温度异重流作用下三峡香溪河库湾春季营养盐时空分布特征研究

Temporal and Spatial Distributions of Nutrients in Xiangxi Bay of the Three Gorges Reservoir under the Influence of Temperature Density Current

三峡蓄水后,香溪河库湾春季出现多次水华现象,其与库湾水动力场及营养盐的时空分布密切相关.本文采用数值方法模拟初期蓄水后温度异重流作用下香溪河的水动力场分布,以守恒物质为示踪剂得到香溪河库湾春季营养盐的时空分布特征.结果表明,初期蓄水后,香溪河库湾流速大幅降低,水体滞留时间增加,上游来流对营养盐的输移作用明显减弱,营养盐易在库湾长时间聚集;同时库湾与长江干流水体温差增大,在异重流作用下,长江干流水体在春季沿表层流入库湾,相应营养盐也随水流带入,干流水体对香溪河库湾营养盐的补充作用明显.异重流的存在对营养盐空间分布影响较大,其影响范围随三峡坝前水位及香溪上游来流不同有小幅变动,通过示踪剂的浓度沿程分布可直观给出香溪河上游来流及长江干流入流在库湾的主要影响区域.研究可为三峡水利枢纽正常运行后库湾支流营养盐的控制和治理提供方法和依据,也可为相似库湾营养盐的分布特征研究提供参考.

Harmful Algal Bloom events have appeared several times in spring in Xiangxi Bay after the impounding of Three Gorges Reservoir （TGR）, this phenomenon is closely related to both the hydrodynamic field and the temporal/spatial distribution of the nutrients in the bay. Numerical simulations were employed to simulate the hydrodynamic field of the Xiangxi Bay under the effect of temperature density current after the initial impoundment of TGR. The characteristics of spatial and temporal distribution of the nutrients （nitrogen and phosphorus ）in spring were obtained by tracer method with conservative tracer. It was found that the changed hydrodynamic field, accumulated nutrients and appropriate temperature generally favor the development of Algal Bloom in Xiangxi Bay. With the substantially decreasing of the flow velocity, the residence time of water in the bay was prolongated, and less nutrient was transported out of the bay by upstream inflow, all these lead to the aggregation of the nutrients in the bay after initial impoundment. With the increasing of the water temperature difference between the bay and the Yangtze River mainstream, the temperature difference between surface and bottom water in the bay was increasing simultaneously, and a temperature density current came in to being. Due to this density current the nutrient was transported from the Yangtze River into the bay along the surface with the mainstream flux. This supplement effect is obvious in Xiangxi Bay. The density current has a great effect on the spatial distribution of the nutrient,the area of influence changes in a narrow range with the variation of both the water level in front of the dam and the upstream inflow. With the tracer concentration distribution along the river/bay, the main areas of influence were identified directly for both the upstream inflow and the mainstream of Yangtze River. This research could provide a basis for controlling and managing the nutrients of tributaries in the bay, and could provide references for the researches on the characteristics of nutrient distribution of the similar bay in the region.