Numerical Simulation Of Water Age And Residence Time In Dahuofang Reservoir Based On EFDC Model

Dahuofang Reservoir is located in the northeast of China and is one of the nine key water sources of Liaoning Province. It makes strategic sense for the economic development and social stability of Liaoning Province to protect this water source. As Dahuofang Reservoir features river-valley reservoir, it may take a long time for the nutrient injected at upstream to reach the dam region, thus the transport time of nutrients has significant impact on the water quality in the reservoir. Therefore, it is important to develop a hydrodynamic model of the reservoir and then to study the influencing factor of transport time in the reservoir, which is also the main objective of this paper.Firstly, we developed a three-dimensional hydrodynamic model of the Dahuofang Reservoir based on the structured Environmental Fluid Dynamics Code (EFDC) with the forcings of river discharge, surface wind stress and thermal flux. The model was calibrated through comparison of modeled and observed surface elevation and temperature. The results demonstrated that the model reproduced the fluctuation of surface elevation and the general trend of temperature generally well.The calibrated model was then used to investigate the impact of river discharge, wind, thermal flux and water level variation on water age (WA) with model experiments. Results demonstrated that river discharge plays a dominant role on water age. In the model runs with only the forcing of river discharge, the water age at spillway are53.93days,126.96days and458.78days under high, mean and low flows, respectively. The water age in the deep channel is smaller than that flanking the deep channel, which exhibits a tongue-shaped pattern. Wind induced current and wind induced mixing has important impact on the transport time of water parcel. The difference of water age along the transverse section is weakened and the water age is completely homogeneous in the vertical direction with the consideration of wind. Comparing the water age under the conditions with and without the consideration of wind, it can be seen that the water age get larger overall. The impact of wind is greater in the deep channel than that flanking the deep channel. Under high and mean flow condition, the impact of wind is greater at downstream than that at upstream. Under low flow condition, the situation is opposite. The thermal flux exerts important impact on transport time through density induced current and thermal stratification. In the model runs with the consideration of thermal flux, the locally horizontal difference of water age get weakened further. The water age at surface layer is smaller than that at bottom layer in the vertical direction and the vertical difference increases further downstream. The impact of thermal flux is greater at bottom layer than that at surface layer. Water level variation has impact on the thermal structure and thus influences the water age. With the decrease of water level, the water age get smaller overall and the vertical difference of water age get weakened. The impact of water level variation is stronger at bottom layer and downstream region than that at surface layer and upstream region.The impact of river discharge on residence time (RT) and local residence time (LRT) was also investigated with the hydrodynamic model. Calculation results demonstrated that the farther away from spillway, the larger the residence time and the smaller the local residence time, and the difference of residence time and local residence time also get larger. The residence time and local residence time showed exponential relationship with river discharge and increased significantly under the forcing of river discharge lower than mean flow.