| Yilong Lake is a typical shallow plateau lake,which is in a semi-enclosed state all year round.The water flow in the lake is basically in a static state,with weak water exchange capacity and long-term accumulation of pollutants,leading to the deterioration of water quality.At the same time,the lake is located in a region facing problems such as lack of water resources and insufficient supply of external water sources,which exacerbates the deterioration of water quality in the lake.In response to the problems of the water environment in the lake,the government has invested in the construction of the water diversion project of the lake,and after the implementation of the measures planned in the 13th Five-Year Plan,the water environment capacity of the lake has increased and the water quality has continued to improve,with the water quality now transitioning from poor V to V.However,the water quality of the lake is greatly influenced by the climate and the spatial and temporal distribution is extremely unstable,so there is a long way to go in the management of the lake.At present,the river channel into Isolong Lake is mainly concentrated in the west of Isolong Lake,which causes the lag of water replacement time in the middle and east areas of the lake,and cannot better achieve the overall water quality to reach the IV water quality standard stably in 2025.In this paper,after combining the measured data and collected information,the MIKE21 software was used to construct a two-dimensional hydrodynamic-water quality model of Isolong Lake,and the model was used to optimize the location of water replenishment points and the distribution of replenishment flow to achieve the optimal improvement effect.The following conclusions were drawn:(1)The flow field of Yilong Lake is influenced by the wind field and affected by the boundary blockage and water balance,forming a circulation of different sizes,the existence of which makes the water in the lake stagnate and pollutants accumulate;secondly,the concentration of pollutants in the water of Yilong Lake gradually increases with the advent of the rainy season,and the concentration changes in the west and middle of the lake are especially significant,showing a spatial distribution pattern of west>middle>east of the lake.(2)The change of the flow rate,the location and the number of water supply points have little effect on the change of the flow field in the lake area.Except for the increase of the flow rate at the entrance and exit,there is no change in other areas.From northwest to southeast,six small and medium-sized circulations are formed in turn.(3)Under the current and planned replenishment,the increase of replenishment increased the exchange capacity of water bodies and the improvement rate of water quality compared with the case of no replenishment,especially in the west of the lake.However,with the increase of the flow into the lake,only the concentration distribution pattern of pollutants in time was changed,and the spatial distribution pattern of concentrations all showed that the west of the lake<the middle of the lake<the east of the lake.(4)In optimizing the location of water recharge points and the number of water recharge points,it is concluded that:introducing water from the north shore,the water quality of Yilong Lake improves better and the proportion of water quality standards is higher;while introducing water from the south shore,compared with the north shore,the water quality improves poorly and the proportion of water quality standards is low;secondly,in the north shore area,the more the water recharge points are set to the east of the lake,the poorer the water quality improvement;at the same time,taking three points of water recharge speeds up the rate of water quality improvement,but The proportion of water quality standards is reduced.Finally,option 3,i.e.,the city river and recharge point 1(see Figure 5.4),was selected as the optimal recharge point.(5)On the basis of Option 3,we further optimize the allocation of water recharge flow rate and conclude that the water quality improvement effect is not good if the flow rate into the lake at recharge point 1 is too large or too small,and the proportion of water quality standard is reduced.After the final comparison and analysis,Option 13(the flow rate of the city river into the lake is 4.5 m~3/s,and the flow rate of recharge point 1 into the lake is 1.5 m~3/s)is the optimal option;and the improvement effect of the optimal option 13 is analyzed,and under its recharge conditions,the replenishment of Yilong Lake basically completes a replacement cycle in 7 weeks,and the COD and TN concentrations in the water body of the lake reach the water quality standard of Class IV in 4 months.For 7 months,except for the high concentration of various pollutants in the wetland area,the COD concentration in the rest of the waters reached theⅢwater quality standard,and the TN concentration has been stable at theⅣwater quality standard. |
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