Coastal Dune Stabilisation in Central Vietnam

Land Use and Climate Change Interactions in Central Vietnam: LUCCiEdition: Water Resources Development and Management, 2017

Ecosystem-based measures to reduce the risk of coastal hazards and adapt to climate change have attracted increasing attention in science, policy, and planning. In the coastal zone of Quang Nam province in Central Vietnam, natural ecosystems such as coastal dunes and mangroves can serve as natural buffers against typhoons, storm surges, waves, and even small tsunamis and protect the shoreline from coastal erosion and sea-level rise. Apart from these protective ecosystem services, intact dune and mangrove ecosystems perform various other regulating services, such as carbon storage and sequestration and groundwater protection, as well as a variety of provisioning and cultural services. Moreover they support a high biological diversity, which forms the basis for secure livelihoods of coastal communities. However, important ecosystem functions and services have already been lost or diminished due to overexploitation and a lack of integrated coastal management approaches. In this chapter, we (1) provide an overview of the current distribution and status of dune and mangrove ecosystems in the coastal zone of Quang Nam province, (2) analyze the actual and potential ecosystem services of and threats to these ecosystems in selected study sites, (3) assess the awareness and preparedness of coastal communities to coastal hazards, and (4) discuss the potential for conservation, restoration, and sustainable use of coastal dunes and mangroves.

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Quang Ngai, a coastal province in central Vietnam, is prone to flash flood caused by high annual rainfall, with 70% (2300mm/yr) falling over three months and steep terrain resulting in high velocity water flows. This often leads to flash floods in mountainous areas and large scale flooding in low-lying areas. The high velocity flows and flooding cause severe erosion on dike, canal and river banks built to protect farm land from flooding in the rain season and sea water intrusion in the dry season. Therefore the stability of these measures provides the local community a protection against flash flood and sea water intrusion at the same time.

The coastal erosion and sedimentation are deeply anxious for many local authorities and communities in Vietnam. The erosion has happened in large scale, and its intensity increased with time. The sedimentation has been expansive for siltation of shipping channels and filling river mouths and lagoon inlets. The deep causes of those are related to coastal evolution, and human impact in both coastal zone and catchment. In many cases, the sedimentation are accompanied and caused by sediments supplied from eroded process neighboring. These disasters have created heavy consequences such as damages of human beings, properties and land; and degradation of coastal environment and ecosystems; unsustainable development, emigration; and unstable thought in life and production of communities. It needs to build national strategy and scientific base for management of these disasters. The control of them needs to be in the framework of integrated coastal management, combining with catchment management. The priority is given to resolutions combined with other benefits, and to combination of controlling coastal erosion and sedimentation. It is also necessary to strengthen legislation and regulations, build database; establish a network of monitoring disasters regularly. The international cooperation must be regards as an important resolution responding to coastal erosion and sedimentation.

ABSTRACT Coastal dunes play an important role in coastal defense along sandy shorelines of the world. The majority of the shorelines experience erosion and this erosion is expected to accelerate under anthropogenic climate change and subsequent sea level rise. This paper investigates the impact of climate change, sea level rise and current management for coastal dunes in the Netherlands. Furthermore the paper discusses the implications of climate change projections for adaptation strategies into the future. Recent climate change scenarios for the Netherlands highlight rising temperature and accelerated sea-level rise. Their combined effects on dune-building processes are expected to be manifested through an increase in erosive forces at the expensive of accretive forces. In the Netherlands, a negative sand balance and inland migration of the beach-dune system has been successfully counteracted in the last decades through the application of sand nourishments. These have enhanced accretion on the one hand and limited erosion on the other hand. Generally, coastal protection has improved despite rising sea levels. Important preconditions that make this sand nourishment strategy possible are: a readily available sand resource that makes exploitation technically and economically feasible; a sound monitoring system supported by solid science; political consensus and a good institutional structure to implement the strategy. In the Netherlands, the necessary preconditions are already in place to successfully adapt to sea level rise. Given the expected accelerated rise in sea level and its potential effects on the dune-beach sediment balance, the annual sand nourishment will need to be intensified to ensure the preservation and integrity of the coastal zone.

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Every year, in the Vietnam Mekong Delta Coastal Zone (VMDCZ), erosions cause approximately 300 ha of agricultural land loss. Therefore, measures for shoreline protection are urgently needed. This paper discusses the impacts of protection measures in the Go-Cong Coastal Zone to prevent erosion/accretion processes, predicted by two numerical models, MIKE21-FM and TELEMAC-2D. Hard and soft measures have been proposed using breakwaters and sandbars, respectively. The simulations show that the erosion/accretion trends provided by both models are similar. For breakwaters, MIKE21-FM provides less accretion than TELEMAC-2D in areas extending over 300 m and 500 m from shorelines. However, for sandbars, MIKE21-FM shows higher accretion within areas extending over 500 m but less than 300 m. Sandbars cause higher accretion in a larger area, extending over 1000 m offshore. The simulation results allow us to propose two alternative measures: (1) a row of several breakwater units will be implanted...

Much of the Dutch coast has been subject to structural erosion. From 1990 onward, sand nourishments have been used under a government policy of dynamic preservation. Annual monitoring and field inspections show that the structural erosion has decreased or even turned into coastal progradation after 1990. The monitoring data concern only morphodynamics and thus supply limited information on system-related geological processes driving the observed changes. Recently acquired ground-penetrating radar (GPR) data help establish the origin of sedimentary elements within the beach-foredune area, determine their decadal-scale preservation potential under the present nourishment policy, and demonstrate temporal and spatial accretion/erosion variability along nourished coasts. GPR images from an onnourished retrograding barrier section show historical storm surge deposits within the eroding foredune and accumulations of natural eolian sediment farther landward. GPR images from a heavily nourished, prograding site show that the accreted foredune and beach consist of nourishment embankments (20%), wind-blown units derived from nourished sand (70%), and progradational beach deposits (10%). The net volume of accretion at this site is approximately 200 m3 /m. Remarkably, almost all sand nourished before 2000 has been washed away, except for embankments constructed in 1990. Analysis of meteorological data suggests that 1999 storm surges are responsible for this erosion. The relative longevity of post-2000 nourishments can be attributed to a combination of shoreface nourishment and favorable meteorological conditions. During a storm surge in 2007, water-lain embankments proved to be more resistant against wave erosion than nourished sand redistributed by wind, indicating the importance of grain size, roundness and packing in the durability of nourishments.

Climate change related disasters such as erosion along riverine and coastal areas of the Mekong Delta in south and the Red River Delta in north are expected to be exacerbated by land subsidence, sea-level rise (SLR), and magnified typhoons. Adaptation to severe erosion is expected to respond to regional circumstances and the demands of local residents. Based on the expectations outlined above, for soft adaptation, attempts were made to conduct perception surveys of local residents, in addition to field surveys of erosion at riverside and coastal areas using an un-crewed aerial vehicle (UAV). Furthermore, for hard adaptation, a proposal is made to conduct pilot field tests at the coast for reinforcing coastal dykes using the combined technique of locally available materials with cost-saving eco-geosynthetics in addition to application of ICT. This paper explains the possibility of smart adaptation combining soft and hard adaptation to reduce severe coastal and riverine erosion in the Vietnamese deltas.

Among the effects of global warming, sea level rise (SLR) and severe typhoons pose the greatest threat to the stability of human settlements along coastlines. Therefore, countermeasures must be developed to mitigate the influences of strong typhoons and persistent SLR for coastal protection. This study assesses climate change impacts on coastal erosion, especially in two projected SLR scenarios of RCP2.6 and RCP8.5. The results show that SLR and severe typhoons lead to the increase of coastal erosion, beach lowering and scour. Moreover, as in projected SLR scenarios, average waves in high tide can cause severe soil erosion at inner slopes and lead to dyke failure by 2060. SLR also increases pore water pressure and causes larger wave attacks on the seadykes making them more unstable in typhoons and storm surges. The paper highlights the need for additional geotechnical engineering measures to protect the coast of Hai Hau district against SLR and severe typhoons. Among the alternatives available for countering these threats, applying soil stabilization and soil improvement combined with geosynthetics are promising strategies for coastal structures. Hybrid structures can be used with earth reinforcement and soil improvement. Additionally, the paper emphasizes the importance of multiple protective adaptations, including geosynthetics and ecological engineering measures against climate change-induced severe erosion on the coast of Hai Hau district.