Abstract This study investigates the role of mechanical layering and fractures on flood-related erosional undercutting and resulting rapid spillway recession. In the summer of 2002, 86 cm of rain fell in an 8-day period across the Guadalupe River drainage basin in central Texas, causing Canyon Lake reservoir to completely fill and overtop the emergency spillway for the first time. The resulting flood incised a gorge into the mechanically layered Glen Rose Formation and caused headward erosion (recession) at the downstream edge of the emergency spillway. Comparison of pre- and post-flood imagery and assessment of flood records indicates that maximum recession localized at the northern end of the emergency spillway where 28 m recession occurred. This recession occurred at an estimated rate of up to 10 m/day during the first ~3 days of the flood, which is among the highest rates of recorded bedrock recession. Analysis of historical photographs, field observations and measurement of erosional undercutting, along with measurements of fracture orientation, fracture spacing, and mechanical rebound are used to understand rock mass characteristics that influenced erosional undercutting and rapid recession of the spillway. Evidence of significant undercutting was observed where incompetent argillaceous wackestone (marl) underlies competent limestone. These results reveal that the greatest amount and rate of recession of the spillway was associated with undercutting and toppling collapse of fracture-bounded limestone blocks. Block size may be a factor in continuation of the process, in that large blocks may accumulate at the base of the scarp and inhibit continued erosional undercutting. Areas where smaller eroded blocks can be carried away by the floodwaters may continue to undercut, facilitating continued recession. The combination of mechanical contrast between layers and natural fractures in competent layers together contributed to exceptionally high rates headward erosion. Observed rock mass erodibility behavior was in the range of medium to high erodibility in limestone with widely spaced fractures that would normally be expected to have very low erodibility comparable to clay (comparable to marl at base of spillway pour-off cliff) – bulk rock mass erodibility in this situation was similar to the most erodibile layer.

中文翻译：

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美国德克萨斯州峡谷湖溢洪道机械分层和天然裂缝对底切和快速向前侵蚀（衰退）的影响

摘要 本研究调查了机械分层和裂缝对与洪水相关的侵蚀底切和导致溢洪道快速衰退的作用。2002 年夏天，德克萨斯州中部瓜达卢佩河流域的降雨量在 8 天内下降了 86 厘米，导致峡谷湖水库首次完全填满并超过了紧急溢洪道。由此产生的洪水在机械分层的 Glen Rose 地层中形成了一个峡谷，并在紧急溢洪道的下游边缘造成了向前侵蚀（衰退）。洪水前后图像的比较和洪水记录的评估表明，最大的衰退位于紧急溢洪道的北端，那里发生了 28 m 的衰退。在洪水的前 3 天，这种衰退估计以高达 10 m/天的速度发生，这是有记录的基岩衰退率最高的一次。历史照片分析、现场观察和侵蚀底切测量，以及裂缝方向、裂缝间距和机械回弹的测量，用于了解影响侵蚀底切和溢洪道快速退缩的岩体特征。在不合格的泥质泥灰岩（泥灰岩）位于合格的石灰岩之下时，观察到明显的底切迹象。这些结果表明溢洪道的最大衰退量和速率与裂缝边界的石灰岩块的底切和倾倒坍塌有关。块体大小可能是该过程继续的一个因素，因为大块体可能会在陡坡底部堆积并抑制持续的侵蚀底切。较小的侵蚀块可以被洪水冲走的区域可能会继续削弱，从而促进持续的衰退。层之间的机械对比和主管层中的天然裂缝的结合共同导致了异常高的向前侵蚀率。观察到的岩体可蚀性在具有大间距裂缝的石灰岩中处于中到高可蚀性范围内，通常预计具有与粘土相当的非常低的可蚀性（与溢洪道倾倒悬崖底部的泥灰岩相比）-散装岩体这种情况下的可蚀性与最易蚀的层相似。层之间的机械对比和主管层中的天然裂缝的结合共同导致了异常高的向前侵蚀率。观察到的岩体可蚀性在具有大间距裂缝的石灰岩中处于中等至高度的可蚀性范围内，通常预计具有与粘土相当的非常低的可蚀性（与溢洪道倾泻悬崖底部的泥灰岩相比） - 散装岩体这种情况下的可蚀性与最易蚀的层相似。层之间的机械对比和主管层中的天然裂缝的结合共同导致了异常高的向前侵蚀率。观察到的岩体可蚀性在具有大间距裂缝的石灰岩中处于中到高可蚀性范围内，通常预计具有与粘土相当的非常低的可蚀性（与溢洪道倾倒悬崖底部的泥灰岩相比）-散装岩体这种情况下的可蚀性与最易蚀的层相似。