UNET simulates one-dimensional unsteady flow through a full network of open channels. Figure 1 illustrates one basic element of a full network problem which is the split of flow into two or more channels. For subcritical flow, the division of flow depends on the stages in each of the receiving channels. These stages are a function of channel geometry and downstream backwater effects.

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Figure 1: The split flow at a channel junction which typifies the full network problem. Here the flow from reach 1 divides into reaches 2 and 3. The volume of flow along each reach depends on the geometry of the reaches and backwater effects.

A second basic element of a full network problem is the combination of flow, termed the dendritic problem. This is considered to be a simpler problem than the flow split, because flow in each tributary is dependent only on the stage in the receiving stream. The full network is the most general problem. It includes single channels, dendritic systems, and fully looped systems (another commonly used term for the full network).

Another facet of the full network model is storage areas; lake-like regions that can either provide water to, or divert water from, a channel. This is a split flow problem, although in this case, the storage area water surface elevation will control the volume of water diverted. Storage areas can be the upstream or downstream boundaries for a river reach. In addition, the river can overflow laterally into the storage areas over a gated spillway, weir, levee, through a culvert, or a pumped diversion.

In addition to solving the one-dimensional unsteady flow equations in a network system, UNET provides the user with the ability to apply several external and internal boundary conditions, including; flow and stage hydrographs, gated and uncontrolled spillways, bridges, culverts, and levee systems. All input, output and calculations are performed in U.S. Foot- Pound Units.

To facilitate model application, cross sections are input in a modified HEC-2 forewater format (upstream to downstream). A large number of river systems have been modeled using HEC-2, and those data files can be readily adapted to UNET format. Boundary conditions for UNET can be input from any existing HEC-DSS data base. For most problems, particularly those with large numbers of hydrographs and hydrograph ordinates, HEC-DSS is advantageous because it eliminates the tabular input of hydrographs and creates an input file which can be easily adapted to a large number of scenarios. Hydrographs and profiles which are computed by UNET are output to HEC-DSS for graphical display and for comparison with observed data.