高山峡谷区古洪水事件重建研究进展

古洪水事件重建是地貌学与环境变化领域的前沿性课题之一。高山峡谷基岩河段是古洪水重建的理想场所,通过系统梳理国内外文献,评述古洪水事件重建研究进展,认为识别古水位标志(palaeo-stage indicators)、系统建立流域尺度洪水综合地貌证据是古洪水重建的前提和基础。而不同类型的古洪水地貌证据中,古洪水滞流沉积物(slackwater deposits,SWD)是最完整的具有确切水位标志指示意义的高水位悬移质沉积物,而其他边滩坝体(bars)是指示洪水路径和水动力条件的低水位标志沉积物。基于这些洪水地貌证据与水位标志物指示,采用多种水力学模型重建古洪水规模。同时采用多种测年方法相互验证建立可靠完整的古洪水事件年代序列。从点到面建立古洪水事件数据库,提出不同成因类型古洪水事件的时空模式,系统揭示特大洪水发生规律及其演化趋势。其中,青藏高原周缘灾难性古洪水事件在形成年代、地貌证据和水文模拟等方面具有鲜明的时空特征与地貌效应,这些对认识全球末次冰期巨型洪水事件具有重要意义。在特定的时空尺度范围内评估极端洪水事件的地表过程与地貌效应,甚至大陆尺度灾难性古洪水研究已经拓展到行星尺度洪水地貌。此外,高山峡谷现代大洪水地貌原型观测对理解古洪水水文过程机制等具有重要的参考价值。

Progress in the reconstruction of palaeoflood events in the mountain canyon valleys around the Tibetan Plateau

Palaeoflood reconstruction is a frontier research topic in the field of geomorphology and environmental change. This paper reviews paleohydrologic techniques and approaches and discusses the hydrological reconstruction and geomorphological effects of palaeoflood events. Paleostage indicators (PSIs) and geomorphologic evidences (e.g., slackwater deposit (SWD)) were identified along the mountain canyon valleys of southwestern China using paleohydrological criteria. Geomorphologic evidences of the palaeoflood events should be systematically investigated at the watershed scale. Palaeoflood SWD, suspended during the high water stage in the slackwater/backwater area, provides a complete sedimentary sequence of PSI; while sand/gravel bars, the bed-load deposits during the flood’s low water stage, and run up deposits above the bar top provide a potential PSI. The magnitude of paleoflood was estimated using multiple one- and two-dimensional hydraulic models in the bedrock reach. Different flood geomorphologic evidences were dated by multiple geochronological methods (e.g.,¹⁴C dating, OSL dating, cosmogenic ¹⁰Be and ²⁶Al exposure age dating). Floods of different origins were checked on a watershed scale. The magnitude-frequency relationship and spatiotemporal pattern of the palaeoflood events were established. The formation age, geomorphic evidence, and hydrological model of the catastrophic paleofloods around the Tibetan Plateau show obvious spatiotemporal characteristics and geomorphologic effects, which has significance for understanding global megaflood geomorphology during the last glaciation. The hydrodynamic conditions and geomorphologic effects of the palaeoflood were assessed on a special scale, and flood geomorphology has been extended from a continental to a planetary scale. In addition, modern large flood investigations can serve as a key reference for understanding the palaeoflood sedimentary processes and hydrological parameters.