Cell Types

Vflo™ contains six different cell types: overland, channel, rated channel, cross-section, reservoir, and base cells. Each cell type is differentiated by specific sets of hydraulic properties, which represent the drainage network components controlling runoff for that specific cell type. For example, overland cells have only the properties of roughness and slope, as these are the primary parameters affecting runoff for land areas. Channel cells, however, have roughness and slope properties as well as channel hydraulics and baseflow.

Runoff generation is determined based on cell type and cell properties. Cell type establishes which set of kinematic equations will be used to calculate runoff, and hydraulic properties values are used to solve those kinematic equations.

Overland cells

Overland cells are used to model land, or areas not usually covered by water. Overland cells are the most common cell types used in Vflo™. Their properties include roughness, slope, and the full set of infiltration parameters. For overland cells, normal depth is governed by Manning’s equation, assuming uniform flow depth over the grid cell.

Channel cells

Channel cells represent channels, streams, and other waterways. Like overland cells, channel cells have the properties of roughness and slope. In addition, channel cells feature channel hydraulic properties. There are three methods for modeling channel hydraulics in Vflo™, represented by three different types of channel cells:

  1. Trapezoidal channel cells, called simply Channel Cells in Vflo™, are useful for creating a channel network where no rating curves or measured channel cross-sections are known. As described below, channel width and channel side slope properties are used to define trapezoidal cross sections for the cell.
  2. Rated channel cells require rating curves developed from field measurements or other hydraulic modeling to model channel hydraulics.
  3. Cross section cells require ordered pairs of distance and elevation to represent measured channel geometry.

Trapezoidal channel cells (called Channel Cells) use channel width and channel side slope to internally define trapezoidal cross sections and generate stage discharge relationships for the cell. Conveyance is computed as the sum of the trapezoidal cross-section and overland flow depth within the cell. An additional property, baseflow, represents the lateral inflow from groundwater. Overall, the properties of basic channel cells include: roughness, slope, channel width, channel side slope, and baseflow.

Either through DEM processing using GIS software, or through AutoBOP, a digital elevation model (DEM) is used to determine the placement of channel cells. It’s important to note, however, that DEM resolution is often coarser than stream channel data, requiring a separate channel map to be used in conjunction with a DEM in order to appropriately assign channel cells. When DEM resolution yields unsatisfactory channel placement results, one of the following three methods may be used:

  1. Using GIS software, a polyline stream shapefile can be converted to a grid and merged with the primary DEM, as described here.
  2. By importing a channel base width map to Vflo™, any cell with a value other than “no data” will be described as a channel cell.
  3. By importing a channel side slope map to Vflo™, any cell with a value other than “no data” will be described as a channel cell.

All channel cells will be assigned the trapezoidal channel cell type (Channel Cells). For areas for which rating curves or cross sections are available, it is necessary to manually select Rated Channel Cell or Cross Section for Cell Type in the Hydraulics panel.

Rated channel cells

Rated channel cells are special channel cells. Instead of using channel width and channel side slope, rating curves are used to simulate the channel geometry. Rated channel cells may be used where complex hydraulics occur due to compound cross sections, roughness elements, or hydraulic structures.

Rated channel cells are assigned using the Hydraulics panel. To enter rating curves, discrete pairs of stage-area and stage-discharge are typed into Vflo™ manually or loaded from a text file. If values of area-stage and stage-discharge are not known, they may be generated from measured cross-section information. Other properties of rated channel cells are: slope, baseflow, and infiltration parameters.

For quick reference, a list of rated channel cells can be generated by selecting View |Tabs | Rated Channel Cell List.

For further information on rating curves, see Loading rating curves and cross sections as well as Tutorial: Establishing rated channel cells and loading rating curves.

Cross section cells

Cross section cells are special channel cells that represent measured channel geometry by ordered pairs of distance and elevation. A rating curve is synthesized internally to represent the stage-discharge relationship. As for rated channel cells, cross section cells may represent areas with complex hydraulics. A base elevation may be entered if flow is assumed to begin at some stage above the lowest cross section elevation value.

Cross section cells are assigned using the Hydraulics panel. Distance-elevation pairs may be keyed in manually or loaded as a text file, and should be entered in units of meters (metric) or feet (US customary). In total, the properties of cross section cells include: roughness, slope, baseflow, base elevation, and cross section pairs of distance and elevation.

For further information, see Establishing cross section cells and loading cross section data.

Reservoir cells

A reservoir cell is used to model a detention basin or an uncontrolled reservoir using storage-stage and stage-discharge curves. The effects of storage are modeled by level-pool routing defined by the storage characteristics entered for the reservoir cell. An additional property, initial storage, indicates the amount of storage available before discharge occurs.

Typically, a reservoir is modeled by a single reservoir cell. If a reservoir is larger than the Vflo™ model cell size, then base cells are used to model the shallow upper reaches of the reservoir. Both controlled and uncontrolled reservoirs may be modeled in Vflo™.

Reservoir cells are assigned using the Hydraulics panel. The initial storage property and storage values entered for storage-stage and stage-discharge curves should be in units of m3(metric) or ft3 (US customary). Stage values entered for storage-stage and stage-discharge curves should be in units of meters (metric) or feet (US customary). Discharge values entered for the rating curves should be in units of cms (metric) or cfs (US customary).

For further information, see Establishing reservoir cells and loading rating curves.

Base cells

The default cell type in Vflo™ is called a base cell. Base cells do not have any parameters other than flow direction. Base cells can be used to model shallow water in wetlands, marshes, and lakes. Base cells are also used to represent reservoirs with shallow upper reaches leading to the main reservoir.

Any flow that passes through a base cell will move through the cell based on the kinematic wave celerity. Specifically, water is propagated according to the kinematic wave speed c2 = gh, where g is the acceleration due to gravity, and h is the water depth. Water depth is time variable depending on discharge rate in the cell, and has no parameters.

Base cells are assigned using the Hydraulics panel.