Sandy Beach, Splashing Waves With The Relationship Between Surface Sediment Grain Size Characteristics And Hydrodynamic Environment

Beach is the coastal zone above mean low tide line and inundated periodically by seawater, while sandy beach is mainly composed of sand, of which the grain diameter isgreater than 0.05mm and smaller than 2mm.Sandy beach is widely distributed and accounts for 13% of the whole length of globalcoast, while in South China sandy beach occupies more than 1/3 coastal area. Sandy beachis not only an important kind of resource for mining and tourism, but also a natural wavebuffer protecting coastal scarps and coastal property from the encroachment of wave. Dueto the influences of the increasing population in coastal areas and the rising globaltemperature, sandy beach is being destructed and polluted quickly by human activities andnatural disasters, especially the rising of sea level and the increase of storm tide. Now it isvery important to enhance the research of morpho-dynamical process and evolvement lawof sandy beach.Based on 25 hours' continuous sampling of surface sediment in swash zone ofLonghaitian sandy beach in Donghai island, this dissertation has studied the differences ofgrain size characteristics between sediments sampled at different parts in swash zone and atdifferent time. The relationship between grain size characteristics of sediment and waterdynamical environment is particularly analyzed. In the end, we find that:(1) The surface sediment in swash zone of Longhaitian is the outcome of erosion onDonghai island and the ebb-tidal delta by tides and waves. The sediment grain sizeconcentrates on coarse sand, middle sand and fine sand, usually with middle sand as modesize and almost no components smaller than 0.125mm. Most samples have uni-modal andsymmetrical frequency curves, with medium standard deviation and moderate kurtosis.They also have proportioned values of C index and M index, with only one segment oncumulative frequency curves on probability scale, showing that the sediments aretransported primarily by saltation.(2) Inside the swash zone, the average grain diameter (Mz) and standard deviation (σ) of the sediment increases from top to bottom. Along the profile of Longhaitian beach, Mzhas a max value at the middle tide line and decreases towards both sea and land as the tidemoves to and fro, whileσdecreases during flood period and increases during ebb period.(3) Inside swash zone, the current has a speed close to zero and minimum energy atthe top, while a max speed and maximum energy at the bottom. Due to the selectivetransportation of sediment by water, finer material was brought to the top of swash zone, soMz increases from top to bottom. A transition zone with extremely-breaking high-energywave, formed at the bottom of swash zone where it borders surf zone, thereforeσincreases from top to bottom.(4) Due to the standing tide wave near Longhaitian sandy beach, at middle tide line,where the current reaches a max speed, a lot of finer material is carried away and settlesdown near high tide line and low tide line and Mz decreases towards both sea and land. Atmiddle tide line high-energy waves in swash zone stirred by high-speed current onlystrengthen the trend above-mentioned. During the flood period, waves move into shallowwater at the same direction of current and smooth because of wave-current interaction, andswash zone also narrows, therefore the sediment become better sorted. During the ebbperiod, waves move into shallow water against the ebb current and steepen because ofwave-current interaction, and the swash zone also widens, therefore the sediment becomeworse sorted.