To use HEC-RAS software to figure out how high floodwaters will go, what key information do you need about the river's shape?

To use HEC-RAS software to calculate floodwater heights, you need comprehensive geometric information about the river's shape, primarily derived from cross-sectional data, longitudinal connectivity, and any hydraulic structures present. The most critical data comes from cross-sections, which are detailed profiles of the river channel and its adjacent floodplains, taken perpendicular to the direction of flow at various points along the river. Each cross-section is defined by a series of station-elevation points. A 'station' represents the horizontal distance from a reference point, often the far left bank looking downstream, while 'elevation' is the vertical height above a common datum, such as sea level. These points precisely map the bed of the river, the banks, and the surrounding terrain that would be inundated during a flood, providing the area available for water flow at different depths. For each cross-section, you also need to specify Manning's Roughness Coefficient, often called the 'n-value.' This coefficient quantifies the resistance to water flow caused by the surface material of the channel and floodplains. A higher n-value indicates greater resistance, such as from dense vegetation, boulders, or rough ground, which slows the water and can increase water surface elevations. Different n-values are typically assigned to the main channel, the left overbank, and the right overbank within a single cross-section, reflecting their distinct surface characteristics. Additionally, bank stations must be identified within each cross-section; these are the points that mark the top edges of the main channel, separating it from the floodplains on either side, which helps HEC-RAS distinguish between main channel flow and overbank flow. The longitudinal information connects these cross-sections. This includes defining the river stationing, which is a system of numerical identifiers typically decreasing from upstream to downstream, assigning a unique location to each cross-section along the river. You also need the reach lengths, which are the average distances between consecutive cross-sections, separately measured for the main channel, the left overbank, and the right overbank, as these paths might not be the same. Finally, any hydraulic structures such as bridges, culverts, or weirs are crucial geometric elements. For these, HEC-RAS requires detailed dimensions, including their width, height, opening sizes, the elevation of their lowest point (invert elevation), and the geometry of any piers or abutments, as these structures significantly constrict flow and affect water surface elevations.