Abstract Sudden breaching of a saddle dam on July 23, 2018, on the perimeter of the Xe Nammoy hydroelectric-power reservoir, recently constructed in the Mekong basin, southern Laos, caused catastrophic flooding that resulted in fatalities and displaced thousands of individuals. This study aims to (1) assess the cause of the dam breach, and (2) understand the hydrologic and geomorphic impact of the flood. Our analysis shows that the collapse of the dam occurred as the reservoir was nearly full, and the water level was rising against the inner slope of the earthen dam. As the water discharged through the breach, we calculate that the reservoir fell ~22 m and lost ~350 million m3, more than a third of the total stored volume on July 23. Inspection of publicly available photographs of the failed dam confirms our interpretation that the failure was structural, involving both piping and rotational slumping, and not due to overtopping of the dam. Failure to properly process the local materials before construction of the earth-core of the dam is the likely cause of failure. Our analysis of satellite imagery, bolstered by field observations, reveals that the flooding inundated an area of ~46 km2 of villages and rural land in the Vang Ngao River, a tributary of the Mekong River basin. By using 1D and 2D hydrological modeling, we calculate a peak flood discharge of ~8500 m3/s, about half the mean discharge of the Mississippi River. The dam failure and concomitant losses are consistent with common inadequacies in this region in assessing potential hazards, challenges of geomorphological-geotechnical engineering when using tropical materials for construction, and emergency planning and warning. Thus, this analysis has important implications for the safer design and construction of the myriad of other dams that are either operating or planned within the multi-national Mekong River basin.

中文翻译：

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2018 年老挝南部湄公河流域（波拉文高原）的大坝溃坝和灾难性洪水

摘要 2018 年 7 月 23 日，最近在老挝南部湄公河流域建造的 Xe Nammoy 水电站周边的一座马鞍水坝突然溃决，造成灾难性洪水，造成人员死亡和数千人流离失所。本研究旨在 (1) 评估溃坝原因，以及 (2) 了解洪水的水文和地貌影响。我们的分析表明，大坝坍塌发生在水库接近满水时，水位逆着土坝的内坡上升。当水从裂缝中排出时，我们计算出水库下降了约 22 米，损失了约 3.5 亿立方米，占 7 月 23 日总蓄水量的三分之一以上。对大坝的公开照片的检查证实了我们的解释，即故障是结构性的，涉及管道和旋转坍塌，而不是由于大坝溢出。大坝土芯施工前未能对当地材料进行适当处理是失败的可能原因。在实地观察的支持下，我们对卫星图像的分析表明，洪水淹没了湄公河流域支流 Vang Ngao 河约 46 平方公里的村庄和农村土地。通过使用一维和二维水文模型，我们计算了约 8500 m3/s 的峰值洪水流量，大约是密西西比河平均流量的一半。大坝倒塌和随之而来的损失与该地区在评估潜在危害方面的普遍不足是一致的，使用热带材料进行建筑、应急规划和预警时地貌岩土工程面临的挑战。因此，该分析对于在跨国湄公河流域内运行或规划的无数其他大坝的更安全设计和建造具有重要意义。