When performing floodplain delineation with HEC-RAS, what specific input parameter directly controls the calculation of water surface elevation for a given flow rate?

The specific input parameter that directly controls the calculation of water surface elevation for a given flow rate in HEC-RAS is the Manning's roughness coefficient, commonly referred to as Manning's 'n' value. This parameter quantifies the resistance to flow experienced by water as it moves through a channel or over a floodplain. This resistance originates from the friction between the flowing water and the wetted perimeter of the channel, including its bed and banks, as well as from form losses caused by channel irregularities, vegetation, and other obstructions. A higher Manning's 'n' value signifies greater resistance to flow, while a lower value indicates less resistance. In HEC-RAS, the model computes water surface elevations by solving the one-dimensional energy equation (or momentum equation) between successive cross-sections. A crucial part of this calculation involves determining the energy losses due to friction. These friction losses are directly proportional to the Manning's 'n' value. For a constant flow rate and given channel geometry, an increase in the Manning's 'n' value will result in greater friction losses between cross-sections, which in turn necessitates a higher water surface elevation to convey the same volume of water. Conversely, a decrease in Manning's 'n' will lead to lower friction losses and, consequently, a lower calculated water surface elevation. Therefore, Manning's 'n' is the primary parameter that dictates the magnitude of friction losses, directly governing the calculated water surface elevation.