| Based on the historical data of Jiaozhou Bay (JZB) and Environment Fluid Dynamics Code Model (EFDC),a comprehensive model of environmental dynamics was established. Validated by the observed data, the model simulated the hydrodynamic, water exchange, sediment transportation and seabed evolution of four individual years. According to the measurement and modeling results, the morphology, dynamic and environmental indicators of JZB were extracted. According to the indicators above mentioned, the evolution of JZB over the last 100 years was investigated systematically from the aspects of geometric shape, hydrodynamic, water exchange and seabed evolution. The impact of human activities on the JZB evolution was discussed emphatically.During the period from 1863 to 1935,the coastline of JZB,predominated by natural evolution, changed little and intertidal flat area decreased quite slowly (decreased by 15.68 km2 in 72 years);thereafter, JZB was dramatically influenced by human activities that the coastline changed a lot, large area of natural intertidal flat was replaced by salt pond, aquaculture area and reclamation, which resulted in intertidal flats and total water body volume decreasing sharply by 70.8% and 9.7% respectively from 1935 to 2005.Shoreline generally advanced seawards, and original tortuous coastline tended to be straight. The historical changes of various coastline types reveal the several historical stages of intensive exploitation and utilization of JZB by human. The area of JZB in 1863 was 567.95 km2,220 km2 larger than that of 2008,and decreased by 38.6% over the last 150 years, in which the shrinking rate after 1935 is approximately 13 times as that of before.The changes of tide, tidal current and residual current of JZB in the past tens of years show that hydrodynamic field has become weak gradually. Due to the reclamation, M2 tidal amplitude and phase in the bay decreased by 2cm and 1 degree from 1966 to 2008,respectively. From 1966 to 2008,the maximum surface flood current velocity decreased by 0.7·m/s (40%), and ebb current velocity declined 0.42·m/s (30%) in bay mouth. The human-induced changes of the coastline position-configuration and nearshore bathymetry have resulted in substantial changes (weakening and disappearing) in the residual current patterns, especially in Qianwan Bay, Haixi Bay, and northeastern JZB.The overall tidal prim of JZB has been reduced by 26% as compared to that in 1928.This is considerably less than the 35% reduction obtained by other studies. The tidal prism, quite similar to tidal flats and JZB area, reduced by 0.25×109 m3 (19%) from 1863 to 2005,in which the reduction rate after 1935 is approximately 250 times as that of before. The distribution of the average residence time (ART) in JZB is controlled by three interactive factors:distance to the bay mouth, structure of the tidal residual currents, and particular characteristics of the intertidal flats. The overwhelming influence of human activities, especially land reclamation, is the main cause of the significant changes in hydrodynamic conditions and water exchange in JZB.The decreasing water-exchange ability corresponds to an increasing ART over the past several decades, particularly after the 1980s.In addition, the influences of the return flow of the bay water from the open sea back into the estuary were quantified by determining the return flow factor for each year. The dominating factor influencing JZB water exchange is the human-induced changes of water area, and consequently the basin volume, tidal prism, return flow factor, and ART. An existing tidal prism model was revised by introducing a mixing factor k, and a simplified formula was developed for JZB.Quantitative analysis of a series of historical bathymetric surveys and the results of numerical model has revealed large changes in morphology and sedimentation from 1863 to 2005 in JZB, China. Evolution of the sea bed could be divided into three stages:natural factor prevailed stage, the combined action of natural and human factors stage and human-dominated stage. The nineties of 20th century is a crucial period of the change of sedimentation in JZB.Before 1990s, the intertidal flat and shallow water area was depositional, and deep water area was erosional. The tidal inlets system maintained well; JZB was in the equilibrium of erosion and siltation during 1990s;After 1990s, the intertidal flat and shallow water area were eroded, however, siltation occurred in the entrance to JZB,deep water areas and ebb-tidal delta. The natural life of JZB is facing the risk of being shorten.The evolution of JZB has a strong stage feature over the last 100 years:natural evolution stage before 1930s and thereafter a evolution stage with human factors involved. And the latter furthermore can be divided into three periods with 1990s as a turning period. These two phases both showed the following evolution with different degrees:geometrical morphology change of JZB caused hydrodynamic field changes, and thereby induced the change of water exchange process and sediment transportation, furthermore led to the change of seabed. While the change of seabed would change the bay geometry in return and accordingly influenced the hydrodynamic field. During this cyclic process, JZB continues to shrink, and will eventually dye out. According to the various aspects of comprehensive evolution process, it is ultimately determined that the coastline in the early 1990s was the "warning limit utilization". JZB at present is already in a dangerous situation.The future of JZB is investigated based on a numerical experiment. Results indicate that water exchange capacity will keep on deteriorating and the whole Bay system will evolve towards into lagoon type, subsequently port channels will be severely threatened. Moreover, tidal inlet stability judgment method for the Bay with bedrock Baymouth is studied preliminarily through the analysis of the sediment net migration near the Baymouth. This method is well applied to JZB, and obtained the erosion-deposition evolution process of seabed and the conversion period accordingly. In addition, according to this article's research and Bay development experience and lessons home and abroad, relevant suggestions on the utilization and protection of JZB are proposed. One of the most important points is that reclamation must be strictly forbidden. Furthermore, proper environmental remediations projects must be carried out in order improve the JZB environment fundamentally. |
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