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首页> 外文期刊>Journal of Volcanology and Geothermal Research >Boundary conditions for damming of a large river by fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming Ⅱ
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Boundary conditions for damming of a large river by fallout during the 12,900 BP Plinian Laacher See Eruption (Germany). Syn-eruptive Rhine damming Ⅱ

机译：12,900 BP Plinian Laacher中的爆炸在爆发（德国）中爆炸河流坝的边界条件。 SYN-BURUPTIVE RHINEDRINEⅡ

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摘要

The Rhine River (Germany) - the largest river in Western Europe - was dammed by pyroclastic material multiple times during the major Plinian Laacher See Eruption (12.900 BP). Dams formed both upstream and downstream of the broad tectonic Lower Neuwied Basin (LNB) which interrupts the narrow Rhine canyon. Here we document upstream damming of the Rhine River at the entrance to the LNB close to the present city of Koblenz due to overloading with tephra fall into the Rhine and its major tributaries, the Moselle and the Lahn. The dam was formed repeatedly during rapid pumice tephra fall events and became breached during breaks in eruptive activity, causing extensive, high-energy flooding throughout the entire basin. The ephemeral Koblenz dams differed significantly from "normal" volcanically-induced darns by being composed principally of washed-together pumice lasts and some driftwood. The porous nature of pumice and its ability to absorb water were crucial factors. Thus, a large volume percentage of the tephra that had fallen into the Rhine floated submerged within the upper part of the water column or swam at the surface. Moreover, the absorption of the river water by the pumice clasts increased the sediment:water ratio of the two-phase flow considerably. We here present a model of dam formation resembling the formation of ice jams. We visualize the Koblenz dams to have been elongate, partly floating and partly grounded, permeable plugs several kilometers long and rising no higher than the flood plain. Damming was most plausibly initiated in the LNB within the area of maximum tephra loading and propagated upstream in a chain reaction similar to the formation of traffic jams. A major dam was finally accumulated at the bottleneck entrance to the LNB, a site combining several favorable conditions: the upstream multi-channel Rhine was confined to a single channel, change of flow direction by 125 degrees, extremely low gradient (0.19%.) starting already 24 km upstream of the bottleneck, constant decrease of flow velocity over several kilometers towards the bottleneck and the Moselle River - largest tributary of the Rhine within the LNB and an important conveyor of additional tephra masses - entered the Rhine only 700 m upstream of the bottleneck. We assume that the Koblenz dams could only have formed and been stabilized by an extremely long "foot region" that extended many kilometers downstream and that was possibly connected to one or several low-rise secondary jams/dams. The backwater of Lake Brohl that was dammed by pyrodastic flows 7 km downstream of the LNB about halfway through the eruption extended upstream into the LNB during the second Plinian stage of the Laacher See Eruption and was probably a major factor contributing to the formation and large size of Koblenz Dam No.4. The Koblenz dams were probably not completely sealed most of the time. This way, the major pre-eruptive Rhine channel received some water. An equilibrium condition was established that allowed the dams to remain stable as long as tephra fell into the Rhine relatively continuously. (C) 2020 Elsevier B.V. All rights reserved.
机译：莱茵河（德国） - 西欧最大的河流 - 在主要的Plinian Laacher中多次受到杀虫材料的损坏，请参见爆发（12.900 bp）。在广义构造下新新婚盆地（LNB）的上游和下游的水坝中断了狭窄的莱茵峡谷。在这里，由于随着Tephra陷入莱茵河及其主要的支流，摩泽尔和达拉恩，我们在靠近当前Koblenz的LNB临近的LNB入口处的莱茵河上游禁止莱茵河的上游渔村渔业。在快速浮法Tepharra秋季事件中反复形成大坝，并且在爆发活动中发生破裂，在整个盆地中造成广泛，高能量的洪水。短暂的Koblenz Dams通过“正常”的火山引起的Darns通过主要由洗涤的浮石和一些漂流木材组成，从“正常”的火山诱导的Darns不同。浮石的多孔性质及其吸收水的能力是至关重要的因素。因此，落入漂浮在水塔在水塔的上部或表面的游泳池内的莱茵河中的Tephara的大容量百分比。此外，浮石泥浆的吸收岩水增加了沉积物：两相流的水比大大。我们在这里提出了一种类似于冰爪形成的大坝形成模型。我们可视化Koblenz Dams一直是细长的，部分漂浮，部分接地，透气塞数漫长，较长，不高于洪水平原。在最大Tephra装载的区域内的LNB中最合理于LNB，在链式反应的上游繁殖类似于交通拥堵。最终在LNB的瓶颈入口处累积了一个主要大坝，该网站结合了几个有利的条件：上游多通道莱茵河被限制在单个通道，流动方向的变化125度，极低梯度（0.19％）从瓶颈上游开始，在瓶颈和摩泽尔河内的几公里内的流速减小 - LNB内最大的支流和额外的Tephra Masses的重要传送带 - 进入莱茵河上游700米瓶颈。我们假设Koblenz Dams只能由一个极度长的“足地区”稳定，在下游延伸了数公里，并且可能连接到一个或多个低层二级卡纸/坝。在LNB的LNB左右下游的Pyrodastic流量达到了7公里的Brohl湖的后水在莱卡的第二个普林尼舞台上延伸到LNB上游，看起来爆发，可能是贡献形成和大尺寸的主要因素。科布伦兹大坝4号。大多数时候，科布伦兹大坝可能并没有完全密封。这样，主要预发莱茵河通道的主要预防莱茵河通道收到了一些水。建立平衡条件，只要Tephra相对连续地落入莱茵河，就允许大坝保持稳定。 （c）2020 Elsevier B.v.保留所有权利。

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