Riverbank erosion and char stability along the fluvial-to-tidal transition zone in the Lower Meghna River and Tentulia Channel in the Ganges-Brahmaputra-Meghna Delta, Bangladesh
Riverbank erosion and char stability along the fluvial-to-tidal transition zone in the Lower Meghna River and Tentulia Channel in the Ganges-Brahmaputra-Meghna Delta, Bangladesh
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DOI：
10.1016/j.geomorph.2023.108692
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发表时间：
2023-04
期刊：
影响因子：
3.9
通讯作者：
L. Valentine;Carol A. Wilson
中科院分区：
文献类型：
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作者：
L. Valentine;Carol A. Wilson
恒河-雅鲁藏布江-梅克纳河三角洲（GBM）已被命名为世界上最脆弱的下沉三角洲之一。它也是最具活力的三角洲之一，拥有世界上最高的沉积物排放量和第三高的水排放量，半日中尺度（2-4米）潮汐。高河流流量和显著的潮汐速度（1-2 m/s）沿着整个下梅克纳河（LMR）形成了一个广阔的河流-潮汐过渡区（FTTZ）。在这个FTTZ，潮汐拉长的通道酒吧，当地称为“焦炭”，迅速演变，由于高泥沙和水排放。查尔沿着LMR有一个总人口超过500万人，因此，评价河流和潮汐过程的相互作用沿着FTTZ是至关重要的，以了解脆弱性的查尔社区之间的洪水和海平面上升。在这里，我们利用一个多方面的方法来评估地貌，沉积学和水文的三个字符纵向间隔内的LMR和它的一个分流-Tentulia通道。十年期土地变化分析表明，FTTZ沿着LMR和Tentulia海峡的上部以4.7平方公里/年的速度获得土地，上游侵蚀最普遍，海岸50公里内发生最多的加积。几乎所有的地层剖面显示细砂和/或中砂在深度（0.5-2米），这意味着三个炭同样容易受到潮汐流和高河流流量的侵蚀。因此，我们的属性增加的稳定性的焦炭在下游方向增加潮汐的影响，而不是沉积学。陆地高程和水位调查显示，随着距离LMR口的距离的减小，相对于海平面的高程降低：从4.05 m（距离口130 km），到2.56 m（距离口100 km），再到1.85 m（距离口70 km）。通过将当地的水位数据与高程调查，我们得出结论，河流，即季风，过程控制的LMR和Tentulia通道的头部汇合处附近的字符的地貌。FTTZ的河流主导区和潮汐主导区之间的转换发生在Char 3的下游（距离LMR口约70 km）。因此，LMR汇合处附近的焦炭社区最容易受到与季风季节相关的河岸侵蚀和洪水的影响，而靠近海岸的焦炭最容易受到与风暴潮和相对海平面上升相关的侵蚀和洪水的影响。
The Ganges-Brahmaputra-Meghna (GBM) delta has been named one of the world's most vulnerable sinking deltas. It is also one of the most dynamic deltas, with the world's highest sediment discharge and third highest water discharge, with semi-diurnal mesoscale (2–4 m) tides. The high fluvial discharge and significant tidal velocities (1–2 m/s) create an expansive fluvial-to-tidal transition zone (FTTZ) along the entire Lower Meghna River (LMR). Within this FTTZ, tidally elongated channel bars, locally called ‘chars’, rapidly evolve due to the high sediment and water discharges. Chars along the LMR have a total population exceeding 5 million people; thus, evaluating the interactions of fluvial and tidal processes along the FTTZ is crucial to understanding vulnerability among char communities with respect to flooding and sea-level rise. Here, we utilize a multi-faceted approach to assess the geomorphology, sedimentology, and hydrology of three chars longitudinally spaced within the LMR and one of its distributaries – the Tentulia Channel. Decadal land change analyses revealed that the FTTZ along the upper LMR and Tentulia Channel has been gaining land at a rate of 4.7 km2/yr, with erosion being most prevalent upstream and most accretion occurring within 50 km of the coast. Almost all the stratigraphic profiles show fine and/or medium sands at depth (0.5–2 m), implying that the three chars are similarly susceptible to erosion by tidal currents and high river discharge. Thus, we attribute the increased stability of chars in the downstream direction to increased tidal influence rather than sedimentology. Land elevation and water level surveys reveal that elevation relative to sea level decreases as the distance to the mouth of the LMR decreases: from 4.05 m (130 km from the mouth), to 2.56 m (100 km from the mouth), and 1.85 m (70 km from the mouth). By incorporating local water level data with the elevation surveys, we conclude that fluvial, namely monsoonal, processes control the geomorphology of the chars near the confluence of the LMR and the head of Tentulia Channel. The shift between fluvial-dominated and tidal-dominated regions of the FTTZ occurs just downstream of Char 3 (∼70 km from the LMR mouth). Therefore, char communities near the LMR confluence are most vulnerable to riverbank erosion and flooding associated with the monsoon season, and chars proximal to the coast are most vulnerable to erosion and flooding related to storm surges and relative sea-level rise.