Stability Of Tidal Inlet In The Sanmen Bay

Tidal inlets are common along littoral drift shores all over the world, and literatures on tidal inlets among barrier coastlines are numerous. In contrast, study on tidal inlets in rocky coast does not draw much attention. Located in the rocky coast of East China Sea, Sanmen Bay is a big tidal inlet system with great values in terms of port and navigation, nuclear station construction, tidal flat cultivation, and environment protection. Several aspects of the Sanmen inlet system are of scientific research significance. First, as a typical tidal inlet system on the rocky coast, Sanmen Bay is an excellent case for study of stability of tidal inlet. Moreover, the latest 50 years saw fast environmental changes happening in Sanmen Bay. Such changes deserve extensive research focusing on the interaction between human activities and coastal evolution.Based on the sub-bottom profile records and lithofacies of drill cores, the thickness of Holocene deposit is estimated as 20-35 m in average, with maximum of 42 m being found in buried ancient channel. The calculated sedimentation rate is about 0.2?0.5 cm/a.18 short cores were collected at different locations in Sanmen Bay. Their lithological character (such as wet and dry density, moisture content) and sedimentary structure were analyzed, and 210Pb?137Cs radioactivity were deteceted in laboratory. Using CIC dating model, the modern sedimentation rates are calculated as 0.6-2.4cm/a for tidal flat,1.9?2.6cm/a in the shallow area and 3.9?8.6cm/a in the deep area of Maotou Channel, respectively.The sedimentation process of inlet system is of typically temporal and spatial variation. Holocene depositional rate is negatively related to water depth, while recent depositional rate is positively related to water depth. Recent depositional rate is ten times larger than that of the Holocene.Derived from the analysis of sea charts and underwater topographic charts since 1930, the geomorphological process show regional and staged feature. In 1930, the recent geomorphology structure formed. From 1930 to 1964, the inlet system was on the stable or slowly accretion condition. Since 1964, the inlet system advance in the faster accumulation rate because of the reclamation of tidal flat. From comprehensive analysis, the geological structure and original topography, tidal current and suspended sediment are main factors influencing the formation and evolution of inlet system. Recently, human activities such as reclamation becomes the main factor and bring about a rapid accumulation by changing coastal boundary and weakening ebb current.The traditional P-A relationships are not fit for a single inlet system. According to the evolutional similarity, the inlet system in Sanmen Bay can be divided into several sub-inlet units, and each sub-inlet unit stands for different spacial scale. Based on sea chart data in 1964 when Sanmen Bay inlets is believed in balanced state, the P-A relationships of inlet systems in Sanmen Bay is calculated as A=2.49x10-4P0.940974, with corelation coefficient being 0.999.Using the above P-A relationship formula, the stability of inlet system in Sanmen bay is evaluated. The different scale of inlets in the Sanmen Bay evolved and readjusted to the above equilibrium P-A relationship under the influence of human activity. Liyang inlet and Qingshan inlet took shorter time to readjust to equilibrium state. Shepan inlet show periodic fluctuation around equilibrium state because upstream Yanzhan tidal flat stretch out and draw back periodically. Maotou inlet readjusted to the equilibrium state slowly than the upstream inlets.According to the future plan for reclamation in Sanmen Bay, the trendency of the inlets system are predicted. Maotou inlet will be in a rapid accumulation state in the next twenty years, and the mean water depth over entrance equilibrium section will be 12.6 m, much less than present water depth of 17.5 m. The estimated maxium water depth in Maotou Trench will be less than 35 m, which may influence the water intake and the diffuse of the warm drain water of nuclear station.