What is Manning's equation used for in site engineering?

Manning's equation is a widely used empirical formula in the field of hydraulics and site engineering for calculating the flow velocity of water in an open channel. The equation takes into account the channel's cross-sectional shape, slope, and roughness, which are essential factors in determining how quickly water flows through it. By applying Manning's equation, engineers can effectively design drainage systems, assess the capacity of channels, and predict how changes in surface materials or channel geometry might affect flow rates.

The other options, while relevant to drainage and stormwater management, do not utilize Manning's equation. For instance, calculating rainfall runoff generally involves different hydrological methods, and determining pipe diameter focuses more on flow rate and pressure rather than velocity alone. Estimating water table depth involves different techniques related to groundwater hydrology rather than open channel flow characteristics. Thus, option C accurately represents the primary purpose of Manning's equation in site engineering.