| The coast of South China has a long and curved shoreline with continuing harbors and sea bays, along which, thousands of kilometers of sandy beaches are distributed. Being situated in the oceanic climate of sub-tropical and tropical zones, the spring and autumn last more than 6 months, and the winter is lack. The change of temperature is not obvious, the humidity is of amenity. Many sandy beaches are developed into excellent tourist scenic spots, people go swimming, take sun bath and sandy bath and have a relax life after work. However, the precious tourist resources have been destroyed in a serious state by now. On one hand, it is caused by natural factors, such as beach erosion in storm events; on the other more important hand, mankind activities directly or indirectly bring on beach erosion and the breakdown of beach dynamical equilibrium. The paper lays stress on the process of morphodynamics of the sandy beaches along the coast of South China aimed at affording the theoretical basis for the protection and continuable use of these sandy beaches.The research includes the following contents: the influence of geological structure on the development of the morphology of the sandy beaches along the coast of South China; the evolvement of sandy beaches under normal hydro-dynamical condition (wave and tide); the response mechanism of sandy beaches to storm-induced wave and tide; the remoulding process of sandy beaches after storm wave and tide; the erosion of sandy beaches and protection measures and the engineering disaster of the sandy beaches. The purpose of the paper is to solve the practical problems of the sandy beaches through the study the process of morphodynamics.The project of the research works consists of the following sub-projects:(1) Survey on the morphodynamics of the sandy beaches along the coast of South China: Twice repeated field surveys on the 7 beaches were carried out in the winter of 2002 and the summer of 2003 respectively, the tide-wave index was introduced in the quantitative analysis of the morphodynamical process of the sandy beaches.(2) The response characteristics of the beaches to the typhoon-induced wave: 12 profiles of beach in different direction along the eastern coast of Xiamen Islands were repeatedly surveyed before and after Typhoon 9914 in the October of 1999, and theresponses of the beaches to the typhoon were studied; The effect of storm surge on beaches flanked the tropical storm Imbudo advancing trajectory were observed in Shuidong harbor and Zhuhai City, Guangdong, in July 2003.(3) Study on the engineering disaster caused by coastal engineering project, taking the Meizhou Port, Fujian as an example: The 3000 tonnage port was built on the spit of sandy beach along the southwest coast of Meizhou Island. It was deposed after construction because of severe silting. The reason of silting was analysed based on the field surveys of wave, current, alongshore sediment transport of the sea area and seasonal change of landform, as well as collection of the historical silting data of the port stakes from August, 1996 to December 1998.(4) Natural beach remoulding process after typhoon attack: Based on the 4 years beach landform profile data of the eastern beach of Xiamen Island, the natural remoulding beach process after Typhoon 9914 attack was studied by use of linearity regression.(5) The protection of coastal erosion: The protection measures of the coast were suggested by use of Silvester's (1984) pocket bay model based on the field survey of Dacheng Bay in May to June,2000 and the collection of historical data.The following conclusions were reached when the projects mentioned above have been finished:(1) The geomorphology of the sandy beach along the coast of South China was controlled by many environmental factors including the geographical location, tectonic background, shoreline direction, marine dynamical conditions and the sand sources. The tectonic movements and sea level changes occurred during the regional geological historical process lay down a basis for the development of large-scale sandy coastal geomorphology. After the postglacial, the sea level was relatively stable, under the recent coastal dynamical conditions, the scouring-silting adjustment made by the actions of supply, migration and accumulation of the sandy beach sediments takes a decisive effect on the state of the beach face. Among them, the wave-tide index, namely K value, is an important indictor to reflect these changes. The K value of sandy beaches along the coast of South China is generally more than 1. Most of the sandy beaches are predominated by wave-controlled geomorphology with rather great gradient of beach face. There is a positive relationship between the K value and seasonal deformation degree of the sandy beach. The grain size of the beach face sediment is closely related to the K value, and also affected to a smaller extent by the seasonal changes of wind wave and the geographical distribution of thevarious shoreline. The mean diameter of sand grain on the beach face of the sandy beaches along the coast of South China is between 0.22-0.80mm, and the starting velocity of the sediment is between 30-40 cm/s, so that distributive range of the grain size on the beach face is an important factor affecting the formation of inner sand bar. When the beach face is composed of fine sand, the sediment is easily started, eroded by storm surge and brought to the nearshore, so that the inner sand bar develops. While the grain size of the beach face is coarser, the inner sand bar is hard to be formed. So, the grain size of the beach face is coarser, the backshore is higher; the grain size of the beach face is finer, the backshore is lower.(2) The response characteristics of the sandy beaches with different shoreline direction to the same typhoon showed quite different after the study on the beach deformation and erosion during 9914 typhoon attacking at Xiamen Island. The strongest deformation and erosion occurred on the beach along the eastern coast of Xiamen Island with the direction vertical to the typhoon advancing trajectory. The outer margin of the normal beach berm on the backshore was eroded and retreated for 25m at maximum. The beach face on the foreshore showed scouring on the upper section and silting on the lower section. The unit width scouring amount on the beach berm and the upper section of the foreshore reached 30m3/m; and the unit width silting amount on the lower section of the foreshore was 17m3/m. The profile type changed from the section of beach berm form to that of sand bar form, showing the double actions of the wave dynamics of typhoon surge and the sudden rising of tidal level. While on the southern beach of Xiamen Island, the profile type basically kept the former state or roughtly occurred the embryonic section of sand bar form. For example, on the sandy beach profile of Xiamen University, the unitwidth scouring amount of the backshore and the upper section of beach face was 7m3/m, and the unit width silting amount on the lower section of beach face was about 2m3/m. During 47 hours of the typhoon attacking, the net alongshore silt discharge for the various coastal section was rather considerable, reaching the range of 0.1-0.4><104m3,and the net silt transport rate was dozens to hundreds of that under the normal wave condition. Because the typhoon intruding Xiamen Island mainly takes the by-E direction, the net alongshore silt transport capacity is the highest on the northern coastal section of the Island, lower on the eastern coastal section, and moderate on the southern coastal section.(3) The research on typhoon Imbudo attacking on the beaches along the coast of Guangdong revealed that the storm did damage to almost all the beaches. Before thetyphoon center landed, the typhoon process (changes of wind velocity and wind direction) on the nearshore flanked the cyclone advancing trajectory caused different wave condition distributive state and storm water piling up, which made greater difference of the storm effect on the beach of the both sides. The topographical change of the beach in Feisha Bay of Gaolan Island located on the right side was obviously greater than that of the beach in Shuidong Harbor located on the left side. The changes also showed different forms. For the former, the foreshore beach face and the seaward side of the bachshore beach berm were strongly scoured with unit width scouring amount of 55m3/m; the mean sealevel (MSL) retreated landward for about 13m with scouring depth of 0.8m; the coastline retreated for 5m with scouring depth of 0.6m. While the landward side of the backshore beach berm showed to be silted up owing to the coastward sediment transport of the overtopping wave with unit width silting amount of more than 0.4m3/m. It indicated the beach made sharp response to the typhoon. At the same time for the latter located on the left side of the typhoon advancing direction , the typhoon wave energy was relatively low and the wave direction was mostly offshore with relatively smaller storm water piling up, so as to cause weaker erosion on the coast and beach. The backshore sand dune was slightly scoured with unit width scouring amount of only 1.5m3/m, which might be resulted from the water piling up by the storm, the scouring by the heavy rain, and the sand blown seawards by the storm gale. The foreshore beach face showed scouring phenomenon in general, but the unit width scouring amount was only 3.0m3/m. While the beach face near the coastline showed weak silting up with unit width silting amount of 0.2m3/m, because the silt scoured and deflated from the backshore sand dune transferred seawards. The profile basically kept the original state and showed relaxing response to the typhoon. After the typhoon passed through, the beach sediments on the both sides all presented the coarsen phenomena, the sorting of the sediment on the higher and middle tidal zones became better and that on the lower tidal zone became worse, but no obvious difference was observed on the both sides.(4) The 4-years' seasonal repeated surveys of the beach profiles on the east coast of Xiamen Island showed the influenced extent to the beaches by the storm is much higher than that by seasonal change. The recovery process of the beach under the frequent wind wave after the storm was different among the different sections along the coast, however, the first 1-2 months was a rapid recovery period, after that, in the 1-2 year's time, much stable beach profile was formed. The seasonal change of the profile was dominated by the seasonal windy wave and the change of tide. The beach with different direction had its own characteristic. The yearly change of the beach had a close relationship with the yearlyfrequent wave condition, and was also related to the beach direction. It was discovered that under the yearly mean wave action, the alongshore silt transport played an important role in the stability of beaches with different direction.(5) In the study on the beach of Dacheng Bay, compared the 1993 chart to the 1960 chart, the coastline of Dacheng Bay obviously pushed forwards to the sea on the eastern section, slightly eroded and retreated on the middle section and basically kept stable on the western section.But the 0m isobath on the most coastal sections was closed up to the coastline except near the Duxi sand spit it pushed seawards. The 0m isobath close to the coastline means that the foreshore width of the beach becomes smaller. Under the by-E prevailing wave action, the coastline of Dacheng Bay shows roughly an embryonic spiral line. But the angle intersected the shoreline of leeward straight line section and the line linking the capes on the both ends, namely p angel, is still 35° by now, and the ratio of the concaved depth (a) to the distance of the capes on the both ends (b) is also merely about 0.30. So that the coastline will be concaved continuously inwards until the alb value is about 0.48; while the P angel will be close to the intersected angle between the prevailing wave peak line and the linking line of the capes on the both ends.To solve the problem of the coast stability firstly needs to conform to the coast environment, so as to reach an equilibrium of the alongshore transport and cross transport of the silt. That is to say, the net silt transport rate of each coastal section along the route must keep basically consistent to make every coast and beach reach a dynamical equibrium of scouring and silting, so that the coastline can be ensured to be stable and constant. It is suggested that the present scientific measures to protect the sandy beach are as follows:To make fully use of the natural protection mechanism, to prohibit the beach from changes of dynamical condition and sediment resource; to simulate natural coast morphology, to construct necessary coastal protection engineering project; to enhance marine consciousness, to carry out rational exploitation of the coast and coastline.(6) The pier construction in Meizhou Island was taken as a typical geological disaster case to analyze and study. The survey result of the surficial sediments show that the sediments on the west and north of the wharf were bad to worse sorting, indicating that they formed under the low-energy hydrodynamical condition. While the sediments at the wharf and on the south of it were medium to better sorting, indicating that they formed under the high-energy hydrodynamical condition. The surficial sediments in the studied area showed a tendency of transfer from south to north. The elevation of the base surface for 23 piercolumns of the pierhead trestle and 14 pier columns of the pier side were surveyed monthly from Aug. 1996 to Dec. 1998. The results showed during the time between June22, 1997-June 15, 1998, severe siltation occurred on the pier side. Except for No. 1-4 pier column where were eroded slightly for 0.1-0.2m, the silting degree increased sucessively from No.5 pier column to No. 14 pier column, and the silting thickness for No. 14 pier column was greatest, reaching 6m, most of the pier columns were in the silting state. The computer procedure was compiled by use of GSTA model and the transport vector at each sampling spot was calculated, so as to analyze and arrange the characteristics of sediment transportation in the whole studied area. It showed that the sediment around the wharf presented the tendency of "concentration" to the wharf from south and north. The field experiment of sediment of sediment trap in summer also proved the phenomena of sediment silting near the wharf. The conclusion was drawn through the study on the silting of the port that before any coastal engineering project, it is necessary to carry through an argumentation of the beach dynamical equilibrium. It is quite often that the beach equilibrium was destroyed when finished a coastal project, resulting in silting or erosion. The wharf of Meizhou Island was silted so quickly after the wharf engineering construction had been finished as to be abandoned, which was a typical case of beach engineering disaster problem.
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