近40年来长江上游干支流悬移质泥沙粒度的变化及其与人类活动的关系

Variation in grain size of suspended load in upper Changjiang River and its tributaries by human activities

以实测资料为基础,研究了长江上游干支流5个代表站即金沙江屏山站、岷江高场站、乌江武隆站、嘉陵江武胜站和长江宜昌站的悬移质泥沙粒度随时间的变化。研究发现,近40年来长江上游不同的水沙来源区和干流控制站的悬沙粒径均呈现出明显的减小趋势。长江上游干支流悬沙粒径的细化趋势,与水库的修建有密切的关系。长江上游的水库均以蓄水发电和灌溉为目标,水库拦截了来自库上游流域的泥沙,其中粗颗粒部分淤积在库内,使得经水库下泄的泥沙显著变细。长江上游干支流绝大多数河流为沙卵石河床或基岩河床,已建水库下游河道均为沙卵石河床,通过水库下游的冲刷得到的细泥沙补给是很有限的。由于床沙中含细颗粒泥沙少,水库下泄的清水无法通过冲刷河床来得到大量泥沙的补给,故经过长距离的水流-河床相互作用后,因水库拦沙而显著变细的悬沙粒径不会因河床冲刷而明显增大。长江上游流域内大规模的水土流失治理收到了明显成效,使重点产沙区的产沙量减小。各种水土保持措施的实施,拦截了较粗的泥沙,也会使进入河道的泥沙粒度变细。运用多元回归分析方法,研究了不同水沙来源区悬沙粒度的变化和两个重点产沙区产沙量的变化,对宜昌站悬沙粒度变化的影响。运用所建立的多元回归方程式。

Based on data from 5 hydrometric stations, Pingshan station on the Jinshajiang River, Gaochang station on the Minjiang River, Wulong station on the Wujiang River, Wusheng station on the Jialingjiang River and Yichang station on the Changjiang River, a study on the temporal variation in grain size of suspended sediment load in the upper Changjiang River has been made. The results show that in the past 40 years, grain size of suspended sediment load of main stem and major tributaries of the upper Changjiang River has a decreasing trend. This decreasing trend can be explained by the effect of reservoir construction and implementation of soil_water conservation measures. The reservoirs in the upper Changjiang River basin, all used for water storage for hydro_electric generation and/or irrigation, trap coarse sediment from drainage area above the dam, and thus, the released sediment becomes much finer than before the construction of reservoirs. The downstream channels are all gravel_bedded or even in bedrock, consisting of little fine sediment, and thus, the released flow can hardly get supply of fine sediment through eroding the bed. Then, after the downstream adjustment, the grain size of suspended sediment is still fine. Large_scale soil_water conservation measures have significantly reduced sediment yield in some major sediment source areas. The relative coarse sediment is trapped, and thus, the sediment delivered to the river becomes finer. A multiple regression analysis has been conducted to determine the influence of sediment fining and sediment yield variation from different water_sediment source areas. Using the two multiple regression equations, the variation in suspended sediment median size entering the middle Yangtze River can be roughly estimated, based on the variations in suspended sediment grain size and sediment yield from different source areas.