Seasonal Variations in Riverine Sediment Transport Timescales in the Pearl River Estuary

Abstract

Understanding sediment transport timescales is essential for predicting morphological evolution, pollutant accumulation, and ecosystem health in estuaries. This study examines seasonal hydrodynamics and sediment transport in the Pearl River Estuary using a well-calibrated numerical model. The results indicate that plume dynamics largely control sediment transport in both the wet and dry seasons. During the wet season, sediments are exported along both estuary flanks with the expanding freshwater plume. Under the combined effects of topography and the Coriolis force, a greater proportion of sediments exits via the confluence of the West Channel and West Shoal. In the dry season, prevailing northeasterly winds suppress sediment export along the East Channel, redirecting most of the riverine sediment westward. Sediment transport timescales, quantified by sediment age, further show that, during the wet season, export via the East Channel requires approximately 30 days, whereas export along the western flank takes about 45 days due to the weaker dynamics over the West Shoal. Reduced river discharge in the dry season increases sediment age overall; offshore delivery within the plume region takes roughly 50 days, while transport via the East Channel may require an additional 30–60 days. Comparative simulations with and without wind forcing reveal that southerly winds during the wet season weaken plume intensity and prolong transport timescales, whereas northeasterly winds in the dry season enhance plume dynamics, accelerating sediment export from the estuary. Collectively, these findings clarify the mechanisms underlying the seasonal variability in sediment transport and provide a scientific basis for estuarine management and engineering.

1. Introduction

Riverine sediment discharge into the coastal ocean is a fundamental component of the Earth’s material cycle, shaping the geomorphology of estuarine deltas and coastal zones [1] and supplying nutrients that underpin the productivity and biodiversity of marine ecosystems [2]. Organic matter and minerals transported with sediments stimulate phytoplankton growth, sustaining the base of the marine food web and supporting higher trophic levels [3]. Additionally, the timescales associated with sediment transport play a pivotal role in depositional processes and the maintenance of ecosystem stability. The duration required for sediment to reach various nearshore regions directly regulates the spatiotemporal distribution of suspended sediments [4], thereby influencing sedimentation patterns and habitat structure. Moreover, sediment transport duration governs the accumulation of heavy metals and nutrients in estuaries due to the cohesive nature of fine particles [5,6]. Therefore, a comprehensive assessment of riverine sediment pathways and transport timescales is essential for interpreting environmental change and guiding management in estuarine and coastal systems.

Riverine sediment discharged into the coastal ocean exhibits pronounced seasonal variability, primarily due to basin-scale precipitation, snowmelt, and anthropogenic activities such as reservoir regulation. For example, the Yangtze River delivers more than 60% of its annual runoff and approximately 80% of its sediment load during the wet season, whereas diminished discharge in the dry season leads to a marked reduction in sediment flux [7]. Similarly, the Mississippi River experiences increased discharge in spring as a result of snowmelt and rainfall, with sediment flux peaking between March and May and constituting over 40% of the annual total. By contrast, summer drought conditions reduce sediment loads to roughly 10% of the yearly flux [8,9]. Historically, the Nile River’s flood season (August–October) was responsible for transporting over 90% of its annual sediment load. However, following the construction of the Aswan Dam, the sediment flux declined dramatically, and seasonal variability was substantially attenuated [10,11].

The hydrodynamic regime of estuaries is highly sensitive to seasonal fluctuations in fluvial discharge, with a substantial influence on sediment transport and morphological evolution. In the Ganges-Brahmaputra Delta, wet-season floods generate supercritical jets that produce intense mixing layers over the continental shelf, thereby facilitating offshore sediment dispersal that can extend to the continental slope [12]. During the dry season, the system becomes tide-dominated, with spring tidal ranges exceeding 4 m. These dynamic tides generate vigorous tidal pumping that remobilize fine-grained particles deposited during the monsoon, advecting sediments landward through flood-dominated pathways [13]. Similarly, in the Colorado Estuary, the strong buoyancy input during the wet season intensifies estuarine circulation, trapping substantial amounts of sediment and promoting mouth bar formation [14]. Under low freshwater conditions, the Colorado Estuary becomes tide dominated, with energetic tides inducing coastal erosion at rates up to 2.5 m per month [15]. Although these studies elucidate key mechanisms governing seasonal sediment transport, targeted investigations into the seasonal variability in sediment transport timescales remain limited.

The Pearl River Estuary (PRE) experiences pronounced seasonal variability in river discharge and sediment load under the influence of the East Asian monsoon. During the wet season (April to September), the river supplies approximately 70–85% of its annual discharge and delivers over 84% of its annual sediment load into the Pearl River Delta. By contrast, from October to the following March, river runoff accounts for only 15–30% of the annual total, and sediment load declines to less than 16% [16]. These fluctuations in freshwater and sediment input significantly alter estuarine dynamics and sediment transport processes. In the wet season, the interaction between substantial freshwater input and saline coastal waters produces strong density gradients and robust estuarine circulation [17]. Sediments consequently accumulate near the density front, with peak suspended sediment concentration (SSC) in the mid-estuary reaching up to 0.3 kg during flood events [18,19]. This sediment trapping results in accretion rates of up to 8 cm per month within the estuary [20]. By contrast, reduced river discharge during the dry season increases the bottom shear stress generated by energetic tidal currents, leading to enhanced erosion, particularly along the main channels [21]. Wind patterns in the PRE also vary seasonally, with southerly winds dominating in summer, at magnitudes of 4–6 m/s [22]. These winds generate surface offshore and compensatory bottom currents, forming a clockwise circulation during the wet season that promotes sediment accumulation on the lower West Shoal and increases accretion rates by approximately 15% [23]. During the dry season, prevailing northerly winds (6–8 m/s) strengthen coastal currents, driving sediment transport southward along the western shore and amplifying channel scouring [24]. Previous studies have addressed sediment flux and morphology; however, the timescales of sediment transport in the PRE have rarely been quantified.

This study aims to systematically compare estuarine dynamics, sediment transport processes, and associated transport timescales between the wet and dry seasons, using the PRE as a representative case. The specific objectives are to: (1) characterize seasonal variations in estuarine hydrodynamics and the distribution of riverine sediments in the PRE; (2) quantify sediment transport timescales by analyzing sediment age; and (3) elucidate the controlling mechanisms governing sediment transport under both wet and dry conditions. By addressing these objectives, this research will advance our understanding of estuarine sediment transport and its coupled hydro-ecological processes, thereby providing a scientific basis for improved estuarine management and sustainable development.

2. Methodology

2.1. Numerical Model

The Environmental Fluid Dynamics Code (EFDC) was employed in this study. This model solves the three-dimensional continuity and Navier–Stokes equations under the free-surface assumption. The Mellor–Yamada level 2.5 turbulence closure scheme is implemented to calculate vertical turbulent viscosity and diffusivity [25,26]. The EFDC utilizes σ (sigma) coordinate systems in the vertical direction and curvilinear, orthogonal grids in the horizontal plane. It is capable of simulating density- and topographically induced circulations, as well as tidal- and wind-driven flows, and the transport of conservative substances, such as salinity and temperature. EFDC has been successfully applied to a wide range of aquatic environments, including rivers, estuaries, lagoons, and reservoirs [27,28,29,30].

The model domain for the PRE is illustrated in Figure 1a. To accurately resolve hydrodynamics and sediment transport, the domain encompasses the entire Pearl River Delta, thereby minimizing uncertainties associated with boundary conditions. The open-sea boundary extends to approximately the 50 m isobath offshore of the delta. The model grid comprises 243 × 294 cells, with the resolution ranging from approximately 100 m within the estuary to 800 m near the boundaries. Vertically, the water column is discretized into 16 sigma layers of equal thickness. The wetting–drying scheme was activated during simulations to improve the representation of hydrodynamic processes in shallow shoal areas. The model was driven by river discharges from upstream hydrometric stations (Wuzhou on the Xijiang River, Boluo on the Dongjiang River, and Shijiao on the Beijiang River, as shown in Figure 1a). Tidal elevation and salinity boundary conditions were derived from a larger-scale hydrodynamic model, which includes the inner continental shelf, estuary, and associated river networks [31]. Initial conditions for the simulation were also obtained from this larger domain model, rendering additional hydrodynamic spin-up unnecessary. The model operated with a time step of 30 s, and water temperature was held constant during the simulation, as stratification in the estuary is primarily controlled by salinity [32].

2.2. Sediment Age

The sediment transport module, developed by Lin and Kuo [33], was integrated into the hydrodynamic model using identical spatial and temporal resolutions. At the riverine boundary, SSC was prescribed continuously to represent riverine sediment input. Given that riverine sediments in the Pearl River are primarily composed of clay and silt, sediment discharged from the upstream area was treated as cohesive. The grain size distribution among the tributaries is similar, typically ranging from 6 and 32 μm, with a median grain size of approximately 8 μm [34]. Previous studies have demonstrated that the suspended sediment in the PRE is predominantly in the form of flocs [35]. Therefore, a relatively high settling velocity (${\omega }_s$ = 0.1 mm/s), as adopted in a previous study [36], was employed. The critical shear stress for erosion was set to 0.15 Pa, in accordance with observations and numerical simulations for the PRE [37]. The erosion parameter, as described in the well-established Partheniades [38] equation, was set to 3 × 10⁻⁵ kg/m²/s based on model calibration.

The concept of age was utilized in this study to quantify sediment transport timescales in the PRE. In this framework, age denotes the elapsed time since a water parcel was last in contact with its source [39,40]. When a tracer is introduced at the upstream boundary, the resulting age at any location reflects the transport time required for the water parcel to travel from the headwater to the location. By introducing the age concentration, a virtual ‘clock’ is assigned to each particle within the simulation, which starts running as soon as the particle enters the region under consideration. At any location in the domain, the age is computed as the ratio of age concentration to tracer concentration [41]. Originally, the concept of age was developed to estimate the transport timescales of dissolved material, and it has been widely applied to many coastal waters around the world [42,43,44]. Subsequently, Mercier and Delhez [4] refined this theory to establish the sediment transport timescales by accounting for the resuspension and settling processes. In their theory, the clock continues to run even when sediment particles settle and are deposited on the seabed. Upon erosion, particles are resuspended with the age being averaged for the bed sediments. The age concentration of sediment particles is governed by the same parameters as sediment transport processes, including settling velocity, critical shear stress, and the erosion parameter.

2.3. Model Scenarios

This study focused on the age of riverine sediments; therefore, the initial conditions prescribed no sediment in either the water column or on the seabed within the estuary. During the simulations, suspended sediment was introduced at the headwater. The clock attached to each particle started to run as soon as the sediments arrived at the four outlets of the PRE (Figure 1b), namely, Humen, Jiaomen, Hongqili and Hengmen. The year 2013 was selected for analysis because river runoff and sediment load during this period were close to the long-term mean values observed for the Pearl River, rendering it representative for assessing seasonal variability in sediment transport and associated timescales. Owing to the influence of the East Asian monsoon, river discharge and SSC exhibit pronounced seasonal variations. In this study, May, June, and July were designated as the wet season (Case 1), characterized by a mean discharge of 14,300 m³/s and an SSC of 0.22 g/L (Figure 2), while October, November, and December were selected to represent the dry season (Case 2), during which the average freshwater discharge and SSC decreased to 2070 m³/s and 0.08 g/L, respectively. We elected to simulate three-month windows within the wet and dry seasons, rather than the full seasons, because a three-month simulation is sufficient for sediment transport to reach a dynamic equilibrium (i.e., the computed SSC remains constant if it is averaged over relevant hydrodynamic timescales such as spring–neap tidal cycles). Previous research has demonstrated that wind also plays a crucial role in modulating estuarine circulation and sediment transport in the PRE [23,24]. To further investigate this effect, two additional simulations were conducted in which wind forcing was excluded during the wet (Case 3) and dry (Case 4) seasons, respectively. This experimental design enabled us to quantitatively assess how variations in wind forcing influence sediment transport in the PRE.

2.4. Model Calibration

The observed water levels from 1 December to 15 December 2013 were used for model calibration (see Figure 1b for the locations of tidal gauges). Calibration of water elevation was conducted by adjusting the bottom roughness coefficient to optimize agreement between the simulated and observed water levels, and the results indicated that a bottom roughness height of 2 mm provided the best fit. Comparisons of modeled and observed water level time series at four stations are presented in Figure 3. Overall, the modeled water levels are consistent with observations, particularly during spring tides, while discrepancies are primarily observed at the flood–ebb transitions during neap tides.

To calibrate tidal currents, salinity, and SSC, available in situ observations collected in the PRE during the summer of 2007 were utilized (see Figure 1b for measurement site locations). Comparisons between modeled and observed velocities and directions are presented in Figure 4. Overall, the model accurately reproduces flow direction and magnitude, although the modeled surface velocities are slightly underestimated. This bias is primarily attributed to the smoothing effect of interpolating terrain data onto the computational grid. The modeled salinity variations and surface-bottom salinity differences generally fall within the range of observed values (Figure 5). Discrepancies are most evident during the latter stages of the flood tide, when salinity peaks. Accurately predicting SSC at individual stations is challenging due to the influence of multiple local processes, such as resuspension, deposition and consolidation. In this study, flocculation was parameterized with a single settling velocity, while the settling of nonflocculated particles was neglected, thus shortening the inferred suspension time of fine-grained sediments in the water column. Due to the substantial differences in bed surface composition between shallow shoals and deep channels, using a uniform critical shear stress typically makes it difficult to accommodate all subregions. The critical shear stress adopted in this study accounts for the influence of sediment cohesion, which can lead to an underestimation of erosion flux in areas where the fraction of cohesive sediment is relatively low. In addition, the impacts of waves were excluded during the simulation. These factors hinder the model’s ability to reproduce the peak values of SSC. Nevertheless, the overall temporal trends of the modeled and observed SSC are consistent (Figure 6).

The model performance was further evaluated quantitatively using the skell score (SS) [45]:

$$SS=1-{\displaystyle \frac{{\sum }_{i=1}^N(X_{mod}-X_{obs}{)}^2}{{\sum }_{i=1}^N(X_{obs}-\overline{X_{obs}}{)}^2}}$$

where $X$ is the variable of interest, and $\overline{X}$ is the mean value. Performance levels were categorized as follows: >0.65, excellent; 0.65–0.50, very good; 0.5–0.2, good; and <0.2, poor. The SS for water level simulations at the selected tidal gauges ranged from 0.91 to 0.98, indicating excellent model performance. SS values for tidal current simulations ranged from 0.53 to 0.87, with better performance near the bottom (SS = 0.73) compared to the surface (SS = 0.61). Salinity simulations also demonstrated higher accuracy near the bottom (SS = 0.84) than at the surface (SS = 0.72). For SSC simulations, SS values varied from 0.49 to 0.69. Overall, the model showed satisfactory reliability and is suitable for assessing sediment transport processes in the PRE.

3. Results

3.1. Seasonal Variations in Estuarine Dynamics and Sediment Transport

We first present the results for Case 1 and Case 2, as these simulations incorporate all relevant physical processes. The residual current and salinity exhibited substantial seasonal contrasts between the wet and dry seasons. During the wet season, residual currents near the Humen Outlet were directed seaward, with higher speeds in deep channels than in adjacent shoals (Figure 7a). Further downstream, the residual velocity increased to approximately 0.35 m/s in the West Channel after encountering freshwater from lateral outlets (Hongqili and Hengmen Outlets). The lateral baroclinic pressure gradient, established by the transverse salinity gradient, acted as a dynamic barrier that confined freshwater from the lateral outlets to the West Shoal [46]. In contrast, on the eastern side of the estuary, residual velocity decreased downstream as it mixed with ambient high-salinity water, while outside the estuary, water movement was predominantly eastward under the influence of southerly winds [47]. During the dry season, salinity across the estuary increased as a result of reduced river discharge (Figure 7b). In the upper estuary, residual currents near the outlets weakened due to diminished buoyancy input. Under prevailing northeasterly winds, residual currents in the western part of the estuary generally shifted southwestward, while in the eastern region, the residual current decreased markedly as a result of lower freshwater discharge via the East Channel. Outside the estuary, westward flow emerged primarily under wind forcing.

The distribution of riverine sediment was primarily controlled by estuarine dynamics. As sediment was supplied solely from upstream during the simulations, the SSC decreased seaward from the upper estuary to offshore in both wet and dry seasons (Figure 7c,d). During the wet season, SSC was nearly uniform across the transverse section in the upper estuary, reaching up to 0.1 g/L. In the middle and lower reaches, elevated turbidity occurred at the junction between the West Channel and West Shoal, where the increased channel width favors the formation of a freshwater plume [48]. The SSC declined eastward, as less riverine sediment was discharged via the East Channel. In the dry season, SSC decreased throughout the estuary, with maximum values of approximately 0.03 g/L. Spatially, turbidity was higher near the Humen Outlet compared to the other outlets. Transverse asymmetry in the lower estuary became more pronounced, with riverine sediments predominantly transported along the western coast, where the SSC was approximately 0.01 g/L. By contrast, the eastern estuary exhibited low turbidity, and SSC outside the estuary was negligible. Deposition of riverine sediment primarily occurred on shoals, consistent with weak hydrodynamic conditions in these areas (Figure 7e,f). During the wet season, sediment mass on the Middle and West Shoals ranged from 1.0 to 10.0 g/m², except in regions adjacent to the lateral outlets. Downstream, seabed sediment mass decreased, displaying a spatial pattern similar to that of suspended sediment. In the dry season, sediment deposition was mainly concentrated downstream of the Humen Outlet, with seabed sediment mass generally below 0.1 g/m². In the lower estuary, sediment accumulation was confined to the West Shoal, whereas seabed sediment in the eastern estuary was scarce.

3.2. Spatial–Temporal Distributions of Sediment Age

The age of a sediment is defined as the time elapsed since it was discharged from the headwaters into the PRE. In general, sediment age was strongly influenced by specific transport pathways. During the wet season, it took approximately 45 days for sediment to be transported from the headwaters to the offshore regions via the West Channel (Figure 8a). A portion of the sediment was diverted to the inner region of the West Shoal, where age increased to 70 days. By contrast, sediment routed through the East Channel could exit the estuary in as little as 30 days. Offshore, along the main plume pathway, sediment age was less than 50 days but increased sharply toward the eastern offshore region. The sediment age increased across most of the estuary during the dry season (Figure 8b), and in the upper estuary, it was further elevated near the outlets due to the weakened seaward current (Figure 7b). Similarly to the wet season, the freshwater plume exhibited the fastest sediment transport rate, where it took only 50 days for sediment to be delivered offshore. By contrast, the sediment age increased by 30 to 60 days when transported via the East Channel compared to the wet season. After being transported outside the estuary, the sediment age increased to more than 100 days.

In both the wet and dry seasons, the spatial distribution of sediment age closely resembled that of suspended sediment (Figure 8c,d), indicating frequent sediment exchange between the water column and the seabed. During the wet season, sediment age on the seabed was 5 to 20 days greater than that in the overlying water column. By contrast, during the dry season, the age difference increased to approximately 10 to 40 days, primarily due to the decreased bottom shear stress [49].

3.3. Vertical Distribution of Sediment Age

Due to processes such as density stratification and estuarine circulation, sediment age exhibits pronounced vertical variability [50]. In this study, a transect along the West Channel was selected to illustrate the vertical distribution of sediment (see Figure 7d for location). During the wet season, both along-channel and vertical salinity gradients were evident (Figure 9a). Specifically, the along-channel salinity gradient was approximately 0.15 psu/km in the lower estuary, increasing to about 1.2 psu/km within the salinity front. The surface-bottom salinity difference reached 9 psu, with the gradient being most pronounced near the surface, indicating vigorous density stratification. The baroclinic pressure gradient, set by the along-channel salinity gradient, drove a landward bottom flow with velocities ranging from 0.05 to 0.2 m/s. In the upper water column, currents were directed seaward, yielding the classical two-layer estuarine circulation [51]. Consistent with the estuarine dynamics, the SSC profile was comparatively uniform near the bottom, while a sharp gradient was present near the surface (Figure 9c). Along the channel, an elevated SSC occurred in the upper transect and at the salinity front, followed by a rapid decline to less than 10 mg/L downstream of the front. The spatial distribution of sediment age was largely controlled by residual current and density gradients (Figure 9e). Under high-discharge conditions, sediment parcels traveled approximately 30 km over 20 days, reflecting the strong seaward residual flow associated with large freshwater inputs. In contrast, in the lower estuary, traversing the same distance required roughly 35 days. Vertically, sediment age differed markedly, with more rapid downstream transport in the upper water column. Elevated sediment ages near the bed primarily resulted from landward bottom currents that efficiently trap sediment in the vicinity of the salinity front [20].

During the dry season, reduced freshwater discharge allows saltwater to intrude further upstream (Figure 9b). This upstream migration shifts both the salinity front and the landward residual currents into the first 20 km of the transect. Within the salinity front, the along-channel salinity gradient decreased to approximately 0.9 psu/km. As in the wet season, elevated SSC values occurred within the salinity front, declining to less than 5 mg/L beyond this front (Figure 9d). Owing to the diminished river discharge, sediment age increases along the transect relative to wet-season conditions. Sediment also resides for longer within the saltwater front, traveling only about 26 km over 40 days (Figure 9f). By contrast, in the downstream region, sediment parcels traverse roughly 38 km in just 5 days. Vertically, suspended sediment moves more rapidly near the surface. Under canonical two-layer estuarine circulation, bottom-layer sediment ages are typically 10–12 days greater than those in the upper water column.

4. Discussion

The results indicate that sediment transport pathways are predominantly governed by the extent and dynamics of the freshwater plume during both the wet and dry seasons. This, in turn, significantly influences sediment transport timescales. The relationship between riverine sediment transport and plume dynamics has been well documented in various estuarine systems. For example, the freshwater plume between the South Passage of the Changjiang Estuary and Hangzhou Bay has been identified as the primary migration pathway for sediment and associated heavy metals, as evidenced by both in situ measurements and numerical modeling [52,53]. Geyer et al. [54] further demonstrated that sediment is transported within the surface-advected plume, while the plume boundary acts as an efficient barrier, preventing further offshore dispersal of sediment. To assess the influence of plume expansion on riverine sediment transport, we calculated the freshwater flux as follows:

$$T=〈{\int }_{-H}^{0}fudz〉$$

(1)

and the suspended sediment transport rate as

$$q=〈{\int }_{-H}^{0}uCdz〉$$

(2)

where the angular brackets denote the tidal average (a 15-day period was used in this study), u represents the horizontal velocity, H denotes the water depth, C denotes the SSC, $f=1-S/S_0$ represents the freshwater fraction, S denotes the salinity at an arbitrary model cell, and S₀ is the salinity of the seawater. Freshwater flux is a commonly used indicator for quantifying plume spread [55,56], while the sediment transport rate serves as an effective metric for evaluating the intensity of sediment movement in coastal waters [57,58].

The freshwater flux and sediment transport rate are illustrated in Figure 10. During the wet season, freshwater from the headwaters split into two main branches. The primary branch flowed along the junction of the West Channel and West Shoal, with the unit-width freshwater flux ranging from 0.8 to 1.7 m²/s (Figure 10a), while the secondary branch was confined to the East Channel, attaining a freshwater flux that was less than half that of the western branch. This disparity in freshwater flux between the two plume branches was governed by the locations of freshwater sources and the increasing width of the estuary. The latter factor can induce a geostrophic balance, causing the flow to be continually deflected westward [59]. Seaward of the mouth, freshwater was transported eastward with a flux of less than 0.3 m²/s. The sediment transport rate closely mirrored the pattern of freshwater flux (Figure 10c). Riverine sediment was initially transported along the estuarine axis, with rates ranging from 80 to 170 g/m/s, and similarly to the freshwater pathways, two distinct sediment transport routes were observed. The sediment transport rate along the eastern edge of the West Shoal was substantially higher than in the East Channel; however, due to weak hydrodynamics over the West Shoal, sediment tended to remain there for extended periods following deposition [49]. By contrast, sediment transported downstream via the East Channel was more constrained, experiencing fewer deposition-resuspension events and thus moving more rapidly, even though the transport rate was less than 35 g/m/s.

During the dry season, both the freshwater flux (Figure 10b) and sediment transport rate (Figure 10d) decreased. Influenced by wind forcing, freshwater turned southwestward upon leaving the headwaters. The freshwater flux within the plume region ranged from 0.5 to 1.1 m²/s and decreased to less than 0.1 m²/s outside the plume pathway. Consistent with plume dynamics, sediment was primarily transported along the western side of the estuary. The reduced sediment transport rate resulted in slower downstream sediment movement, which in turn increased sediment age compared to the wet season.

Numerous previous studies have demonstrated that wind forcing plays a significant role in modulating plume structure, thereby altering sediment transport dynamics [60,61]. In the PRE, wind characteristics—including both magnitude and direction—vary due to the prevailing monsoon. The results of the present study also reveal that sediment movement and transport timescales exhibit distinct seasonal variations, which are partially driven by changing wind conditions. To explicitly examine the effects of wind on sediment transport, we present the results for Case 3 and Case 4 in this section. Compared to Case 1, tidally averaged salinity decreased within the estuary, particularly in the western sector (Figure 11a). The residual current increased in the upper estuary and along the eastern edge of the West Shoal, while slightly decreasing in offshore regions. Comparisons between Case 1 and Case 3 suggest that the prevailing southerly winds during the wet season inhibit the offshore spread of freshwater. This outcome primarily results from the opposing directions of wind and flow, directly weakening the surface residual current; additionally, southerly winds induce Ekman transport, which promotes the onshore accumulation of surface water, further restricting the seaward expansion of low-salinity water [48,62]. When wind forcing is excluded, the mass of both suspended and seabed sediments increases in the lower estuary (Figure 11c,e). Specifically, the 0.01 g/L SSC contour extends approximately 6 km further downstream in the western estuary, while the eastern offshore region experiences a slight decrease in SSC due to reduced eastward currents. Similarly, the area where seabed sediment mass exceeds 1.0 g/m² also expands.

In the absence of wind, the primary transport pathway of the freshwater plume shifted eastward during the dry season (Figure 11b). Residual currents weakened near the Humen Outlet and in the plume region, with a maximum value of only 0.2 m/s. These results indicate that northeasterly winds during the dry season can enhance the westward expansion of the freshwater plume, consistent with previous findings [63]. Corresponding to alterations in subtidal currents, the westward spread of suspended sediment diminished, causing regions of high SSC to shift toward the West Channel (Figure 11d). Conversely, suspended sediment was transported further downstream along the East Channel. Without wind forcing, sediment primarily accumulates within the West Channel rather than on adjacent shoals (Figure 11f). Seaward of the mouth, the region of net sediment deposition expands, with substantial sediment accumulation observed in the eastern offshore area.

Excluding wind forcing from the model produced marked changes in sediment age. Compared to the results with wind forcing, suspended sediment moved more rapidly along the western estuary during the wet season, (Figure 12a), with riverborne material requiring less than 40 days to exit the estuary within the plume. Although sediment age remained higher on the West Shoal, it was generally 5 to 10 days lower than when wind forcing was present, while in the eastern part of the estuary, sediment age showed a slight increase due to the weakened residual currents along the East Channel. The reduction in sediment age on the seabed was particularly pronounced in comparison to the case with wind forcing (Figure 12c), and these results suggest that southerly winds during the wet season can inhibit sediment transport out of the estuary, thereby increasing sediment age during this period.

During the dry season, the exclusion of wind forcing resulted in an overall increase in the age of both suspended and seabed sediments throughout the PRE. An additional 30 to 50 days was required for sediment to be transported outside the estuary within the plume region (Figure 12b). The increase in sediment age in the eastern portion of the estuary was generally less than 15 days. Sediment age on the seabed followed a similar pattern to that of suspended sediment but was approximately 20 to 35 days greater (Figure 12d). Collectively, these findings indicate that northeasterly winds during the dry season enhance the development of the freshwater plume and facilitate seaward sediment transport, thereby reducing the transport timescales for sediment exported from the estuary.

Our results demonstrate that sediment age varies with river discharge, wind forcing, and tidal dynamics. Prior studies have further shown that sediment age is strongly influenced by sediment properties and estuarine morphology. For example, Mercier and Delhez [4] reported that the age of coarser sediments was typically about 3 days greater than that of fine-grained sediments, owing to higher settling velocities and the correspondingly stronger role of sedimentation. In the tidal York River, sediments with low critical shear stress were more readily eroded, which reduced seabed sediment age [64]. Estuarine deepening associated with navigation projects can intensify estuarine circulation and enhance sediment trapping, thereby increasing sediment age [65]. Because fine sediments, through their cohesive behavior, act as key carriers of nutrients and heavy metals, sediment retention time plays a pivotal role in water quality and estuarine ecosystem function. In the Elbe Estuary, the prolonged residence of sediment within the turbidity maximum has been associated with the lowest dissolved oxygen concentrations [66]. Observations further indicate that heavy metals such as Pb and Cd can be enriched by a factor of 2–3 near the salinity front relative to other estuarine regions, reflecting contrasts in sediment transport timescales [67]. Globally, riverine sediment loads have declined in recent decades due to human activities, including dam construction and sand mining [68]. In many estuaries, this reduction has promoted local sediment erosion and shortened sediment retention times [69], with ecological consequences that warrant further investigation to better support estuarine management.

In this study, the sediment age across the wet and dry seasons was investigated by setting uniform values for settling velocity and critical shear stress. These assumptions, in combination with the exclusion the influence of waves, led to the underestimation of local sediment erosion. In response, sediment tended to remain for a longer time on the seabed and eroded with a relatively higher sediment age [70]. In the PRE, a major flood can deliver a large sediment load with substantial deposition within the estuary [19], and the seabed may require several years to return to its pre-flood state. The effects of sedimentation and resuspension during this period on the overall sediment transport timescales warrant further investigation in the future. In addition, the waves in the PRE also undergoes distinct seasonal variations. The averaged significant wave height ranges between 1.0 and 2.5 m, propagating from the north or the northeast during the dry season. In contrast, the wave height during the summer is typically below 0.5 m, and they predominantly from the south or southeast [22]. Wave dynamics significantly influence water column mixing, the expansion of freshwater plumes, and sediment resuspension, inevitably altering the sediment transport timescales [71,72]. In future studies, we will incorporate wave simulation into the current numerical model to more accurately represent seasonal variations in sediment transport timescales.

5. Conclusions

In this study, seasonal variability in sediment dynamics and transport timescales in the PRE was examined using a well-calibrated numerical model. The results demonstrate that sediment transport is dominantly controlled by plume dynamics in both wet and dry seasons. During the wet season, riverine sediment is transported seaward and subsequently deflected southwestward downstream, influenced by the widening of the estuary. The development of the freshwater plume significantly inhibits the eastward dispersal of sediment originating from lateral outlets. As a consequence, a substantial portion of sediment is retained around the West Shoal, requiring approximately 45 days to exit the estuary due to diminished hydrodynamic forces in that region. By contrast, sediment exported via the East Channel traverses the estuary more rapidly, taking approximately 30 days. During the dry season, reduced river discharge leads to an overall increase in sediment age, with offshore delivery within the plume region taking up to 50 days. Additionally, sediment age in the eastern estuary is extended by 30–60 days due to the preferential development of the river plume along the western flank. Wind effects were quantitatively assessed by comparing model simulations with and without wind forcing. The analysis indicates that southerly winds during the wet season weaken the freshwater plume, thereby prolonging sediment transport timescales, whereas dominant northeasterly winds in the dry season enhance plume development and facilitate more rapid sediment export. Sediment transport timescales serve as an important indicator for assessing estuarine ecological conditions, and to further advance research on this topic, future work should also consider the impacts of extreme floods and wave forcing.