WMS:MODRAT Edit Parameters

Edit MODRAT Parameters Dialog

Attributes or parameters for all MODRAT hydrograph stations are defined and/or edited using the Edit MODRAT Parameters dialog, shown below. This is a dynamic dialog that will change depending on what the user has selected when the dialog is brought up. This dialog is accessed by selecting the Edit Parameters command from the MODRAT menu or by double-clicking on a basin, reach, or relief drain when MODRAT is the active model in WMS.

If a subarea, reach, or relief drain is selected before issuing the Edit Parameters command, then the data for that object is loaded for editing. When a hydrograph station is selected (subarea/reach/relief drain), only the items which correspond to the selected hydrograph station are active, all others are inactive. For example, when a subarea is selected, all items under the Basin Data heading are active and all items under the Reach Data heading are inactive.

Once the MODRAT Parameters dialog appears, it will remain open until the user selects the Done button. This enables the user to switch between basins, reaches, or relief drains quickly and define the necessary parameters without closing and opening the dialog for each hydrograph unit.

General Features

Location

The number entered in the Location field of the MODRAT Parameters dialog indicates the place in computational order the subarea or reach occupies. Computations in MODRAT begin at location 1 and continue sequentially. The location assignment is generally done automatically in WMS; for more information on numbering a watershed model, see the MODRAT Numbering the Tree section.

Lateral

The letter entered in the Lateral field of the MODRAT Parameters dialog indicates the line to which the subarea or reach is connected. The main line in any MODRAT model is line A; laterals may be labeled B through Z. Reaches with two letters indicate a confluence point of two or more reaches. The lateral assignment is generally done automatically in WMS; for more information on lettering a watershed model, see the MODRAT Numbering the Tree section.

Basin Parameters

MODRAT Parameters dialog when a basin is selected

Area

Enter the tributary area, in acres, of the subarea. If the WMS graphical modeling tools have been used to define the watershed from digital data, the area of each subarea will be automatically computed and will appear in this text field. If not, the user must manually enter the area of each subarea.

Time of Concentration/Tc (min)

Enter the time of concentration of each subarea, in minutes. Normally, this number is between 5 and 30 minutes. LACDPW has developed several regression equations to compute the time of concentration of a subarea, which are embedded in WMS. The Compute Regression Tc button will launch the Time Computation dialog where users can compute Tc using the LACDPW regression equations.

Soil Type

Determine the soil type most representative of each subarea, between 2-172 for normal soil conditions and 202-372 for burned soil conditions. Soil type maps from LACDPW are used to find which soil is predominant in the subarea. This procedure has been automated in WMS; a digital soil map may be imported into WMS and overlaid on the watershed to be modeled. WMS will then calculate which soil covers the majority of the area of a subarea. This function is part of “mapping MODRAT attributes”; a full description can be found in the Map MODRAT Attributes section. If the watershed being modeled has not been graphically defined in WMS, enter the appropriate soil number based on visual inspection.

Impervious Area

Define the percent of the subarea that is impervious to infiltration. The percent impervious generally depends on the land use in the basin; as with soil type, land use maps have been developed to allow definition of this parameter through graphical analysis. WMS can determine the percent impervious by mapping MODRAT attributes, or the user may enter this number independently.

Rainfall Depth

Enter the depth of rainfall, in inches, on the subarea. The depth of rainfall on any subarea in the Los Angeles area can be determined from isohyetal maps developed by LACDPW (maps for the 2, 5, 10, 25, 50, 100, and 500-year are available). These isohyetal maps are available in ARC/INFO® Grid format for import into WMS. WMS can determine the rainfall depth in any subarea from these rainfall grids through the Map Attributes command in the MODRAT menu.

Along with defining a rainfall depth, a temporal distribution series must also be defined. The Define Series button launches the XY Series Editor in WMS. This editor allows importing or manually entering a temporal distribution for the rainfall in the subarea. The temporal series typically used for LA County can be imported from the file LACDPWStorm1500min.xys, found in the /MODRAT/Rain/ folder of the WMS installation.

Outlet and Diversion Parameters

MODRAT Parameters dialog when an outlet is selected.

Length

The length, in feet, of each reach in a watershed is needed to perform routing computations in MODRAT. When the watershed has been graphically delineated in WMS, the length of each reach is automatically calculated and assigned. Otherwise, the user must calculate and enter the length of each reach manually.

Slope

The slope of each reach is needed to perform routing in MODRAT. If the watershed to be modeled has been graphically delineated in WMS, this value will be calculated and assigned to each reach. If not, enter this value manually.

Manning's Roughness Coefficient, n

Enter the Manning’s roughness coefficient, $n$. The default value for MODRAT is 0.014. Any other roughness coefficient value deemed appropriate may be entered.

Routing/Conveyance Type

Each reach in an MODRAT model must be assigned to be one seven conveyance types. Certain parameters must also be defined to describe the geometry of the conveyance. Boxes in which to enter these parameters will appear when a conveyance type is selected. The conveyance types are:

• Variable – if this type is selected, MODRAT will begin flood routing in a street section, change from street to pipe when the flow depth reaches the curb height, from pipe to rectangular channel when a pipe diameter of 8 feet is exceeded. Routing will continue in the rectangular channel until a maximum depth of 13 feet is reached.
• Mountain channel – no additional parameters need to be defined for this conveyance type.
• Natural valley channel – no additional parameters need to be defined for this conveyance type.
• Street channel – the street width (feet) and the curb height (inches) must be selected when using this option.
• Circular pipe – the diameter of the pipe (feet) of proposed or existing drains may be entered; otherwise, leave this at zero and MODRAT will compute the needed diameter of this conveyance type.
• Rectangular channel – the roughness of the channel sidewalls and either the base width (feet) –OR– the depth of the channel (feet) are needed for this conveyance type.
• Trapezoidal channel – channel side slope, maximum peak velocity (ft/s), side wall roughness, and either the base width –OR– depth of the channel are needed for MODRAT routing computations.

Related Topics

• Mapping Parameters from GIS Data Files
• Job Control Parameters
• Renumbering the Tree
• I/O
• Computing Tc