Hydrodynamic Controls on Seasonal Circulation Modes and Sediment Convergence in a Monsoon-Driven Asymmetric Inlet

Abstract

Tam Quan Inlet, a monsoon-driven asymmetric entrance on the south-central coast of Vietnam, has experienced persistent shoaling and severe downdrift erosion despite jetty construction and repeated maintenance dredging. This study investigates the unresolved linkage between seasonal circulation reorganization, inlet-directed sediment convergence, channel infilling, and southern-beach erosion. A coupled Delft3D-FLOW/WAVE model, constrained by field observations from May 2022 and November–December 2022, was used to diagnose hydrodynamic controls and compare alternative management layouts. The model satisfactorily reproduced the dominant variability of water level, wave conditions, and depth-averaged currents during calibration and independent validation, providing a suitable basis for process diagnosis and comparative layout assessment. The simulations identify four recurrent circulation modes: a cape-crossing north-to-south longshore jet, flow acceleration and deflection near the southern jetty, a northeast-monsoon recirculation cell that promotes inlet-directed convergence from the southern beach, and a partial summer reversal under SE-sector waves. These modes explain why shoaling persists after one-sided intervention and why the southern shoreline functions simultaneously as an eroding downdrift beach and a seasonal sediment source to the inlet. Among the tested layouts, PA2 most effectively concentrates flow through the inner throat while relocating sediment retention to an external storage basin, supporting controlled trapping and periodic bypassing. The results support a sediment-balanced management strategy that integrates controlled trapping, maintenance dredging, and sediment bypassing to improve navigation reliability and reduce the sediment deficit along the downdrift shoreline.

1. Introduction

Tam Quan Inlet is a strategically important maritime gateway on the south-central coast of Vietnam, where navigational access, storm-shelter function, and adjacent shoreline stability are tightly coupled [1,2,3,4]. In wave-exposed monsoonal settings, inlet reliability depends not only on maintaining a navigable throat but also on the seasonal interaction among entrance geometry, wave climate, littoral sediment transport, and engineering intervention [5,6,7,8,9]. More broadly, tidal-inlet systems respond to coupled controls from morphology, hydraulics, littoral transport, and human modification; persistent shoaling is therefore rarely a purely local dredging problem [5,7,10,11,12].

Over the past few decades, Tam Quan Inlet has experienced persistent and recurrent shoaling, particularly after major engineering intervention at the entrance. Since approximately 2010, repeated infilling of the entrance channel has continued despite the construction of an approximately 850 m shore-connected southern jetty and recurrent maintenance dredging [2,4]. Reported operational estimates indicate annual accretion within the access channel and inlet-entrance sector on the order of 10⁵ m³/year [4,13], creating persistent constraints on navigation and storm-shelter operation. Concurrently, the downdrift southern shoreline has undergone severe erosion, expressed as shoreline retreat, beach narrowing, and local damage to revetment structures [13]. The coexistence of chronic entrance shoaling and downdrift erosion indicates that Tam Quan should be interpreted as a coupled inlet–shoreline morphodynamic system rather than as an isolated navigation channel problem [3,5,7,10].

Previous studies have shown that Tam Quan Inlet is highly sensitive to seasonal wave forcing, sediment redistribution, and structural modification [1,2,3,4,14]. However, these studies did not yet synthesize the recurrent circulation modes through which seasonal wave forcing, inlet asymmetry, and one-sided engineering intervention combine to maintain chronic shoaling after construction of the southern jetty. In particular, the physical linkage between circulation reorganization, inlet-directed convergence, and the coupled occurrence of entrance infilling and downdrift erosion has remained insufficiently resolved within a calibrated and independently validated process-based framework.

Addressing that gap is important for both scientific interpretation and engineering management. In morphodynamically asymmetric inlets influenced by headlands and shore-connected structures, long-term evolution is governed not only by net sediment supply but also by the seasonal reorganization of nearshore circulation into patterns that favor inlet-directed transport, localized trapping, and repeated sediment convergence near the inlet mouth [1,2,5,7,8]. This also helps explain why earlier structural intervention did not eliminate shoaling: the southern jetty modified the transport pathways, but it did not remove the circulation mechanisms that continue to route sediment toward the entrance and trap it near the navigation corridor.

Tam Quan Inlet is subject to pronounced monsoonal seasonality. Energetic NE–ENE waves dominate the winter monsoon, whereas lower-to-moderate energy SE–SSE waves are more characteristic of the summer regime [1,9]. The alternation between these seasonal wave climates implies that the inlet does not respond through a single, time-invariant transport configuration. Instead, distinct forcing regimes activate different circulation modes with different implications for sediment routing, entrance shoaling, and sediment deficit along the southern shoreline. A process-based explanation of chronic shoaling at Tam Quan therefore requires explicit examination of how seasonal forcing controls recurrent circulation patterns and how those patterns govern the location and persistence of sediment-trapping zones.

Clarifying these controls is important not only for morphodynamic interpretation but also for engineering assessment and management. In the management of engineered inlets, practice has increasingly moved beyond repeated dredge-and-dispose operations toward sediment-balanced approaches, including sediment bypassing, nearshore placement, beneficial reuse of dredged sediment, and controlled sediment-retention measures that seek to maintain navigability while reducing littoral sediment loss [11,12,15,16,17]. For a site such as Tam Quan, the practical value of these approaches depends on whether the principal trapping zones can be diagnosed in relation to the governing hydrodynamic mechanisms and whether alternative structural layouts can relocate retention outside the navigation corridor without compromising inlet function [7,12,15,16,17].

Against this background, the present study investigates the hydrodynamic controls on seasonal nearshore circulation, sediment convergence, and layout performance at Tam Quan Inlet, a morphodynamically asymmetric and monsoon-driven entrance along the central coast of Vietnam. Specifically, the study aims to (i) identify the recurrent seasonal circulation modes that develop under representative monsoonal wave conditions; (ii) examine how these modes govern inlet-directed sediment transport, localized trapping, and chronic entrance shoaling; (iii) evaluate whether alternative structural layouts can shift sediment retention away from the navigation channel while improving the potential for sediment sharing with the eroding southern shoreline; and (iv) use Escoffier-type screening as a first-order hydraulic constraint on throat admissibility. By integrating field-supported numerical modeling, independent calibration and validation, circulation-based sediment-transport diagnostics, and comparative layout screening, the study provides a process-informed basis for sediment-balanced inlet management at Tam Quan.

2. Materials and Methods

2.1. Study Area

Tam Quan Inlet is located on the south-central coast of Vietnam, in Hoai Nhon Ward, Gia Lai Province; this area belonged to Binh Dinh Province before the 2025 administrative merger (Figure 1). The inlet forms the maritime gateway to the Tam Quan fishing-port system and associated anchorage. The entrance is strongly asymmetric: Truong Xuan Cape and a rocky shoreline bound the northern side, whereas a shore-connected southern jetty and associated dike system constrain the southern side. The gap between these boundaries forms a relatively wide entrance throat through which incident waves can penetrate directly into the mouth and entrance channel.

This asymmetric geometry is central to the site response. Under energetic wave conditions, refraction and diffraction around Truong Xuan Cape and the southern structure reorganize wave energy and current pathways in the nearshore zone. The inlet therefore behaves as more than a passive receiver of littoral drift; it functions as a dynamic sediment-convergence zone in which longshore and cross-shore transports are modulated by local circulation structures.

The cape partly shelters the entrance from direct northeast wave attack, whereas the southern structure modifies the active transport corridor approaching from the south and southeast. At the broader planform scale, the coastal sector adjacent to Tam Quan exhibits a relatively stable northern shoreline, a shoaling-prone entrance sector, and a persistently erosional southern shoreline. This spatial contrast indicates that the inlet mouth may act as a sediment-convergence zone linked to the surrounding shoreline response [1,2,8,18].

Previous investigations at Tam Quan have documented several principal sediment sources and transport pathways around the inlet, including four main components (Figure 2): (1) southward longshore transport from the northern coast under NE–ENE wave conditions; (2) sediment supply from the river during higher-discharge periods; (3) northward longshore transport from the southern shoreline under southwest-monsoon conditions; and (4) cross-shore exchange associated with wave-driven onshore transport and storm action [1,2,3,4,8,14]. These pathways should not be interpreted as independent or stationary routes; rather, their relative importance varies seasonally as wave direction, wave energy, inlet exchange, and the asymmetric entrance geometry modify the nearshore circulation field.

Morphological evidence indicates sustained erosion along the southern shoreline, approximately 1–3 km downdrift of the inlet. Field observations document revetment toe scour, foreshore narrowing, and erosional scarps, while profile comparisons between 2017 and 2019 show upper-beach retreat and lowering of foreshore elevations (Figure 2). At the same time, repeated shoaling has been observed within the entrance and navigation corridor, requiring recurrent maintenance dredging and reducing the reliability of navigation and storm-shelter access. Field photographs provide additional evidence of the coupled occurrence of entrance shoaling and downdrift southern-shoreline erosion (Figure 3). Together, these features indicate a sediment-deficit condition spatially linked to inlet trapping and interruption of natural bypassing.

Taken together, the site configuration, observed morphological response, and inferred sediment pathways support the interpretation of Tam Quan as a coupled inlet–shoreline morphodynamic system. The entrance geometry favors wave penetration and circulation reorganization, while Truong Xuan Cape and the shore-connected southern jetty impose strong spatial asymmetry on sediment routing. This setting motivates the process-based modeling framework developed in the following sections, in which seasonal circulation modes, sediment convergence, and alternative layout performance are examined under representative monsoonal forcing conditions.

2.2. Methodology

2.2.1. Datasets and Pre-Processing

Bathymetric and hydrodynamic datasets used in this study were compiled from two field campaigns conducted in May 2022 and November–December 2022, together with existing engineering surveys, navigational charts, and offshore bathymetric products. The compiled dataset includes topographic-bathymetric maps at scales of 1:1000–1:2000 acquired during 2017–2022, a 1:5000 bathymetric survey completed in 2023, Vietnamese navigational charts at a scale of 1:25,000, and offshore bathymetry from the GEBCO grid (GEBCO_2023; GEBCO Compilation Group, 2023), which was used only for the offshore background beyond the zone of direct inlet influence. These sources collectively cover the inlet, adjacent shoreline, nearshore zone, and offshore approach. The 2022 campaigns were designed to support model setup, calibration, and independent validation under two distinct seasonal observation windows. The station framework was kept consistent between campaigns, with repeat measurements at established locations where re-deployment was required.

The topo-bathymetric surface for the Delft3D model was assembled from datasets of different spatial extent, survey period, and resolution. Highest priority was given to the most detailed local surveys in the inlet throat and the adjacent morphodynamically active nearshore zone, because these sectors exert the primary control on wave penetration, current reorganization, and sediment convergence at the entrance. The 1:25,000 navigational charts were used mainly to extend coverage across the transitional nearshore waters, whereas GEBCO data were retained only for the offshore background beyond the zone of direct inlet influence.

All topographic, bathymetric, and observed water-level data were reduced to a common national vertical reference prior to interpolation and model implementation; the adopted reference in this study was the Hon Dau datum. Water-level observations from the harbor tide station were tied to the national elevation system through surveyed control benchmarks, allowing simulated and observed water levels to be compared within a consistent vertical framework. All datasets were transformed to UTM Zone 48N referenced to WGS84 before grid generation.

Where datasets overlapped, the merged bathymetric surface was constructed by prioritizing the most detailed available source, particularly within the inlet and adjacent nearshore sectors, and then applying grid averaging followed by smoothing to obtain a numerically consistent DEM for model input. This procedure was adopted to preserve the principal geometric controls governing inlet response, including the entrance throat, Truong Xuan Cape, the southern jetty sector, and the outer nearshore approach, while reducing numerical artifacts associated with abrupt transitions between datasets of different origin, survey period, and map scale (Figure 4).

Hydrodynamic observations used for model evaluation comprised wave, current, and water-level measurements collected during the 2022 field campaigns. In May 2022, wave-current data were measured at stations S1 and S2 using FlowQuest 1000 instruments acoustic Doppler current profilers (LinkQuest Inc., San Diego, CA, USA), while suspended-sediment samples at S1 were used to support interpretation of sediment-mobility conditions (

Table 1. Field measurements used for calibration and validation.

Station ID	Coordinates (WGS84)	Period	Instrument	Variables	Interval	Deployment	Purpose
S1	14°34′9″ N, 109°4′10″ E	23 May 2022, 16:00–30 May 2022, 15:00	FlowQuest 1000 acoustic Doppler current profiler	Wave parameters, currents, and SSC	1 h (waves, currents); 3 h (SSC)	Fixed near-bed deployment on frame	Calibration
		26 November 2022, 13:00–3 December 2022, 14:00	FlowQuest 1000 acoustic Doppler current profiler	Wave parameters, currents, and SSC	1 h (waves, currents); 3 h (SSC)	Fixed near-bed deployment on frame	Validation
S2	14°34′1″ N, 109°4′42″ E	23 May 2022, 16:00–30 May 2022, 15:00	FlowQuest 1000 acoustic Doppler current profiler	Wave parameters and currents	1 h	Fixed near-bed deployment on frame	Calibration
Tide station	14°34′38″ N, 109°4′5″ E	26 November 2022, 10:00–3 December 2022, 14:00	Tide-gauge water-level record	Water level	1 h	Fixed in the harbor/inlet area; referenced to Hon Dau datum	Validation

). In November–December 2022, wave-current observations were repeated at S1, and water level data were obtained from the harbor tide-gauge record within the inlet area. Together with the compiled topo-bathymetric dataset and engineering-layout drawings, these observations were used to support process diagnosis and comparative evaluation of the proposed alternatives.

Because the DEM was assembled from surveys acquired at different times, some residual uncertainty remains in rapidly evolving shoals and nearshore bars. However, the morphodynamically active inlet-control zone was represented using the highest-priority local datasets available and is therefore considered adequate for the process-based and comparative objectives of this study.

2.2.2. Hydrodynamic Forcing and Representative Scenarios

Hydrodynamic forcing at Tam Quan is governed primarily by a pronounced monsoon regime. Winter conditions are associated with more energetic NE-ENE waves, whereas summer conditions are more commonly characterized by lower-to-moderate energy SE-SSE waves [1,9] (Figure 5b,c). This seasonal shift alters not only the potential magnitude and direction of longshore sediment transport but also the nearshore circulation pattern around the inlet, including jet acceleration, flow separation, and recirculation in the lee of Truong Xuan Cape and the southern jetty.

The tidal regime is irregular diurnal, consistent with the south-central coast of Vietnam. Reported tidal ranges are approximately 1.2–1.8 m under typical conditions and around 0.5 m during neap conditions. Although tidal water-level fluctuations remain important for water exchange through the inlet, available evidence indicates that the representative hydrodynamic conditions relevant to chronic shoaling are more strongly controlled by wave forcing and wave-induced circulation than by tidal asymmetry alone [9,19].

Tam Quan Inlet is also connected to a relatively small and steep river system with strongly seasonal discharge under monsoon conditions. River inflow may become more important during flood periods; however, during the dry season and the southwest monsoon, discharge at the inlet is generally low, and the system tends to adjust toward a low-flow morphology. This tendency is further reinforced by upstream flow regulation within the Lai Giang system, including reservoir storage on the An Lao tributary. Under these non-flood conditions, hydrodynamic and sedimentary processes at the inlet are therefore interpreted to be governed predominantly by marine forcing, particularly incident waves and wave-induced nearshore circulation, rather than by sustained fluvial discharge. Accordingly, the representative forcing conditions considered in this study were defined to capture the dominant seasonal wave regimes responsible for circulation reorganization, sediment convergence, and recurrent shoaling at the inlet mouth [1].

Hydrodynamic boundaries were prescribed using time-varying tidal water levels reconstructed from TPXO8.0 harmonic constituents, together with representative upstream discharges for the seasonal screening runs [20]. Because river discharge at Tam Quan is comparatively small under non-flood conditions, constant inflows of 5 m³/s and 10 m³/s were adopted for the representative dry- and wet-season cases, respectively. This simplification is consistent with previous studies indicating that the recurrent shoaling problem at Tam Quan is governed predominantly by marine forcing, while river contribution is secondary outside flood periods [1,4].

Offshore wave forcing was extracted at point P (14.5° N, 109.5° E) from the NOAA WAVEWATCH III dataset using a multi-year time series for 2013–2025 [21]. The dominant seasonal wave regimes were identified from this record by statistical analysis over the two principal monsoonal windows, with the northeast monsoon defined for November–March and the southeast monsoon for May–September. Representative forcing conditions were then selected in terms of significant wave height, peak period, and mean wave direction to diagnose circulation reorganization under the principal seasonal states. Four incident-wave sectors were considered: NE, ENE, SE, and SSE [9,22] (

Table 2. Representative wave scenarios used for circulation diagnosis and engineering screening.

Scenario	Hs (m)	Tp (s)	Direction (°)	Use in This Study
NE	2.65	8.0	45	Dominant winter condition; circulation diagnosis and layout screening
ENE	2.00	8.0	60	Winter-sector circulation diagnosis
SE	1.50	6.0	135	Dominant summer condition; circulation diagnosis and layout screening
SSE	1.25	6.0	157	Summer-sector circulation diagnosis

). The numerical setup and boundary conditions used for circulation diagnosis and layout screening are summarized in

Table 3. Numerical setup and boundary conditions used for circulation diagnosis and layout screening.

Item	Setting/Value
Hydrodynamic framework	Delft3D-FLOW coupled with Delft3D-WAVE (Delft3D 4.03.01)
Grid configuration	Nested structured grids
Outer computational grid	200 × 100 cells
Inner computational grid	500 × 500 cells
Local minimum resolution	~10 m in the inlet-control zone
Vertical discretization	1 layer (depth-averaged)
Offshore wave forcing source	Representative wave conditions at point P (14.5° N, 109.5° E)
Representative wave scenarios	NE, ENE, SE, SSE
River discharge	5 m3/s and 10 m3/s, constant in time
FLOW time step	1 min
Bed roughness	Manning n = 0.026
Representative sediment size	D50 = 200 µm
Sediment type	Non-cohesive sandy bed
Morphodynamic run duration	~2 weeks per representative scenario
Morphological updating	Bed updating enabled
Morphological acceleration factor	MorFac = 1
Calibration period	23–30 May 2022
Validation period	26 November–3 December 2022

. Of these, the NE and SE cases were retained for comparative engineering screening because they represent the dominant winter and summer forcing regimes governing the recurrent hydrodynamic response of the inlet.

In this configuration, the scenario set serves two complementary purposes: first, to identify the circulation mechanisms responsible for sediment convergence under representative monsoonal forcing; and second, to compare how alternative layouts modify those mechanisms under the two dominant seasonal states. The model is therefore used here as a process-based and comparative framework, rather than as a deterministic reconstruction of the full long-term sequence of hydrodynamic events [23].

2.2.3. Coupled Delft3D-FLOW/WAVE Model Setup

This study uses a process-based numerical framework based on the coupling of Delft3D-FLOW and Delft3D-WAVE (Delft3D 4.03.01; Deltares, Delft, The Netherlands) to simulate water levels, depth-averaged currents, wave transformation, and wave–current interaction at Tam Quan Inlet [23,24,25,26]. The coupled Delft3D-FLOW/WAVE framework provides an integrated representation of tidal exchange, depth-averaged circulation, wave transformation, wave–current interaction, sediment transport, and morphodynamic response in coastal and inlet environments. Its nested curvilinear structured-grid capability is well suited to resolving the offshore-to-nearshore wave-transformation pathway, the inlet throat, adjacent beaches, and near-jetty circulation gradients at Tam Quan Inlet. These features provide a practical balance of physical detail, computational efficiency, diagnostic transparency, and reproducibility for comparative inlet-layout assessment.

For the present study, these capabilities are particularly relevant because the morphodynamic behavior of Tam Quan is controlled by the interaction among incident wave penetration, flow contraction through the inlet throat, circulation deflection near the southern jetty, and sediment convergence in the entrance sector. The coupled framework, therefore, allows the model to represent not only the separate effects of waves and currents but also their combined influence on seasonal circulation reorganization and sediment-routing pathways. Rather than being used as a deterministic long-term forecasting tool, the coupled system is applied here as a process-based diagnostic and comparative framework for testing how alternative engineering layouts modify wave exposure, current structure, and potential sediment-retention zones under consistent boundary-forcing conditions. The FLOW module resolves tidal exchange, mean circulation, and representative river inflow, while the WAVE module resolves the transformation of offshore wave forcing toward the nearshore and inlet mouth. Accordingly, the model is used to identify forcing-dependent circulation mechanisms, evaluate the relative tendency for sediment to enter the navigation corridor or be retained in external basins, and compare the hydrodynamic and morphodynamic responses of the proposed layouts.

A nested-grid modeling strategy was adopted to balance computational efficiency with the spatial resolution required to resolve wave transformation, circulation reorganization, and sediment trapping at Tam Quan Inlet [23,25]. The outer grid comprised 200 × 100 cells and transferred offshore forcing toward the detailed nearshore domain, whereas the inner grid comprised 500 × 500 cells and resolved the inlet throat, adjacent beaches, and the circulation gradients associated with Truong Xuan Cape and the southern jetty (Figure 6). Local refinement in the inlet-control zone was approximately 10 m, allowing the model to capture the main flow contraction, wave penetration, and near-jetty circulation features. The hydrodynamic model was implemented in depth-averaged form using a single vertical layer. The model-domain configuration shown in Figure 6 was redrawn and modified based on the authors’ original model grids and updated geospatial datasets. A preliminary version of the model-domain configuration was previously reported by Tung and Duc Anh [26], whereas the present configuration is used here as part of the expanded calibrated and independently validated process-based analysis.

This modeling strategy was intentionally process-based rather than predictive of site-specific long-term evolution. Representative morphodynamic simulations were therefore used to compare the relative tendency for shoaling in the navigation corridor and for sediment retention in external basins under the dominant seasonal wave climates. The results are interpreted as comparative mechanistic evidence for layout selection within the limits of the available field record and model assumptions, rather than as deterministic forecasts of long-term morphological change.

2.2.4. Morphodynamic Simulation and Comparative Indicators

Using the representative forcing scenarios defined above, morphodynamic simulations were conducted to compare how the existing condition and the alternative layouts modify shoaling and external sediment retention at Tam Quan Inlet. Bed updating was enabled throughout the simulations, and the bed was represented as a non-cohesive sandy system using a representative median grain size of D₅₀ = 200 µm, consistent with the detailed project-level model configuration and with the documented occurrence of fine-to-medium sand in the Tam Quan nearshore system [3]. While it is acknowledged that spatial grain-size sorting may occur under actual field conditions, the single representative grain size was adopted to represent the highly mobile fine-sand fraction that primarily contributes to channel infilling at Tam Quan Inlet. A potential implication of this simplification is that sediment-transport rates may be locally overestimated in shallow nearshore zones where coarser native sediments are present, while local sorting and armoring effects are not explicitly resolved. However, because the same sediment representation is applied consistently across all tested layouts, the comparison of relative shoaling tendency within the navigation corridor remains appropriate for the process-based and comparative objectives of this study.

Each representative morphodynamic simulation was run for approximately two weeks under the selected monsoonal forcing scenarios. The objective was not to reproduce the full long-term evolution of the inlet directly, but to resolve the short-duration bed response associated with the dominant seasonal forcing states and to compare how the alternative layouts modify the location, magnitude, and spatial organization of deposition and scour. In the adopted model configuration, the morphological acceleration factor was set to MorFac = 1. The simulated bed-level changes are therefore interpreted as direct short-duration morphodynamic responses to representative forcing conditions, rather than as accelerated predictions of long-term morphological evolution [23].

For comparative assessment, the morphodynamic results were evaluated in terms of bed-level-change patterns and integrated depositional volumes within the defined analysis zones. The navigation corridor was treated as the primary shoaling zone, whereas the outer basins and designated trapping sectors were treated as sediment-retention zones. Positive bed-level change within these areas was used to compare the relative tendency for sediment to accumulate either within the navigation channel or outside it in the intended retention zones. This framework allows the alternatives to be assessed according to whether they reduce direct channel infilling and shift the dominant retention pattern away from the navigation corridor [23,27,28].

Because the representative morphodynamic simulations were designed to compare bed-response patterns under the dominant seasonal forcing states [29], the corresponding volumetric results are presented here as annualized equivalent measures on a common temporal basis. These annualized values are not intended to represent deterministic yearly sedimentation rates; rather, they provide normalized comparative indicators for evaluating the relative tendency of each layout to reduce channel shoaling and promote external sediment retention under the adopted forcing framework. This interpretation is particularly important because the simulations resolve short-duration responses to representative seasonal conditions rather than the complete sequence of waves, floods, storms, dredging operations, and morphological feedbacks that would control long-term evolution.

The interpretation of these morphodynamic outputs is constrained by the design of the simulations. Uncertainty in the compiled bathymetry, the simplification of continuous wave climates into stationary representative scenarios, the use of a single representative grain size, and the restricted simulation duration may influence the absolute volumetric estimates [30]. For this reason, the results are used primarily to compare relative layout performance under consistent boundary conditions, rather than to provide deterministic long-term forecasts of inlet evolution or yearly sedimentation volumes [31]. Because the simulated responses are governed by the specific inlet geometry, wave climate, and sediment conditions at Tam Quan, the quantitative volumetric results are site-specific and should not be transferred directly to other inlet systems without local recalibration [32].

To connect process-based hydrodynamic interpretation with layout evaluation, this study adopts a framework consisting of four groups of indicators [7,27]. Hydrodynamic indicators describe wave penetration into the inlet, longshore-current intensity around Truong Xuan Cape, and local flow acceleration or deflection near the southern structure. Sediment-transport indicators identify potential convergence zones and the relative tendency for sediment either to enter the navigation corridor or to be retained in outer basins. Management-oriented indicators evaluate whether a given layout shifts the dominant sediment-retention zone away from the navigation corridor and creates a practical source area for bypassing toward the eroding southern beach. Geometric indicators assess whether the proposed inner-throat configuration maintains sufficient hydraulic concentration and remains consistent with equilibrium-based inlet behavior.

2.2.5. Calibration and Validation

Model calibration was carried out for 23–30 May 2022 using measured wave parameters, current velocities, and supporting hydrodynamic observations from the field campaigns. Independent validation was then conducted against the November–December 2022 dataset without changing the calibrated parameter set. The field campaigns used for model calibration and validation were briefly reported in an earlier conference contribution by Tung and Duc Anh [26]. In the present study, the original field and model datasets were reprocessed and replotted using a revised figure layout to support a substantially expanded process-based analysis of seasonal circulation modes, sediment convergence, and engineering-layout response. Model performance was assessed using root mean square error (RMSE) and the Pearson correlation coefficient (R) for water level, significant wave height, peak wave period, mean wave direction, and depth-averaged current velocity.

The calibration–validation procedure was designed to demonstrate that the model reproduced the dominant temporal variability, forcing magnitudes, and circulation scales required for mechanistic interpretation and comparative layout assessment. Accordingly, emphasis was placed on skills sufficient to resolve the principal hydrodynamic controls on wave penetration, flow reorganization, and inlet-scale circulation, rather than on exact reproduction of every short-term fluctuation in the field record. This level of agreement is appropriate for the process-based and comparative scope of the present study. The corresponding performance metrics are summarized in

Table 4. Summary of model skill during calibration and independent validation.

Variable	Calibration (May 2022)	Validation (November–December 2022)	Assessment
Water level	RMSE = 0.05 m; R = 0.89	RMSE = 0.03 m; R = 0.95	Good reproduction of phase and amplitude
Significant wave height	RMSE = 0.15 m; R = 0.82	RMSE = 0.23 m; R = 0.80	Good reproduction of energetic variability
Peak wave period	RMSE = 1.29 s; R = 0.85	RMSE = 2.25 s; R = 0.82	Higher uncertainty during energetic winter conditions
Mean wave direction	RMSE = 3.25°	RMSE = 6.28°	Directional trend captured; larger winter scatter
Depth-averaged current velocity	RMSE = 0.02 m/s; R = 0.72	RMSE = 0.03 m/s; R = 0.75	Adequate for circulation-mode analysis

.

2.2.6. Escoffier-Type Hydraulic Assessment and Layout Definition

Based on the existing planform, field observations, and previous studies, the outer Tam Quan Inlet may be regarded as only weakly constrained geometrically. Under such conditions, exchange flow through the inlet throat tends to remain insufficiently concentrated, while wave penetration into the inlet and adjacent channel remains significant [2,4,19]. This setting favors sediment mobilization, redistribution, and inward sediment intrusion from the nearshore zone, thereby contributing to the persistent shoaling observed within the navigation corridor. Accordingly, the selection of a hydraulically appropriate inlet-throat width is treated here as a key control variable, because it influences flow concentration, wave penetration, and the overall tendency for channel infilling [33].

Within this framework, the hydrodynamic, sediment-transport, and geometric indicators are used to identify the mechanisms controlling circulation and shoaling, while an Escoffier-type cross-sectional analysis provides a supplementary hydraulic assessment of inlet-throat stability at the cross-section scale [6,34,35]. Its purpose is to define an initial hydraulically admissible throat scale, expressed first as an equilibrium cross-sectional area and then as a corresponding range of reasonable throat widths. These values are not treated as final design dimensions. Rather, they provide initial hydraulic guidance for formulating candidate layouts, which are then tested against the numerical modeling results and the broader indicator set established for each case. The maximum velocity in the inlet channel, accounting for both tidal forcing and upstream river discharge, is determined from the analytical solution for a lagoon–inlet system [34,36,37].

$${\widehat{u}}^4+2u_f{\widehat{u}}^3+\mu {\widehat{u}}^2-\nu =0$$

(1)

where $\mu$ and $\nu$ are the tidal-forcing terms, $\widehat{u}$ is the dimensionless inlet velocity, and $u_f$ is the velocity contribution associated with river inflow. The tidal component was based on a characteristic ocean tidal amplitude of $a_0$= 0.8 m, while $u_f$ was specified using representative discharge conditions.

To represent the seasonal hydrological regime, the analysis first adopts $Q_{dry}=5{\mathrm{m}}^3/\mathrm{s}$ and $Q_{wet}=10{\mathrm{m}}^3/\mathrm{s}$ based on previous local modeling and technical assessments [13]. Additional discharge scenarios are then introduced to reflect measured hydrodynamic states from field surveys, including $Q=6{\mathrm{m}}^3/\mathrm{s}$ during the dry season and $Q=50{\mathrm{m}}^3/\mathrm{s}$ during the wet season [19]. The inclusion of these measured values provides an observational benchmark for comparing the Escoffier curves with recorded peak velocities of 0.49–0.58 m/s before evaluating the proposed layouts. Taken together, the discharge range from $Q=0\mathrm{t}\mathrm{o}Q=50{\mathrm{m}}^3/\mathrm{s}$ allows inlet stability to be examined across a sufficiently broad spectrum of flow conditions, from low-flow periods to monsoon-driven high-flow states.

The inlet-hydraulic coefficients, including Manning roughness $n_m=0.023$ and the entrance and exit loss coefficients $k_{en}$ and $k_x$ were standardized on the basis of previous local studies, and the full set of hydraulic and geometric inputs is summarized in

Table 5. Parameters of the Tam Quan Inlet system used in calculating Escoffier’s curves and inlet stability.

Parameters	Value
Bay surface area, $A_b$ (km2)	1.1
Effective inlet-channel length, $L$ (m)	860
Ocean tidal amplitude, $a_0$ (m)	0.8
Tidal period, $T$ (h)	12.42
Manning coefficient, $n_m$ (s m−1/3)	0.023
Coefficient for entrance loss, $k_{en}$ (-)	0.05
Coefficient for exit loss, $k_{ex}$ (-)	0.95
River flow discharge, $Q$ (m3/s)	0–50
Acceleration due to gravity, $g$ (m/s2)	9.81
Cross-sectional surface width, $B$ (m)	400

. To maintain physical consistency under the energetic monsoon-dominated conditions of central Vietnam, the stability parameters are not adopted directly as universal constants but are adjusted to the regional setting. The velocity exponent is set to $n=5$, a value consistent with established sediment-transport formulations for tidal inlets [35] and specifically applied to equilibrium topographical models [38]. This higher exponent is used here to represent the strong non-linear sensitivity of sediment-transport capacity to seasonal variations in ebb-flow strength at Tam Quan [19].

Furthermore, to reduce calibration arbitrariness, the transport-efficiency coefficient is set to $k=0.015$. According to phenomenological similarity criteria [35], this coefficient should reflect the specific dynamic balance between local littoral sediment input and ebb-tidal transport capacity. Therefore, $k$ was derived through an inverse modeling approach constrained by local in situ measurements [19]. By anchoring the theoretical Escoffier stability curve to the measured maximum ebb-flow velocities of 0.49–0.58 m/s [19], $k=0.015$ places the measured ebb-flow velocities below the estimated stability threshold. In the present study, these parameters are therefore treated as regionally calibrated values for comparative hydraulic assessment, rather than as universal constants [3,39]. Accordingly, the calibrated value of $k=0.015$ should be interpreted as site- and region-specific for the energetic, monsoon-dominated sandy-inlet conditions of central Vietnam and should not be transferred directly to other inlet systems without local recalibration using site-specific sediment supply, tidal-prism, and velocity data.

The equilibrium velocity $V_e$, representing the threshold required to maintain the inlet against wave-driven shoaling, is defined following Escoffier-type stability theory [35]:

$$V_e={\displaystyle \frac{\pi A_C^{\frac{1}{q}-1}}{T{C}^{\frac{1}{q}}}}$$

(2)

where $C$ is the sediment-transport capacity constant that represents the balance between littoral sediment input and the ebb-flow transport capacity of the inlet. In this study, the Van Rijn formulation is used to estimate the representative littoral sediment-input term under the northeast- and southwest-monsoon regimes, rather than to determine $C$ directly as an independent empirical constant [6,40]. The calculation is driven by the NOAA wave dataset extracted at point P for 2013–2025, ensuring consistency with the wave-forcing framework used in the preceding sections. The geometric parameters used in the stability assessment are derived from the available bathymetric surveys and satellite imagery (Figure 7). The effective inlet-channel length is defined as the distance from the hydraulic center of the lagoon to the throat section, thereby representing the frictional resistance of the navigation corridor. The hydraulically active lagoon surface area, $A_b$ was defined as the water surface area contributing to tidal storage, with the first-order tidal prism estimated as $P\approx A_b\times 2a_0$. Together with the boundary conditions listed in Table 5, these parameters define the existing-condition baseline. Under this baseline, the present inlet entrance remains relatively wide and weakly constrained, with $A_{c,PA0}\approx 1250{ \mathrm{m}}^2$, producing velocities lower than the estimated stability threshold and therefore consistent with the chronic deposition observed at the site. This comparison supports the interpretation that the existing flow regime does not provide sufficient hydraulic flushing capacity to offset monsoon-driven sediment input without implying that the analytical model provides a unique or fully calibrated prediction of inlet stability.

The stability analysis indicates that the equilibrium cross-sectional area $A_{eq}$ varies from approximately 100 m² during the southwest monsoon to approximately 300 m² during the northeast monsoon. The northeast monsoon is adopted as the controlling assessment condition because it delivers the dominant sediment input responsible for chronic shoaling at the inlet [4]. On this basis, the candidate layouts are evaluated with the objective of maintaining the throat cross section close to the northeast-monsoon equilibrium scale of $A_{eq}\approx 300{ \mathrm{m}}^2$, while recognizing that this value provides a hydraulic target range rather than a unique design dimension.

It should be emphasized that the Escoffier analysis yields an equilibrium throat cross-sectional area rather than a unique inlet width. In practice, $A_{eq}$ is used here as a hydraulic criterion to guide the selection of a reasonable throat-width range, such that the resulting configuration may sustain sufficient flow concentration to reduce shoaling tendency. The selected width is therefore not treated as a direct geometric conversion from $A_{eq}$, but as an initial hydraulic estimate that must be checked against the numerically simulated wave field, circulation pattern, and sediment-transport response of each layout [34]. Thus, the Escoffier analysis provides a first-order hydraulic constraint that strengthens, rather than replaces, the process-based numerical comparison.

On this basis, two management-oriented layouts are formulated for comparative assessment under identical forcing conditions, denoted PA1 and PA2. PA1 is an anti-shoaling layout consisting of a northern wave-reduction and sand-control breakwater, a sheet-pile training structure connected to the inner channel, and a southern sand-control dike tied to the existing southern jetty (Figure 8a). Its objective is to reduce direct wave penetration, weaken inlet-directed sediment transport from the north, and shift deposition outside the navigation corridor. PA2 is a sediment-trapping layout that retains the northern and southern control elements of PA1 while adding a low-crested sill, an outer trapping basin, and a flow-guiding structure intended to retain sediment within a more controlled outer basin before it enters the navigation corridor (Figure 8b). Accordingly, PA1 and PA2 are not evaluated as final engineering designs but as comparative process-based layouts used to test whether hydraulic concentration, wave reduction, and external sediment retention can be improved relative to the present condition. The digitized engineering layouts implemented in the numerical screening runs are shown in Figure 9.

In summary, the Escoffier concept is not used here as a final design method. Instead, it serves as an initial hydraulic appraisal tool for distinguishing between an excessively wide, deposition-prone throat and a more hydraulically admissible concentrated throat. The final comparison between PA1 and PA2 is therefore based not on the cross-sectional analysis alone, but on the combined evidence from the initial Escoffier guidance and the numerical-model results for wave penetration, circulation, sediment transport, and the spatial distribution of sediment retention. This combined framework preserves the hydraulic significance of the equilibrium-area analysis while ensuring that layout preference is evaluated through the simulated hydrodynamic and morphodynamic response.

3. Results

3.1. Model Performance and Suitability for Process Diagnosis

The coupled Delft3D-FLOW/WAVE model reproduced the dominant temporal variability of water levels, significant wave height, and depth-averaged currents during both calibration and independent validation, indicating that it resolves the hydrodynamic scales required for process diagnosis at Tam Quan Inlet. Because the validation simulation was conducted without further parameter adjustment, the results provide an independent test of model transferability across contrasting seasonal conditions.

Water level showed the strongest overall agreement, with a calibration RMSE of 0.05 m and correlation coefficients increasing from 0.89 during calibration to 0.95 during validation (Figure 10d and Figure 11d). Significant wave height was also reproduced satisfactorily, with RMSE values of 0.15 m and 0.23 m, and corresponding correlations of 0.82 and 0.80, for the calibration and validation periods, respectively (Figure 10a and Figure 11a). Although errors in wave parameters were larger than those for water level, the model captured the principal energy fluctuations and the relative contrasts among the representative seasonal forcing cases. Peak wave period and mean wave direction showed greater uncertainty, particularly during energetic winter conditions, but remained adequate for resolving the dominant wave-transformation patterns controlling nearshore circulation reorganization (Figure 10b,c and Figure 11b,c).

Errors in depth-averaged currents remained small, on the order of 0.02–0.03 m/s, and are considered sufficient for the present objective (Figure 10e and Figure 11e). Here, model performance is judged not by exact reproduction of every short-term fluctuation, but by the ability to reproduce the dominant forcing magnitudes, phase relationships, and circulation structures relevant to sediment routing. On that basis, the model skill is adequate to support diagnosis of the recurrent seasonal modes identified below, including the cape-crossing longshore jet, flow deflection near the southern jetty, the winter recirculation cell promoting inlet-directed convergence, and the partial summer reversal under SE-sector waves.

3.2. Seasonal Wave Exposure and Recurrent Circulation Modes

Model results show that the present entrance of Tam Quan remains highly exposed to incident wave forcing because the opening between Truong Xuan Cape and the southern jetty is sufficiently wide for wave energy to penetrate directly into the mouth sector under the representative winter- and summer monsoon conditions (Figure 12). As a result, the inlet does not behave as a hydraulically sheltered throat; instead, it functions as an energetically open transition zone in which wave transformation continues to reorganize the nearshore flow field within and immediately outside the entrance. Under NE and ENE winter monsoon wave conditions, significant wave heights of about 1.2–1.5 m persist within the entrance gap before attenuating farther landward, whereas under SE and SSE summer monsoon wave conditions, the wave field at the mouth is weaker but remains sufficient to sustain active circulation and sediment mobilization (Figure 12). This persistent wave exposure is fundamental to the morphodynamic behavior of the system because it enables the development of recurrent circulation patterns that control sediment delivery toward the inlet and navigation corridor.

The simulated depth-averaged currents reveal four recurrent circulation modes that together explain the persistence of shoaling at Tam Quan. A preliminary interpretation of inlet sedimentation and associated nearshore circulation was previously reported [26]; in the present study, this interpretation is redrawn, expanded, and reorganized into a mode-based framework linking seasonal wave forcing, circulation reorganization, and sediment convergence. Mode 1 is a cape-crossing north-to-south longshore jet generated under NE–ENE winter waves (Figure 13a,b). In this mode, flow accelerates around Truong Xuan Cape and continues southward toward the inlet approach, thereby routing sediment from the northern shoreline toward the entrance sector. Mode 2 is the structural acceleration and deflection of this jet near the southern jetty (Figure 13a,b). Rather than allowing uninterrupted downdrift transport, the existing southern structure intensifies and redirects the current close to the mouth, increasing the likelihood of sediment retention near the entrance and navigation corridor.

Mode 3 is a persistent winter recirculation cell that develops along the southern side of the entrance under energetic NE forcing (Figure 14a). This circulation cell interacts with inlet exchange to create a zone of inlet-directed convergence so that sediment released from the eroding southern beach is not simply exported farther south but is seasonally redirected back toward the inlet margin and trapping zone. This mode provides the clearest hydrodynamic explanation for the coupled occurrence of southern-beach erosion and chronic entrance shoaling. Mode 4 is a partial summer reversal under SE–SSE forcing, in which the nearshore exchange pattern is reorganized and flow develops from the southern sector toward parts of the entrance under a weaker forcing structure (Figure 14b). Although less energetic than the winter modes, this summer configuration remains morphodynamically relevant because it sustains sediment exchange toward the inlet under an alternative seasonal pathway.

Taken together, these four modes indicate that Tam Quan does not operate under a single transport pathway but under a set of forcing-dependent circulation states that repeatedly favor sediment convergence in or near the entrance. Chronic shoaling is therefore best interpreted as the cumulative outcome of recurrent seasonal circulation reorganization under persistent wave penetration at the mouth, superimposed on the asymmetric entrance geometry and the one-sided hydraulic control imposed by the southern jetty (Figure 12).

This process diagnosis provides the physical basis for the proposed engineering layouts. In this context, PA1 is formulated primarily as an anti-shoaling layout, combining a northern breakwater, a training wall, and a southern control structure to weaken Mode 1 and Mode 2, reduce wave penetration into the mouth sector, and shift sediment retention away from the inlet throat and navigation corridor. PA2 retains the wave-reduction role of the northern breakwater but places greater emphasis on controlling Mode 3 and Mode 4 through enhanced external sediment retention, using a low-crested sill, an external trap, and associated guiding structures to intercept and retain sediment outside the entrance before it can be advected into the channel. In both layouts, dredging and sediment bypassing should be interpreted as complementary management measures within the same process-based framework, allowing sediment trapped outside the entrance to be beneficially transferred to the eroding southern beach.

On this basis, the following sections examine PA1 and PA2 not simply as structural configurations, but as alternative process-based responses to the diagnosed hydrodynamic controls, with the shared aim of weakening circulation patterns that favor inlet shoaling, reducing wave penetration into the entrance sector, and relocating net sediment retention away from the navigation corridor.

3.3. Hydrodynamic Response of Alternative Layouts

The hydrodynamic response under the representative NE and SE forcing scenarios shows that both PA1 and PA2 modify the inlet environment in a manner consistent with their intended engineering functions, namely to reduce energetic intrusion into the entrance sector, maintain hydraulic effectiveness within the navigation corridor, and promote calmer outer zones favorable for controlled sediment retention. The diagnostic extraction points used for the wave- and current-response comparisons are shown in Figure 15.

Under NE forcing, both layouts substantially reduce wave penetration into the inlet entrance and adjacent inner channel relative to the present condition (PA0). The clearest reductions occur at the northern outer-basin point DB and at the inner-channel point C2, indicating that the proposed structures effectively weaken direct wave transmission into the inlet-control zone (Figure 16a,d). At DB, the mean extracted wave height decreases from 1.20 m under PA0 to 0.35 m in PA1 and 0.40 m in PA2 (

Table 6. Mean extracted wave height at diagnostic points under NE forcing.

Point	Present Condition (PA0)	PA1	PA2
DB	1.20	0.35	0.40
C1	1.40	0.80	0.80
C2	0.75	0.30	0.30
SP	1.00	0.50	0.50

). At C2, wave height decreases from 0.75 m to 0.30 m under both layouts (Table 6). At the seaward channel point C1, both PA1 and PA2 reduce mean wave height from 1.40 m to 0.80 m, while at the southern lee-side point SP, the corresponding reduction is from 1.00 m to 0.50 m (Table 6, Figure 16c). These results indicate that both alternatives reduce wave energy not only within the entrance sector but also in the intended outer retention areas.

Under SE forcing, the wave-reduction effect remains evident but is generally weaker than under NE conditions, reflecting the more open southern approach of the inlet under this seasonal wave regime. Even so, both layouts continue to suppress wave energy effectively at the outer storage point DB, where mean wave height decreases from 1.30 m under PA0 to 0.35 m in PA1 and 0.40 m in PA2 (

Table 7. Mean extracted wave height at diagnostic points under SE forcing.

Point	Present Condition (PA0)	PA1	PA2
DB	1.30	0.35	0.40
C1	1.40	0.80	0.80
C2	1.00	0.80	0.80
SP	0.50	0.45	0.45

). At C1, both layouts again reduce wave height from 1.40 m to 0.80 m (Table 7). At C2, however, the reduction is more limited, from 1.00 m under PA0 to 0.80 m under both layouts, indicating that the summer approach remains more exposed than the winter-controlled inner sector (Table 7). At SP, the reduction is modest, from 0.50 m to 0.45 m (Table 7; Figure 17). Overall, the SE results confirm that the proposed layouts still provide a calmer hydrodynamic environment in the outer basin and entrance sector, although the degree of sheltering is smaller than in the dominant NE monsoon condition.

The current-response diagnostics are broadly consistent with the wave-field modifications. Under NE forcing, both layouts reduce current magnitude within the outer storage zones and lee-side retention areas while maintaining hydraulically active flow through the channel approach. This contrast is favorable from a morphodynamic perspective: lower current velocities in the designated outer basins increase the likelihood of controlled deposition, whereas sustained flow in the channel helps preserve hydraulic concentration where self-clearing capacity is most needed. The strongest reduction occurs in the southern lee-side zone, where current velocity decreases from about 0.15 m/s under PA0 to approximately 0.02 m/s under both PA1 and PA2 (Figure 18b). In the channel sector, differences between the two layouts are smaller, although PA2 tends to maintain slightly stronger flow near the seaward entrance than PA1 (Figure 18c,d).

A similar pattern is observed under SE forcing (Figure 19). Both layouts continue to reduce hydrodynamic activity in the outer storage zones while preserving, and in some locations slightly enhancing, flow through the channel approach relative to the more quiescent depositional areas. This is an important result because the objective of the alternative layouts is not to suppress sediment motion everywhere, but to reorganize the hydrodynamic field so that sediment transport is directed away from the navigation corridor and toward zones where deposition can occur in a more controlled manner.

Taken together, the wave and current diagnostics show that both PA1 and PA2 improve the hydrodynamic functioning of the inlet relative to PA0. However, PA2 provides a somewhat more favorable hydraulic configuration because it combines effective wave reduction with a clearer separation between the active channel and the outer low-energy retention zone. These hydrodynamic differences provide the physical basis for the contrasting morphodynamic responses discussed in Section 3.4, particularly with respect to channel shoaling and the relocation of sediment retention to external basins.

3.4. Morphodynamic Response and Preferred Layout

Consistent with the hydrodynamic response described in Section 3.3, the representative morphodynamic simulations under NE forcing show that the present condition (PA0) remains prone to sediment accumulation within and immediately around the navigation corridor. Under this configuration, bed-level change is distributed continuously across the inlet mouth and adjacent channel, indicating persistent coupling between entrance-scale circulation and inlet-directed sediment convergence.

Both PA1 and PA2 reduce this tendency by shifting a larger proportion of deposition toward designated outer retention zones. However, the degree of relocation differs between the two layouts. In PA1, deposition within the channel is reduced, but the main depositional pattern remains partly connected to the mouth region. In PA2, the depositional zone is more clearly displaced outward, forming a better-defined external storage basin and reducing the likelihood of sediment re-entry into the navigation corridor.

This difference is reflected in the simulated bed-level-change patterns shown in Figure 20. Compared with PA0, both PA1 and PA2 reduce morphodynamic activity within the navigation corridor, but PA2 shows the clearest spatial separation between the protected channel and the dominant depositional zone. From an engineering perspective, this distinction is important because the preferred configuration is not simply the one that reduces deposition locally at the inlet mouth but the one that most effectively relocates sediment retention to areas where accumulation can be controlled and managed.

The volumetric results are consistent with this interpretation. Within the navigation corridor (zone S1) (Figure 21), the annualized equivalent shoaling volume decreases from 78,852 m³/yr under PA0 to 34,252 m³/yr under PA1 and 26,308 m³/yr under PA2 (

Table 8. Computed annualized equivalent shoaling volume within the Tam Quan navigation corridor for PA0, PA1, and PA2.

No.	Scenario	Shoaling Volume (m3/yr)
1	Present condition (PA0)	78,852
2	PA1	34,252
3	PA2	26,308

). Both layouts therefore substantially reduce direct channel infilling relative to the present condition, with PA2 yielding the lowest shoaling volume among the three cases considered. Here, the reported m³/yr values provide an annual-equivalent basis for comparing the three configurations under a consistent forcing and evaluation framework. Their principal role is to highlight relative differences in morphodynamic response among the tested layouts, particularly with respect to channel infilling and the redistribution of sediment retention toward external storage zones.

A complementary pattern is observed in the outer retention zones (S2–S5) (Figure 21). Under PA0, no meaningful controlled external retention zone is formed. Under PA1, the total retained sediment volume in the designated outer zones reaches 240,695 m³/yr, with the largest contribution occurring in S4. Under PA2, the total retained volume increases to 266,084 m³/yr, distributed across S2–S5 and including additional storage in S5 (

Table 9. Computed annualized equivalent retained sediment volume in designated external storage zones for PA1 and PA2.

No.	Zone	PA1	PA2
1	S1	–	–
2	S2	49,908	51,908
3	S3	33,626	37,626
4	S4	157,161	155,161
5	S5	–	21,389
6	Total retained volume (m3/yr)	240,695	266,084

). Although the retained volumes in individual sub-zones differ only moderately between PA1 and PA2, PA2 produces the highest total external retention while simultaneously giving the lowest shoaling volume in the navigation corridor. This combined response indicates a more effective hydraulic and morphodynamic separation between the active channel and the principal sediment-storage area.

These integrated values should be interpreted as comparative annualized indicators, not as exact forecasts of yearly sedimentation. Their main value lies in demonstrating the relative tendency of the layouts to reduce channel infilling and relocate sediment retention to predefined outer basins under a common forcing and scaling framework. When interpreted together with the spatial bed-level-change patterns, the results consistently indicate that PA2 provides the most favorable overall morphodynamic response among the three cases considered.

On this basis, PA2 is identified as the preferred layout. Relative to PA0, it provides the largest reduction in channel shoaling, and relative to PA1, it produces a clearer and more extensive external storage pattern. In practical terms, this makes PA2 more consistent with the broader objective of sediment-balanced inlet management, in which sediment is redirected away from the navigation corridor and retained in accessible outer basins that can support subsequent dredging, recovery, or bypassing operations.

3.5. Escoffier Stability and Equilibrium-Based Interpretation

The comparative hydrodynamic and morphodynamic results are consistent with a first-order equilibrium-based interpretation of the Tam Quan Inlet. Under the present condition (PA0), the entrance remains relatively wide and weakly constrained, allowing continued wave penetration into the mouth sector and limiting hydraulic concentration within the active throat. This configuration is consistent with the persistent shoaling tendency observed in the navigation corridor.

In this study, the Escoffier-type analysis is used as a first-order hydraulic reference to support interpretation of throat concentration and shoaling tendency, rather than as a final design criterion. From this perspective, both PA1 and PA2 improve upon PA0 by increasing entrance control and reducing hydraulic dispersion across the outer mouth.

However, the two alternatives differ in how effectively this hydraulic control is organized. In PA1, the entrance becomes more confined and wave intrusion is reduced, but the transition between the mouth and the outer depositional area remains only partly structured. As a result, the hydraulically active throat and the dominant sediment-retention zone are still not fully separated.

In contrast, PA2 is more consistent with the equilibrium-based interpretation. The layout combines improved throat concentration with a clearer external sediment-storage zone so that hydraulic activity is maintained where self-clearing potential is required, while deposition is encouraged in a more weakly forced outer basin. This behavior is consistent with the hydrodynamic results, which show clearer separation between the active channel and the low-energy outer basin, and with the morphodynamic results, which show the lowest shoaling in the navigation corridor together with the highest total external retention among the tested layouts.

Taken together, these results indicate that PA2 provides the most favorable overall configuration among the three cases considered. Relative to PA0, it reduces the hydraulic openness of the entrance and weakens the conditions that favor direct channel infilling. Relative to PA1, it achieves a clearer hydraulic and morphodynamic separation between the navigation corridor and the principal sediment-storage zone. The equilibrium-based interpretation therefore supports the broader conclusion that the preferred layout is the one that improves throat concentration while relocating sediment retention away from the active channel, which in the present study is best represented by PA2.

4. Discussion

4.1. Mechanistic Interpretation of Chronic Shoaling

The results indicate that chronic shoaling at Tam Quan Inlet is maintained by recurrent circulation reorganization rather than by a single, stationary longshore-transport pathway. Under NE-sector forcing, wave transformation around Truong Xuan Cape promotes a cape-crossing longshore jet directed from north to south, while the southern jetty locally accelerates and deflects the flow near the inlet approach. The interaction between these two controls produces a hydraulically asymmetric entrance in which sediment can be routed toward the mouth from the northern sector and subsequently retained near the navigation corridor or within adjacent low-energy recirculation zones.

A second mechanism is the winter recirculation cell that develops in the lee of the southern jetty under energetic NE forcing. This cell provides a hydrodynamic link between the eroding southern shoreline and inlet-directed sediment convergence. Sediment released from the downdrift beach can therefore be partially redirected toward the inlet margin, helping to explain why southern-shoreline erosion and channel infilling occur concurrently. Under SE-sector waves, the partial circulation reversal does not remove this linkage; rather, it reorganizes the sediment-routing pathway so that the southern sector can still contribute to inlet-directed transport under a different seasonal circulation state. The persistence of shoaling after one-sided intervention therefore reflects the continued operation of circulation-driven convergence mechanisms, rather than simply insufficient dredging capacity.

These findings indicate that the Tam Quan Inlet should be interpreted as a coupled inlet–shoreline morphodynamic system in which entrance geometry, seasonal wave direction, and shore-connected structures jointly control sediment pathways. The contribution of this study is to identify these recurrent circulation modes within a calibrated process-based modeling framework and to show how they explain the observed coexistence of entrance shoaling, structural flow deflection, and downdrift shoreline erosion. This interpretation provides the physical basis for the sediment-balanced management implications discussed in the following section.

4.2. Implications for Sediment-Balanced Management

From an engineering perspective, inlet stabilization at Tam Quan should not be assessed solely in terms of reduced wave height within the entrance. More relevant questions are where the dominant sediment-retention zone is located, whether hydraulic concentration is maintained within the active throat, and whether retained sediment can be recovered and returned to the downdrift coast. In this sense, the present results show that effective management depends not only on reducing energetic intrusion into the entrance but also on controlling the location of sediment storage.

For Tam Quan, the most favorable management direction is to shift sediment retention away from the navigation corridor and into controlled external basins that are more suitable for planned recovery or bypassing. Both PA1 and PA2 move the system in this direction, but PA2 performs more effectively because it provides a clearer hydraulic and morphodynamic separation between the active channel and the principal storage zone. This reduces the likelihood of direct re-entry of retained sediment into the navigation corridor and is therefore more consistent with a sediment-balanced approach to inlet maintenance.

The PA2 layout would not eliminate the need for maintenance dredging; rather, it would change the expected role and location of dredging within a sediment-balanced management strategy. Under the present condition, maintenance dredging is mainly required within the navigation corridor because sediment is deposited directly in the active channel. Under PA2, the dominant retention zone is shifted toward the designed external trapping basin, so dredging would be expected to focus more on the controlled storage area and less on emergency removal from the navigation corridor. Although the present simulations do not define a final operational dredging frequency, the reduced annualized equivalent shoaling volume in the navigation corridor indicates that PA2 may reduce the frequency of emergency channel dredging or lengthen the interval between channel-maintenance events. This operational implication should be refined through longer-term morphodynamic simulation and post-implementation monitoring.

More broadly, the results suggest that effective stabilization in morphodynamically asymmetric, wave-exposed inlets should be based on controlling where sediment is retained rather than attempting to suppress sediment motion everywhere. In practice, this implies combining throat concentration, wave energy reduction, and controlled external trapping so that dredging and bypassing, where required, can be incorporated into a more systematic sediment management strategy.

4.3. Scope and Limitations

This study was designed as a process-based and comparative assessment of hydrodynamic controls on circulation reorganization, sediment convergence, and engineering-layout performance at Tam Quan Inlet. Accordingly, the numerical simulations were not intended to provide final design-stage optimization or deterministic long-term morphodynamic prediction. Instead, they were used to diagnose the dominant forcing-dependent mechanisms and to compare the relative responses of the existing condition and alternative layouts under a consistent set of representative monsoonal scenarios. The simulated bed-level changes and annualized equivalent sediment volumes should therefore be interpreted as normalized indicators of relative layout performance, rather than as direct forecasts of future yearly sedimentation, dredging demand, or bypassing volumes.

Several sources of uncertainty influence the quantitative interpretation of the results. First, the topo-bathymetric surface was assembled from local surveys, navigational charts, and offshore bathymetric products with different spatial resolutions, acquisition periods, and levels of detail. Although the highest-resolution local surveys were prioritized in the inlet throat and adjacent morphodynamically active nearshore zone, residual uncertainty may remain in rapidly evolving shoals, nearshore bars, and shallow beach sectors. Second, model calibration and validation were constrained by two field-observation windows. The model reproduced the dominant variability of water level, wave conditions, and depth-averaged currents sufficiently for the process-diagnostic objectives of this study; however, the available monitoring stations do not fully resolve the spatial variability of wave–current interaction and sediment transport within the inlet-control zone. Third, the representative forcing scenarios simplify the continuous variability of monsoon waves, tides, river discharge, storms, and flood events into selected seasonal conditions. This approach is appropriate for isolating the dominant circulation mechanisms, but it does not reproduce the full event sequence controlling long-term inlet evolution.

The modeling framework also involves assumptions that constrain the level of inference. The hydrodynamic model was implemented in depth-averaged form, which is suitable for resolving inlet-scale circulation modes and layout-induced changes in flow organization but does not explicitly represent vertical flow structure, stratification, or three-dimensional turbulence near structures. The morphodynamic simulations used a single representative grain size and a short simulation period with MorFac = 1. As a result, the simulations capture direct short-duration bed responses to representative forcing but do not fully resolve longer-term feedbacks associated with grain-size sorting, storm clustering, flood pulses, episodic dredging, progressive channel adjustment, or seasonal bathymetric recovery. River discharge was also simplified in the representative scenarios; therefore, flood-dominated sediment delivery and river–marine interactions during extreme events remain outside the present scope.

These limitations define the appropriate interpretation of the results but do not alter the main process-based conclusions. The model results provide a consistent basis for identifying recurrent circulation modes, diagnosing sediment-convergence pathways, and comparing the relative effectiveness of PA1 and PA2 in relocating sediment retention away from the navigation corridor. However, the reported sediment volumes, dredging implications, and preferred operational measures are site-specific and should not be transferred directly to other inlet systems without local bathymetric data, hydrodynamic forcing, sediment characteristics, and model recalibration. Future work should extend the present framework through longer-term morphodynamic simulations, additional spatial monitoring, storm and flood scenarios, variable sediment-size representation, and post-implementation monitoring to refine dredging intervals, bypassing volumes, and operational design.

5. Conclusions

This study demonstrates that chronic shoaling at Tam Quan Inlet is governed primarily by recurrent seasonal circulation reorganization within a morphodynamically asymmetric entrance, rather than by net alongshore sediment transport alone. Because the opening between Truong Xuan Cape and the southern jetty remains highly exposed to incident monsoonal waves, wave energy penetrates directly into the mouth sector and sustains four recurrent circulation modes: (i) a cape-crossing north-to-south longshore jet, (ii) structural flow acceleration and deflection near the southern jetty, (iii) a winter recirculation cell that promotes inlet-directed sediment convergence from the southern beach, and (iv) a partial summer reversal under SE-sector forcing. These modes show that the inlet and the downdrift southern shoreline operate as a coupled morphodynamic system, in which shoreline erosion and entrance shoaling are dynamically linked through recurrent sediment recirculation rather than representing separate management problems.

This process diagnosis has direct engineering implications. For asymmetric, wave-exposed inlets of this type, effective stabilization should not rely solely on isolated anti-shoaling structures or repeated dredging within the navigation corridor. The governing management objective should instead be to separate the active navigation channel from the dominant sediment-retention zones while preserving sufficient hydraulic concentration within the inlet throat. Within this framework, the preferred layout, PA2, provided the most favorable response among the tested alternatives. It reduced the annualized equivalent shoaling volume in the navigation corridor from 78,852 m³/yr under the present condition to 26,308 m³/yr, while shifting deposition toward designated external storage zones with a total retained volume of 266,084 m³/yr. This result indicates that the most effective strategy is not to suppress sediment motion everywhere but to control where sediment is retained and to manage trapped material as a potential resource for bypassing or beneficial placement along the eroding downdrift shoreline.

From a practical perspective, the PA2 layout could support a more sediment-balanced inlet-maintenance strategy. By relocating the dominant sediment accumulation from the navigation corridor to a controlled external trapping basin, PA2 may reduce the need for emergency channel dredging, improve the reliability of navigation and storm-shelter access, and allow maintenance activities to focus more on a designated storage zone. These outcomes are relevant to the operation of the local fishing-port system and to the livelihood security of coastal communities that depend on reliable inlet access. Environmentally, controlled trapping and planned reuse of sediment could reduce uncontrolled sediment disposal and help partially restore sediment supply to the eroding southern beach. Economically, concentrating maintenance dredging in a more accessible and predictable external retention area may improve the efficiency of inlet maintenance and reduce recurrent disruption caused by channel infilling.

More broadly, the Tam Quan case provides process-based insight for the management of other asymmetric, monsoon-dominated inlets affected by headlands, shore-connected structures, and seasonal wave forcing. Stabilization should begin with diagnosis of recurrent circulation modes and sediment-convergence pathways, followed by integrated assessment of wave reduction, flow guidance, hydraulic throat concentration, and controlled external trapping. The Escoffier-type screening used in this study provides a supplementary hydraulic constraint, indicating that layout selection should remain consistent with first-order inlet-throat efficiency and self-cleansing capacity. Although the morphodynamic simulations are interpreted comparatively rather than as deterministic long-term forecasts, the consistency among the circulation diagnosis, hydraulic screening, and comparative layout response demonstrates that process-informed sediment-balanced management can provide a more robust and sustainable basis for inlet stabilization than repeated navigation dredging alone. Future work should refine the preferred layout through longer-term morphodynamic simulations, storm and flood scenarios, post-implementation monitoring, dredging-cost assessment, and environmental evaluation of sediment bypassing or beneficial sediment reuse.