Final answer:
The sizing data for horizontal storm drain piping depends on the flow rate of the water, which is determined by the average velocity and the cross-sectional area of the pipe.
Step-by-step explanation:
The sizing data for horizontal storm drain piping is based on the flow rate of the water. The flow rate is determined by the average velocity of the water and the cross-sectional area of the pipe. According to the equation Q = Av, where Q is the flow rate, A is the cross-sectional area, and v is the average velocity, the larger the cross-sectional area or the greater the velocity, the higher the flow rate will be. Therefore, the sizing of the storm drain piping will depend on the desired flow rate of the water.