Numerical Simulation Of Water Refill Scheme In Lihu Lake Based On EFDC Model

Lihu Lake is a typical shallow water urban lake formed by Taihu Lake stretching inland.As an important landscape lake in Wuxi city,due to the impact of human activities,Lihu Lake once became one of the most serious eutrophication lakes in Taihu Basin.Through a series of ecological comprehensive regulation measures adopted by Wuxi municipal government in recent years,the eutrophication degree of Li Lake has been effectively controlled and the water environment has been improved obviously,but the sensory indexes such as water transparency and suspended matter concentration have not changed significantly.In order to improve the water transparency of Lihu Lake,Wuxi city plans to build water purification project to treat the water from Taihu Lake for ecological water replenishment.In this paper,the convection and diffusion module of EFDC Model was used to establish the relationship between suspended matter concentration and transparency,and to simulate the change rule of suspended matter concentration with time under different water refill conditions,so as to predict the improvement effect of ecological water refill on transparency of Lihu Lake.The conclusions are as follow:（1）The Changguang River takes in water,and the Meiliang Lake and Caowangjing Pump stations carry out ecological water refill for Lihu Lake.Under three flow conditions（7.5,15 and 22.5 m3/s）,there are dead water areas in Lihu Lake,and the local water exchange effect is bad.And the effect of zone A is the worst,mainly because the inlet of Meiliang Lake pumping station is close to the west side of bank of the Lihu Lake,and the water replenishment source directly enters the pumping station diversion channel after passing through bank of the Lihu Lake,so there is a large dead water area in zone A.To ensure the effect of water exchange in zone A,diversion walls can be set to guide clean water around zone A.（2）In the case that no diversion wall is set in zone A,three inlet flow conditions（7.5,15,22.5m3/s）are simulated,and the water refill effect is about three outlet flow distribution ratios（1:1,1.5:1,2:1）.The results show that under the same inflow flow,the water refill effect is better when the pumping ratio of Meiliang Lake and Caowangjing pumping stations is 1.5:1.Under this ratio,the average pollutant concentration in the whole lake decreases rapidly,and the time for clean water to reach the two outlet points is balanced.From the point of view of zoning effect,the main performance is B>C>D>A,zone A and zone D two areas of water quality improvement effect is poor,mainly there is a large range of dead water area in zone A and zone D.（3）In the case of diversion wall in zone A,the ratio of left and right water outlet is 1.5:1,and the water refill effect of the two diversion wall schemes is simulated under three flow conditions（7.5,15 and 22.5m3/s）.The results show that after setting water retaining wall in zone A,both schemes can greatly improve the effect of water exchange in zone A and shorten the time of water replenishment.However,the improvement effect of zone D is still poor,and after setting water retaining wall in zone A,and a new small area of dead water will be formed in zone B near the west side of bank of the Lihu Lake,which has A certain impact on the effect of water exchange in zone B.（4）The effect of two flow conditions（15 and 22.5m3/s）and two diversion wall schemes was simulated for the operation mode of water refill after the predicted precipitation level of 0.5m.The results showed that the running mode of the pre-precipitation level could shorten the water replenishment time by 3 or 5 days,but the running mode of the pre-precipitation level could affect the lake ornamental performance.（5）After the diversion wall is constructed in zone A of Lihu Ecological water supply project,the water quality can reach the class Ⅲ standard on the whole,and the transparency will be raised to about lmeter.But the effect of water exchange in local dead water area is not ideal,so other engineering measures must be taken to strengthen it.In terms of the design flow,two sizes of 15 and 22.5m3/s are recommended,which need to be demonstrated economically.The outlet water ratio of 1.5:1 is better.