Research On Some Key Techniques Of Decision Support System For Coastal Erosion In The Yangtze Delta

In the past, the huge amount of sediment load from the Yangtze River contributed to the successive accretion of the Yangtze Delta. However, contemporarily global warming and the construction of large hydraulic engineering works in the catchment and its estuary significantly decelerate the trend, and the delta degradation has been observed since 2003. Considering the combined influence of the planning dams and the South-to-North Water Diversion Project, the anticipated coastal erosion problem will be even worse. Since the Yangtze Delta is one of the most important economic regions in China, a long-term coastal spatial planning will be an inevitable issue for local governments. While, most coasts in the Yangtze Delta are typically muddy that are characterized by extremely complex land-sea interaction. In view of the complexity and uncertainty of human and natural dimension under the circumstances of global change, the need for integrating human and computer intelligence necessitates a Decision Support system (DSS) for the risk assessment of coastal erosion. In this thesis, we specially focused on some key techniques for the DSS. It mainly includes four aspects:(1) we develop an orthogonal transect scheme based on topographic gradient to trace the realistic changes of non-straight shoreline, and furthermore investigate the muddy non-straight shoreline dynamics of Chongming Dongtan in Shanghai using mapping based shoreline evolution models. Also, a shoreline interpolation was conducted in order to make the orthogonal transects trace the more realistic shoreline evolution. Based on the eCognition object-oriented image segmentation method, the shorelines of Dongtan are extracted from 6 scenes of Landsat-TM satellite images of 1987,1990,1995,1998,2003, and 2006, respectively. The results derived from the orthogonal transect scheme indicate that the shorelines of Dongtan are divided into the eroding segment in the south and accreting segment in the east and north. The eroding shoreline segments basically maintain unaltered and the accreting shoreline segments continue to expand by gradually decreasing speed. The maximum accretion rate and the maximum erosion rate are respectively 247.2m/a and 22.0m/a. The "suppressed" shoreline segments lie in the region of the boundaries of the island shadow. The results obtained appear coherent to the true situation, and can be utilized as a reference of decision making for coastal management.(2) Few rivers in the world are influenced so markedly by hydraulic engineering works as Yangtze River. An effective decrease in sediment discharge to the estuary has been induced since 1960s. As one of the only few naturally accreting tidal flats in Yangtze estuary, Chongming Dongtan would definitely modify its shoreline dynamics in response to the decrease. Based on Landsat-TM satellite images of 1987,1990, 1995,1998,2002,2006, and 2008, the proposed orthogonal transect method is used to obtain the shoreline change rate. According to the calculated average shoreline change rate, we divide the shoreline of Dongtan into four segments, erosion segment, rapid accretion segment, stabilization segment, and general accretion segment. They have different response patterns to the reduced sediment discharge. Furthermore, we provide a quantitative assessment of the impact of sediment discharge by regression analysis. The results show that, during 2003-2005, the Three Gorges Dam (TGD) and possible cascade effects contributed about 15% of total decrease of average shoreline change rate since 1987, while the anticipated impact of the South-to-North Water Diversion Schemes would be 5-10% of the TGD. And a critical sediment discharge at Datong Station, about 0.24/0.29×108 tons/year, controlling the transition of accretion to erosion in Dongtan is found. Without actions taken, Dongtan would reach the limits to growth within coming 20-50 years or a shorter span of time.(3) A new integration approach of beach evolution model and GIS was developed to evaluate the longshore sediment transport rate. By partitioning calculation units and modifying one-line model to meet complex physical setting, the integrated scheme was established. Firstly, the shoreline change rates are derived from EPR method based on GIS, where the shoreline indicator is selected as Om-isobath from digitized nautical charts. Then, the closure depth was obtained using the available information about the research results in literatures and the bathymetry of a specific coast. Finally, the longshore sediment transport rates can be calculated inversely in calculation units by substituting those values of the shoreline change rates and the closure depth into the modified one-line model. The estimated longshore sediment transport rates would include the accumulated effect of episodic events, sediment runoff transport, cross-shore sediment transport, and sediment exchange between flats and channels, etc., which were usually difficult to model. In experiments, the longshore sediment transport rates are computed for a portion of Nanhui Beach of Shanghai, respectively, by the proposed approach and topographic method. The results present that the total relative error is 8.77%. And some suggestions are also given to improve the proposed approach.(4) We present a Decision Support Framework (DSF) for the risk assessment of coastal erosion in consideration of the potential management problems and challenges for economic development in the coastal zone of the Yangtze Delta. The framework consists of four major components:integrated database, GIS-based risk assessment models, visualization toolkit and scenarios generator. Especially, we developed a GIS-based risk assessment model for the muddy coasts of Yangtze Delta. Firstly, four primary indicators including administration unit, population density, geological types and coastal characteristics were used to segment the shorelines into the relatively homogeneous units. Then, the DPSIR (Driving forces, Pressure, State, Impact, Response) framework was used to analysis physical and socioeconomic factors asscociated to the coastal erosion of the Yangtze Delta, and constructed an indicator system that includes 10 vulnerability indicators, i.e., coastal elevation, coastal slope, longshore sediment transport rates, shoreline change rates, tidal range, significant wave height, relative sea level rise, intertidal width, intertidal vegetation type and intertidal vegetation zone width, and 3 impact indicators, i.e., population density, land use type and ecological hotspots. Subsequently, depending on the nature of each of these variables, they were assigned ranks ranging from 1 to 5, with 1 representing minimum vulnerability/hazard and 5 representing maximum vulnerability/hazard. Before evaluating the vulnerability and hazard, these variables were weighted based on their relative importance in determining the coastal erosion vulnerability and hazard. An Analytic Hierarchy Process (AHP) was employed for the variables weightings. Finally, the Vulnerability Index (VI) and Hazard Index (HI) were evaluated based on a linear model. And the Risk Index (RI) was calculated by a simple weighted average of VI and HI. The assessment results show the validity of the approach. Accordingly, the DSF will make the specialized data and information more accessible to managers and other stakeholders, and has an extensive capability to facilitate communication and synergetic work between humans and computers. In this way, the proposed DSF is expected to make manager make more scientific decision.