Role of morphological variability in the evolution of nearshore sandbars
Role of morphological variability in the evolution of nearshore sandbars
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形态变异在近岸沙洲演化中的作用
DOI：
10.1016/j.coastaleng.2012.05.005
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发表时间：
2012
影响因子：
4.4
通讯作者：
M. Stive
中科院分区：
文献类型：
--
作者：
M. Smit;A. Reniers;M. Stive
使用基于深度平均形态过程的双近岸沙洲系统模型的计算已被用于验证这样的假设，即具有小变异性的测深比具有明显新月形图案的测深更容易适应新的水动力条件。计算结果用于研究这样一种假设，即如果这些条件持续一段较长的时间，近岸水深测量倾向于向与同时存在的恒定水动力强迫相匹配的裂口通道模式发展。在每次计算中，在两个连续的恒定水动力条件下，初始沿岸均匀的双条形测深，加上一个小的随机床面扰动。对于每一组条件，四种不同的计算显示了较晚的过渡时刻的影响——因此更明显的进化模式——对第二种条件的适应水平。过渡到第二种状态后，由于过渡时刻水深的不同，会产生不同的水动力环流。根据现有特征在过渡时刻的明显程度，这些环流要么加强现有的水深模式，要么允许水深模式演变为新的撕裂通道模式，其间距类似于从一开始就应用第二种条件时所发生的间距。由于水动力条件的变化通常比适应时间快得多（至少以天为单位），因此观测到的裂口通道距离不太可能与同时存在的水动力条件下的预期长度尺度相匹配，这与现场观测结果是一致的（例如Holman et al., 2006）。因此，可以得出结论，近岸模式是由之前的形态（以及之前的水动力）和当前当地水动力条件的结合形成的，其次是沉积物特征等因素。
Computations using a depth-averaged morphological process-based model of a double nearshore bar system have been used to test the hypothesis that bathymetries with small variability adapt more easily to new hydrodynamic conditions than bathymetries with distinctly imprinted crescentic patterns. The computations are used to investigate the assumption that nearshore bathymetries tend to evolve toward a rip-channelled pattern matching concurrent constant hydrodynamic forcing, if these conditions prevail for an extended period of time. In each computation an initially alongshore uniform double barred bathymetry, seeded with a small random bed-level perturbation, was forced by two sequential constant hydrodynamic conditions. For each set of conditions, four different computations show the effect of a later transition moment – and thus more distinctly evolved patterns – on the level of adaptation to the second condition. After the transition to the second condition, different hydrodynamic circulations occur due to differences in the bathymetry at the transition moments. Depending on how pronounced the existing features were at the moment of transition, these circulations either reinforce the existing bathymetric pattern or allow the bathymetry to evolve to a new rip-chanelled pattern with a spacing similar to the one that occurs if the second condition had been applied from the start. As hydrodynamic conditions generally change more rapidly than the adaptation time, which is at least in the order of days, it is highly unlikely that observed rip channel distances match length scales expected for concurrent hydrodynamic conditions, consistent with field observations (e.g. Holman et al., 2006). It is therefore concluded that nearshore patterns are formed by a combination of both the antecedent morphology – and thus antecedent hydrodynamics – and the current local hydrodynamic conditions, next to factors like sediment characteristics.