To measure flow in an 8-inch diameter city sewer line using the Manning formula, which additional data is necessary?

The correct choice emphasizes the importance of pipe length and invert elevation when using the Manning formula for measuring flow in a sewer line. The Manning formula is used to estimate the flow rate in open channels or partially filled conduits based on the cross-sectional shape and size of the flow area, the slope of the channel, and the roughness coefficients of the materials involved.

Knowing the pipe length is significant because flow characteristics can change along the length of the pipe due to various factors, such as changes in slope or roughness. The invert elevation is equally critical as it helps to determine the hydraulic gradient along the pipe. The gradient impacts both flow velocity and flow rates; therefore, knowing the elevation allows for accurate calculations using the formula.

Other parameters mentioned, like pipe roughness and water depth, play important roles in the Manning equation, yet they alone do not provide a complete picture without context about the length of the control section and elevation differences. In essence, both length and elevation are vital to accurately assess the hydraulic behavior of the flow in the sewer line.