|近十余年长江上游大量兴建大型水库，蓄水、调节和拦沙对中下游河川径流和泥沙产生了深刻影响.河川径流减少，径流季节提前，伏秋（特别是10月）流量显著降低、变差系数增大，97%严重干旱频率情景变成80%~85%.同时，宜昌和出海输沙量分别减少93%和70%，中下游河槽冲刷下降1~3 m，三峡蓄水后仅13年清水冲刷幅度和范围已超过三峡预期30年的冲刷上限，目前仍呈加速趋势.干流各站平均水位下降2~4 m，与此同时河道同流量洪水位反而升高.水库调节是水位降低的主要原因，河道冲刷更加剧水位降低，当前水位变化对防洪和生态都不利.汛后流量和干流水位提前降低使洞庭湖和鄱阳湖（两湖）提前干枯、松滋等"三口"入湖水量减少，伏秋高热季节两湖生态环境面貌发生了根本变化.汛后流量减少甚至显著增加长江大通十月流量小于15000 m3/s几率和上海长江水源受咸潮影响风险，10月咸潮入侵变成最严重时段必须引起高度重视.我们认为，径流和径流节律变化是当前长江生态环境最主要问题之一.建议以"水资源工程"重新定位上游大型工程、以"水资源优先"优化流域管理和切实回归既定三峡工程运行原则等统一调度和改善中下游水情；通过水库挖泥等措施修复长江物质通量，抑制中下游剧烈冲刷和稳定河流格局；加强中下游蓄滞洪区等防洪能力建设，为最大限度降低上游水库防洪和蓄水压力创造条件；主要通过改善上游水库调度维护两湖环境条件，"引清水入洞庭"和"增加供水设施建设"加强两湖适应能力.这是长江修复和保护重点.
|关键词: 径流节律 水库调节 河道冲刷 生态效应 河流保护
|Effect of dams on the regime of the mid-lower Yangtze River runoff and countermeasures
ZHOU Jianjun, ZHANG Man
Department of Hydraulic Engineering, Tsinghua University, Beijing 100084, P. R. China
|In the past about two decades, mega-reservoirs impounded in the upper basins have profoundly altered the runoff and its regimes in the lowland reaches of the Yangtze River. Annual, autumn (Sept.-Nov.) and October flow discharge via the Yichang gauge station downstream of the Three Gorges project (TGP) have decreased by 11%, 29.6% and 40%, respectively, with remarkable higher coefficient of variation. The rare scenery of dry-year in frequency 97% has become 80%-85% comparing with their previous counterparts before 1990. On the other hand, sediment load downstream Yichang reduced by 93%, while by 70% across the Datong gauging station to the estuary. As a result of clear-water flow, scouring and incision in the mid-and lower reaches exceeded 2.1 km3 in total and 1-3 m in main channel elevations, surpassed the maxim limit previously predicted to take place after 30 years running of the TGP, and still the incision is accelerating ahead according to investigation. Mainly, because of regulations and also riverbed incision, water stages all way along the lowland reaches are lowered ahead of time by 2-4 m, that is the criminal leading the riparian shoals and major lakes, the Dongting and Poyang, dried up in advance, correspondingly. Also, because of earlier drying up and stage lowering, diversion of water from the river to the Dongting diminishes, the flow across Datong in Oct. is greatly reduced with the frequency of less than 15000 m3/s discharge greatly increased that is of key for the resisting of saltwater and the safety of drinking water for Shanghai. Moreover, at the same time of stage lowering, main channel flood level of specific discharge is elevated that means flood safety reduction. The present variation in runoff regime has changed the natural fluvial features and the environmental conditions in the lowland reaches and to low the impact of hydro-project should be listed as key restoration for the Yangtze River protection. We suggest to reposition the role of upstream hydro-projects to run the key reservoir firstly using to water as objecting goals, not electricity; to respect law-designated TGP principles of operation in its planning stage; to restore the sediment and other fluvial fluxes functionally via reservoir dredging; to strengthen the flood-coping capacity, in particular the flood detention requirement by the planning of the TGP to easy the tensions of both flood and refilling of water in autumn; and to improve the environmental conditions of the Yangtze connecting lakes, mainly through upstream reservoirs operation optimizations and it should also be supported through diverting more clear water from the river to Dongting Lake and improving the water supply facilities for Poyang Lake as auxiliary measures.
|Key words: Runoff regime regulation downstream scouring eco-consequence river protection