A Study On The Propagation Characteristics And The Energy Change Of The Instability Of Longshore Currents

In recent years,due to the economic development of coastal area,coastal construction and coastal environmental protection issues gradually received people's attention,coastal current problems also become a hot object of study.Based on existing longshore current studies in straightness slope,this paper studied the propagation characteristics and energy changes characteristics in perpendicular direction to the shoreline of the instability of longshore currents.These provide certain basis for the instability of longshore currents and nearshore low frequency motion,because it has some significance for further study in probing into the complicated nearshore hydrodynamic characteristic.In this paper,the results of study have some certain reference value to the transport of coastal pollution and sediments.Around the above,this paper mainly does the following work:1.Firstly,based on the principle of correlation analysis,the paper analyzed the propagation characteristics of the instability of longshore currents.For all the wave situations(the experimental terrains are divided in to two kinds,respectively are 1:100 slope and 1:40 slope;the kinds of incident wave respectively are irregular waves and regular waves),the correlation analysis of the flow velocity time histories of two current meters between two different vertical ranges under two slopes would be done,and the maximum correlation coefficient and the corresponding delay time would be obtained.Then by determining the relatively large correlation coefficient and the relatively short delay time,the deviation angle of the instability of the longshore currents will be received by means of triangle rule.Thus,the direction of propagation of the instability of the longshore currents will be received,and it is necessary to analyze the cause of the offset.The results show that the deviation angle of the instability of longshore currents was about 15° to 30° under irregular waves,and the angle increases with the periods and wave heights of the incident waves,and the slopes also have some influence,the deviation angle under 1:40 is obviously bigger than 1:100.The results also show that the deviation angle of the instability of longshore currents was about 15° under regular waves,the periods and wave heights of the incident waves have some influence in this angle,but the deviation angle under 1:100 is obviously bigger than 1:40.The reason why instability of longshore currents have a deviation angle rather than be parallel to shoreline is affected by vertical circulation system under the actual situation,and on the bottom of bed,the backflow from seabed is mainly,and in the nearly surface,the quality transport flow is mainly.2.The instability of the longshore currents' energy changes characteristics in perpendicular direction to shoreline was studied by the maximum entropy spectrum method,and the influence of energy changes caused by wave situations of incident waves was also analyzed.All the wave situations would be done the maximum entropy spectrum analysis in order to get the maximum entropy spectrum of the flow velocity time histories of every current meter vertical to shoreline,then three optimal frequencies components which includes the instability of the longshore currents was calculated by the least square method.Then to get the amplitude spectrum of three best frequencies,the regression method of trigonometric function was used.According to the theory of wave energy,the energy of the instability of the longshore currents was studied by amplitude change.And furthermore the energy relationship with two other optimal frequencies fluctuations was analyzed.At last,based on the similarity criterion and the influence to edge wave of theinstability of the longshore current,whether the other two low frequency components are edge waves was determined by analyzing the low frequency components in the flow velocity histories.The results show that,when the distance to shoreline is very close,because of the shallow water depth and the existence of the instability of longshore currents,the energy of the other two low frequency wave motions will be larger than the instability of longshore currents.Then with the increase of the distance,the instability of longshore currents exist,the energy specific value of the other two low frequency wave motions and the instability of longshore currents show a trend of increase,and reaches the maximum peak at the moment that the average longshore currents also get the maximum strength,then becomes a trend of decrease.This kind of change trend is the same as the instability of longshore currents.In addition,the incident wave height and period also have some degree of effects on these two low frequency wave motions' energy,but there is no clear trend of increase or decrease.About the judgment of edge waves,only the maximum frequency fluctuation(0.03(0.04)Hz)among three under the regular waves is the edge wave.3.Based on the wavelet cross-spectrum method,the time-frequency characteristics of the instability of the longshore current were studied.And all the wave situations(irregular waves and regular waves)would be studied.Through the wavelet cross-spectrums to analyze that how the incident wave height,period,and the terrains influence their cross-spectrums.The results show that,the three best frequencies are all varies according to their own periods,and all of them have a high correlation degree,among them,the correlation degree of instability of longshore currents reaches up to 0.8,and has a stable fluctuation.These show that there exists a stable instability of longshore currents near the breaking wave point.It also finds that the incident waves 'period and height have little influence on correlation,but the slope has much.And on the whole,the absolute values of correlation peak values under irregular waves are bigger than regular waves.