| Tidal flats have important ecological service functions and are a potential land resource.Over the past four decades or so,there has been a tendency for the speed and intensity of coastal reclamation in China to increase in order to resolve human-land conflicts and the need for economic construction.At the same time,the ecological environment and land resource use problems brought about by excessive mudflat reclamation in some areas are becoming increasingly prominent.In the case of stable sediment sources,the coastline dynamics of muddy tidal flats are characterized by high vertical siltation and horizontal expansion;over time,the tidal flats in front of the embankment will gradually recover to their original height and width,so the choice of a rate of reclamation appropriate to the rate of tidal flat siltation has important implications for the natural recovery of tidal flats and the development of tidal flats planning and protection strategies.This study uses GIS and RS techniques to analyze the spatio-temporal variation of the coastline in the past 30 years in the three coastal and provincial administrative units involved in the national strategy ‘Changjiang River Delta Integration',namely,Jiangsu Province,Shanghai and Zhejiang Province: in terms of spatial pattern,the fastest rate of mudflat reclamation was achieved at Heng Sha East Beach in Shanghai,followed by the central Jiangsu Province and the southern shore of Hangzhou Bay in Zhejiang Province,with the slowest rate of coastline propulsion at 5.60 m/a in the northern Jiangsu Province among all coastline segments.In terms of temporal pattern,the central Jiangsu Province,Chongming East Beach and the southern shore of Hangzhou Bay were the first to carry out mudflat reclamation between 1985 and 1995;between 1995 and 2015,almost all coastline cultivation activities were frequent,with artificial coastlines continuously replacing natural ones.Changes in the coastline over the past three decades have been mainly due to socio-economic development and humanterrestrial conflicts that have accentuated the transformative effect of human activities on the natural coastline.The appropriate rate of mudflat reclamation was calculated the by combining two factors,sedimentation rate and slope of tidal flat,and a suitability assessment model was proposed based on the ratio of the appropriate rate of reclamation and the actual rate of coastline advancement.The evaluation showed that from 1985 to 2015,the areas where the rate of mudflat reclamation is too fast are Sheyang Estuary,Xinyang Port and Doulong Port in Jiangsu Province,and the coastal sections of Xiayangtu and Shepan Island in Sanmen Bay,and Wenzhou Bay in Zhejiang Province;the areas where the rate of mudflat reclamation is faster are Qidongzui,Chongming Dongtan,Chunhu Town and Tuci Town in Xiangshan Bay and Nanyangtou in Taizhou Bay;the areas where the rate of polder reclamation is slightly faster are Linhong Estuary,Dafeng Port,Chuandong Port,Nanhui Dongtan in Shanghai,Andong Shoal in Zhejiang Province and Beiyangtu in Taizhou Bay.Reclamation at slightly,quickly and excessively fast rates disrupt the balance of geomorphic dynamics in tidal flats and are prone to create irreversible effects on the ecological environment,which should be strictly controlled.Finally,the Delft3D model was used to investigate the evolution of the tidal flats under different sediment sources,tidal differences and initial slope.It was found that the tidal flats showed a continuous siltation trend with a steepening slope under a stable sediment supply,and the steepening trend was more pronounced with a larger sediment supply,and the flats showed an upward convex trend.Tidal bank profiles are more prone to steepening under the influence of moderate tidal range,and erosion occurs in the lower part shortly after a macro tidal range.The gently sloping tidal flats are more prone to sediment accumulation and the slope changes are more pronounced than the steeper sloping tidal flats. |
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