Study For Impacts Of Contiguous Development In Bohai Bay On Coastal Mudflat Hydraulic Environment And Evolution Process

Coastal mudflats are not only important wetland resources but also important land resources. Coastal mudflats in Bohai Bay located in the east of China, a large densely population and economically developed area, are land resources reserved which is urgently needed to develop in China. In recent decades, the development intensity of coastal mudflats in Bohai Bay increases gradually, showing an contiguous development trend. It has been noted that mudflat development and construction of coastal engineerings can affect mudflats and surrounding environment, an environmental impact assessment must be done before each project. But less systematic studies on cumulative impacts of multiple projects, especially large scale contiguous development, on coastal mudflats’ hydrodynamics, sediment, and evolution, are documented.In this paper, different periods of remote sensing data, bathymetry data, and cross-sectional observation data are used to analyze the evolution process of coastal mudflats during the past50years, especially during the last10years. Results show that, with the siginifcant decreasement of the riverine sediments, silting rate of mudflats decreases in this area, and even erosion appears in some parts, inducing larger grain size of mudflat surface sediments, and steeper slope of cross shore profiles. With the development intensity increasing, the width of intertidal mudflats narrowes, and areas of mudflats between0to-5isobath decreases dramatically.Based on observation and analysis of tidal currents, waves and grain size of sediments, quantitatively analyses on comparison of tide action and wave action, tidal shear stress, wave incipient depth, sediment incipient shear stress have been conducted. It is found that scouring and silting of mudflats between0to-5isobath is influenced by combined effects of tidal currents and waves:sediments are stirred by waves and transported by tidal currents.A tidal wave mathematical model is established for Bohai Bay, by which the variation of tidal wave system in past70years, and furthermore the impacts of contiguous development of mudflat in Bohai Bay in the last10years on tidal wave system, are simulated and analyzed for the first time. Results show that, compared with1938, M2tidal amphidromic point near the mouth of Yellow River remaines unchanged on the whole, but the tidal range in western Bohai Bay especially the nearshore areas has increased; however, with the increasement of mudflat development intensity, cumulative effects of contiguous development appear:not only tidal range increases, but also M2tidal amphidromic point near the mouth of Yellow River moves so that the position of this amphidromic point in2012moves11.73kilometers to the southeast side compared with that in2006, and moves13.35kilometers to the southeast side compared with that in2003. If long-term reclamation plan in Bohai Bay (2030) has been implemented, this amphidromic point would move further10kilometers to the southeast side in comparison with that in2012.Based on two-dimensional hydrodynamic model of Bohai Bay, the impacts of contiguous development of mudflats in Bohai Bay on sectional tidal volume in different area, tidal current velocity, tidal current forms and residual current, are numerically analyzed. Results show that, for a single project, the effects are limited within a certain range around the project, but contiguous development can affect larger areas of the bay or even the entire bay. Take the inner bay cross-section calculation as an example, flood and ebb tidal volume in2012reduce by about10%compared with that in2003. If long-term reclamation plan in Bohai Bay (2030) has been implemented, flood and ebb tidal volume would reduce by about18.5%compared to that in2003. From2003to2012, although within0to-5m isobath tidal velocity fluctuates, a decreasing trend of overall velocities appears, with the reducing amplitude of0.2m/s or less. If long-term reclamation plan in Bohai Bay (2030) implemented,. the reducing amplitude can reach0.2-0.4m/s, Construction of projects can affect tidal current forms significantly around them, with the increasement of reclamation scale, the range of tidal current forms changed also increases gradually.This is the first systematic study on cumulative impacts of contiguous development in Bohai Bay on coastal mudflats. New understandings from this study will be an important foundation for rational development and sustainable use of coastal mudflats in Bohai Bay, and has tremendously significance for promote the construction of Surrounding Bohai economic circle, balance between development of regional economic and of social, resources and environment.