Like other hydrologic routing methods, the variable storage coefficient (VSC) is based on the continuity relation. It does not apply empirical or calibrated parameters. It calculates channel storage based on average channel characteristics, and travel time based on Manning’s relation. It can be used for artificial and natural channels with three roughness coefficients in the same cross-section.

In Visual OTTHYMO, the three routing commands of the original HYMO model are lumped in a single command ‘Route Channel’. The VSC routing cannot be used when backwater effects are significant. In such cases, a fully dynamic model (e.g. EXTRAN should be used).

The updated RouteChannel command in VO 5.1 is used to route hydrographs through channel cross-sections with a defined channel and floodplain using the variable storage coefficient (VSC) method. The low flow channel section and a separate floodplain section are defined with X and Y co-ordinates.

The RouteChannel command in VO5.1 has been split to calculate channel routing in both a “low flow channel” and “floodplain” portions of the cross section with the routing effects then being combined for the final output. The user can define channel and floodplain lengths and slopes.

For circular or rectangular pipes, ROUTE PIPE command should be used. The command sizes the pipe to the minimum diameter necessary to avoid surcharging. For design, the user should increase the size to the next standard diameter.