Study On The Geomorphological Characteristic And Controlling Factors In The Southern Bohai Strait

The Southern Bohai Strait, lying in the north-western region of ShandongPeninsula, is the demarcation area of Yellow sea and Bohai Sea. As the most abundantgeomorphologies area in Shandong Peninsula, the study area contains not only longsandy beach, islands such as Sang Island, Miao Island, Changshan Island and so on,the nature scenic-mirage, but also magical legend, human landscape, for example,Penglai Pavilion and so on. With the increasing of marine development activities,changing and destroying were happened to some important geomorphologies, thatleaded to a series of chain reaction.Based on distribution characteristics of surface sediments, water depth andtopography, coastal and submarine geomorphologies etc., this article provided acomprehensive analysis of coastal and submarine geomorphologies feature.Contrasting of satellite remote sensing images, beach profiles, water depth andtopography, study the evolution characteristics of the coasts, beach profiles andgeomorphologies. Then according to tectonic movement, tidal current characteristics,wave conditions, sediment movement etc., the dynamic mechanism of formation andevolution of the geomorphologies in this area were researched.The main conclusions of the study were as follows:(1) The distribution of surface sediments in the study area shows regionalcharacteristic. The rough particles containing gravels distributed mainly in the area ofthe cape of Qimu Island, the south of Sang Island, the east of Huangshui River, theoffshore of Penglai, the south of Nanchangshan Island and Daheishan Island,Dengzhou Shoal and Dengzhou Channel. The surface sediments in other areas weremainly fine particles.(2) The main geomorphological types in the study area included tidal currentscouring trough, underwater shoal, underwater erosion-accumulation slope,underwater erosion slope, underwater accumulation slope and accumulation plain etc.The geomorphological type in the areas around islands was mainly underwatererosion slope. Tidal current scouring trough developed between islands as well as between the islands and land. Large underwater shoal remained on the west side of thestrait. Underwater erosion-accumulation slope appeared on the southern offshore area.From offshore to open sea area, the geomorphological type transforms fromunderwater accumulation slope to accumulation plain.(3) The distribution of beach types in the south of the study area also showsregional characteristics. In the west region, the type of the beach on the north side ofQimu Island is dissipation type. In the middle region, the type of the beach on thesoutheast of Sang Island and the east of Luanjiakou are a transition type betweendissipation and reflection, the former is rhythmic bar-beach, and the latter is ridge andrunnel beach. In the east region, the coast on the west of Penglai is experiencingserious erosion and the underwater slope is steep in offshore area. The type of thebeach is reflection type. The beaches transform from low-energy dissipation type tohigh-energy reflection type from west to east in the south of the study area witherosion intensity from gentle to serious.(4) The comparison of coastlines extracting from remote sensing images ofdifferent periods showed that coastlines of Sang Island and Miaodao archipelago arestable. The variation of southern sandy coastlines are complex, which having both thespatial and the periodic characteristics. See from the spatial variation feature, wefound that:①The coasts in the north of Qimu Island, the southeast of Sang Islandand the east of Huangshui River are open and straight without sheltering of islands.These coasts eroded seriously in the past years which affected directly by the wave.②Coast at the southwest of Sang Island sheltered by the island is dynamicequilibrium with slightly erosion or accretion.③With the existing of DengzhouShoal, the wave broke under the shoal and the energy weaken obviously. This effectprotects the sandy beach behind the shoal which appears dynamic equilibrium in theoriginal condition. See from the time variation feature, we found that:①The erosionintensity and speed of coasts in the north of Qimu Island, the southeast of Sang Islandand the east of Huangshui River were reduced obviously after2000.②Coast in thesouthwest of Sang Island was changed by artificial buildings such as breakwaters, andshowed deposition trend after2000.③Coasts in the west of Penglai eroded seriouslywhich affected by the sand exploitation activities on Dengzhou Shoal after1984.(5) The depth comparison from navigation charts of different periods shows that:①Water depth and topography of open sea in the study area changed a little, and kept a dynamic equilibrium with slightly erosion or accretion since1975.②The depth andtopography in the areas around islands, between island and land, on Dengzhou Shoal,offshore and near artificial buildings changed obviously.③The northern side ofislands facing the waves were mostly eroded. The area among Miaodao archipelagoshowed accretion because of the sheltering of islands. Channels between islands orisland and land were apparently eroded.④The topography changing features ofsandy coasts in the south part of the study area showed apparently regionalcharacteristics. That showed dynamic equilibrium with slightly erosion or accretion inthe middle region, but seriously erosion in the west of Penglai which located in theeast region of the study area.(6) The marine geomorphologies in the study area formed by the tectonicmovements in the earlier stage and the ocean dynamic effects during later stage. Themodern marine geomorphologies were influenced mostly by interaction of fluent andwave. The sandy beach and sea cliff in the southern area is controlled by the wavecondition. The trough between islands and land is controlled by the tidal current. TheDengzhou Shoal and the area around Miaodao archipelago which have coarse particlesediment and erosion geomorphologies are controlled by the interaction of fluent andwave.(7) The sources of sediment include rivers supply, coastal erosion and submarineerosion, in which the coastal erosion sediment is the major one. The large-scalegeomorphologies, such as Miaodao archipelago, Dengzhou Shoal and DengzhouChannel and so on, is the most important factor that keep the stability of thetopography and geomorphologies environment. The destroy of Dengzhou Shoal in thelate of1980s by the sand exploit activities which damaged the balance of topographyand geomorphologies and resulted in transforming or rebuilding, was the fundamentalreason of the topography and geomorphologies features variations.