Study On The Stability Of Artificial Gravel Beach At Macro-tide And Hydrodynamic Coast

Coastal erosion,as a kind of coastal geohazard,has become increasingly serious all over the world because of natural process and human activities such as global climate change,sea level rise,reduction of sediment supply,and coastal engineerings.Beach nourishment,which is considered an ecological measure,now is becoming an important choice to alleviate coastal erosion and protect the coast.Artificial gravel beach,as a new attempt in China,has incomparable applicability in macro-tide and strong hydrodynamic regions.Based on the field investigation of the artificial gravel beach in Tianquan Bay,Xiamen,and previous topography data,hydrological data and research achievements,the beach profiles and coastlines were obtained in different times to analyze the pattern of morphological change.The morphology change and erosion mechanism of the artificial gravel beach are elaborated from different angles by calculating the grain size parameters,morphological parameters and longshore transportation rate and analyzing the changes of the beach under the action of macro-tide and hydrodynamic after completion and the deviation between measured values and calculation results,to establish and revise the formula of transport rate.The conclusions are listed as follows:(1)The characteristics of the profile morphology of the artificial gravel beach shows as follows:margin of beach berm,shoreline and the boundary of gravel and sand suffered from the significant erosion,and the width of beach berm narrowed down;berm crest line was formed with the obvious lifting of berm edge;the beach slope becomes larger and varies in a wide range,the slope in eastern side is steeper than that in the western side.In spring and summer,the western side of the gravel beach varies greatly,and the intensive erosion area occurs in the bottom.The erosion area of the gravel beach is obviously enlarged in autumn and winter,and both sides of the drainage pipe suffer most serious erosion,on the contrary,the berm area is silted due to the replenishment of gravel.(2)Grain size curves in different areas of the artificial gravel beach are close to normal distribution,only some areas are positive skewness or negative skewness.The average sphericity in each sections increases with the decrease of the gravel size,in other word,the sphericity increases when the beach altitude reduces.Along the direction of the sediment transportation,the gravel roundness increases gradually with the sediment particle size decreases.(3)The artificial gravel beach was filled with gravels of 57130 m3 in total.The calculation result of annual gravel sediment transport rate by field analysis and theoretical calculation is 1000～1500m3/a,the beach should be maintained every 5 to 6 years.Recycle the gravels from the deposition area in downstream and re-lay on the erosion area is a good way to maintain the stability of the artificial gravel beach.