Satellites HY-1C and Landsat 8 Combined to Observe the Influence of Bridge on Sea Surface Temperature and Suspended Sediment Concentration in Hangzhou Bay, China

Abstract

We analyzed the influence of a cross-sea bridge on the sea surface temperature (SST) and suspended sediment concentration (SSC) of Hangzhou Bay based on landsat8_TIRS data and HY-1C data using an improved single window algorithm to retrieve the SST and an empirical formula to retrieve the SSC. In total, 375 paired sampling points and 70 transects were taken to compare the SST upstream and downstream of the bridge, and nine transects were taken to compare the SSC. The results show the following. (i) In summer, when the current flows through the bridge pier, the downstream SST of the bridge decreases significantly, with a range of 3.5%; in winter, generally, the downstream SST decreases but does not change as obviously as in summer. The downstream SSC increases obviously. (ii) The range of influence of the bridge pier on the downstream SST is about 0.3–4.0 km in width from the bridge and that on the downstream SSC is approximately 0.3–6.0 km. (iii) When the current flows around the pier, a portion of the flow is dispersed in upward and downward directions; the downward flow generates local scour. When the scouring at the front end of the pier stops, the upward flow behind the pier brings the sediment and the bottom cold water downstream, causing the downstream SST to decrease and the SSC to increase. (iv) The other portion passes around the pier, which generates a wake vortex. Once a wake vortex is released, a low-pressure center appears, sucking the sediment and the bottom cold water to the downstream sea surface, reducing the downstream SST and raising the SSC. (v) The range of reduction of the SST downstream of the bridge is shorter than the range of increase in the SSC. This is because the wake vortices have an effect in the 0.3–4.0 km downstream but not in the 4.0–6.0 km. Therefore, the SST and SSC are affected within the range of 0.3–4.0 km by wake vortices, while in the 4.0–6.0 km region, the SSC is still high due to the transport of sediment by currents.