Abstract Dam-failure floods typically involve greater peak discharge than the largest meteorological flood at a basin. Determining the geomorphic effectiveness of extreme flooding caused by a breach mechanism provides insight into the role of flood scale on the resulting processes and landforms. Here, we present a geomorphological and hydraulic analysis of the 1959 Vega de Tera (NW Spain) dam-break flood, a worldwide notable dam-failure incident that released a flow of 7.8 106 m3 that caused the death of 144 people at Ribadelago before reaching Lake Sanabria (9 km down valley). This watershed-lake connection provides a comprehensive analysis of an extreme sediment delivery event in the context of a millennial long lake depositional record. One-dimensional unsteady flow computation shows a peak flow hydrograph attenuating from 13,000 m3 s−1 to 5150 m3 s−1, that reached a maximum flow depth of 34 m and velocity of 30 ms−1. Spatial variation of erosional and depositional landforms are related with local flow hydraulics: i) in steep sectors flow regime was supercritical (shear value up to 11,200 Pa) and produced up to 30 m deep bedrock channel erosion; ii) at the boundary of steep and flatten sectors, transition to subcritical regime generated large plunge pools (up to 6000 m2 and 15.2 m in depth); iii) in low-gradient sectors low shear stress gave rise to depositional landforms, namely gravel bars with dam boulders up to 3 m long, and a debris cone with coarse gravel and expansion sand bars. The depositional landforms amount for a total volume of ca 2.11 106 m3 in the Tera valley (37% in the gorge and 63% in the floodplain). The dense, energetic sediment-laden flow reached Lake Sanabria forming a debris cone close to the mouth and caused an underwater hyperpycnal current, depositing a ~ 10 cm-thick sandy-silt layer all over the two distal subbasins. The estimated volume of the deposited fine sediments in the lake ranges between 200,000 and 368,000 m3. The lake record shows that this was the largest flood in the basin during the Holocene. Previous to the dam break, the sediment connectivity between the Sanabria watershed and the lake was limited because of the “staircase” topography and the presence of small glacial depressions filled with sediments since deglaciation. Even during the flood, the great majority of the sediments were deposited along the flood pathway, and only a small percentage (10–20%) reached the lake. Although the hydraulics of the Tera River were not changed after the flood, the newly formed pools in the watershed could diminish the connectivity between the river and the lake in the future, as some new sedimentation areas (pools) were generated acting as natural dams and thus decreasing sediment input to the lake.

中文翻译：

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山区极端洪水期间的地貌影响、水力学和流域湖连通性：1959 年西班牙西北部 Vega de Tera 大坝溃决

摘要 溃坝洪水通常比流域最大的气象洪水涉及更大的峰值流量。确定由破坏机制引起的极端洪水的地貌有效性可以深入了解洪水规模对由此产生的过程和地貌的作用。在这里，我们展示了 1959 年 Vega de Tera（西班牙西北部）溃坝洪水的地貌和水力分析，这是一场世界性的重大溃坝事件，它释放了 7.8 106 m3 的流量，在到达 Ribadelago 之前导致 144 人死亡。萨纳布里亚湖（沿着山谷向下 9 公里）。这种分水岭与湖泊的联系在千年长湖沉积记录的背景下提供了对极端沉积物输送事件的综合分析。一维非定常流计算显示峰值流量从 13 衰减，000 m3 s-1 到 5150 m3 s-1，达到最大流深 34 m 和速度 30 ms-1。侵蚀和沉积地貌的空间变化与局部流动水力学有关：i) 在陡峭的扇区中，流态是超临界的（剪切值高达 11,200 Pa），并产生了 30 m 深的基岩通道侵蚀；ii) 在陡峭和平坦部分的边界处，过渡到亚临界状态产生了大型跌水池（高达 6000 m2 和 15.2 m 深）；iii) 在低梯度区域，低剪切应力产生了沉积地貌，即具有长达 3 m 的大坝巨石的砾石坝，以及带有粗砾石和膨胀砂坝的碎屑锥。Tera 河谷的沉积地貌总量约为 2.11 106 m3（峡谷中 37%，漫滩中 63%）。浓密的，充满沉积物的高能流到达萨纳布里亚湖，在靠近河口的地方形成一个碎屑锥，并引起水下超重流，在两个远端子盆地上沉积了约 10 厘米厚的沙质淤泥层。湖中沉积的细粒沉积物的估计量在 200,000 至 368,000 立方米之间。湖泊记录显示，这是全新世盆地内最大的洪水。在大坝溃决之前，由于“阶梯”地形以及冰川消退后存在充满沉积物的小冰川洼地，萨纳布里亚流域与湖泊之间的沉积物连通性受到限制。即使在洪水期间，大部分沉积物也沿洪水路径沉积，只有一小部分（10-20%）到达湖泊。