Abstract 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 pumiceous tephra fall events and 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 dams by consisting principally of washed-together pumice clasts 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 many 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 comparable 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°, extremely low gradient (0.19‰) starting already 24 km upstream of the bottleneck, constant decrease of flow velocity over many 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 pyroclastic flows 7 km downstream of the LNB about halfway through the eruption extended further and further 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 enabled the dams to remain stable as long as tephra fell into the Rhine relatively continuously.

中文翻译：

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在 12,900 BP Plinian Laacher See Eruption（德国）期间，通过沉降物筑坝大河的边界条件。莱茵河大坝二期

摘要 莱茵河（德国）——西欧最大的河流——在普林尼亚拉赫湖大喷发（12,900 BP）期间多次被火山碎屑物质筑坝。大坝的上游和下游形成了广阔的下新维德盆地 (LNB)，它中断了狭窄的莱茵河峡谷。在这里，我们记录了莱茵河上游在靠近现在科布伦茨市的 LNB 入口处筑坝的情况，这是由于落入莱茵河及其主要支流摩泽尔河和拉恩河的火山灰超载。大坝在火山灰快速坠落事件中反复形成，在喷发活动中断期间被破坏，导致整个盆地发生大范围、高能量的洪水。短暂的科布伦茨大坝与“正常”火山诱发的大坝有很大不同，主要由冲刷在一起的浮石碎屑和一些浮木组成。浮石的多孔性质及其吸水能力是关键因素。因此，大量落入莱茵河的火山灰漂浮在水柱的上部或游到水面。此外，浮石碎屑对河水的吸收显着增加了两相流的沉积物：水比。我们在这里展示了一个类似于冰塞形成的大坝形成模型。我们想象科布伦茨大坝是细长的，部分漂浮，部分接地，可渗透的塞子长数公里，不高于洪泛区。筑坝最有可能在 LNB 中最大的 tephra 负载区域内启动，并以类似于交通拥堵的形成的连锁反应向上游传播。在LNB的瓶颈入口处终于堆积了一座大坝，这个站点结合了几个有利条件：上游多渠道莱茵河被限制在一个单一的渠道，流向改变125°，极低坡度（0.19‰）开始在瓶颈上游 24 公里处，流向瓶颈和摩泽尔河（LNB 内莱茵河的最大支流和额外火山灰的重要输送机）在数公里内不断降低流速，在仅 700 米处进入莱茵河瓶颈。我们假设科布伦茨大坝只能通过一个极长的“脚部区域”形成并稳定下来，该区域向下游延伸了数公里，并且可能与一个或几个低层次要堵塞/水坝相连。布罗尔湖的回水被 LNB 下游 7 公里处的火山碎屑流拦住，大约在喷发中途，在 Laacher See 喷发的第二个普林阶阶段，它越来越向上游延伸到 LNB 中，这可能是促成形成的主要因素和大型科布伦茨大坝 No.4。大多数时候科布伦茨大坝可能没有完全密封。通过这种方式，主要的喷发前莱茵河通道接收了一些水。建立了一种平衡条件，只要火山灰相对连续地落入莱茵河，水坝就能保持稳定。