Water Circulation System of a Sunken Plaza Based on the Concept of a Sponge City Utilizing Groundwater

The use of a natural and low-impact way of managing surface water and groundwater in a sunken plaza is meaningful for urban ecological development. To achieve water storage, purification, and recycling, this study investigated a water circulation system in a sunken plaza based on the groundwater and surface water transformation and combination. A joint water-stop curtain system was constructed with underground continuous walls, RC wall, building main structure, adjacent support structure, and waterproof stratum at the bottom of the main structure. On the basis of the stratum data and parameters of the study area, a three-dimensional (3D) groundwater model was built using FEFLOW 7.5 comparing the actual groundwater level monitoring data from the field pumping test with the simulation data, the accuracy of the model was verified, and the water volume change of the sunken plaza was calculated. The low permeability of the curtain system calculated by a field test and the numerical model proved that the pumping activities in the sunken plaza had extremely low impact on the groundwater level outside the plaza. On the basis of this result, the pumping and seepage wells and water quality and quantity observation wells were designed scientifically and rationally. The purification and circulation control of water were realized in combination with rainwater, drainage, landscape, paving, and other facilities. According to the circulation system designed in this study, the annual water recycling capacity of the sunken plaza can reach more than 60,000 m3, and the total suspended solids (TSS), chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) indicators in the rainwater can be reduced by more than 15% on average. The sunken plaza area forms a small spongy body that realizes water circulation, economic benefits, and ecological functions for the first time using groundwater storage. The developed method can be used for the water circulation and ecological construction of sunken plazas with similar geological environments.