The Geomorphological Evolution Of Nourished Beach And Its Effect On Aeolian Sand Transport

Beach nourishment has become the main method for coastal ecological restoration.The original beach terrain can undergo significant and rapid changes.Beach nourishment not only resists coastal erosion,but also produces coastal aeolian sand effect,which differ from the characteristics of aeolian sand effect in natural beach,generating a form of coastal aeolian sand movement under human activities.Therefore,studying the characteristics of beach geomorphological evolution and its impact on aeolian sand effect can help guide the prevention and control of aeolian sand activities caused by beach nourishment,and the design of beach nourishment project in strong wind areas,providing theoretical support and reference significance for the prevention and control of coastal aeolian sand disasters and coastal restoration.This article takes the north nourished beach of Changle Airport in Fujian province as the research object,and analyzes the geomorphological characteristics and evolution of the beach,the influencing factors and potential mechanism of the wind speed variation at the edge of the nourished beach berm,and the variation characteristics and main influencing factors of the aeolian sand transport rate in beach nourishment by means of field observation and numerical simulation.The aeolian sand transport mode in beach nourishment is systematically summarized.The main research conclusions are as follows:By analyzing the changes in nourished beach geomorphology,including beach profiles,dry beach widths,foreshore slopes,single-width remaining sand,and surface sediment grain size,studies have revealed that the beach geomorphology in the study area underwent significant change during the first three months after beach nourishment and showed a stable trend over a period of one and a half years.After beach nourishment by beach berm sand-filling,the wind speed will increase at the edge of beach berm.The elevation of nourished beach berm and the nourished slope are the main controlling factors for the increase of wind speed near beach berm.When the position of the beach berm and the nourished slope are constant,the wind speed at the berm edge has a positive correlation with the height of the nourished berm(NBH),and increases linearly with the height.When the NBH remains unchanged,the wind speed at the berm edge is positively correlated with the nourished slope(NBS),and increases linearly with the latter.A coupling relationship between the wind speed amplification rate at nourished berm edge and the two main influencing factors(NBH and NBS)was established based on MLR analysis.The coupling effect of NBH and NBS on wind speed was illustrated.However,compared with the NBH,NBS has a greater impact on the variation of wind speed at the nourished berm edge.The aeolian sand transport rate on the dry beach in beach nourishment exhibits a logarithmic growth trend from the high water level to the break slope,while the abrupt terrain causes a sharp decrease in the sediment transport rate from the break slope to the nourished berm edge,followed by an increase in the sediment transport rate again.The fetch effect caused by the abrupt terrain of the slope-break to berm is the main influencing factor of aeolian sand transport in beach nourishment in the study area.Different from the typical beach wind-sand flow development pattern(i.e.,growth-saturation-weakness pattern),the aeolian sand transport on the dry beach that is maintained by beach berm sand-filling mainly exhibits a transport pattern of growth-unsaturation/saturation-weakness-re-growth.