波流共同作用下珊瑚礁冠层附近平均流分布及阻力特性试验

Laboratory study on mean flow and drag characteristics around thereef canopy under the combined effect of wave and current

本文通过物理模型试验研究了波流共同作用下珊瑚礁冠层附近平均流的分布特征以及阻力特性,分析了典型波浪工况下无潮流、正向潮流和反向潮流分别作用下平均流速、摩阻流速、阻力系数的沿礁变化规律。结果表明:无潮流时礁前斜坡及外礁坪上存在海底回流且在礁缘附近回流最强,在礁坪上冠层附近平均流表现为向岸流,且该流沿礁向海岸方向持续增大。相较于无潮流时,正向潮流作用下冠层内外均为向岸流,在礁坪上冠层内外的向岸流显著增大;反向潮流作用下冠层内外均为离岸流且在礁缘处达到最大,该离岸流在礁坪上逐渐减小然后趋于稳定。无潮流时礁坪上摩阻流速呈小幅波动;相较于无潮流时,正向潮流、反向潮流影响下礁坪上摩阻流速显著增大,其中正向潮流影响下增幅更大;无潮流时礁坪上水力粗糙度沿礁减小,正向潮流和反向潮流影响下水力粗糙度普遍有幅度不等的增加。三种工况下礁坪上的阻力系数均沿礁整体呈下降趋势,相较于无潮流时,正向潮流和反向潮流影响下礁坪上的阻力系数显著增大,且正向潮流作用时增幅更大。

In this study,mean flow profiles and reef drags characteristics around the reef canopy under the combined effect of wave and current have been investigated,followed by analyses on the cross-reef variations of mean velocity,friction velocity and drag coefficient with flooding and ebb tidal currents,as well as without tidal currents under a typical regular wave condition.In the case of no tidal current,wave-driven seaward current appears both on the fore-reef slope and on the outer reef,and it reaches a maximum near the reef edge.The mean current around the canopy on the reef flows shoreward,and continues to increase across the reef flat.Under the influence of the flooding tidal current,the mean current in and above the canopy moves shoreward,and its magnitude on the reef flat is significantly higher compared to that without tidal currents.Under the influence of the ebb tidal current,the mean current in and above the canopy moves seaward,and it reaches its maximum near the reef edge.Afterwards,the mean current gradually decreases and becomes steady on the reef flat.On the reef flat,the friction velocity fluctuates slightly in the case of no tidal current.The combined influence of wave and current results in a significant increase in the friction velocity,which is greater in the presence of flooding tidal current.The hydraulic roughness parameter decreases across the reef flat in the absence of tidal currents,and its magnitude on the reef flat increases when there is a flooding or ebb tidal current.The drag coefficient displays a decreasing trend for the three waves and current scenarios.Compared to the case without tidal currents,the drag coefficient increases notably with both flooding and ebb tidal currents,and such increase is more significant with the flooding tidal current.