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Influence of anthropogenic activities
Human activities along the coast (including land reclamation, port development, wind-farms im-poundment), within river catchments and watersheds (dam impoundments, river diversion, ir-rigation) and offshore (dredging, sand mining, oil and gas extraction) in combination with the above mentioned natural forces often exacerbate coastal erosion. Furthermore, since the ongoing climate change may impact the frequency and intensity of extreme events such as typhoons, cli-mate change could also influence the erosion rate and sediment transport through the wave and current actions.
Increases of erosion rates are particularly observed in Asia. The Jiangsu Province in China suffers from erosion rates as high as 85 m y-1 (Bilan, 1993). The author attributes these rates to river damming and diversion that lead to decrease in sediment supply to the coast, and to the clearing of mangrove forests which makes coastal areas more susceptible to hazards. Also due to mangrove clearing, 30% of the Malaysian coastlines are estimated to be undergoing fast erosion (Othman 1994). Thampanya et al. (2006) measured a net erosion rate of 1.3 to 1.7 m y-1 along the southern coast of Thailand and attributed it to combines effects of dam impoundments, clearing mangrove forests and coastal land subsidence. Since the early 20th century, southern Vietnam has been eroded at a rate of » 50 m y-1, mostly due to waves and currents actions on a vanishing mangrove vegetation (Mazda et al., 1997; Cat et al., 2006).
The hydro-sedimentary processes detailed above (SPM transport, erosion, deposition, floccula-tion, ETM) are key factors triggering the sediment dynamics in the Red River Delta and Gulf of Tonkin system, from estuarine and coastal to regional scales. The following section presents the characteristics and current knowledge regarding hydro-sedimentary dynamics of our study area.
Regional settings and scientific questions in our study area : the Red River Delta to Gulf of Tonkin system
This section highlights the regional settings of the study area, which extends from the Red River Delta (RRD) to the Gulf of Tonkin, called more globally GoT afterwards. First, a general overview of the characteristics of the GoT is presented. Then, the focus is made on the shelf hydrodynamics, and lastly the hydrodynamics and SPM dynamics in the estuarine and coastal areas are presented. The scientific questions raised by the presentation of the study area and to which this thesis will respond are presented in boxes.
General description of the Gulf of Tonkin
The GoT covers an area of 115000 km² from about 16– 10’-21– 30’N and 105– 30’-111– E (Fig. I.9). This crescent-shape semi-enclosed basin, also referred as Vinh Bac Bo in Vietnamese or as Beibu Gulf in Chinese, is 270 km wide and 500 km long and lies in between China to the North and East, and Vietnam to the West. It is characterized by shallow waters as deep as 90 m and is open to the South China Sea (SCS, or Vietnamese East Sea) through the South of the Gulf and to the East through the narrow Hainan Strait. This latter, also known as Quiongzhou Strait, is on average 30 km wide and 50 m deep and separates the Hainan Island from the Zhanjiang Peninsula (mainland China). The bottom topography in the GoT and around Hainan Island is rather complex, con-stantly changing, especially along the coastlines, and partly unknown. Furthermore, the Ha Long Bay area counts about 2000 islets, also known as notched, sometimes no bigger than a few hun-dred square meters. Lastly, the Red River Delta (RRD) coastlines is composed of low-lying plains and tidal flats (Tong Si Son, 2016).
Both Red River and RRD are located in a tectonically active region, and the middle and lower parts of the river flows along the NW-SE elongated fault-bound structure of the Cenozoic Song Hong Basin (Wetzel et al., 2017). Geological, geomorphological and archaeological studies sug-gest that the evolution of the delta itself was closely related to the GoT sea-level changes during the Holocene (12000 years BP to present) (Hoekstra and Weering, 2007). In early Holocene and in response to the inundations attributed to this period, the delta propagated into the drowned val-ley. In mid-Holocene, the sea-level rising (2 to 3 m above the present sea level), allowed widespread mangrove development on the delta plain, as well as the formation of the famous marine notches in Ha Long Bay and Ninh Binh areas (Hori et al., 2004). In recent Holocene (4000yrs BP), the for-mer delta plain emerged as the results of sea-level lowering, and the delta changed into the present wave and tide-influenced delta (Tanabe et al., 2003). The northern part of the RRD, which is tide-dominated, developed from 2000 to 1000 years BP. About 1000 BP, depositional processes shifted southwards, leading to the construction of the southern delta which appears to be dominated by fluvial and wave-induced morphological features. The actual shape of the RRD is therefore the product of thousands of years of fluvial sediment inputs but also, the product of reworking waves and currents, whose relative importance varies in time and space (van Maren et al., 2007).
At steady state, deltas are naturally slowly sinking due to sediment deposition. Natural subsidence rates in deltas generally range from 1 to 10 mm y-1 (Jelgersma, 1996; Stanley and Warne, 1998), even though accurate measurements remain rare. In combination with global rising sea-level, the subsidence phenomena may be enhanced by anthropic activities such as oil and gas extraction, sand mining, urban constructions, groundwater removal, decreasing sediment load by sediment trapping in reservoirs upstream or diversion of freshwater for households and land-uses (Syvtski et al., 2009). Accelerated subsidence has been documented in many deltas and can locally reach up to 300 mm y-1 (Haq, 1997). Ericson et al. (2005) classified 40 deltas throughout the world based on an assessment of the effective sea-level rise (ESLR) defined by the combination of eustatic sea-level rise, the natural rate of fluvial deposition and subsidence, and the accelerated subsidence due to groundwater and hydrocarbon withdrawal. The RRD appeared to be only subjected to eustatic sea-level rise, whereas neighboring deltas were subjected to sediment trapping (e.g. due to reservoirs) or accelerated subsidence (e.g. by groundwater use). These results should be taken into consideration with a careful attention since the methodology described in this study relies on a number of assumptions, including a uniform eustatic sea-level rise. However, is it known that the SCS sea-level rising rate is 3 times faster than the global mean rate (Dieng et al., 2015; Piton and Delcroix, 2018).
The GoT climate, as well as the South China Sea (SCS) region climate, is mainly governed by two alternative monsoon cycles, characteristics of subtropical climates. A southwest (SW) monsoon during boreal summer and a northeast (NE) monsoon during boreal winter (Wyrtki 1961; Wang and LinHo, 2002). A detailed view of this reversal of wind direction from winter to summer in the GoT is shown in Fig. I.10 a.
The winter monsoon lasts from fall to winter with almost homogeneous NE wind direction over the GoT (Fig. I.10 b, e), with mean speed peaks in winter of » 7 m s-1. In spring, the winds relax with mean maximum values » 4 m s-1 and change direction into becoming easterlies (Fig. I.10 c). The summer monsoon starts from June and lasts through September, and implies W-SW wind direction with peaks of » 5 m s-1 (Fig. I.10 d).
The winter monsoon originates from the Siberian High (Wang and He, 2012) and thus brings cold and dry conditions; whereas the summer monsoon originates from the Southern Pacific and In-dian Ocean (Lau and Yang, 1997), and thus bring warm and wet conditions over the GoT. Conse-quently, the GoT shows a strong seasonal rainfall variability: driest from January to April, with net water flux of » 3 mm d-1, and wettest in Summer and Fall (» 6 mm d-1) (Fig. I.11).
Figure I.11: Climatological monthly precipitation (in mm d-1) over the GoT for the period 2005-2017 from ECMWF.
At shorter time-scales, the GoT atmospheric circulation is under the influence of typhoons (or tropical cyclones), which hit northern Vietnam about 4-5 times every year and predominantly from June to November, when the ITCZ (Intertropical Convection Zone) is positioned in the SCS. The frequency of typhoons affecting the coast of Vietnam has increased during the second half of the 20th century (Thanh et al., 1997). Typhoons are usually generated over the western Pacific and are known to cause great destruction and death as they move over lands. The deadliest one occurred in 1881, killing 300000 persons while swiping away Haiphong harbor. Besides the se-vere impacts in terms of human loss, typhoons brings along high water levels, large waves, strong winds and heavy rain that can lead to flooding, dikes and sand dunes erosion, as well as saltwater intrusion in land crops (Vinh et al., 1996; Quynh et al., 1998). Furthermore, typhoons are asso-ciated with strong impulse of momentum, heat and water exchanges between the ocean and the atmosphere and thus can induce strong intraseasonal variations in the wind-driven circulation and in the thermohaline structure of the SCS upper layer (Chu and Li, 2000; Lin et al., 2003; Tseng et al., 2010). In the SCS, Typhoon Ernie (1996) triggered significant SST cooling and sea surface de-pression in the wake of the storm (Chu et al., 2000). Typhoon Pabuk (2007) largely intensified the near-bottom current along its route (from the Luzon Strait to the Hainan Strait) (Liu et al., 2011). Wang et al. (2014) also observed a strong deep SCS circulation induced by typhoon activities, which suggests that effects of typhoons can penetrate into the deep ocean.
Lastly, at interannual time scales, the SCS region is dominantly influenced by the El Niño South-ern Oscillation (ENSO) phenomena. Several studies have been conducted on interannual (ENSO) variability of the SCS region in terms of sea surface temperature (Huynh et al., 2016; Liu et al., 2014; Tan et al., 2016; Yang et al., 2015: Wang et al., 2006), sea level (Peng et al., 2013; Rong et al., 2007), surface winds (Huynh et al., 2016), precipitation (Juneng and Tangang 2005; Vinh et al., 2014; Räsänen and Kummu, 2013; Räsänen et al., 2016), cyclone frequency occurrences (Camargo and Sobel, 2005; Wu et al., 2005), river runoff (Räsänen and Kummu, 2013; Xue et al., 2011) and upwelling variability (Da et al., 2019). Furthermore, many studies suggested that the SCS sum-mer monsoon is modulated by ENSO such that El Niño/La Niña phases weaken/strengthen the summer monsoon with a later/earlier onset (Lau and Yang 1997; Chou et al., 2003; Liu et al. 2012; Dippner et al., 2013). While ENSO effects have been largely studied over the SCS, no studies (in english) have focused on ENSO effects over the GoT.
In addition, the Madden Julian Oscillation (MJO) and the Indian Ocean Dipole (IOD) also influ-ence the SCS interannual variability, and thus possibly the GoT interannual variability. MJO con-tributes to » 10% of the intra-seasonal anomalous precipitations over the SCS in summer (Zhang et al., 2009) while the IOD may impact the onset of the summer monsoon during non-ENSO years (Yuan et al., 2008). Again, skewing through literature, no studies have focused on the impacts of MJO and IOD over the GoT.
Both typhoon-induced variability and ENSO induced variability have been studied on various at-mospheric and oceanic parameters over the SCS, but remain undocumented in the GoT. In this low elevated and densely populated region, it is essential to understand and estimate regional im-pacts of short-term extreme events like typhoons to interannual variability factors like ENSO.
Red River – Taking its name from its brown-reddish colour, the Red River has its source in the mountains of Yunnan (China), where it is known as the Yuan River. It then flows into Vietnam and is renamed the Thao River. Once in Vietnam, it receives two major tributaries: the Da and the Lo Rivers, and continues flowing through seven Vietnamese provinces (Fig. I.12 a, b). With a total length of 1150 km and a watershed of about 169000 km² (Fig. I.12 a), the Red River system is the second largest river system of Vietnam and still is an important commercial and transporta-tion route. Downstream the confluence of the Da, Thao and Lo Rivers, the main river bifurcates into numerous distributaries feeding the Red River Delta and enters the GoT through nine main mouths (Fig. I.12 b).
Table of contents :
I.1 Complexity of hydro-sedimentary processes along the estuary – coastal ocean – open ocean continuum
I.2 Regional settings and scientific questions in our study area : the Red River Delta to Gulf of Tonkin system
I.3 Objectives and thesis outlines
II Study of the seasonal and tidal variability of the hydrology and suspended particulate matter in the Red River estuary with observations
II.1 Summary of Chapter II
II.2 Article under review in Journal of Marine Systems Seasonal and tidal variability of the hydrology and suspended particulate matter in the Van Uc estuary, Red River, Vietnam
III Bathymetry and bottom friction contributions to the tide representation in the Gulf of Tonkin
III.1 Summary of Chapter III
III.2 Article under review in Geoscientific Model Development: Sensitivity study on the main tidal constituents of the Gulf of Tonkin by using the frequency-domain tidal solver in T-UGOm
IV Hydrodynamics of the estuary – coastal zone – open sea continuum in the Gulf of Tonkin
IV.2 Material &Methods
IV.3 Configuration Optimization
IV.4 Evaluation of model results over the Gulf of Tonkin
IV.5 GoT surface circulation and fluxes
V Conclusion and future work
V.1 Results summary