Time-Frequency Domain Analysis And A Study Of Pollutant Transport Under The Effect Of The Instability Of Longshore Currents

With the increasing development of the coastal economy, the environmental pollution problems of ocean have holden people’s wide interests, especially coastal zone and off-shore area were affected by environmental pollution more seriously. The shear instabilities of longshore current has important influences on the transport of coastal pollutant and sediment. The research on the the instability of longshore currents and the pollutant transport rule under the effect will provide theoretical reference for the prevention of ocean environmental pollution. Base on these aspects, the main content includes these parts as follows:(1)First, temporal and spatial variation characteristics of under regular waves were investigated by using the wavelet transform theory. By analyzing the wavelet energy spectrum of the shear instabilities of longshore current, focus on analyzing the temporal and spatial variation characteristics of the shear instabilities of longshore current which were observed under the conditions that the incident waves were regular waves and the topographies were two flat slopes which gradient were 1:40 and 1:100.(2)Second, use the space transform method to transform coordinate of dye patch movement image collected by CCD camera, then use this image generated by this approach to study the transportation of pollutant under regular waves of the shear instabilities of longshore current. The transport velocity of pollutant in longshore and cross-shore direction of each case were calculated, and the variation trends of the pollutant concentration center were analyzed.(3)Third, use the dye patch movement image behind coordinate transformation, the transportation of pollutant under irregular waves of the shear instabilities of longshore current were investigated. The transport velocity of pollutant in longshore and cross-shore directions of each case were calculated first, and then the variation trends of the pollutant concentration center were analyzed.By analyzing the wavelet energy spectrum of the shear instabilities of longshore current velocity time series under 15 wave conditions, the results show that when the incident waves are regular waves, the shear instabilities of longshore current can also be observed, but its temporal and spatial variations are not obvious. This was probably because of comparatively shorter time series of the experiment that resulted in the shear instabilities of longshore current had not fully been developed. The variation of the shear instabilities of longshore current over time is continuous once it has been generated; the shear instabilities of longshore current are more dominant in the energy spectrum in longshore direction than cross-shore direction; an increase in the slope will reduce the generated area of the shear instabilities of longshore current; and the shear instabilities of longshore current frequency, caused by the shear instability, changes around 0.015 Hz with a gradient of 1:40 or a gradient of 1:100.In regular waves, by calculating the transport velocity of pollutant in longshore and cross-shore directions, analyzing the variation trends of the pollutant concentration center, the results show that the transport velocity of pollutant in longshore direction is 0.1~0.2m/s, the transport velocity in cross-shore direction is 0.01~0.06m/s, obviously, the transport velocity in longshore direction is faster than in cross-shore direction, and in the case of the same incident wave period, the pollutant transport velocity increases with the increase of wave height; the direction of pollutant movement main along the direction parallel to shoreline, there is no movement in the offshore direction; in the same period and different height of incident wave, the produced range of the shear instabilities of longshore current in crossshore direction increased obviously with the increase of wave height.In irregular waves, the transport velocity of pollutant in longshore direction in 1:100 slope is about 0.04~0.11m/s, the transport velocity in cross-shore direction in 1:100 slope is about 0.01~0.05m/s; in 1:40 slope, the transport velocity in longshore direction is about 0.1~0.3m/s, in crossshore direction the transport velocity is about 0.01~0.06m/s. No matter in 1:100 or 1:40 slope, the transport velocity increased with the increase of wave height in the same period of incident wave. The transport velocity in 1:40 slope is generally faster than in 1:100 slope. The analysis results of the variation trends of the pollutant concentration center are same as the analysis results of regular waves.