To calculate the water flow from a 10,000L tank using Torricelli's formula, the velocity can be determined with v=√(2gh), where h is the height of water. For flow rate, the equation Q=Av is used, with A representing the cross-sectional area of the pipe. When multiple tanks are involved, such as three 10,000L tanks draining into a 6-inch main pipe, the individual flow rates from the 2-inch pipes can be summed to find the total flow into the main drain. The flow speed will remain consistent, but the total volume will depend on the combined flow rates from all tanks. Accurate calculations require considering the capacity and configuration of the entire system.