Impacts Of Reclamation On Tidal Dynamics In Hangzhou Bay

Hangzhou bay (HZB), with a typical funnel-type area of covered more than 4,800km2, is the estuary of Qiantang River (QR) and closely connected with Changjiang Rriver in its north. Besides, it’s linked to Ease China Sea through a branch of water passages isolated by different islands, such as Shengsi, Daqu, Changtu, Daishan and Zhoushan. Since 1960s, continuous reclamations happened in the estuary of QR and HZB coast, which had dramatically changed its hydrodynamics and morphology. What’s more, sediment transport and pollutant diffusion process would be inevitably impacted, which may also lead to the change of flushing and silting state near sea bed. Most importantly, these effects induced by anthropogenic activities are unrecoverable. Therefore, it’s necessary to evaluate the outcome of the past decades reclamations.Based on the past document and Landsat MSS\TM\ETM data, the evolution of coastal line of HZB in four periods (1962,1973,2005,2015) was analyzed. Then a 3-D unstructured high resolution FVCOM hydrodynamics model was built and calibrated. With the model and coastal line data collected from remote sensing, three different numerical simulation scenarios were conducted to hindcast HZB tidal dynamics in 1962 (before large scale reclamation),2005(the reclamation between 1962 and 2005) and 2015 (last decade reclamation). Tidal level, tidal current, residual current were compared and analyzed to investigate the change of inner hydrodynamics. The main conclusions are as following: Remotes sensing data shows that the south coastal line was severely changed while north did not change too much. For instance, the distance between Ganpu and Cixi was decreased 17 percent from 1962 to 2015, so did the distance between Zhapu and Andong. Reclamation conducted by human is the main reason for the result. The 3-D hydrodynamic model simulates the tidal characteristics well in the domain. The result shows the tidal phase was not simultaneously in the upper limit of HZB and its mouth. Besides, the tidal energy flux in the water passages between Zhoushan Islands shows a high value, which is worthy to be more investigated.The comparison between each other experiment discovered several major results. The simulation shows the water level in spring high tide at the upper limit of HZB is 0.5m higher in 2005 than in 1962, which was close to observation data. While it did not change too much in the middle part of HZB. The reclamations in the last decade lead to the spring high tide water level rise around 0.2m/s in the middle part of HZB but has little effect near the upper limit of bay. All the reclamations made little difference to the low tide. Besides, reclamation has no relationship with the water level in the area of HZB mouth and open sea.The shape of tidal current ellipse inside the bay evolved along with change of coastal line, which showed a decrease changed trend from the upper limit of HZB to its mouth. And the main tidal channel shifted a little to north from the transversal current profile. The velocity increase in the upper limit of HZB while decrease near the tidal flats in south coastal line.The residual current increased noticeable in the upper region of HZB, particular near the cross line between Ganpu and Cixi. The max residual current region evolves along with coastal line. All the surface residual current, middle layer residual current, bottom residual current, show similar trend but the deeper, the less change happened due to bottom frication.