Analysis Of The Characteristics In Nearshore Seabed Topography Evolution And It’s Applacation

Coastal areas have important support role in the social and economic life, and where a majority of important infrastructures and industries located. As the interface of oceans and inlands, the socioecological environment of coastal areas is quite sensitive and prone to anthropogenic impacts. Offshore seabed, an important part of the coastal area, is basically formed with discharge from the upstream and the hydrodynamic conditions of water and sediments. But recently there is a growing awareness that offshore seabed is significantly influenced by human activities such as hydroelectric dams and deep-water channels. The seabed erosion and accretion, which shapes the offshore topography and the coastline, is a good reflection of the exchange pattern of water and sediments as a direct or indirect result of large-scale projects such as reclamation, construction of wharves, harbors, channels, and flood seawalls. Therefore, regarding its importance and threats faced with, it is quite necessary to carry out in-depth research on analyzing the offshore erosion-accretion patterns and take effective strategies for the safety of seawalls and sustainable development within the coastal areas.This paper focuses on an optimal method to analyze the terrain evolution characteristics of offshore seabed on basis of conventional methods for hydrodynamics, geology, sedimentology, particularly as well as mathematical statistics and geostatistics in ArcGIS. The offshore seabed of Shanghai Petrochemical Complex (SPC) in the north of Hangzhou Bay is selected as our study site. Major work of this research includes:error identification and elimination of underwater topography data measured by shipborne equipment; comparison and optimization of spatial interpolation method in the geostatistical analyst package; safety assessment of petrochemical seawalls in the light of seabed erosion-accretion characteristics and evolution mechanisms. Four critical conclusions are obtained:(1) Voronoi topographic map created with cluster analysis method in the geostatistical analyst package is useful to identify the discrete gross error in the topography data. Moreover, regarding the spatiotemporal graduality and continuity in the data, each Voronoi topographic map was built with two adjacent years’ elevation data, thus the ’ribbon’ error can be eliminated effectively.(2) Through cross validation, four widely used spatial interpolation method in the geostatistical analyst package were compared. Results show that Experience Bayesian Kriging (EBK) interpolation is more accurate if all theoretical models and parameters are set as default in GIS. But after models fitting and parameters optimization using variogram analysis in GS+, the precision was improved for Ordinary Kriging (OK), Universal Kriging (UK), Simple Kriging (SK), which was obviously superior to that of EBK. Moreover, SK is slightly better than OK and UK, since it is more sensitive to the optimization of parameters of theoretical models.(3) As for the study site SPC, the evolution of offshore seabed is dominated by erosion in areas deeper than 15m in the long term (1972-2015). The terrain in the middle of Jinshan Trough remained relatively stable, but the entire trough extends lengthwise and broadens southward obviously in the open area on both sides. The annual net erosion rate reached 7.57cm/a during 2002-2012. In general, the development of Jinshan Trough has a close relationship with the sedimentary strata, but the erosion-accretion pattern is significantly affected by varying conditions of water and sediments due to human activities in the short term.(4) Safety assessment considered the flood control capability of seawalls, the stability of Jinshan Trough and the seabed erosion-accretion within the offshore shallow waters. First, the current seawalls has a promising ability to prevent severe floods. Second, most regions of Jinshan Trough (64%) are in a steady state of erosion-accretion, but the entire trough is moving slightly away from the coastline especially in the west of the sixth coastal levee and the city beach. Third, through measuring the distance between ’dot’ and ’line’, the average seabed width between the coastline and the 10 m/15 m isobaths was calculated, which exhibits that the topography evolution within the offshore shallow waters is dominated by accretion in areas shallower than 15m. Both the 10 m and 15 m isobaths is closed to the coastline. But the seabed width is much smaller along the sixth coastal levee and the city beach where erosion dominates, which is a big threat to seawalls safety and deserve close attention from municipality.Two main innovations of this thesis are:(1) In terms of the geostatistical analysis, this thesis contributes to the body of knowledge to identify and eliminate errors in underwater topography data measured by shipborne equipment, using spatiotemporal Voronoi topographic maps which accommodate graduality and continuity of the original data. Besides, a simple but effective tool developed based on GIS can provide quick assessment result of seawall safety.(2) In terms of the application output, this thesis can provide useful reference, such as the quantity and density of measuring points, for strategies of measuring the seabed topography on basis of the terrain features and accuracy requirements. According to the major findings, it calls for more points within regions covered by a complex terrain, but less within flat and uniform areas. In addition, the method for safety assessment of seawalls can be duplicated elsewhere beyond Shanghai, and provide scientific reference for the safety management of seawalls.