1. Introduction
Wetland resources in China are widely distributed and diverse in type, with significant regional differences. The Yellow River Delta wetland is the most intact and expansive important wetland in China’s warm temperate zone. Its unique hydrodynamic-sediment-vegetation coupling process and geomorphological evolution mechanism have long been an important frontier in international coastal wetland ecology and geomorphology research.
Coastal salt marshes are typical coastal wetland ecosystems, mainly distributed in the upper intertidal zone, periodically inundated by tides, and inhabited by salt-tolerant plants such as
Suaeda salsa,
Spartina alterniflora,
Tamarix, and
Phragmites [
1,
2]. Recent studies highlight that marsh loss manifests through diverse pathways (drowning, edge erosion, pond expansion), yet integrated models capturing these multi-mechanism feedbacks remain limited [
3]. Salt marshes not only provide ecological functions such as water purification, blue carbon sequestration, and biodiversity maintenance [
4,
5] but also effectively mitigate extreme marine disasters like storm surges, forming natural coastal protection [
6,
7].
Salt marsh plants play a key role in capturing sediment and maintaining geomorphological stability, earning them the title of “ecosystem engineers” [
8]. They exhibit strong adaptability to sea-level changes and environmental disturbances [
1]. However, when the rate of sea-level rise exceeds the sedimentation rate and the sediment budget is in negative balance, the salt marsh system faces degradation risks, manifested as subsidence of marsh surfaces, increased edge erosion, and reduced vegetation cover [
9,
10,
11]. Their resilience is highly contingent on sediment supply and relative sea-level rise (RSLR) rates, with salt marshes particularly vulnerable to drowning under high RSLR [
3]. In recent years, the area of coastal wetlands worldwide has continued to decrease under the dual impact of climate change and human activities [
12,
13].
A particular concern is the disturbance caused by invasive species to local ecosystems.
Spartina alterniflora was introduced to China in the 1990s as a plant for stabilizing tidal flats. Initially, its distribution was sporadic, but since 2010, it has expanded explosively, with an average annual expansion rate of 25% [
14]. By 2020, its distribution area had exceeded 6000 hectares. This expansion has significantly compressed native vegetation habitats, leading to a decline in biodiversity and seriously threatening the stability of salt marsh ecosystems [
15,
16]. Critically, simplistic eradication strategies may overlook functional trade-offs, as
Spartina alterniflora can provide significant wave attenuation and carbon storage services depending on its spatial position [
17]. Furthermore, large-scale removal efforts face significant challenges in monitoring effectiveness and predicting resultant sediment dynamics [
18,
19].
Changes in salt marsh vegetation not only affect ecological stability but also significantly regulate hydrodynamics and sediment processes. Vegetation cover increases water flow resistance, accelerates wave energy dissipation [
20], and effectively promotes the settling of suspended sediments, which is a crucial mechanism for maintaining wetland elevation and mitigating sea-level rise. However, the removal of invasive species or vegetation degradation will alter sediment dynamics. Simulations suggest that when wetland area is reduced by 25%, its sediment capture capacity decreases by 50% [
21]. A study based on the Delft3D model also shows that after vegetation removal, wetlands exhibit reduced sedimentation rates and localized erosion under the influence of storm surges [
22]. Current predictive capabilities are hampered by insufficient integration of key processes and high-resolution observational data [
23,
24]. This gap limits our ability to forecast geomorphological consequences of management interventions under changing climates [
3,
23].
In this context, focusing on intertidal saltmarsh ecosystems, it is essential to systematically investigate the impacts of vegetation changes on coastal wetland disaster prevention and geomorphological evolution, given the critical regulatory role of saltmarsh vegetation in hydrodynamics and sediment transport. To this end, this study developed a numerical simulation model coupling hydrodynamics, sediment, and vegetation, through quantitatively simulating the hydrodynamic responses and geomorphological changes before and after the management of Spartina alterniflora, in conjunction with local vegetation restoration engineering practices, aiming to reveal the ecological-geographical synergistic evolution patterns of salt marshes.
4. Discussion
The study of coastal wetland ecological-geomorphological coupling processes has deepened, with various numerical models being applied to systems such as salt marshes and mangroves [
31,
32,
33,
34]. Among them, the equilibrium model based on relative elevation-vegetation biomass [
35] has made significant progress in describing the static relationship between vegetation and topography, but it struggles to capture vegetation dynamics (establishment, growth, expansion, degradation) and its real-time feedback with hydrodynamic-sediment processes. The two-dimensional hydrodynamic-sediment-vegetation coupling model developed in this study, based on the Delft3D-FLOW wave-tidal dynamics coupling, sediment transport, and topographic evolution processes, interacts with the biomass accumulation and vegetation growth dynamics simulated in MATLAB (version 2013 or higher), aiming to more dynamically simulate the response of the Yellow River Delta salt marsh system to multiple stresses (such as sea-level rise, sediment reduction, and invasive species management). The following discussion focuses on the key findings of the model results, comparisons with existing knowledge, model limitations, and their implications for management.
4.1. Simulation Verification, Comparative Assessment, and Feedback Mechanisms of Hydro-Sedimentary Processes and Geomorphic Changes
4.1.1. Simulation Validation and Comparison
The model successfully reproduces the characteristic of fine-grained sediments being transported to farther land areas due to their low settling rate [
36], showing quantitative consistency in sediment spatial distribution patterns with simulations from the salt marsh of Plum Island Sound, USA [
37]. The sensitivity analysis of sediment settling velocity (
Figure 4b) indicates that an increase in settling rate (e.g., from 0.1 mm/s to 0.9 mm/s) significantly enhances the sedimentation intensity in the lower intertidal zone, with an average elevation increase of approximately 0.7 m. This result is consistent with the findings of Best et al., indicating that an increase in sediment settling rate promotes sediment accumulation in lower tidal areas while inhibiting further accumulation in higher tidal areas [
9]. The simulation results further confirm that, under the background of a sharp reduction in sediment flux from the Yellow River [
38] and coarsening of sediment particle size, the resistance of the delta salt marsh wetland to sea-level rise and wave erosion significantly decreases. For example, the simulation shows that when the suspended sediment concentration at the open boundary is extremely low, the average elevation of the lower intertidal zone is only −1.07 m, significantly lower than −0.39 m when sediment supply is abundant. This strongly supports the critical role of sufficient sediment supply in maintaining salt marsh stability [
39].
4.1.2. Response Mechanisms of Morphodynamic Evolution in Tidal Flat-Saltmarsh Systems to Driving Factors
Based on the simulation results from
Figure 4 and
Figure 5, the morphological evolution of the tidal-flat-saltmarsh system in response to different driving factors is primarily controlled by the dynamic interplay between hydrodynamics and sediment transport. In terms of tidal range, a smaller tidal range limits the inundation of the saltmarsh platform and water exchange, causing wave energy to dissipate primarily in the lower intertidal zone, intensifying erosion. Simultaneously, it promotes sediment accumulation at the tidal limit, forming a steep transitional zone that helps maintain a clear tidal channel network and local microtopographic variations. A higher tidal range increases tidal current velocity and extends the inundation time in the middle and upper zones, promoting sediment deposition in this area and contributing to the formation of a gentle transitional zone. However, the lower intertidal zone may experience stronger scouring erosion. At the same time, the strong tidal energy associated with a higher tidal range can destabilize tidal channels, leading to the blurring of saltmarsh morphology, overall flattening, and driving the landward migration of the saltmarsh. In terms of settling velocity, a lower settling velocity allows sediment to be easily transported landward by water flow, leading to increased erosion in the lower intertidal zone and the landward movement of sediments. This limits the overall sedimentation rate of the saltmarsh but helps maintain the hydrodynamic activity and erosive power of tidal channels, promoting channel development. A higher settling velocity, on the other hand, promotes rapid sedimentation in the lower intertidal zone, increasing its stability. However, this rapid sedimentation quickly raises the intertidal flat and blocks tidal channel passages, reducing the number of channels or even causing them to become infilled. In terms of external suspended sediment concentration, low concentrations limit the sediment supply to the entire wetland system, slowing down the morphological development process. High concentrations, on the other hand, provide abundant sediment sources, leading to increased sedimentation across the system, flattening the slope, intensifying internal deposition in tidal channels, and limiting the complexity and connectivity of the channel morphology. Regarding sea-level rise, in the absence of sufficient sediment compensation, it increases relative water depth, enhances hydrodynamic forces, and extends inundation time, making it difficult for the sedimentation rate to counteract the combined effects of erosion and rising sedimentation baselines. This not only causes the entire tidal flat to uniformly lower and the wetland inundation area to expand but also increases the erosive power of hydrodynamics on the saltmarsh platform while inhibiting vegetation colonization. Ultimately, this leads to a decline in the overall elevation of the saltmarsh and degradation and disappearance of tidal channel networks and other spatial structural features, and, in extreme cases, it may cause the structural collapse of the saltmarsh platform. These response mechanisms collectively reveal the vulnerability and adaptability of the tidal-flat-saltmarsh system when facing environmental changes.
4.2. Biogeomorphic Feedbacks in Saltmarsh Systems: Drivers, Responses, and Restoration Challenges
4.2.1. Tidal Creek-Vegetation Feedbacks and Vegetation Patterning
The model reveals the feedback relationship between the development of the tidal channel network and the spatial pattern of vegetation: the expansion of tidal channels limits the lateral spread of vegetation, leading to the fragmentation of continuous vegetation patches [
40]. As shown in
Figure 3, the distribution range of
Spartina alterniflora in the lower intertidal zone is significantly greater than that of
Suaeda salsa, but due to the expansion of tidal channels, the vegetation patches become fragmented, resulting in a significant reduction in its coastal protection function.
4.2.2. Response Mechanisms of Vegetation Coverage to Driving Factors
The response pattern of vegetation coverage to driving factors revealed in
Figure 6 reflects the complex interplay between geomorphological processes, hydrological stress, and the vegetation niche in the saltmarsh ecosystem. The tidal range has been confirmed as the core factor controlling the distribution of saltmarsh vegetation [
41] and the system’s resilience. The simulation results of this study (
Figure 6a) clearly show that as the tidal range increases (from 0.4 m to 2.2 m), the overall vegetation coverage in the saltmarsh decreases dramatically (from 67% to 17%), particularly in the areas above sea level, where coverage drops sharply from nearly complete to 40%. This is primarily due to the dual stress effects associated with a large tidal range: enhanced hydrodynamic conditions intensify the scouring of the upper intertidal vegetation roots, significantly increasing the likelihood of physical damage. The intensified inundation process significantly extends the immersion time in the lower intertidal zone and raises the water level beyond the critical inundation height for vegetation, triggering hypoxic stress that exceeds its physiological tolerance threshold, thereby disrupting normal physiological functions and ecological adaptability. This indicates that a larger tidal range significantly inhibits vegetation establishment and maintenance in the intertidal zone by increasing inundation frequency and hydrodynamic disturbances. The increase in settling velocity exhibits a nonlinear effect: its enhancement promotes rapid sediment deposition and elevation of the intertidal zone’s lower part, creating new ecological niches for vegetation to expand seaward, thereby increasing coverage below sea level. While the above-sea-level areas remain stable due to lower stress, excessively high settling velocity may cause the intertidal flat to rise too quickly or result in drastic morphological changes, destroying habitats or hindering colonization, leading to a decrease in coverage. This suggests the existence of an optimal balance between settling velocity and local hydrological conditions. The increase in external suspended sediment concentration significantly enhances vegetation coverage in the lower intertidal zone by promoting sediment accumulation, while having little impact on the above-sea-level areas with lower stress. An increase in the relative sea-level rise rate has an overwhelming negative effect: it directly leads to the continuous reduction in the area of intertidal flats suitable for vegetation growth and causes the inundation depth and duration of the previously sub-sea-level areas to rapidly exceed the survival threshold, leading to complete vegetation degradation. The accompanying increase in erosion further destabilizes habitat stability. In summary, saltmarsh vegetation is highly sensitive to changes in geomorphological and hydrodynamic driving factors, which shape vegetation patterns by regulating intertidal flat elevation, inundation time and height, and the intensity of hydrodynamic stress.
4.2.3. Core Dilemmas and Geomorphic Risks in Spartina Alterniflora Control
In recent years, the rapid expansion of
Spartina alterniflora has garnered widespread attention due to its impacts on wetland ecosystems. A large body of research has shown that this invasive species significantly reduces biodiversity in coastal wetlands and poses a serious threat to ecosystem health [
42,
43]. This study quantitatively evaluates the geomorphological risks associated with different removal strategies. Simulation results clearly indicate that, in the absence of effective alternative protective measures, the strategy of rapidly removing Spartina alterniflora carries high risks. The rapid removal strategy leads to significant erosion of the leading edge of the mudflat, a phenomenon consistent with the sudden elevation drop in vegetation loss areas observed in long-term monitoring records [
44]. In contrast, the strategy of using native
Suaeda salsa for restoration significantly slows down the erosion rate in the saltmarsh area. Therefore, management decisions should be based on the specific conditions of the area, assessing the hydrodynamic disturbance intensity that may result from removal, and implementing corresponding engineering interventions to maintain shoreline stability, thus avoiding new ecological disasters caused by the management measures. At the same time, it is important to recognize the limitations of the native dominant species
Suaeda salsa in terms of its protective efficacy and sediment-promoting capabilities. Its ecological function restoration and enhancement require time and the support of supplementary measures.
4.3. Eco-Geomorphic Response and Degradation Mechanisms Under Extreme Sea-Level Rise Scenarios
4.3.1. Hydrodynamic Impacts and Eco-Geomorphic Responses
Under the context of high relative sea level rise rates, the observed spatiotemporal variation pattern of bed shear stress (
Figure 8) reveals the key processes of tidal-wave-topography interactions. Seawater preferentially invades along tidal channels due to the natural flow path created by the low-lying topography of the tidal channels, explaining the early stress increase observed on the inner side of the tidal channel. The rising tide extends the range of wave action to higher tidal flats, directly causing a significant increase in stress at the upper intertidal zone. The upward expansion of high bed shear stress zones reflects the process of wave energy input advancing landward with increasing water depth. Tidal channels, acting as deep troughs, effectively guide wave energy into the saltmarsh, allowing it to affect more landward areas. At maximum tide, the upper intertidal zone becomes the main stress zone, indicating that this area is subject to the strongest hydrodynamic forces at this time. During ebb tide, the enhanced along-channel shear force in the tidal channel is caused by concentrated drainage in the channel, which facilitates the transport of sediment seaward. Stress peaks observed near the tidal channel mouth during ebb tide may be related to wave breaking or the concentration of energy release caused by the falling water level. The results from
Figure 8 and
Figure 9 collectively indicate that waves are the primary physical driver that initiates and exacerbates scouring degradation in the saltmarsh region. Waves are not only the direct source of bed shear stress, but more importantly, their propagation through tidal channel topography and energy release on the saltmarsh platform significantly increases stress levels in the saltmarsh area, directly triggering and sustaining the scouring erosion process. Tides primarily regulate this degradation process indirectly by controlling water level changes and tidal channel flow.
4.3.2. Coupled Ecosystem-Geomorphic Degradation Under Sea-Level Rise Forcing
The three-phase elevation evolution model of the saltmarsh shown in
Figure 10 reveals the competitive relationship between sea level rise rate and sedimentation rate. Early erosion: In the early stages of sea level rise, the increase in relative water depth and enhanced hydrodynamics surpass the sedimentation rate in the saltmarsh area, causing erosion in the lower intertidal zone. Mid-term recovery: Under conditions with external sediment supply, the increased suspended sediment concentration and prolonged inundation time may promote some degree of sedimentation, leading to an overall elevation of the bed and recovery of the saltmarsh elevation. Late-stage degradation: However, when relative sea level rise (RSLR) continues to exceed the sedimentation rate, sediment deposition cannot compensate for the combined effect of subsidence and enhanced hydrodynamics, leading to a significant increase in inundation frequency and duration, causing overall degradation of the saltmarsh area. The observed retreat in the distribution of
Suaeda salsa in
Figure 11 is its direct response to inundation stress. Although
Figure 10 shows some elevation of the tidal flat, the rate of elevation is much lower than RSLR, meaning that the relative water depth and inundation duration faced by the vegetation continue to increase. This “ insufficient elevation gain” is the core environmental pressure driving continuous vegetation loss. The loss of vegetation further weakens the functional integrity of the saltmarsh ecosystem. On one hand, the reduction in vegetation leads to a significant decline in its wave attenuation and sediment-trapping ability, allowing wave energy to more easily penetrate the saltmarsh, which not only exacerbates erosion in the upper intertidal zone but also hinders the capture and deposition of new sediments. On the other hand, the weakening of root-binding stability significantly reduces sediment stability, making the tidal flat more susceptible to erosion, thus forming a vicious cycle of “erosion leading to vegetation loss, and vegetation loss triggering further erosion.” Under the combined effects of continuous inundation pressure from sea level rise, the increasingly enhanced hydrodynamic environment, and the ecological functional decline due to vegetation loss, the saltmarsh geomorphic structure is disrupted, and the ecosystem falls into continuous degradation. Ultimately, if there is insufficient sediment input or effective mitigation measures, the saltmarsh ecosystem will struggle to maintain its original structure and function, facing the risk of collapse.
4.4. Model Limitations
The construction and application of this model have limitations, which must be carefully considered when interpreting results and guiding management. Firstly, while the generalized topography used can capture the overall behavior of the system, it is difficult to accurately reflect the complex micro-topography of the real tidal flats. This may lead to biases in predicting feedback intensity between tidal channels and vegetation at the local scale, erosion hotspots, and sedimentation patterns and may underestimate the potential impact of spatial heterogeneity on the overall system stability. Secondly, the simplification of the vegetation module is a key limitation, as the model does not account for inter-species competition and the lag effects of biomass accumulation. This limits the accuracy of long-term vegetation succession dynamics and final community structure predictions. Furthermore, the model primarily focuses on mid-term dynamic time scales, which imposes significant constraints and makes it difficult to fully capture the cumulative effects of accelerated sea-level rise, increased frequency of extreme weather events, and long-term slow feedback from the ecosystem. At the same time, while the use of the time acceleration factor (MF) can effectively reveal macro trends, it may smooth out the instantaneous effects of high-frequency dynamic events and their impacts on landforms and ecosystems. Future research should explore more refined time coupling strategies to address this limitation. Overall, this model is more suitable for evaluating the relative effectiveness and risks of different management strategies in the near to medium term.
4.5. Management Implications
Based on the quantitative simulations and discussions in this study, the following specific management recommendations are proposed for the protection of the Yellow River Delta salt marsh wetland and the management of Spartina alterniflora. The primary task is to optimize the Spartina alterniflora removal strategy: strictly avoid implementing rapid large-scale removal in key protective shoreline sections, prioritize progressive, zonal rotation removal or integrate simultaneous ecological engineering, and use models to pre-assess local hydrodynamics and sediment environment change risks. Secondly, efforts should be focused on improving sediment utilization efficiency: actively explore the resource utilization of dredged sediment from estuarine channels to alleviate the pressure of insufficient sediment entering the sea. Strengthening the monitoring network and implementing adaptive management are crucial: establish a long-term high-resolution monitoring network in key areas and dynamically adjust management strategies based on real-time data and model updates. In management decision-making, a systematic trade-off between short-term ecological benefits and long-term protective risks must be made: discuss the short-term ecological recovery benefits of removal and the potential long-term risks of weakened coastal protection functionality. In areas with sediment scarcity and insufficient alternative protection, it may be practical to retain some Spartina alterniflora as a transitional protection strategy while accelerating research to cultivate locally adapted communities with stronger protective functions. Finally, the long-term threat of sea-level rise must be given high priority, given the enormous risk of salt marsh ecosystem collapse under high relative sea-level rise rates. Management strategies should be forward-looking. Continuous monitoring of sea-level rise and regional subsidence rates, evaluating the long-term sustainability of sediment supply, and actively implementing adaptive measures to enhance system resilience and promote natural or artificial sediment accumulation are essential.
5. Conclusions
This study developed a coupled hydrodynamic-sediment-vegetation model to systematically explore the impact of Spartina alterniflora management and Suaeda salsa restoration on the geomorphological evolution and vegetation distribution of coastal wetlands. It also analyzed the regulatory mechanisms of various dynamic environmental factors (including tidal range, sea level, sediment supply, and sediment characteristics) on geomorphological patterns and vegetation spatial distribution. The study shows that the management of Spartina alterniflora significantly altered the sediment deposition patterns in saltmarsh wetlands, leading to increased local erosion and weakening the overall stability of the wetland system. Further analysis of the wetland morphology after Suaeda salsa restoration revealed that tidal range variation significantly influences the erosion-deposition patterns of the intertidal zone by adjusting the spatial distribution of wave energy. Under low tidal range conditions, wave energy concentrates in the lower intertidal zone, inducing erosion and forming a steep slope transition zone; in contrast, under high tidal range conditions, the tidal current’s sediment transport capacity increases, promoting sediment accumulation in the middle and upper intertidal zone. Sediment settling velocity determines the distribution of sediment deposition: low settling velocity drives sediment landward, maintaining the development of tidal creek systems, while high settling velocity promotes rapid sedimentation in the lower intertidal zone and accelerates the closure of tidal creek channels. Changes in suspended sediment concentration regulate the rate of geomorphological evolution: under low concentration conditions, elevation growth is limited; under high concentration conditions, it promotes saltmarsh expansion and tidal creek sedimentation. Coastal wetland vegetation exhibits high sensitivity to tidal range, sediment dynamics, and sea-level rise, particularly in submergence-prone areas. Sediment settling velocity nonlinearly influences vegetation expansion, with the system reaching an optimal state under moderate settling conditions. The study also revealed the potential threat of sea level rise to saltmarsh wetlands, particularly when sedimentation rates are insufficient to offset relative sea level rise, posing a risk of ecosystem collapse.
It is important to emphasize that the model used in this study has certain limitations, which may affect the accuracy of predictions regarding local complex landform-vegetation feedback and long-term succession dynamics. This uncertainty must be considered when interpreting the results and guiding management practices. Therefore, future research should focus on more detailed field observations, quantitatively verifying the key processes predicted by this model, conducting sensitivity analysis of uncertainties in key parameters such as vegetation and sediment, and quantitatively assessing the long-term ecological and geomorphological effects of different Spartina alterniflora removal schemes and sediment resource management strategies. At the same time, the model’s capabilities need to be expanded to include interspecies competition mechanisms and long-term ecological accumulation feedback, in order to enhance the prediction capabilities for wetland restoration pathways and final states.
In conclusion, future coastal wetland protection and restoration strategies, particularly those targeting Spartina alterniflora management, must carefully balance ecological restoration goals with coastal protection functions. Under the dual pressures of insufficient sediment supply and sea level rise, prioritizing the stability of critical coastal sections is crucial. The risks revealed by the model should be fully considered, avoiding rapid large-scale removal in vulnerable areas and actively exploring and optimizing proactive interventions such as engineering-based sediment supplementation to enhance the resilience and sustainability of wetland ecosystems.