| Coastal zone is an important and valuable resource for human society and an active zone for land-sea interaction and human activities.Sandy beaches occupy more than one-third of the global coastline,with high social-economic and ecological values.Beach erosion has been presented globally in recent decades due to the impacts of global climate change,sea level rise,and more frequent and severe storms.The contradiction between the decreasing beach resources and the increasing demand for beach recreation has emerged,which requires scientific and sustainable exploitation of beach resources and,if necessary,proper restoration or nourishment.However,the frequency of storm varies from region to region and the characteristics of beaches vary,making beach nourishment methods highly variable,with many having significant site specificity,resulting in more complicated beach morphodynamics.Embayed beach is the most common type of sandy coast in the world,and it is of great scientific significance and practical value to study the morphodynamic process of the embayed beach under the impacts of storm events.This can not only enrich the study of beach erosion mechanism caused by storm events,but also provide a theoretical basis for the decision on beach nourishments to cope with storm erosion of embayed beaches.Focusing on different embayed beaches exposed to frequent storms on Zhujiajian Island in Zhoushan City,Zhejiang Province:Dongsha(sandy beach,with seawall and nourishments),Dashali(sandy beach,with seawall),Qiansha(sandy beach),Wushitang and Xiaowushitang(gravel beaches,with seawalls),this study analyzed surficial sediment characteristic,beach topography,shoreline position and nearshore hydrodynamics,using field measurement,Argus video monitoring and XBeach numerical simulation to reveal the morphodynamics of embayed beaches under various influences of storm events with different intensity scales.The main results are the followings.(1)Nearshore hydrodynamics,topographical and sediment variations of embayed beach under the influence of typhoon Tapah were characterized.The hydrodynamics in the nearshore were enhanced by the typhoon event,which in turn had a significant impact on beach sediment and geomorphology,with beach erosion and sediment grain size coarsening being the main characteristics of the typhoon impact on Dongsha beach.The storm surge caused by a single typhoon“Tapah”made the nearshore high tide level during neap tide be significantly higher,and almost equal to that during spring tide.The peak significant wave height(Hs)during typhoon“Tapah”can reach 4.49 m in the embayment.The flow velocity near the bottom also increased due to the typhoon,and the flow direction became irregular under the typhoon.The highest suspended sediment concentration near the bottom can reach 9.07 kg/m~3(3.40 times that of the calm spring tide).The enhanced nearshore hydrodynamics resulted in the erosion of the subaerial beach,with a value of-18.58 m~3/m.(2)The patterns of beach morphological change and sediment variation under the impacts of different typhoon events were discussed.The effects of various typhoons on the topography and sediment grain size characteristics of Dongsha beach are similar but with different response.The duration and the severity level of typhoon events are the important factors affecting the beach erosion degree,while the high tide level during the typhoon controls on the upper position limit of profile erosion.Similarly,all the profiles had the flat shape after typhoon and the significant alongshore accretion/erosion variability.The profile volumetric changes are different,showing that the average profile erosion of the beach was the largest(-72.69 m~3/m)after typhoon“Talim”,while the smallest erosion(-8.01 m~3/m)was observed at Dongsha beach under the influence of typhoon“Danas”.The higher typhoon storm power index,the larger beach erosion induced by typhoon.All the beach profiles showed significant alongshore variation in volumetric change,with more erosion occurring in the southern and central parts of Dongsha beach.The pre-typhoon beach conditions,typhoon characteristics,and tide levels during typhoon are the probable reasons for the differences in beach response.(3)The response differences of embayed beaches in various types with different human interventions to the strong hydrodynamics during typhoon season were compared and concluded.Profiles of five embayed beaches behaved differently during the typhoon season,while sediment characteristics all showed the coarsening in grain size and offshore trend in transport.Location and migration of sand-mud transition(SMT)can be good indicators of hydrodynamic energy level and beach accretion/erosion pattern.Under the strong hydrodynamic conditions during the typhoon season,gravel beaches were more stable than sandy beaches,and the sandy beach without human activity was less erosive and less variable.The beach profiles on embayed sandy beaches were mainly characterized by the flattening of the beach berm and the decreasing in elevation,while the gravel beach profiles were more stable.During the typhoon season,the largest change of beach sediment occurring at the southern part of Dongsha beach where was disturbed significantly by the human intervention,while the most stable sediments being at Wushitang beach.The SMTs of embayed sandy beaches were stable or had seaward migration during the typhoon season,and their offshore distances are positively correlated with the nearshore significant wave height and headland extent,while negatively correlated with the profile volumetric change of the beach and tidal range.(4)This study revealed the seasonal evolution pattern of embayed beach associated with storm quantities and pointed out that implementing the small scale and short-interval nourishment timely after the storm event is an effective method to maintain this kind of beach.Argus video monitoring system is an effective technique to monitor beach responses to storm events and beach nourishments.During the 13-month video monitoring period,19 storm events with dirrerent severity levels occurred in the nearshore of study area.Analyses of image data show that beach eroded during autumn(-59.71 m~3/m)and winter(-0.23 m~3/m)when storms were intensive,while accreted in spring(139.97 m~3/m)and summer(14.40 m~3/m).10 beach nourishment projects all effectively increased the dry beach area and compensated for storm erosion,while the concentration of nourishment sediment placements resulted in a continuous counterclockwise rotation of Dongsha beach.To better compensate for storm erosion,the implementation of nourishment projects should take the erosion/accretion pattern and the timing order in consideration.(5)A beach morphodynamic model based on XBeach was established,the nearshore wave and topographical change of beach was simulated,and the control of multiple factors on the change of beach morphology was determined.Using the model to increase the range of the profile to 1400 m offshore,it is found that the beach profile showed a significant pattern of upper profile(over the mean sea level)erosion and lower profile(under the mean sea level)accretion under typhoon“Tapah”,and the changes of the profile elevation in different cases without nourishment all stopped approximately at the sand-mud transition(400 m offshore).Significant wave height,tide level,sediment grain size and beach nourishments have different impacts on the morphological changes of the beach profile under the storm,with significant wave height having the largest influence.The significant wave height controls on the magnitude of the profile erosion and the tide level dominates the locations of erosion and accretion.Facing storms with high significant wave heights,the different settings of sediment grain size ranging from fine to medium sands did not result in significant differences in the beach profile morphology.Differences in the cross-shore location of borrowed sediment placements resulted in variations in profile morphological changes,and the offshore distance of the stopping point in profile elevation change was farther in the cases with nourishment implementation compared to those cases without nourishments. |
PDF Full Text Request