Vulnerability Assessment Of The Coastal Wetlands In The Yangtze Estuary, China To Sea-level Rise

Sea-level rise (SLR) caused by global climate change will have important impacts on coastal ecosystems. The coastal wetlands, as important parts of coastal ecological environment, provide valuable ecosystem services, including climate regulation, flood regulation, water purification and coastal protection. The coastal wetlands ecosystems occurring at the transition zone between land and ocean are particularly sensitive to SLR. To study the impacts of SLR caused by global climate change on the coastal wetlands, assess the vulnerability of coastal wetlands to SLR and propose feasible and practical mitigation strategies are scientific basis for securing coastal wetlands ecosystems.The dominant vegetation of the coastal wetlands in the Yangtze Estuary mainly includes Phragmites australis, Scirpus mariqueter and Spartina alterniflora. Zonation of coastal wetland along the elevation gradient from the seawalls to the sea (Phragmites australis-Spartina alterniflora-Scirpus mariqueter) is widely recognized in the Yangtze Estuary, due to the adaptability difference of coastal wetlands ecosystems to habitat elevation and inundation duration. In this paper, taking the coastal wetlands in the Yangtze Estuary as a case study, the spatio-temporal change of wetlands vegetation in recent 20 years was analyzed by satellite images. The main impacts of SLR on the coastal wetlands are to change habitat elevation, and further affect the survival and growth of wetlands vegetation. The lowest distribution elevation of three types of wetlands vegetation in the Yangtze Estuary were gained by field experiment, which provided scientific basis for the vulnerability assessment of the coastal wetlands to SLR. The main impacts pathway, process and consequence of SLR on the coastal wetlands were analyzed by the Source-Pathway-Receptor-Consequence (SPRC) conceptual model and the vulnerability assessment model of coastal wetlands to SLR was built. Based on the IPCC vulnerability definition and the SPRC assessment model, an indicator system for vulnerability assessment on the coastal wetlands to SLR was developed, in which the rate of SLR, habitat elevation, daily mean inundation duration, land subsidence/uplift rate and sedimentation rate were selected. A process-based quantatively assessment method was established by quantifying each indicator, calculating the vulnerability index and grading the vulnerability index on the GIS platform. Vulnerability assessment, based on the projection of SLR rates from the present trend (0.26 cm/yr), IPCC AR4’s AlF1 scenario (0.59 cm/yr) and IPCC AR5’s RCP8.5 (0.98 cm/yr), were performed for three time periods:short-term (2010-2030), medium-term (2010-2050) and long-term (2010-2100). Finally according to the vulnerability assessment results, mitigation measures were proposed to mitigate the impact of SLR on the coastal wetlands ecosystems. The main studies were as follows:1 The spatio-temopral change of coastal wetlands in the Yangtze Estuary was analyzedBased on the satellite images, the spatial distribution of the coastal wetlands in the Yangtze Estuary in 1990,2000 and 2011 was obtained by processing data, previous studies and field verification. The total area and regional area of wetlands vegetaion in 1990,2000 and 2011 were calculated. The total area of coastal wetlands in the Yangtze Estuary declined during 1990-2000, and the area of P. australis saltmarsh significantly declined. The total area of coastal wetlands in the Yangtze Estuary increased during 2000-2011, and the expansion rate of S. alterniflora was higher than P. australis and S".mariqueter.2 The lowest distribution elevation of wetlands vegetation in the Yangtze Estuary was obtainedIn this study, based on the field experiment the survival and growth of P. austrails, S. alterniflora and S .mariqueter planted at different elevation were observed and measured in Chongming Dongtan. According to the experiment results, the lowest distribution elevationof P. australis, S. alterniflora and S".mariqueter saltmarsh were fit, being 2.9 m,2.5 m and 1.9 m, respectively. The reseach result provided scientific basis to assess the vulnerability of coastal wetlands to SLR.3 A conceptual model of "Source-Pathway-Receptor-Consequence, SPRC" was established for vulnerability assessment of coastal wetlands to SLRThe SPRC model was designed to solve what happened, what’s the pathway, what’s the receptor and what’s the impact, In this study, the main impact pathway, process and consequence of SLR on the coastal wetlands were analyzed by SPRC model and the vulnerability assessment model of coastal wetlands in the Yangtze Estuary to SLR was built. SLR (S) caused by climate change has important impacts on the coastal wetlands habitats(R) by crust vertical movement and sedimentation (P). According to the geomorphological characteristics and the type of the coastal wetlands in the Yangtze Estuary, the coastal wetlands habitats were divided into 10 kinds of receptor. The main impacts of SLR on the coastal wetlands are to change habitat elevation, and further change inundation duration of habitat. If the inundation duration exceeds the tolerance range of the habitat, it will affect survival and growth of wetlands vegetation, change the ecosystem structure and function, and finally result in habitat degradation and loss (C). Moreover built of seawalls at costal zone prevent coastal wetlands to migrate landward, and further exacerbate the impacts of SLR on the coastal wetlands, which results in large area of wetland habitat loss.4 An indicator system and a quantitative methodology for vulnerability assessment of the coastal wetlands to SLR were developed.Based on the IPCC vulnerability definition and the SPRC assessment model, an indicator system for vulnerability assessment on the coastal wetlands to SLR was developed, in which the rate of SLR, habitat elevation, daily mean inundation duration, land subsidence/uplift rate and sedimentation rate were selected. A process-based quantatively assessment method was established by quantifying each vulnerability indicator, calculating the vulnerability index (according to the impact process of SLR on the coastal wetlands) and grading the vulnerability index (according to the inundation duration difference of wetlands habitats) on the GIS platform.5 A spatial assessment on the vulnerability of coastal wetlands to SLR was realized.Vulnerability assessment was performed by calculating the vulnerability index and grading the vulnerability index on the GIS platform for three time periods:short-term (2010-2030), medium-term (2010-2050) and long-term (2010-2100), based on the projection of SLR rates from the present trend (0.26 cm/yr), IPCC AR4’s AlFlscenario (0.59 cm/yr) and IPCC AR5’s RCP8.5 (0.98 cm/yr). The results indicate that under the present SLR trend, the coastal wetlands are mainly in low vulnerability, the percentage amounting to 6.6%,9.8% and 8.1% in 2030s,2050s and 2100s, respectively. The coastal wetlands in low vulnerability are mainly located at the southern and southeastern coasts of Chongming Island. Under AlF1 scenario, the coastal wetlands are mainly in low vulnerability in 2030s and 2050s, amounting to 9.0% and 9.5%, respectively, the percentage in moderate and high vulnerability significantly increases in 2100s. The coastal wetlands in low vulnerability are mainly located at the southern and southeastern coasts of Chongming Island, western coasts of Hengsha Island and the coasts of Jinshan Biantan. The coastal wetlands in moderate and high vulnerability are mainly located at the southern and southeastern coasts of Chongming Island, the southern coasts of Jiu duansha and the coasts of Jinshan Biantan. Under RCP8.5 scenario, the coastal wetlands are mainly in low vulnerability in 2030s, and in low and moderate vulnerability in 2050s. The percentage in moderate and high vulnerability significantly increases in 2100s, amounting to 11.5% and 13.2%, respectively. The coastal wetlands in low vulnerability are mainly located at the eastern coasts of Chongming Dongtan, the southern coasts of Jiu duansha and the coasts of Jinshan Biantan. The coastal wetlands in moderate vulnerability are mainly located at the northern coasts of Chongming Dongtan and the coasts Nanhui Biantan. The coastal wetlands in high vulnerability are mainly located at the southern and southeastern coasts of Chongming Island, the western coasts of Hengsha Island and the coasts of Jinshan Biantan.6 Mitigation and adaptation measures were put forward to cope with the possible impacts of SLR on the coastal wetlandsBased on the vulnerability assessment of the coastal wetlands, practical and effective mitigation measures were proposed. (1) Scientific management of sedimentation:carrying out coastal engineering can increase sedimentation and promote coastal wetlands development, which are important options to mitigate the impacts of SLR on the coastal wetlands. (2) Controlling land subsidence:the rate of land subsidence can be reduced by limiting groundwater extraction and controlling excessive construction of large engineering. Meanwhile in the process of development and utilization of the coastal wetlands resources, the subsidence effect of new sedimentary soil should be considered, and several measures should be carried out to reduce subsidence rate of coastal wetlands and mitigate the impacts of SLR on the coastal wetlands. (3) Recreating and extending the coastal wetland:using dredged sediments to recreate mudflat and promote coastal wetlands development seaward, and re-establish wetland pioneer vegetation to promote sedimentation and extending new wetlands are all effective soft management options to mitigate the impacts of SLR on the coastal wetlands. (4) Controlling reclamation:reasonable control of reclamation coastal wetlands in the Yangtze Estuary is promise to make the plan, design and implement of reclamation follow the natural succession law of coastal wetlands, which is an effective measure to mitigate the impacts of SLR on the coastal wetlands.