Sea Dyke Research Articles

Wave dissipation by vegetation for coastal defense in the Western Mekong Delta under climate change

The combination of the Mekong Delta dyke-mangrove system effectively protects the hinterland against flooding and erosion in extreme conditions for decades since they could dissipate waves and currents efficiently. However, literature and satellite data indicate that severe erosion happens along the Western sea-dykes of the Ca Mau coast caused by increased intensity of storms due to climate change. State-of-the-art wave transformation models are used to predict the influences of wave propagation and wave overtopping discharge over sea dykes and/or associated with mangrove ecosystems under extreme typhoons, which cooperates with both high – frequencies (HF) and low – frequencies (LF) waves. Results confirm that the HF wave quickly dissipates in shallow water, whereas the LF wave is dominant and penetrates over a longer distance. At dyke toe, the wave height through vegetation is substantially diminished, approximately ten times less than that in the sea-dyke solely. However, the modelling wave overtopping discharge seems to be overestimated compared with the standard-based formula.

Estimation of Shoreline Changes of Can Thanh Townlet and Long Hoa Commune in Hochiminh City

Can Gio is considered as the “green lung” of Ho Chi Minh City with the mangrove ecosystem with many investment projects as well as livelihood models associated with the strength of mangrove forest. Currently, in Long Hoa commune and Can Thanh town, the “Can Gio sea encroachment project” is being implemented with the goal of creating an urban area of sea encroachment tourism, resort and business tourism with events, conferences and seminars. In the process of construction, infrastructure expansion, service development and dense population concentration, it is inevitable that the shoreline change processes will be affected, especially within these two coastal areas. This study presents a method of using the integrated DSAS tool in ArcGIS together with Sentinel 2A image data to assess the shoreline change of Long Hoa commune and Can Thanh town from 2015 to 2022. By extraction shoreline through the MNDWI index, the results show that the process of accretion and erosion were intertwined, except for the sections of the shore that have been reinforced with sea dykes. It is evident that in the land at Dong Tranh Cape, southwest of Long Hoa commune and northeast of Can Thanh townlet, the process of erosion is taking place, while in the southwest, the process of accretion is predominant. The results of this study can be considered as a reference for coastal development in Can Gio sea area in the present and in the future.

Study on Several Structural Solutions for Sea Dyke Construction Including Wave Deflecting Block at the Sea with Large Waves in Vietnam

Sea dykes play an essentially important role in disaster prevention. Sea dyke is one of the important constructions that always are noted at the highest level of protection. Therefore, these protections and improving the quality of sea dykes is considered a particularly important task. Sea dykes are directly affected by sea waves, during the rainy and stormy season in particular, so many segments of Dyke Sea have been degraded and decreased effective coastal protection, easily eroded, broken, and sea water intruding inland, causing cause a lot of damage. The phenomenon of coastal erosion has caused serious socio-economic consequences such as loss of residential land, threatened to destroy sea-dyke works, separate traffic routes. In addition, it also causes social consequences for coastal communities. To reduce the above consequences and to ensure the stability of sea-dyke works, synchronous and long-term strategic solutions is required. Therefore, it is necessary to study solutions to improve the quality of sea dykes. Thus, the topic of the research is "Study on several structural solutions for sea dyke construction including wave deflecting block at the sea with large waves". That is also the goal of this article.

Strengthening climate-resilient development and transformation in Viet Nam

Climate change is presenting an ongoing and eminent threat to various regions, communities and infrastructure worldwide. In this study, the current and future climate impacts faced by Viet Nam due to Tropical Cyclones (TCs), specifically wind and surge, are evaluated, and different adaptation measures to manage this risk are appraised. The level of wind and storm surge risk was assessed focusing on three categories of assets: residential houses, agriculture, and people. The expected damage to these assets was then evaluated based on their exposure to the hazard under current and future climate scenarios. Physical adaptation measures such as mangroves, sea dykes, and gabions, and financial adaptation measures such as risk transfer via insurance were applied to the expected future risk and evaluated based on a socio-economic cost–benefit analysis. The output will give decision-makers the ability to make more informed decisions, prioritize the most cost-effective adaptation measures and increase physical and financial resilience. The results indicated significant TC exposure in future climate scenarios due to economic development and climate change that almost doubles the current expected damage. Surge-related damage was found to be many times higher than wind damage, and houses had more exposure (value in total) than agriculture on a national scale. The physical adaptation measures are successful in significantly reducing the future wind and especially surge risk and would form a resilient strategy along with risk transfer for managing TC risks in the region.

NGHIÊN CỨU SỰ PHÂN BỐ CỦA LOÀI CÓC KÈN (Derris trifoliata LOUR.) Ở XÃ GIAO LẠC, HUYỆN GIAO THỦY, TỈNH NAM ĐỊNH

Derris trifoliata is a species which participate in mangrove ecosystem. Species’s distribution depends on many factors, the most important of which is soil structure. Derris trifoliata grows well on clay loam soil or sandy loam soil. Firm ground is less favorable for true mangrove species, so the abundance found along sea-dykes, coastal areas and aquaculture lagoons is much higher than in mangrove core zone. On sandy soil or light sandy loam soil, when sand percentage is high, plants grow poorly with very low density. Study results show that Derris trifoliata can grow on many different types of substrates, but mostly on soil with sand percentage from 39 to 50%, clay from 39 to 45% and limon about 10%. Derris trifoliata grows near sea dykes and aquaculture lagoons, the nearer it is towards sea, the lower abundance and density is. Other ecological factors such as humus content, total nitrogen, C:N ratio, and salinity also affect the distribution of Derris trifoliata but to a lesser extent. In places with high density of Derris trifoliata, although the amount of humus in the soil is not much, the average nitrogen content is high because Derris trifoliata belongs to Fabaceae family, nodules lumps formed on their roots by Rhizobium can fix nitrogen from atmospheric nitrogen.

Study on the impact of the tide on the sea dyke stability in Quang Ninh province

The different reasons causing the instability of slope, one of the causes affecting the stability of slope of the dyke body is the impact of the water level in front of them and the unstable seepage. Especially in the current climate change, the change of water level on the flow system (rivers, streams, …) and sea is greatly affected, which is reason causing the instability of slope of the dyke body. Therefore, this paper will analyze the impact of the tide on the dyke stability, which contributes to supplementing and completing the design and audit method of the dyke slope stability. The resutls are applied analysis and calculation on some typical sea dyke in Quang Ninh province.

A Formula for the Relationship between Penetration Depth, Viscosity of Asphalt Grouting and Diameter of Stone, Apply for Designing Asphalt-Grouted Stone Revetments

Where suitable rock of only a limited size is scarce, asphalt grouting can be used to provide an impermeable armour layer of greater resistance to wave action than the component rock alone. Asphalt-grouted stone revetments provide a flexible and durable cover layer which can resists abrasion, is indifferent to chemical attack by salt water etc.The paper presents the research on the relationship between the depth of asphalt grouting penetration with the diameter of stone and asphalt viscosity to apply for designing asphalt-grouted stone revetments to protect sea dykes.

Simulations of future typhoons and storm surges around Tokyo Bay using IPCC AR5 RCP 8.5 scenario in multi global climate models

ABSTRACTThe object of the present study is to numerically evaluate the characteristics of future potential typhoons and storm surges around Tokyo Bay under a RCP8.5 scenario usinga simple one-way model system composed of ARW-WRF and FVCO. In order to evaluate the effect of the expected future increase in sea surface temperature (SST), air temperature (AT) and relative humidity (RH) four types of cases were considered in the 2041–2060 and 2081–2100 horizons. The results show how higher SST will likely lead to more intense typhoons and storm surges, though higher AT in troposphere and tropopause will somehow moderate this effect. In addition, a simple method to estimate the most influential wind direction that can generate surge elevations in Yokohama, Tokyo and Funabashi was examined by using the results of a numerical simulation. Finally, a simple methodology to calculate the required design height of sea walls and dykes in Tokyo Bay was developed, which shows how current design levels will be insufficient to maintain the present acceptable risk levels in the area.

Field Surveys and Numerical Simulation of the 2018 Typhoon Jebi: Impact of High Waves and Storm Surge in Semi-enclosed Osaka Bay, Japan

Typhoon Jebi made landfall in Japan in 2018 and hit Osaka Bay on September 4, causing severe damage to Kansai area, Japan’s second largest economical region. We conducted field surveys around the Osaka Bay including the cities of Osaka, Wakayama, Tokushima, Hyogo, and the island of Awaji-shima to evaluate the situation of these areas immediately after Typhoon Jebi struck. Jebi generated high waves over large areas in these regions, and many coasts were substantially damaged by the combined impact of high waves and storm surges. The Jebi storm surge was the highest in the recorded history of Osaka. We used a storm surge–wave coupled model to investigate the impact caused by Jebi. The simulated surge level was validated with real data acquired from three tidal stations, while the wave simulation results were verified with observed data from four wave monitoring stations. The high accuracy of the model demonstrates the usefulness of numerical simulations to estimate the heights of storm surges and wind waves at specific locations, especially where no monitoring stations are available. According to the simulation, the significant wave height was nearly 13 m in the entrance of Kii Strait between Tokushima and Wakayama and 4 m inside Osaka Bay. During the field survey, we encountered collapsed sea dykes, which were obviously damaged by high waves. In fact, the storm surge reached only 1.7 m above the normal tidal level at Kobe, Hyogo, which was not extremely high. Hence, the combination of storm surge and high waves can explain the extent of destruction in Hyogo, such as the failure of an inland floodgate and a stranded large vessel over the breakwater, which were observed during the field survey. We emphasize the importance of adequate coastal designs against high waves even in semi-enclosed bays, as they seem to have been underestimated when the typhoon disaster risk management was conducted.

THAY ĐỔI ĐƯỜNG BỜ BIỂN TRÀ VINH TỪ NĂM 1989 – 2014

The coastline of Tra Vinh, about 65 km long from the mouth of Cung Hau to Dinh An, is composed of loose sediment, which is easily affected by natural factors and human activities. Based on satellite image analysis combined with survey and field measurements, the changes in coastline between 1966 and 2014 were identified. The results show that in this period the coastline had an average deposition rate of about 5–10 m/year. Dong Hai was the strongest deposition area with maximum rate of 40 m/year, average 28–30 m/year. The eroded coastline alternated with an average erosion rate of about 5–8 m/year. From 1990 up to now, many human constructions have been carried out along the coast of Tra Vinh, contributing to the impact of changing the shoreline. In the area of My Long Nam, Dong Hai and Long Vinh, the mangrove planting project was performed which helped to create continuous accretion of coastline with average rate of 17–33 m/year. In the period of 2009–2014, sea dykes in Hiep Thanh commune and Con Truong, Truong Long Hoa commune were built to prevent coastal erosion. During 2009–2014, Hiep Thanh coast and Truong Long Hoa coast were eroded at an average rate of 8–20 m/year and 6–10 m/year, respectively. After appearance of the dykes, the survey results show that in the 2014–2015 period, the shoreline of Hiep Thanh and Con Trung which has sea dykes was no longer eroded, but the adjacent coastline was more eroded at an average rate of 14–38 m/year. In Dan Thanh commune, where Duyen Hai electric center and Quan Chanh Bo canal were constructed, from 2009 to 2014, the coastline was eroded with the average rate of 12–24 m/year. In 2014–2015 period, coastal erosion was stronger at an average rate of 36–45 m/year.

Lifetime wave overtopping risk analysis of sea defences subjected to changing operational conditions

Abstract Sea defences are at increasing risk of wave overtopping due to changing hydraulic conditions caused by climate change and deteriorating structural resistance. This paper presents a method for assessing wave overtopping risk of coastal flood defences subjected to changing operational conditions. The joint probability of hydraulic parameters such as sea water level and significant wave height is investigated using the extreme value analysis approaches taking into account rising sea level in the future. Meanwhile, structural resistance degradation such as the crest level settlement of earth sea dykes is considered in overtopping risk analysis by utilising a state-based stochastic deterioration model. Time-dependent probability of wave overtopping failure is estimated from the associated limit state equation, and the effect of hydraulic parameters and crest level settlement on the risk of overtopping failure is investigated. A risk cost optimised maintenance strategy is then determined by balancing the risk of overtopping failure and the costs for structural maintenances. The results for the numerical example of a typical earth sea dyke show that the probability of overtopping failure will increase significantly due to changing hydraulic parameters and losing crest level, thus appropriate maintenance will be needed to reduce the risk during the lifecycle.

Lifetime wave overtopping assessment of coastal defences under changing environments

Coastal defence structures become more vulnerable due to changing environments such as sea level rise caused by climate change, hence requiring accurate time-variant reliability analyses in order to provide optimum maintenance strategies. This paper presents a method for assessing wave overtopping risk of the coastal defences subjected to changing operation conditions. The joint probability of sea water level and wave height is used to assess future hydraulic conditions on the basis of the UK climate projections. Structural resistance degradation of coastal defences such as the crest level settlement of earth sea dykes is also considered in the process of risk analysis and failure probability evaluation. A case study of a typical earth sea dyke section affected by changing environments is used to demonstrate the effectiveness of the proposed method. The results show that the overtopping failure probability of the earth sea dykes will increase significantly due to changing environments, and appropriate maintenances are needed to reduce the risk of the wave overtopping failure.

Reliability analysis and optimum maintenance of coastal flood defences using probabilistic deterioration modelling

This paper presents a method for time-dependent reliability analysis, future performance predictions and optimum maintenance strategy of coastal flood defences such as earth sea dykes subjected to changing operation conditions. The reliability analysis takes into account both the changes of hydraulic variables such as water level and wave height due to climate change and the deterioration of structural capacities such as crest freeboard loss and soil seepage length degradation over time. A probabilistic time-dependent semi-Markov process is adopted for estimating the transition probabilities on the basis of available field data through routine inspections and expert judgements. Limit state equations are established for evaluating time-dependent probability of failures associated with wave overtopping through the flood defence crest and piping in the underlying water conductive soils. A multi-objective optimisation method is then employed to find optimum solution space for asset inspection and maintenance by minimising both costs for inspection and annual risk of structural failures. The results for a typical earth sea dyke at Thames Estuary show that the probability of failures associated with wave overtopping and soil piping will increase significantly over time, thus appropriate maintenance strategy is required to reduce the risk caused by changing operating conditions.

Sea Dyke Rehabilitation and Climate Change in Dutch and Japanese Contexts

The purpose of this paper is to analyze the relationship between climate change and the need to rehabilitate sea dykes. Sea dykes are a critical component of coastal infrastructure and national flood prevention systems and are increasingly susceptible to a number of failure mechanisms under climate change conditions. This paper will explore case studies of sea dyke rehabilitation and climate change in both the Netherlands and Japan. Both countries have urban areas within close proximity to coastal areas and have constructed sea and river dykes as part of their national flood prevention plans. The International Panel on Climate Change published a report in February 2012 stating that mean global temperatures are going to increase by 1 to 3 degrees Celcius by 2050, which will affect global weather conditions. The characteristics of climate change which most affect sea dykes include the frequency and severity of storms as well as global sea level rise. These trends increase the risk of dyke failure modes such as overtopping, micro instability, and erosion of non-reinforced inner slopes. Techniques for rehabilitation both proven and proposed will be discussed with a particular focus on methods for implementation as well as the policy framework of these projects.

Human activities and coastal erosion on the Kien Giang coast, Vietnam

By 2009, the Kien Giang coast, Vietnam, had experienced significant coastal erosion and mangrove degradation. Recent mitigation strategies, developed through policies, plans and mangrove planting programs have not been successful, in part because the causes of coastal erosion were not adequately identified. This paper investigates the relationship between human activities and coastal erosion in Kien Giang province. This study used mixed methods to understand the causes of coastal erosion with an emphasis on human activities. In this investigation, local communities were involved as co-investigators to explore the causes of coastal erosion in Kien Giang province. While natural factors (adverse effects of climate change and sea level rise) have been widely reported as main causes of coastal erosion, human activities initially were not recognised by local communities as significant contributors to coastal erosion and mangrove degradation. Human activities such as poor aquaculture pond construction, poor construction of new and upgraded sections of the sea dyke system, mangrove afforestation using only a single species, mangrove cutting for commercial and domestic uses, and construction of local boating channels, and the interaction of anthropogenic activities and physical processes are significant contributors to erosion. The study resulted in the awareness of the impact of community activities on the coast being improved. Knowledge gaps and necessary policy changes are identified.

System development for storm surge hazard assessment based on WebGIS for Tianjin Binhai New Area

It is imperative to develop a risk assessment system for quickly predicting storm surge disaster due to the vulnerability of Tianjin Binhai New Area. The flood routing model with user-defined breaches was firstly established based on the seed spread algorithm in order to achieve a rapid forecasting of storm surge flood information. Furthermore, fuzzy mathematics was utilized to identify the storm disaster grade, and the hazard mapping was conducted to visually obtain the hazard spatial and temporal distribution. Finally, the flood routing visualization method was proposed based on numerical simulation of storm surge to achieve the reappearance scene of dynamic evolution process. The developed system can play a vital role in the management and decision-making of sea dyke mitigation engineering in Tianjin Binhai New Area.

ASSESSMENT OF CHANGES OF EXTREME WAVE CONDITIONS AT THE GERMAN BALTIC SEA COAST ON THE BASIS OF FUTURE CLIMATE CHANGE SCENARIOS

Information on possible changes in extreme wave heights is needed to determine the future effectiveness and safety of coastal and flood protection structures. In this study, an assessment of possible changes in the extreme wave heights at selected locations along the German Baltic Sea coast has been carried out on the basis of numerical simulations of waves in the western Baltic Sea and regional climate model data for the past and the future (1961-2100). The future climate change signal of significant wave heights with a return period of 200 years mainly depends on: (i) the location, (ii) the climate change scenario run, (iii) the time period of the comparison, and (iv) the approach adopted to calculate the wave climate. The results show increases of up to +0.5 m and decreases of up to -0.5 m of the extreme wave heights. The increases might have considerable effect on the constructional design of coastal and flood protection structures-such as breakwaters, sea dykes, and vertical walls-because the extreme wave heights are used as input parameters for the design of the structures.

Mathematical modeling of coastal marine environments using observational data for coastal management

Coastal development affects human life and economic activity. Given a necessity to develop coastal areas, there is a need for a method by which to understand and quantitatively assess and predict changes to coastal marine environments. In this paper, we propose a mathematical modeling for coastal marine environments using observational data. In particular, we establish probabilistic graphical model based on the data-driven statistical model for Saemangeum coast of Korea, where land reclamation work has been taking place. The derived model consists of latent and observation variables and their causal relationships. Ocean currents occurred by water exchange appear to be the key factor influencing the coastal marine environments in the artificial lake of Saemangeum coast. Hence, coastal water quality in the coastal management is the major concern by stakeholders. Using the proposed model we were able to compute the followings to demonstrate the usage of our proposed model: First, if the lake were to be entirely cut off from sea water exchange (which takes place through sluice gates in the sea dyke to the open sea), coastal water quality may deteriorated to approximately 37.5% of its current quality. Secondly, in order to maintain a minimum acceptable coastal water quality in the artificial lake, permitting its use in industrial supply and agriculture, currents of about 0.6 m/s are required for sea water exchange. This approach will assist in coastal management by supporting decision-making, policy planning, and the establishment of strategies for sustainable coastal development and conservation of coastal marine environments.

해안제방 시공 중 해수유입에 대한 차수보강 사례분석

In this study, the causes and countermeasures for the leakage of a sea dyke under construction are analyzed. In general, the seabed ground is clearly divided from the embankment but a lot of parts show abnormal zones with low resistivity from the results of electric resistivity survey. Hence the causes of the leakage are considered as following: three-dimensional shear strain behavior, irregular compulsory replacement of the soft seabed ground with low strength and quality deterioration of the waterproof sheets during the closing process. The improvement method is determined by considering the constructability in the seawater and its velocity condition, durability, economic feasibility, similar application cases and so on. Consequently, a combination of low slump mortar and slurry grouting and injection method is selected as an optimum combination. Mixing ratio and improvement pattern are determined after drilling investigation and pilot test. The improvement boundary is separated into general and intense leakage area. The construction is performed with each pattern and the improvement effects are confirmed. The confirmed effects with various tests after completion show tolerable ranges for all of the established standards. Finally, various issues such as prediction of length of the waterproof sheet, installation of it against seawater velocity, etc. should be considered when sea dykes are designed or executed around the western sea which has high tide difference.

Time-dependent reliability analysis of coastal defences subjected to changing environments

This paper presents a method for assessing the risk of wave run-up and overtopping of existing coastal defences and for analysing the probability of failure of the structures under future hydraulic conditions. The recent UK climate projections are employed in the investigations of the influence of changing environments on the long-term performance of sea defences. In order to reduce the risk of wave run-up and overtopping caused by rising sea level and to maintain the present-day allowances for wave run-up height and overtopping discharge, the future necessary increase in crest level of existing structures is investigated. Various critical failure mechanisms are considered for reliability analysis, i.e., erosion of crest by wave overtopping, failure of seaside revetment, and internal erosions within earth sea dykes. The time-dependent reliability of sea dykes is analysed to give probability of failure with time. The results for an example earth dyke section show that the necessary increase in crest level is approximately double of sea level rise to maintain the current allowances. The probability of failure for various failure modes of the earth dyke has a significant increase with time under future hydraulic conditions.