Hydraulic Properties

Hydraulic properties represent the drainage network components controlling runoff for a specific cell. 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.

The complete set of hydraulic properties includes: flow direction, roughness, slope, channel width, channel side slope, baseflow, base elevation, initial storage, rating curves (area-stage, stage-discharge, or storage-stage curves), and channel cross sections (entered as ordered pairs of distance and elevation). These properties are controlled through the Hydraulics panel. In addition, infiltration properties controlled through the Infiltration panel include: hydraulic conductivity, wetting front, effective porosity, soil depth, initial saturation, abstraction, impervious, and evapotranspiration.

The set of properties required to represent the hydraulics for a particular cell depend on cell type. Overland cells, for example, require only information for roughness and slope, while channel cells require information for these properties as well as for channel width, channel side slope, and baseflow.

Generally, hydraulic properties values may be entered in Vflo™ two ways:

1. A parameter map may be developed using GIS software and imported as an ESRI ASCII grid (see Tutorial: Formatting parameter maps in GIS). In this way, values for the particular hydraulic property are populated for each cell in the Vflo™ model grid. For example, a roughness map can be developed using ArcGIS, output as an ASCII file, and imported into Vflo™. Roughness fields will be populated for each cell, and will be saved within the basin overland properties, or BOP (*.bopx, *.bop) file.
2. A property may be manually entered for a cell or cells through the Hydraulics and Infiltration panels (see Editing cell properties).

If gridded data is available for the modeled area, it is advisable to develop parameter maps using GIS software and to import the maps to Vflo™. If gridded data is not available, however, then some parameters, such as roughness and infiltration properties, may be assigned constant values that are representative of local watershed or sub-area conditions.

Cell properties may be edited at any time using the Hydraulics and Infiltration panels.

Flow direction

The flow direction property applies to all cell types in Vflo™ and establishes the routing of runoff generated for a cell. Flow direction options are: east, southeast, south, southwest, west, northwest, north, northeast, and none (generated runoff is not routed to another cell). Flow direction is derived from a high resolution digital elevation model (DEM) using GIS software, or established by manually drawing a flow direction map in Vflo™. Once flow directions are established, they may be edited by manually changing the flow direction selection in the Hydraulics panel (see Editing cell properties).

Roughness

The roughness property is a measurement of hydraulic roughness. It represents the degree to which land use (or, more specifically, the soil surface, vegetative material, rocks, and other factors associated with specific land use categories) creates frictional drag over the soil surface. Hydraulic roughness is important for parameterizing a Vflo™ model, as roughness controls the velocity of surface runoff. All cell types except for base cells and rated channel cells have a roughness parameter.

Typically, roughness values are ascribed to cells in a Vflo™ model in accordance with land use information for the area. If land use data is available for the modeled area, roughness values for a Vflo™ model may be developed from available land use datasets using GIS software. Land use descriptions are reclassified as roughness values, and the roughness map is then imported as an ESRI ASCII grid to Vflo™. If gridded data is not available for defining hydraulic roughness values, then constant values may be assigned for a watershed or sub-areas that are representative of the local conditions.

Roughness values are Manning’s roughness coefficients, or coefficients used in the Manning equation to determine hydraulic roughness. As such, they have no units. The minimum possible value is 0.01.

Slope

Slope represents the elevation gradient from one cell to another. Slope affects runoff flow rate. Slope is derived from a digital elevation model (DEM), which represents the spatial distribution of elevations. Using a high resolution DEM is important for yielding representative slope values. The coarser the DEM resolution, the flatter slopes become. The impact of DEM resolution on runoff simulation depends on how much relief there is in a basin.

Slope is expressed as a decimal fraction in Vflo™, not as a percentage or degrees from horizontal. The minimum value is 0.00001. All cells except base cells and reservoir cells have a slope parameter.

See here for instructions on developing slope maps in GIS.

Channel width

Channel width is a property of channel cells. Channel width represents the width of the channel at its base. Channel width and channel side slope are used together to determine the hydraulics of channel cells. Trapezoidal cross sections are formed from channel width and channel side slope information and used to generate a stage-discharge relationship for the cell. Channel width maps may be generated using GIS software, or values may be entered directly through the Hydraulics panel of Vflo™. One approach to developing channel width maps using GIS is to derive a geomorphic relationship between drainage area and channel bottom width. Otherwise, channel width can be determined from as-built plans or surveys, or deduced from local knowledge about channels.

Channel width should be entered in units of meters (metric) or feet (US customary), with a minimum possible value of zero. Note that a channel cell’s channel width property is not associated with the cell width, or cell size of the Vflo™ model.

Channel side slope

Channel side slope is a property of channel cells representing run/rise, or the ratio between the change in distance over the change in elevation for channel sides. Channel cells use channel side slope along with base width to define trapezoidal cross sections for the cell. If geomorphic relationships between side slope and drainage area are available, a distributed map of side slope can be computed based on flow accumulation using GIS. Channel side slope can also be determined from as-built plans or surveys, or deduced from local knowledge about channels. See here for instructions on developing channel side slope maps in GIS.

Channel side slope is a value without units, as it is a ratio; it’s important to note, however, that the run (horizontal change) and rise (vertical change) values must be in the same units, such as meters/meters (metric) or feet/feet (US customary). For example, a channel with a horizontal change of 60 feet and a vertical change of 20 feet would indicate a side slope of 60 feet/20 feet, or 3. In addition, note that channel side slope is the ratio of horizontal change/vertical change, the inverse of how slope is commonly stated (vertical change/horizontal change). This is to accommodate channels with vertical side walls, which would be undefined if the typical definition of slope as vertical change/horizontal change were used. The minimum possible value for channel side slope is zero (a vertical side wall).

Baseflow

Baseflow represents the lateral inflow from groundwater and/or outflow to groundwater. Baseflow is a property of channel cells, rated channel cells, and cross section cells. Baseflow is assigned as a steady state property of a given cell in Vflo™, rather than being calculated as a function of inputs or otherwise simulated. Positive baseflow means a stream is gaining water from the groundwater. Negative baseflow represents outflow or loss to the groundwater or streambed subsurface. When Vflo™ is solved, baseflow values contribute to soil profile saturation. When the soil profile is saturated, runoff is generated.

There are three methods for entering baseflow values:

1. A baseflow map may be developed in GIS and imported to Vflo™ (see here for instructions).
2. Baseflow may be manually entered for a cell or cells using the Hydraulics panel (see Editing cell properties).
3. Baseflow may be entered in terms of total lateral inflow over a channel reach (see the Entering baseflow page for instructions).

In AutoBOP, GIS maps, and the Hydraulics panel, baseflow should be entered in units of cms/m (metric) or cfs/ft (US customary). To enter baseflow in terms of total lateral inflow over a channel reach, change in baseflow is entered in units of cms (metric) or cfs (US customary).

Base elevation

Base elevation is a property of cross section cells. A base elevation may be entered if flow is assumed to begin at some stage above the lowest elevation value of the cross section. Base elevation is manually entered using the Hydraulics panel. It should be input in units of meters (metric) or feet (US customary).

Initial storage

Initial storage is a property of reservoir cells used to model the storage volume available before discharge occurs. Initial storage is manually entered using the Hydraulics panel in Vflo™. Initial storage should be input in units of m³ (metric) or ft³ (US customary), with a minimum value of zero.