The role of monsoon-induced wave action in driving mangrove loss in deltaic settings remains underexplored in comparison with the role of anthropogenic activities. Here, we reveal that the Ganges-Brahmaputra-Meghna Delta (GBMD), the world's largest mangrove ecosystem within a monsoon-dominated region, exhibited an increased trend in total mangrove area at a rate of 133.3 +/- 6.7 ha yr(-1) from 1988 to 2022, despite a landward retreat of the mangrove shoreline at 5.98 +/- 1.56 m yr(-1). Monsoon-driven wave action is the primary driver of mangrove loss, with sea-level rise and tropical cyclones acting as critical amplifiers that exacerbate wave-driven erosion. In contrast, tidal currents promote sediment redistribution into channels, backshore areas, and around barrier islands and sandbanks, thereby fostering mangrove colonization and largely compensating for mangrove loss. Our findings highlight how southwest monsoon-induced waves drive mangrove loss, shedding light on the mechanisms underlying mangrove degradation in wave-dominated coastal areas.

Global deltaic mangroves are threatened by accelerated sea-level rise. However, the extent to which accelerated sea-level rise drives landward mangrove migration remains difficult to verify in modern deltas, where concurrent seaward expansion is often driven by sediment delivery. Part of the complication is that interplay of anthropogenic activities and sediment-hydrodynamics in deltas obscures the mangrove response to sea-level rise. Here we analyze the dynamics of the Mekong Delta mangrove shorelines over 496 km through 1992–2024. We find that, despite high rates of relative sea-level rise (RSLR), there was a net mangrove gain of approximately 46300 ha, with three-quarters occurring landward and one-quarter seaward. The observed landward migration was statistically associated with RSLR and occurred where accommodation space was available, while seaward expansion was linked to river sediment supply and modulated by local hydrodynamic processes. Disparities in landward migration between estuarine and coastal regions underscore challenges to mangrove resilience, reflecting how the mangrove response to RSLR is mediated by anthropogenic constraints. This study provides critical insights to inform adaptive management of mangroves under rising sea levels.

Mangrove forests are globally acknowledged for stabilizing coastlines, reducing wave energy, and protecting coastal habitats and adjacent land uses from extreme events. However, most regions experience alarming mangrove loss against natural and human disturbances. This study profiles dynamic changes in mangrove cover and shoreline migration along the Yangon estuary using Landsat imagery and machine learning approach from 1988 to 2023. Mangrove cover declined from 1175 ha in 1988 to 531 ha by 2011. It then increased to 5470 ha by 2023, resulting in a net gain of over 4000 ha. Concurrently, shoreline analysis using the mangrove vegetation line, indicates 92 % seaward progradation along the coastline. The western shoreline recorded mean accretion and erosion rates of +35.6 m/yr and -1.7 m/yr, while the eastern side showed more dynamic rates of +79.6 m/yr for accretion and -29.1 m/yr for erosion. Key findings highlight mangroves' ability to keep pace with the relative SLR, aquaculture as the dominant driver of post-2008 mangrove loss, and underscore the roles of sedimentary variation and high sediment availability, extensive tidal flat existence, and coastal sheltering in supporting recent mangrove expansion. While further studies are needed, these insights offer a valuable foundation for future conservation and management efforts.

While mangrove forests in river-dominated estuaries have been extensively studied, wave-dominated estuarine mangroves have received relatively little attention. Here, a comprehensive analysis of mangrove forests in the Beilun Estuary, a typical wave-dominated estuary and the transboundary estuary between China and Vietnam, was conducted using a machine learning of continuous change detection and classification algorithm. The results revealed that the total area of mangrove forests in the Beilun estuary increased by 11.2 % during the period from 1986 to 2000, followed by a slight decline from 2000 to 2022. Over the past 37 years, a total of 568.4 ha of mangroves were lost, primarily on the landward side of the estuary's western part. However, this loss was offset by a gain of 688.1 ha, with newly established mangroves predominantly located behind the western barrier islands, as well as in the central and eastern parts of the estuary. Meanwhile, the shoreline of mangrove forests in the western part remained stable, while the central and eastern parts of the estuary presented seaward expansion at a slow rate of 2.1 ha/yr and 1.7 ha/yr, respectively. These findings suggest that the southeast waves limited the seaward expansion of mangrove forests in western estuary, while the combined action of tidal currents and waves resuspended and transported sediments to fill the tidal creeks, providing habitats for mangroves. Barrier islands formed by strong waves create a sheltered environment for mangrove growth. Land-use conversion to aquaculture ponds is the primary driver accounting for the loss and fragmentation of mangrove forests. However, the sea-level rise, at an increasing rate of 3.38 mm/yr, has not impeded the seaward expansion of mangroves in the Beilun Estuary. This research provides a profound perspective on dynamic changes in mangrove forests within a typical wave-dominated estuary, which can serve as a crucial reference for the future conservation and restoration strategies in a global scale.

Lakes, as vital components of the Earth's ecosystem with crucial roles in global biogeochemical cycles, are experiencing pervasive and irreparable worldwide losses due to natural factors and intensive anthropogenic interferences. In this study, we investigated the long-term dynamic patterns of the Tonle Sap Lake, the largest freshwater lake in the Mekong River Basin, using a series of hydrological data and remote sensing images between 2000 and 2020. Our findings revealed a significant decline in the annual average water level of the lake by approximately 2.1 m over 20 years, accompanied by an annual average reduction in surface area of about 1400 km2. The Tonle Sap Lake exhibited episodic declines in water level and surface area, characterized by the absence of flooding during the flood season and increasing aridity during the dry season. Furthermore, the shoreline of the lake has significantly advanced towards the lake in the northwestern and southern regions during the dry season, primarily due to sedimentation-induced shallowing of the lake edge depth and decreased water levels. In contrast, lake shorelines in the eastern region remained relatively stable due to the constructed embankments for the protection of the cultivated farmland. While the seasonal fluctuations of the Tonle Sap Lake are regulated by regional precipitation in the Mekong River Basin, the prolonged shrinking of the lake can be mainly ascribed to intensive anthropogenic activities. The interception of dams along the upper Mekong River has resulted in a decrease in the inflow to Tonle Sap Lake, exacerbating its shrinkage. Moreover, there are minor impacts from agricultural land expansion and irrigation on the lake. This study highlights the driving forces behind the evolution of Tonle Sap Lake, providing valuable information for lake managers to develop strategies aimed at conserving and restoring the ecological integrity of the Tonle Sap Lake.

The Mekong River is one of the world's most important continental rivers, and provides invaluable water re-sources for the survival and social development of nearly 70 million inhabitants of Southeast Asia. However, little work has been carried out on the multiforcing-induced hydrological behavior of the Lower Mekong River -delta (LMR) over the past 100 years. Here, the secular variations in water discharge were analyzed for seven gauging stations along the main stream of the LMR from the 1920s-2019, to identify the riverine hydrological mechanisms under different dynamic conditions. The results show that the water discharge delivered by the LMR to the delta decreased on both 50-year and centennial scales. Meanwhile, the daily water discharge of the LMR-delta has become more evenly distributed intra-annually, leading to "no flood in the flood season, no drought in the dry season" in this reach. While regional rainfall dominates the long-term variations in the water discharge in the LMR, episodic tropical cyclones can induce a gradual increase with enlarged spatial divergences in water discharge from the mountainous region to the delta by bringing heavy rainfall. Moreover, although the operation of the mega dams in the Upper Mekong Basin has significantly influenced the distribution of the downstream water discharge in the dry season, minor impacts can also be found in the wet season. The water consumption for irrigation was increased from the upstream to the downstream basin, which may contribute to the severe reduction in the downstream water discharge, even though typhoons induced much more runoff along the downstream reach. Considering future climate change in Southeast Asia with potential upward trends in anthropogenic stressors, it can be expected that the water discharge of the LMR-delta could worsen, and urgent countermeasures that address water resource management should be strongly considered by decision-makers in the affected countries of the Mekong River-delta.

Climate change has induced sea-level rise and a high intensity of storms, which create high nearshore waves. These caused severe mangrove degradation and erosion along the coastal wetland areas in the Mekong Delta in Vietnam. Mangroves in the coastal wetland foreshore can withstand only some certain design storm waves and grow under several certain submerged conditions. Therefore, reducing waves and shallowing wetland elevation for recovering mangroves and protecting them in an early birth state is important. Bamboo or melaleuca fences have been used as a nature-based solution to reduce waves and currents approaching the shore for these above purposes along Vietnamese Mekong deltaic coasts. This paper investigates wave transmission through the bamboo fence system and assesses its effectiveness in protecting the mangroves. Waves were simultaneously measured at two locations for comparison: in front of and behind the fences. The result shows that the wave reduction by the fences is considerable, and sedimentation occurs rapidly in the shelter areas behind the fences, which is highly favorable for the recovery and growth of mangroves. Next, the empirical formulae have been proposed for relationships between the wave transmission coefficient of the fence and the dimensionless wave-structures parameters, such as the relative water depth, the wave steepness, and the fence freeboard. The findings create a basic technical reference for designing a naturally friendly-based solution by using bamboo and/or wooden fences in coastal protection generally and protecting mangroves specifically. The outcome of the research contributes to narrowing an existing gap in Vietnamese design guidelines for coastal wetland protection and also facilitates the use of locally available eco-friendly materials for coastal management along the Vietnamese Mekong delta coasts.

Mangrove forests (MFs), which occur along tropical and subtropical coastal zone, are among the most productive and richest carbon-rich ecosystems in the world. However, MFs have experienced great losses worldwide due to the impacts of intensive human activities and relative sea level rise. Here, the recent dynamic changes in the MFs of the Red River Delta (RRD), the second largest delta in Vietnam, were quantified using the random forest algorithm based on 328 remote images obtained by Landsat TM/ETM+/OLI during 1986-2019. The results show obvious increasing trends in the MF area with a change rate of 39 ha/y from 1,655 ha in 1986 to 2,944 ha in 2019. The change rates in the northern RRD (NP) and southern RRD (SP) were 8.44 ha/y and 30.62 ha/y, respectively, even though MFs in the tropics worldwide encountered significant losses. Further, approximately 1,091 ha of MF was converted into aquaculture ponds during 2000-2006 in the interior of the RRD, while the shoreline of MFs in the NP and SP continually expanded seaward to the northeast and southwest at average rates of 38.9 m/y and 17.9 m/y, respectively. Moreover, although the fluvial sediment supply declined dramatically from 199.57 to 16.41 mt/y during 1986-2017, the sediment supply was not responsible for the present seaward expansion of MFs. Wave action drove sediments to overwash barriers, and tidal dynamics carried this sediment into tidal channels, depositing sediments behind the barriers and on both sides of tidal channels with continuing MF growth. In addition, the conversion of MFs into aquaculture ponds by human activities was responsible for the massive losses of MFs in 2000-2006, but these impacts were alleviated as conservation policies were implemented by local governments in subsequent years. This study indicates that the growth, loss and recovery of MFs are affected by various factors, and it is vital to map the variations in MFs to better implement regional ecological restoration programs for coastal MFs.

Local and global loadings, which may cause the local damage and/or global failure and collapse of offshore structures and ships, are experimentally investigated in this study. The research question is how the elasticity of the structural section affects loading during severe environmental conditions. Two different experiments were undertaken in this study to try to answer this question: (i) vertical slamming impacts of a square flat plate, which represents a plate section of the bottom or bow of a ship structure, onto water surface with zero degree deadrise angle; (ii) wave impacts on a truncated vertical wall in water, where the wall represents a plate section of a hull. The plate and wall are constructed such that they can be either rigid or elastic by virtue of a specially designed spring system. The experiments were carried out in the University of Plymouth’s COAST Laboratory. For the cases considered here, elasticity of the impact plate and/or wall has an effect on the slamming and wave impact loads. Here the slamming impact loads (both pressure and force) were considerably reduced for the elastic plate compared to the rigid one, though only at high impact velocities. The total impact force on the elastic wall was found to reduce for the high aeration, flip-through and slightly breaking wave impacts. However, the impact pressure decreased on the elastic wall only under flip-through wave impact. Due to the elasticity of the plates, the impulse of the first positive phase of pressure and force decreases significantly for the vertical slamming impact tests. This significant effect of hydroelasticity is also found for the total force impulse on the vertical wall under wave impacts. Graphic abstract Hydroelasticity effects on water-structure impacts: a impact pressures on dropped plates; b impact forces on dropped plates; c , d , e , f wave impact pressures on the vertical walls; g wave impact forces on the vertical walls; h wave force impulses on the vertical walls: elastic wall 1 vs. rigid wall (filled markers); elastic wall 2 vs. rigid wall (empty markers)

Current offshore engineering design codes for wave loading are based on pure water, but wave impacts often involve aerated flow, either as a result of air entrapment as the wave overturns or due to impact with a wave that is already broken. Therefore, it is necessary to develop a new understanding of how design codes should incorporate water-air mixture for wave impact. This experimental study investigates aeration effects on wave impacts by means of slamming impacts of a square flat plate onto a pure and aerated water surface, with zero degree dead-rise angle. The (low) aeration level between 0% and 1.6% was applied and the drop velocity varied from about 1 m/s to 7 m/s. There was a significant reduction in the peak impact loads (both pressure and force) for impact in aerated water compared to that in pure water. There was also a significant reduction in the first phase of the pressure and force impulse for aerated water. The variation in impulsive loadings is less sensitive than peak loadings for impacts. The implication for design is that maximum instantaneous loads may be conservative in the presence of aerated water.

Saltwater intrusion is one of the major issues for coastal water resources management in the world since it has great impact on flora, fauna and the people that live along the estuary. Therefore, it is necessary to properly evaluate the process of saltwater intrusion of space and time. This paper aims to simulate saltwater intrusion in the Luy river basin in Binh Thuan province and propose measures to minimize its unwanted impacts in short-term and long-term. New Generation River modeling package, MIKE HYDRO River, in combination with advection-dispersion module was adopted to investigate the salinity intrusion. The model was well calibrated and validated based on surveyed data in June 2017 and March 2018. The salinity intrusion during validated period is the most severe since it is at the end of the dry season and during the spring tide. Especially, salinity had been measured along the Luy river in order to not only determine correctly the length of the river affected by salty water, i.e. the location with salinity concentration of 0.75‰ – where there is no risk of salinization to the plants, but also to validate the saltwater intrusion model in term of longitudinal profile. The result reveals that saline penetrates deeply into the Dong and Luy rivers, e.g. more than 8 km of seawater wedge length in the later one in present situation. Saline will intrude severely into the Dong, Luy, Mao and Oai Son rivers with the length of 13, 27.6, 10.8 and 13.1 km respectively, if considering low flow in dry season and sea level rise under RCP4.5 scenario in 2050. For minimizing unwanted impacts of salinity intrusion, suitable solution, i.e. structural measure, was selected. Precisely, two anti-salt weirs in the Dong and Luy rivers were proposed to be constructed at about 3 km from the estuary.

Coastal shoreline in the Mekong Delta in Vietnam have been eroded significantly in the past decade due to climate change and sea level rise. Many measures have been used to protect the shoreline from erosion, which could be solid-structure measures and natural-based solutions. A solid structure is nowadays unsuitable for both economy and foundation reasons in silt coastal areas whereas the natural-based solution is becoming more concerned and priority. Mangrove planting is the most environmental-friendly and sustainable soft-structure measure for reducing waves along the coast. However, new mangrove plants are impossible to grow in such severely eroded coasts. Therefore, using a bamboo fence to reduce wave energy and to increase sedimentation for new mangrove growth is becoming a suitable solution along Mekong Delta coastlines. Non-hydrostatic model SWASH (Simulation Waves till Shore) is applied in this study to simulate waves transmitting through bamboo fences. Results show that the fence porosity has a significant effect on wave transmitting through a bamboo fence. There is about 11% to 72% reduction of wave height for the fence porosity of 50%. In addition, the fence with porosity of 90% results in a maximum of 29% wave height reduction. Finally, the transmission coefficient decreases with increasing the relative fence thickness.

Aeration effects on wave loading are of considerable importance for offshore design. This paper describes experimental work to investigate four types of wave impact on a truncated vertical wall (representative of a plate in an FPSO vessel), in pure and aerated water. Investigations showed that high aeration and flip-through wave impacts are the most severe impact types and should therefore be considered for offshore structure design. It was also observed that there is a significant reduction in peak impact loads (both pressure and force) for impacts in aerated water compared to those in pure water. However, there was almost no reduction in impulsive loadings in aerated water compared to those in pure water, and therefore maximum instantaneous loads may be conservative in the presence of aerated water, in the implementation for offshore structure design. This paper is a companion paper to “Aeration effects on water-structure impacts: Part 1. Drop plate impacts”.

In Vietnam, there has been a growing demand for the application of risk analysis, risk-based decision making, and risk management in various industries and sectors of government. Although risk-related concepts have been explored in various sectors, there has not yet been any framework or guidance on how risk should be quantified and what would be the acceptable risk/risk criteria. In this chapter, an overview of concepts for risk analysis and risk management is given. Risk assessment and evaluation, basic risk-related concepts, and their developments are discussed. Methods and applications to establish an acceptable risk level and safety criteria will be reviewed. The present framework of acceptable risk level and risk-based optimal safety for flood defenses worldwide is summarized. Possible applications of the methods to the current situation and conditions in Vietnamese are discussed. Special attention is paid to how a risk-based framework could be applied to Vietnam's conditions in determining the acceptable level of risk of flooding at the national scale. The proposed criteria will be tested and applied in the assessment of flood risks in the low-lying coastal regions in Nam Dinh. Safety standards are explored by considering acceptable levels of individual and societal risk in the view of its current protected value and socioeconomic developments.

Abstract Sea dikes in Vietnam have appeared to be vulnerable to attacks from the sea during typhoons. At many places, dikes were breached due to damage of the inner slope caused by wave overtopping. Making much higher dikes could prevent this but that will create new problems with the existing infrastructure and spatial planning in the area. Therefore, an attempt is being made to make new dikes not (much) higher than the existing ones (MSL + 5-6 m). In all cases, of course, the dike should be higher than the design water level which has typical values in the order of magnitude of MSL +3-4 m. The main remaining issue then is the wave overtopping.

Mekong delta is severe-vulnerable for fluvial flood. Though frequency of coastal flood is low frequency, we should know the risk. This paper shows the extreme heights of storm surges calculated by extreme theory and hindcast of storm surge heights past 60 years. Based on the extreme heights, probabilistic flood area was calculated by physical process model of shallow water flow. And more, impact of rising sea-level is also considered with storm surges. The results were quantified as area of flood and affected population. Reducing of capacity of dykes due to rising in sea-level also was shown at the end of this paper.

Some of the important elements to be considered by the designer in the field of water defences and hydraulic structures include the determination of the maximum environmental loads such as maximum wave height, maximum water level and maximum river discharge at the locations of interes t. These variables are usually described by statistical distribution functions. Th e parameters of these distribution functions can be estimated by various methods based on observed datasets. The main point of interest is the behaviour of each method f or predicting the extreme values. The calculated value should be as close as possible to the true value and its uncertainty should be as small as possible. In this paper, firs tly, an overview is given of data management techniques, which are needed to further extreme value analysis. Secondly is focused on statistical methods to model the occurre nce probabilities of extreme values. The methods is applied for the prediction of extrem e waves along the Dutch North Sea coast and extreme storm surges along the Vietnamese South China Sea coast.