Study On The Influence Of The YuQiao Reservoir ’s Pre-reservoir On The Groundwater Level In The Surrounding Area Of Lihe River

In order to solve the problem of eutrophication in Yuqiao reservoir,a pre-reservoir project has been constructed to purify the water quality.Cutting off the corner and bending straightening,a check gate has been built.Water level raises up 1m or so,serpentine backwater area along the river back is about 15 km.The groundwater level on both sides of the river in the lower reaches of the Lihe River has been raised,which has broken the original water balance and changed the local soil conditions,resulting in a series of ecological impacts.Using numerical simulation method,a study on river-groundwater system is carried on in this paper.Based on hydrodynamic river model HEC-RAS and 3D groundwater models MODFLOW,a two-way coupling model is established.Flow residual is reduced by adjusting the water balance with application of the Picard cycle.Through field surveys and measurements from May 2017 to October 2017,5groups of measured data were obtained,which were used in the parameter calibration and model verification.The validation results manifeste that correlation coefficient of the simulated and measured value is over 0.97,Nash–Sutcliffe efficiency is over 0.95,the system flow residuals decreases by 0.6m~3/s.The coupling model was then applied into the research of river-groundwater system and then eight sets of working conditions are proposed in combination with the water supply and hydrological conditions of the Lihe River basin.The conclusions are as follows.(1)For the Lihe River,the elevation of river water level is about 1.0~0.0 m,and the backwater area is about 13~16km along the channel.(2)For the groundwater,the water level from the entrance of reservoir to the Guohe Bridge has been raised obviously,where the maximum lifting range is1.0~0.9m and the influence area of both sides of the channel is about 1.0~1.7km.The water level from the Guohe Bridge to the Wanggezhuang Bridge is gradually decreasing from west to east,where the maximum lifting range is 0.9~0.0m and the influence area is about 0.1~1.5km.There is no change in the water level from the Wanggezhuang Bridge to the Dongliangzihe Bridge.At the same time,the groundwater depth in the study area is generally greater than 2.00 m.When the water is diverted from Luanhe River to Yuqiao Reservoir,it is close to 1.80m.Therefore,soil(secondary)salinization is very unlikely.However,to take long-term flow diversion scheme is not recommended.(3)For the water balance,seepage recharge from river to groundwater has increased and discharge from groundwater to river has decreased.Besides,the diving evaporation has increased.(4)For the relationship between river and groundwater,the groundwater level is higher than the river water level in the upper reaches.And in the lower ones,the groundwater level is closer to the river water level.The river and groundwater level are both lifted and the difference of them is reduced.But overall,the groundwater level is still slightly higher than the river’s,indicating that it is still dominated by the discharge from groundwater to surface water.