Hydraulic properties of channels and cross sections are computed differently in HEC-HMS and HEC-1.

General Channel Properties

In HEC-HMS, properties are computed from the physical properties of the channel, whereas in HEC-1 the properties are computed with formulas based on the kinematic wave assumption. In HEC-HMS the cross-sectional flow area is computed as:

and the ratio of wave speed, c, to flow velocity, v, is:

where w is the bottom width, z is the side slope, and y is the flow depth. Velocity is computed using Manning's formula and the properties of the cross section.
In HEC-1 the cross-sectional flow area is computed as:

and the wave speed, c, is computed as:

where  and m are kinematic parameters based on the channel shape.

Eight Point Cross Sections

In HEC-HMS the flow depth for a given discharge is determined from the cross section properties and then area, top width, and wave speed are computed for that depth using the cross section properties. In HEC-1 the discharge, area, top width, and wave speed are computed for 20 depths and stored in a table. During the routing process, area, top width, and wave speed were interpolated from the table for each discharge value. HEC-HMS guarantees that the area, top width, and wave speed are computed exactly for each discharge value.