流速断面法在长江古流量估算中的应用

Application of velocity-cross section methods for paleodischarge in the Yangtze River

河流古流量的估算可以定量化研究流域的水文、气候变化特征。根据收集的多个地质钻孔资料恢复出长江南京段3个古河槽横断面,对古深槽沉积物样品进行14C与ESR年代测定以及沉积相的分析,判定-50～-90 m的古深槽形成于末次冰期最盛期。根据古河床沉积物粒度,利用泥沙起动公式计算古河槽不同深处的垂线平均流速,然后利用垂线平均流速和垂线间的过水面积计算单宽流量,最后累加恢复末次冰期最盛期的平滩流量。其计算结果为10 000～12 000 m3/s,与现在1月份多年平均流量相当。该时期的纵坡降为4×10-4～5×10-4,与现在长江上游屏山—宜宾段相当。

The reconstruction of river paleodischarge can determine the quantitative hydrologic and climatic characteristics of the drainage basin. Three paleochannel cross-sections of the Yangtze River at Nanjing section are restored by borehole data. Based on ^14C and ESR （Electron Spin Resonance） ages and analysis of sedimentary facies, the paleochannel between the -50 - -90 m existed in the last glacial maximum （LGM）. According to the sediment sizes, the right sediment incipient formulae calculate the average vertical velocity at different depths. Then, the average vertical velocity multiplies by the corresponding area to get the unit paloedischarge. Finally, adding all of the unit paleo- discharge reconstructs the paleodischarge in the LGM. The paloedischarge in the LGM is about 10 000 - 12 000 m^3/s that is equal to the present January average annual discharge. Longitudinal gradient in the LGM is about 4 × 10^-4 - 5 × 10^-4 and is the same with the present upper Yangtze River at Pingshan-Yibin section.