The popular theory for the instability of longshore currents is the linear shear instability theory.It is assumed that the instability mode with the largest growth rate dominates the instability of longshore currents. However,the assumption cannot be used to some case from the experimental results.In the present paper,milti-mode unstable process（or behavior） has been analyzed which is in good agreement with our previous experiment results.Velocity profile has great effects on the growth mode of the instability of longshore currents.Specially, variations can lead to multi-mode of the instability of longshore currents,viz.more than two peaks occur in growth rate curve.In present paper,we analyze the effect of frontshear and backshear simultaneous variations and individually of the both on the multi-mode of the instability of longshore currents using analytic velocity profiles.The analysis shows that the wave development（or phenomena,formation,evolution） in our experiment can be attributed to the frontshear instability.%目前对沿岸流不稳定研究比较成熟的理论基础是线性不稳定理论,该理论假定增长率最大的不稳定模式决定着沿岸流的波动特性,但实验分析结果表明对有些情况下该假定难以解释实验中观测到的情况.利用沿岸流不稳定运动的多模式理论分析了实验中观测到的情况.结果表明沿岸流速度剖面对于沿岸流小稳定运动模式影响很大,特别是速度剖面的变化会导致不稳定模式出现多个模式的情况,即增长率曲线出现多于两个峰的情况.用解析的速度剖面分析了速度剖面前剪切和后剪切同时变化及单独变化对于沿岸流不稳定多模式运动的影响.结果表明对于坡度为1：100的平直斜坡的情况用前剪切不稳定理论可以解释其观测到的现象.
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