Design Of Tombolo-shape Drainage Tip And Its Influence On Morphology Of Beach

Beach is a kind of valuable resource in the coastal area.However,with the rapid development of coastal areas and the increasing demand for coastal drainage,more and more culverts have been designed and placed in beaches,which may inevitably affect the original landform and stability of the beach,especially for the unreasonable designs.As a new type of drainage tip,the tombolo-shape design can provide a new method for beach.At the same time,studying on the influence of the tombolo-shape drainage tip to the stability of the beach geomorphology will enrich the theories of beach dynamics and coastal engineering.Based on the special project of the research and application of beach management technology on island tourism beaches(201405037),several surveys of topography and beach profiles were carried out on near the drainage tips of Xiamen Convention and Exhibition Center,combining with the analysis of some historical satellite images from 2010 to 2015 and the numerical simulation in the research area,the process of the beach geomorphology change was obtained before and after the construction of tombolo-shape drainage tip.The main conclusions are as follows:(1)Impact of different types of drainage tips on beach geomorphology.According to the shape of the tips,the drainage tips along the beaches of Xiamen Island were classified to analyze the variation of coastlines by satellite images comparison and the beach profiles change both from the upstream and downstream of the tips.It turns out that the tombolo-shape drainage tip has shown an obvious advantage in protecting and stabilizing the beach compared with the traditional ones,such as spur dike,which may result in severe erosion in the downstream of the tips.(2)Beach profile change after construction of tombolo-shape drainage tips.the changes of the beach berm width,beach slope and the unit sand volume were analyzed by beach profile investigation near the tombolo-shape drainage tips.Analysis result of the measured data shows that:the height of the beach surface is reduced,the beach slope became steep and the beach berm retreated severely in the area without protection of drainage tip.While in the protected area,the elevation of the beach rised,the beach slope is just slightly reduced and the unit sand volume become larger than before.(3)Shoreline change after construction of tombolo-shape drainage tips.The beach shoreline changes near drainage tips and the sand coverage changes were monitored repeatedly,and the sand samples collection and grain size analysis were carried out.Analysis result shows that the drainage pipe is always covered by sand,one year later,part of the tips were covered gradually,and the sand coverage on tips is above 70%when reach a steady state finally.The characteristic of sediment distribution near the tips is that the closer to the tips,the finer the grain size,but the sorting and skewness near the two tips are quite different.(4)Study on the stability of partial beach near the tombolo-shape drainage tips under different water level condition.Because of the strong tide and weak hydrodynamic environment in Xiamen Bay,a further study on the effect of tidal range to the local beach stability near the tombolo-shape drainage tips was carried out.Based on the wave field model of the Xiamen Bay,a small area model of waves and sediment was established.,the range and extent of erosion and deposition in the local beach near the tips it calculated by controlling different tide levels condition with this model,and also the influence of the tidal condition on the erosion and deposition of the local beach is analyzed.Finally,a relevant solution was put forward to solve the problem of clogged nozzle according to the calculation results.