Delineating suspended sediment concentration patterns in surface waters of the Changjiang Estuary by remote sensing anal
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Delineating suspended sediment concentration patterns in surface waters of the Changjiang Estuary by remote sensing analysis LI Jing1∗ , GAO Shu1 , WANG Yaping1 1
Ministry of Education Key laboratory for Coast and Island Development, Nanjing University, Nanjing 210093, China
Received 22 May 2009; accepted 14 December 2009 ©The Chinese Society of Oceanography and Springer-Verlag Berlin Heidelberg 2010
Abstract Three Landsat TM imageries (taken on 18 May 1987, 4 August 1998 and 28 July 2007) were used as the data source to identify the spatial and temporal variations of the suspended sediment concentration (SSC) in surface waters of the Changjiang Estuary. Atmospheric correction was carried out to determine the water-leaving reflectance using the FLAASH module. A regression equation between surveyed SSC and suspended sediment index was chosen to retrieve the SSC from the Landsat TM images. In addition, tidal harmonic analysis was performed to calculate tidal conditions corresponding to the acquisition time of satellite images. The results show that the SSC spatial patterns are similar to the in situ observation results, which show the highest SSC in the region of turbidity maximum zone in the Changjiang Estuary. For the period of 1987 to 2007, the SSC pattern is controlled mainly by tidal dynamic conditions and wind speeds, rather than sediment discharges from the river. Key words: suspended sediment concentration, Landsat TM image, tidal conditions, the Changjiang Estuary
1 Introduction The suspended sediment concentration (SSC) is an important indicator to reflect the sediment transport and resuspension processes, which connects the hydrodynamics with geomorphological conditions (Zuo et al., 2006). The fine particles containing a large amount of nutrients, organics and heavy metals can affect the ecosystem and environment within an estuary and over the adjoining oceanic waters (Gao et al., 2002). The estuary has been affected by both river runoff and tidal currents. The SSC in the estuary is controlled by multiple factors including freshwater and sediment discharges of the river, tides and waves. The change of SSC in the estuary can indicate the changes of sediment sources and transport processes or dynamic conditions; such changes have different temporal scales, and they jointly affect the magnitude and distribution pattern of the SSC. Large rivers (e.g. the Changjiang River), have huge freshwater and sediment discharges to the ocean. Since the early 1950s, more than 48 000 reservoirs
and dams have been built in the drainage basin of the Changjiang River which reduces the sediment discharge to the estuary, especially after the building of the Three Gorges Dam (Lu et al., 2006; Gao and Wang, 2008). Since the 19th century the Changjiang River delta has a general trend of continuous accretion, creating new land for Shanghai, one of the most prosperous regions in China (Wu et al., 2006). Yet during the past decades the decrease of sediment discharge resulted in a transition from accretion to coastal erosion (Li et al.,
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