Residual flow, bedforms and sediment transport in a tidal channel modelled with variable bed roughness
Residual flow, bedforms and sediment transport in a tidal channel modelled with variable bed roughness
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DOI：
10.1016/j.geomorph.2017.08.029
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发表时间：
2017-10-15
期刊：
影响因子：
3.9
通讯作者：
Robins, P. E.
中科院分区：
文献类型：
--
作者：
Davies, A. G.;Robins, P. E.
在陆架海和河口中，海床对潮汐流的摩擦影响通常通过规定的、空间/时间不变的阻力系数来模拟。在实践中，海床表现出相当大的可变性，特别是空间上的可变性，这在原则上应包括在模拟中。海底粗糙度（ks）的局部变化改变水流强度，从而改变局部沉积物输移速率。使用空间/时间变化的k的效果在这里参考潮汐通道（梅奈海峡，N.威尔士），其中底形的变化已被监测使用多波束测量。该通道不仅表现出强大的潮汐流，但也是剩余的诱导流，在这里被用作诊断，以评估各种床粗糙度配方测试中的Telemac模型。潮汐模拟已经进行了恒定的和时间/空间可变的ks，和预测的剩余流量被证明是敏感的这些表示。对于一个平均的春季小潮（SN）周期与变量ks，平均余流计算为525米（3）秒（-1），与观测一致。该残余流可以通过在0.15 m至0.3 m范围内施加恒定的ks值来恢复。结果表明，由于中部河道的沙丘，海床的整体有效粗糙度小于最大局部粗糙度的一半，但比整个河道的空间平均KS值大约50%。使用变量ks对M-2单独潮汐进行的模拟产生了491 m（3）s（-1）的稍小（7%）的剩余流量。使用振幅增强73%的等效M-2'潮汐来调和这些估计。对k（s）的主要贡献是沙丘，沙丘是使用货车Rijn（2007）的公式模拟的，受到额外的“历史效应”。发现模拟的k（s）近似等于沿沿着河道中段断面观察到的沙丘高度，而不是预期高度的一半。这是由于反向潮流中的底形的非平衡性质，表现出较短的波长和更对称的轮廓比沙丘在稳定流。(C)2017 Elsevier BY. All rights reserved.
The frictional influence of the seabed on the tidal flow in shelf seas and estuaries isusually modelled via a prescribed, spatially/temporally invariant drag coefficient. In practice, the seabed exhibits considerable variability, particularly spatially, that should in principle be included in simulations. Local variations in the seabed roughness (ks) alter the flow strength and, hence, local sediment transport rates. The effect of using a spatially/temporally varying k, is assessed here with reference to a tidal channel (Menai Strait, N. Wales) in which the variability of the bedforms has been monitored using multi-beam surveying. The channel not only exhibits strong tidal flow, but also a residual induced flow that is used here as diagnostic to assess various bed roughness formulations tested in a Telemac model. Tidal simulations have been carried out with both constant and temporally/spatially variable ks, and the predicted residual flow is shown to be sensitive to these representations. For a mean spring neap (SN) cycle with variable ks, the average residual flow is calculated to be 525 m(3) s(-1), consistent with observations. This residual flow can be recovered using imposed, constant values of ks in the range 0.15 m to 0.3 m. The results suggest that the overall, effective roughness of the seabed is less than half of the maximum local roughness due to the dunes in mid-channel, but more than the spatially-averaged ks value in the channel as a whole by about 50%. Simulations carried out with an M-2-alone tide using variable ks produce a somewhat smaller (by 7%) residual flow of 491 m(3) s(-1). The use of an equivalent M-2' tide of amplitude enhanced by 73% reconciles these estimates. The main contribution to k(s), is made by dunes which are modelled using Van Rijn's (2007) formulation subject to an additional 'history effect'. The modelled k(s), is found to equal approximately the observed height of the dunes along mid-channel transects rather than half the height as expected. This is attributed to the non-equilibrium nature of the bedforms in the reversing tidal flow, which exhibited shorter wavelength and more symmetrical profiles than dunes in steady flow. (C) 2017 Elsevier BY. All rights reserved.