On reversal of wave-generated longshore currents at tidal frequencies on dissipative beaches contiguous to a mesotidal estuary, S.E coast of Nigeria

IF 2.6 3区 地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY
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Abstract

Reversals in wave-generated longshore currents in surf zones occur at different temporal and spatial scales. Along coastlines with tidal channel openings with well-developed mouth bars or ebb-tidal deltas, the reversal mechanism is often attributed to the mouth bar – induced refraction of the shoaling waves. This reversal mechanism is characterized by convergence of longshore currents from the adjoining surf zones at the mouth of the tidal channel. Simultaneous half-hourly monitoring of wave-generated longshore currents over 50 successive (daylight) semi-diurnal tidal cycles in beach surf zones adjoining the Qua Iboe River estuary, S.E coast of Nigeria showed the above reversal pattern during flooding stage only. The converse pattern, where the surf zone longshore currents diverged away from the mouth of the estuary, was observed during ebbing stage. Both surf zones showed flow direction inversion with respect to each other, with velocity vector correlation coefficient r > − 0.8 in over 80 % of the data set. Instances of comparable flow direction (<10 %) were also recorded. Tidal processes are implicated in the documented results. Direction-averaged longshore current velocities, typically in the 15–60 cm/s range, attained highest values in both surf zones at about spring tide phase. Also, tidal cycle-residual longshore current maximum and minimum velocities occurred close to spring and neap tide, respectively. Only 30 % of the residual velocities were eastward directed in the up-drift surf zone as against 80 % in the down-drift counterpart. Given the prevailing southwesterly waves, the present results negate the assertion that reversal in longshore current direction in this offset shoreline setting is exclusively a consequence of wave refraction by mouth bar morphology. The reversing pattern of the longshore currents over a tidal cycle is well explained by incorporating interacting effects of shoaling waves with tide-induced oscillations in water level as well as the estuarine flow.

尼日利亚东南海岸潮间带河口毗连消能海滩上潮汐频率下波浪产生的长岸流的逆转
波浪在冲浪区产生的长岸流逆转发生在不同的时间和空间尺度上。在潮汐通道开口处有发达的口岸或退潮三角洲的海岸线上,逆转机制通常归因于口岸引起的浅滩波折射。这种逆转机制的特点是来自潮汐通道口毗邻冲浪区的长岸流汇聚在一起。在尼日利亚东南海岸 Qua Iboe 河河口附近的海滩冲浪区,对波浪产生的沿岸长流进行了 50 个连续(白天)半昼夜潮汐周期的半小时同步监测,结果表明只有在洪水期才会出现上述逆转模式。相反,在退潮阶段,冲浪区的长岸流偏离了河口。两个冲浪区的流向相互反转,80%以上的数据集的速度矢量相关系数为 r > - 0.8。此外,还记录到流向相当的情况(10%)。记录的结果与潮汐过程有关。方向平均长岸流速通常在 15-60 厘米/秒之间,在两个冲浪区都达到了最高值,大约在春潮阶段。此外,潮汐周期残余长岸流的最大和最小流速也分别出现在春潮和落潮附近。在上漂冲浪区,只有 30% 的残余流速是向东的,而在下漂冲浪区,则有 80% 的残余流速是向东的。考虑到盛行西南波,本研究结果否定了在这种偏移的海岸线环境中,沿岸长流方向的逆转完全是由口杆形态对波浪的折射造成的这一说法。潮汐周期中沿岸长流的逆转模式可以很好地解释由潮汐引起的水位摆动和河口流的相互作用。
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来源期刊
Marine Geology
Marine Geology 地学-地球科学综合
CiteScore
6.10
自引率
6.90%
发文量
175
审稿时长
21.9 weeks
期刊介绍: Marine Geology is the premier international journal on marine geological processes in the broadest sense. We seek papers that are comprehensive, interdisciplinary and synthetic that will be lasting contributions to the field. Although most papers are based on regional studies, they must demonstrate new findings of international significance. We accept papers on subjects as diverse as seafloor hydrothermal systems, beach dynamics, early diagenesis, microbiological studies in sediments, palaeoclimate studies and geophysical studies of the seabed. We encourage papers that address emerging new fields, for example the influence of anthropogenic processes on coastal/marine geology and coastal/marine geoarchaeology. We insist that the papers are concerned with the marine realm and that they deal with geology: with rocks, sediments, and physical and chemical processes affecting them. Papers should address scientific hypotheses: highly descriptive data compilations or papers that deal only with marine management and risk assessment should be submitted to other journals. Papers on laboratory or modelling studies must demonstrate direct relevance to marine processes or deposits. The primary criteria for acceptance of papers is that the science is of high quality, novel, significant, and of broad international interest.
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