| Beach is a sensitive area of the land-ocean interaction, with the social and economic development, the growing demand for beach leisure, to protect the beach, especially the development and protection of the coastal major tourist areas Boao Jade Belt Beach in Hainan, has become one of the important subjects for the coastal researchers. It is necessary to study the beach morphodynamics and its response of tropical cyclones and other severe weather.In this paper, the author uses the data of surface sediments sampls getted in August2010, December2010and March2011, as well as the beach profile and surface sediments before and after the strong tropical storm "Nock-ten" in July2011, and discusses the distribution characteristic of surface sediments, sediment transport trends and the response characteristics of tropical cyclones in Boao beach. Main results and conclusions are as follows:1) The overall characteristics of the sediment grain size. The beach median diameter Φ50ranged-1.67to3.36Φ, an average of1.36Φ (medium sand); sorting coefficient σ1ranged0.29to1.55Φ, an average of0.72Φ (moderately sorted); skewness Ski ranged-0.55to0.59, an average of-0.02(nearly symmetrical); kurtosis KG ranged0.69to2.87, an average of1.10(mesokurtic).2) Longshore changes of sediment grain size. The beach was divided into southern and northern segments. Thinning trend to both sides of the beach sediment from the estuary, M2of the sediment from～0Φ in estuary to the south increased to0.95Φ (the southern tip of the beach) to the north increased to2.23Φ (northern end point), reflecting the river sediments Discharges and Suspended transport to both sides. The northern section of beach sediment (Φ50=2.13±2.68Φ) fine than the southern section (Φ50=0.95±2.62Φ), the northern section of sediment sorting (σ1=0.78±0.77Φ) was poor than the southern section (σ1=0.68±0.70Φ). The main reason was great reefs located in the offshore of north beach, induced wave energy significantly less than the south beach.3) Cross-shore changes of sediment grain size. Boao beach lateral variation of sediment grain size is more complex, but on the whole, with thinning trend in surf belt.4) Seasonal changes of sediment grain size. In the most of cross-section and the majority of sampling points, sediment grain size had the obvious seasonal variation, but the thickness of the changes in regularity. It showed little seasonal changes in the overall. For example, the whole study area,199±49sampling points Mz average of1.35Φ,1.46Φ and1.27Φ.5) Comparison of the topography and sediments of the beach before and after storm. During Strong tropical storm "Luotan", Boao Beach experienced obvious erosion and deposition, the maximum erosion thickness was2.13m, the maximum deposition thickness was1.16m.19section141measurement points average was net erosion and net erosion thickness is0.20m. Beach profile changed complexly after tropical cyclone affection. The most of the beach berm were shortened or eroded disappear; some of the profile changed small; Individual profile accumulated. Beach, located in south of Wanquan River mouth, responded stronger to tropical cyclone the than the north one. The main reason was great reefs located in the offshore of north beach, induced wave energy significantly less than the south beach. Surface sediment in backshore responded weekly to tropical cyclone, and grain size parameters'value changed small (ΔMzmax=0.18; Δσ/max=0.21Φ; ΔSkimax=0.23;ΔKGmax=0.08), but it is changed strong in foreshore (ΔMzmax=1.98Φ;Δσ/max=0.760; ΔSkimax=0.78; ΔKGmax=1.13). The main reason was that beach morningglory herb and other psammophytic vegetation protected the backshore. Grain size parameters of surface sediment changed complexly in foreshore. There were armoring belt of surface sediment in foreshore, and the grain size was bigger than before the storm, but it appeared different parts of foreshores.6) Cross-shore sedimen transport trends. The beach, nearing Wanquan River mouth, was affected by river runoff, and Jade Belt Beach spit segment has emerged as a trend of offshore transport, faced with the erosion threat; in its northernmost, as beach was sheltered by reef and headland, the tends of sediment transport were to the shore; at its southern end of the beach, it showed offshore transport trends in storm season, and to shore transport trends in other seasons. The Boao beach sediment transport trends mainly affected by wave. Individual profiles showing different transport trends from surrounding beaches were affected by human activities.In a word, sediment grain size of Boao beach exist complex spatial and temporal changes, but the medium sand characteristics did not change. It is essential for the enxironmental quality of the beach tourism. |
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