Numerical Simulation of Longdao River Water Quality Improvement Project

According to the construction design, Longdao River water quality improvement demonstration project, a tributary of Wenyu River and located in the suburban area of Beijing where an international exposition center is under construction, mainly includes construction of secondary river channel, wetland river channel in the upper part of the River and in the lower part construction of meandering secondary river channel with deep pools and shoals and ladder pools, vegetation buffer zone etc. In the middle of the River, there connects Luomadong and Luomaxi lakes, where dredging of riverbed is considered.

After collecting its riverbed topography, information of operation modes, hydrology, water qualities and decontamination capacity data of Longdaohe demonstrative project of water quality improvement, a two-dimensional numerical model was set up on the basis of generating fine grids to simulate spacial-temporal variations of water qualities with different disposal levels (different degradation coefficients) and different diversion flow rates in 330 days. The water quality indexes are represented by chemical oxygen demand (COD), ammonia nitrogen (NH3-N) and total phosphorus (TP).

Taking TP as an example, the main conclusions drawn from the simulation are as follow:

When the average TP from Wenyu River was 2.06 mg/l and the comprehensive degradation coefficient was 5*10^-6 1/s, the calculated average outlet concentration was reduced to 0.30 mg/l and the average reduction rate was up to 85.3%. However, when the comprehensive degradation coefficient is 5*10^-7 1/s and 5*10^-8 1/s, the exit TP concentration is reduced much less, with an average reduction rate of 12.1% and 2.2%, respectively. The corresponding exit average concentration is 1.81 mg/l and 1.99 mg/l, respectively.

The simulation also shows that most of the water surface of Longdao River demonstration project, especially most water is located in the lower part of Longdao River, and degradation of COD, NH₃-N and TP in the river is mainly completed in this part.

When the average concentration of TP is 2.06 mg/l and the water diversion flow rate is 0.25 m³/s, TP concentration at outlet is 0.14 mg/l and the removal rate is 93.2%. While the flow rate is increased to 0.5 m³/s, the outlet concentration is 0.37 mg/l, and removal rate is 82.2%.