北门江河口径流变化对河口段水环境的影响

Influence of runoff variation on water environment in Beimen River estuary

北门江河口上游水库的修建将导致丰水期水库下泄流量有所减少,枯水期水库下泄流量有所增加,这些变化将会引起北门江河口处的咸潮上溯距离和冲淡水区域变化等生态环境问题。以北门江河口为研究区段,构建基于MIKE21的平面二维水动力模型和二维盐度扩散模型,以咸潮上溯和冲淡区为研究对象,分析河口上游修建水库后,在丰水期、平水期、枯水期径流改变的情况下,北门江河口地区咸潮上溯和冲淡区变化规律。结果表明:(1)水库建设后,枯期下泄流量与天然河道中的流量相差不大,对枯期的咸潮上溯距离无显著影响;丰水年6月(汛期)建库后因为下泄流量减少幅度较大,咸潮上溯距离最大增加约1 790 m;平水年6月建库前后流量减少量少于丰水年,建库后咸潮上溯最大增加距离小于丰水年,为1 270 m。(2)水库建设后,由于汛期下泄淡水流量降低,与建库前相比,丰水年6月新英湾海域的冲淡区面积变化最大,在最低潮位时,新英湾海域中冲淡区面积占比减少17.3%。研究成果可为水利枢纽工程对河口地区咸潮上溯和冲淡区的影响评价提供理论依据。

After the construction of reservoir in the upstream of Beimen River estuary,the discharge of reservoirs will reduce in high flow period and increase in low flow period. These changes may cause ecological and environmental problems such as the variations of salty tide up-tracing distance and diluted zone at the Beimen River estuary. In this paper,a two-dimensional hydrodynamic model and a two-dimensional salinity-diffusion model based on MIKE21 are constructed for the Beimen River estuary. The variation of the salty tide distance and the diluted zone are simulated and analyzed in the high flow period,normal flow period and low flow period after the construction of the upstream reservoir. The results show that： After the construction of reservoirs,the discharge volume in dry season is not significantly different from that of natural river,so the reservoir has no significant effect on the up-tracing distance of saltwater; while the discharge decreases greatly in June（ flood season） in high flow year,and the maximum up-tracing distance of salty tide increases about 1790 m. In normal flow year,the discharge reduction in June is less than that in high flow year,and the maximum increased distance of saltwater tide up-tracing is less than that in high flow year by 1270 m. After reservoir construction,due to discharge decrease from the reservoir,the variation of the diluted zone in Xinyingwan Bay is the largest in June in high flow year compared with that before the construction. In the lowest tidal level,the diluted area of the bay is reduced by 17. 3%. The above research results can provide a theoretical basis for the evaluation of the impact of water conservancy project on the up-tracing of salty tide and variation of dilute areas in estuarine areas.