沙坝地形上沿岸流不稳定运动模式变化特性

Variations of the instabilities of longshore currents on sand barred beaches

为研究沙坝地形上沿岸流不稳定运动特征模式变化特性,建立了考虑底摩擦影响的不稳定模型,并用该模型计算了两种沙坝地形（坡度分别为1∶40和1∶100）上的不稳定特征模式,这些模式内容包括不稳定运动波长、传播速度、不稳定增长率和扰动速度场。将这些结果与忽略底摩擦计算结果进行了比较,分析了底摩擦对不稳定特征模式的影响。上述研究表明沙坝地形上不稳定运动传播速度约为时均沿岸流最大值的0.71~0.79;1∶40沙坝地形上产生不稳定运动所用的时间比1∶100地形上更短;底部摩擦可以很大程度上抑制沿岸流不稳定运动的产生;一般观测到的（准）韵律形、新月形沙坝都在后剪切区（紧邻时均沿岸流最大值向海方向区域）产生;不稳定运动在沙坝处运动最强,并且坡度越缓,不稳定运动沿岸方向波长越长。

In this paper, a hydrodynamic numerical model including bottom friction is developed to calculate the in- stabilities of longshore currents on two barred beaches with respective slopes of 1：40 and 1：100. This model enables to estimate wavelength, propagation velocity, growth rate and perturbation velocity field of the instabilities. Here, the effects of the bottom friction on the instabilities are analysed according to the beach slope. The results show that the propagation velocity of the instabilities ranges between 0.71--0. 79. Furthermore, the instabilities develop faster on the 1：40 slope beach than on the 1：100 slope beach. It is also shown that the bottom friction has a great effect on the inhibition of the instabilities and that backshear is a key parameter in the instabilities development. Therefore sand bars can often be observed in this zone. Finally, the strongest energy of the instabilities generally occurs at the crest of bar and the milder the slope, the longer the wave length of the instabilities.