landaddy, therefore when the current velocity is elative small during slack and ncptides, the advefunction is comparable to the hardni preregarding water (specially near the river bell, so the internal for structure will belly, once the internal flow struct is changed, the up-try of sandy will be enhanced near the boom. This the sortion of d theta (salt) will elindekih The design forces haryancy effect on turbulence. This theclising of andelsleeping. The fing of high-in layer near the button. Manwhile, invelit ply with an incing SSC near the button, therefore the hindered sealing effect enhances the bottom turbidity and promotes the density stratification (including sediment and salinity density). Then again, sharper stratification will induce stronger turbulence damping and higher bottom SSC. The abovementioned snow ball effect is the primary micro mechanism of fine sediment dynamics in an estuary. (iv) In addition, with an increasing SSC near the bottom, local bed resistance will be decreased due to drag reduction, tidal stirring is amplified and the erosion capacity is enhanced accordingly. Therefore the aforementioned snow ball effect might be terminated under the competition between stratification and high-turbidity induced tidal amplification. Chapter 7 applied the above findings into the large domain model of the whole Yangtze Estuary and investigated the typical effect of seasonally varying river discharge, wind climate and mean sea level on the seasonal variation of ETM. From observation and modeling data, we concluded evidently that (i) Both tidal energy and density stratification enhance saltwater intrusion; (ii) Four independent factors (river flow, wind, mean sea level and water temperature) determining the seasonal sediment regime are identified; (iii) River discharge impacts the pattern of residual currents; (iv) Seasonally varying wind effect alters the longshore currents ; and (v) Seasonally varying mean sea level affects the saltwater intrusion length in the DNC. In this study, in short we highlight that multiscale physical processes jointly characterize the current and sediment regime in a fine sediment estuarine system ...