Synthetic aperture radar (SAR) is an active instrument which is used to create images of an object. Underwater bottom topography can be retrieved indirectly by measuring variations of the sea surface roughness in the SAR images, although the microwaves cannot penetrate into the water. In this paper, we present a new simple method for bathymetric mapping in the shallow sea. Based on the radiometric correction, sea surface roughness is derived using SAR images. These results are then used for water depth inversions based on the Alpers-Hennings (AH) model, supported by a few true depth data points (sounding data or chart data). This method is used to bathymetric mapping of two areas in the Subei shoal. The study results of the two cases show that the trend of the inversion and true depths match well. The retrieval accuracy depends on the true depth data points. In a case, the true depth data is the sounding data, the relative errors between the inversion and true depths is less than 20%. In the other case, the true depth data is the chart data. And the result is worse, because the measure time of the chart is 1979, and the time of SAR images are 2000s. The bottom topography was changed. The proposed method has two advantages in that it does not require environmental parameters and it is relatively simple to operate.
Coastal zone is the interaction area between the ocean and the land, and it is one of the most important residential areas of human. Coastal area management and planning is necessary in utilizing coastal space and resources. Coastal zone changed rapidly in recent decades in Tangshan, China. In this research, a total of 11 Landsat images were selected for studying the coastal change in Tangshan during the last 35 years. Results showed that the coastline length increased by 114.05 km, while land area increased by 449.76km2 from 1975 to 2010. The main period of coastal increasing in Tangshan occurred during 2005–2010, and the primary area changes happened in Caofeidian District and Jingtang Port. The main reason of the rapid coastal changes in Tangshan was the human activities of industrial and commercial district construction and harbor construction.
According to SAR imaging mechanism of underwater bathymetry, a 3-D hydrodynamic model based on SELFE (Semi-implicit
Eulerian-Lagrangian Finite Element) is applied to demonstrate the temporal variations of the channel bathymetry
changes. A microwave radar imaging of oceanic surface’s program of M4S is used to simulate the variation of
normalized radar cross section (NRCS) induced by the ocean surface current. The simulation is carried out to study the
effect of water depth changes and current variations on SAR imaging of channel bathymetry. The depth of channel is
defined between 5 m to 17 m with an interval of every two meters. Six time of tide are used in the current variation
simulation. The NRCS variation demonstrated in the simulated images are compared with in situ data, historical
bathymetric maps and calibrated SAR images. All the four kinds of images manifest similar changes of channels, which
proves that there is agreement between the simulation model used in this paper and other data. Simulation results also
show that bigger depth change induces bigger NRCS variation. In the time of maximum ebb tide, simulated relative
NRCS is bigger than others.
Coastline change, erosion-accretion evolution, and their relationship in the Pearl River Estuary (PRE) of China over the past 25 years are analyzed using six remote-sensing images from 1986 to 2011 and two nautical charts. Due to land reclamation in the period from 1986 to 2011, the total length of the PRE coastline increased by 149.2 km, which is equivalent to a growth of 0.57% per year, and the coastal land increased by 251.76 km 2 , which is equivalent to a growth of 0.23% per year; in addition, water depth change showed a trend in that foreshores became shallower, while deep channels became deeper. Areas where the coastline extended seaward had deposition to some extent, except for deepwater ports. Human activities played an important role in coastline change and erosion-accretion evolution in the PRE, which intervened with the natural variation of coastline and erosion-accretion. In addition, pollutants from the reclamation land became the major factors of coastal water pollution, which may significantly influence the environment of the PRE in a negative way.