岷江上游全新世古洪水事件沉积记录与水文重建研究

Southwest River is one of the highest hydroelectric dam densities in China. Discharge of the Minjiang River is the largest River in the upper Yangtze River. Gauged flood data series are too short to make informated decisions regarding the design flood of the large hydroelectric engineering and transport engineering along the upper Minjiang River. Palaeoflood investigations using cross-disciplinary technology can provide better estimates on the magnitude and frequency of rare floods that have occurred in a river basin in long time spans. However, it is important to identify nature flood stage indicators from flood deposits and microtopography on the mountains canyons valley of the upper Minjiang River. Modern flood deposits and modern flood stage indicators were systemically observed and measured along the upper Minjiang River. Sedimentary and microtopographic features of flood water level marks were surveyed and analyzed. Palaeoflood slackwater deposits (SWDs) were identified by a series of palaeoflood sedimentological criteria in the fieldwork. Palaeoflood events recorded by SWDs at the multiple sites were demonstrated by geophysical and geochemical proxies including grain size distribution, geochemical elements, and mineralogical analysis. Palaeoflood chronology was established by radiocarbon and optically stimulated luminescence methods. Holocene palaeoflood peak discharges at the multiple palaeoflood sites were estimated by step backwater method in the stable bedrock. The veracity of palaeoflood estimation could be critically examined and evaluated by modern large flood and the relationship curves between the largest flood discharges and drainage for global maximum known events. The value of Holocene palaeoflood data provides the physical evidence of rare flood and limits on the largest magnitude of normal flood along the upper Minjiang River. These results are important in dealing with engineering problems and understanding the driving mechanism of Holocene extraordinary floods in the southwest China.

西南河流是我国水资源和水能源开发重点，岷江作为长江上游水量最大的支流，其上游仅有80年实测洪水数据，而重大水利工程建设急需“千年一遇设计洪水、万年一遇校核洪水”。从新兴学科全新世古洪水研究途径，将会有效延长工程水文学实践当中超长尺度河流洪水水文资料序列。其中，如何在岷江上游高山峡谷准确识别具有确切自然水位标志的沉积物与微地貌是个关键。本项目在探究现代洪水自然水位标志与洪水沉积之间的关系，掌握最接近自然水位标志的洪水沉积和微地貌特征，建立流域尺度古洪水沉积野外宏观特征与理化指标判别体系的基础上，通过岷江上游古洪水全面系统野外调查，梳理全新世以来的古洪水事件，在基岩峡谷稳定河段重建其水文学指标，并用现代大洪水数据等途径对参数校核，用OSL和14C技术建立洪水事件年代序列，分析其发生频率。本项目的研究可为工程建设、防洪减灾以及揭示西南河流全新世特大洪水发生规律方面提供重要的基础数据和科学依据。

本项目成果极大丰富了岷江上游万年尺度特大洪水信息，重新界定了流域稀遇大洪水上限，初步厘清了不同成因类型的古洪水地貌与沉积特征，为今后西南季风区河流古洪水研究奠定了重要基础。古洪水滞流沉积物（SWD-PSI）高水位悬移质沉积，位于干支流交汇处或者河谷展宽处，以粉细沙为主；边滩坝（Bars）作为低水位高能量推移质沉积，以砂砾石为主。高山峡谷区古洪水容易受到复合链生型灾害影响，从流域尺度多地点多断面联合分析论证，构建了岷江上游古洪水事件年代序列为14-10 ka,8.5-6.5 ka，6.5-4.5 ka,4.5-3.5 ka四个时段，其中主要集中在中全新世晚期（7-4 ka）。岷江茂县段中全新世（6-4 ka）极端洪水事件,利用逐步回水模型重建规模为6500-10500 m3/s，古溃决洪水规模为25000 m3/s；岷江源早全新世冰缘湖（14-10 ka）外泄过程古洪水事件；茂县渭门乡堰塞湖早全新世（13-8 ka）多期次溃决洪水事件，叠溪堰塞湖顶部OSL年代为中全新世（8-5 ka），不同湖积阶地指示多期次古溃决洪水事件。综合区域气象水文资料，利用洪水地貌证据地球化学元素和重矿物含量沿程变化分析，有效区分岷江中全新世溃坝型与暴雨型洪水沉积。尤其是利用泥质层状钙华重建识别出青藏高原东缘早中全新世7次强季风降雨事件导致的洪水频繁期（flood-rich periods）。这些成果为理解西南河流特大洪水发生规律以及重大工程防洪减灾提供重要的基础数据和科学依据。