Define Watershed Layers

This dialog box is the interface to assign specific Ground Covers to closed polylines in a drawing, based on their drawing layers.  There are 3 modes of operation; Rational Method, SCS Method, and HydroCAD. 

With the dialog set to Rational Method, the runoff coefficients are the C-Factors in the Rational Equation Q = C*I*A.  Q is flow, I is rainfall intensity and A is area.  The Rational Method is often used for urban and residential flow analysis.  For example, building layers can be assigned a high runoff coefficient (C factor) such as 0.85 and wooded areas would be assigned a low runoff coefficient such as 0.20.  

With the dialog set to SCS Method, the coefficients are set to values from 0-100, so Roofs might be 85 and Woods 20.  A Soil Type must also be specified for each Ground Cover.

With the dialog set to HydroCAD, the runoff coefficients are not set at all, but are added when the data is exported to HydroCAD.

With the Rational or SCS Methods, the runoff coefficient area polylines are used to determine the weighted runoff coefficients for drainage areas in conjunction with other Carlson Hydrology commands such as Watershed Analysis and Edit Sewer Structure. The runoff coefficient polylines are automatically clipped by the drainage perimeter polyline to find the coefficient sub-areas within the drainage perimeter.  Therefore, it is important to close all polylines, use distinct layers for features that have distinct runoff values, and to assign a runoff coefficient to the unassigned, "remainder" areas.  It is also important to enclose areas beyond the site with closed polylines and assign runoff coefficients to those layers to account for the off-site water entering the site.

Looking at Rational Method first, the initial dialog box would look something like this:



Adding or editing a layer brings up the next dialog:



The Library button brings up the next dialog, where new Ground Covers can be added from the Rational Method library.


For each layer, an area name and runoff coefficient are assigned and can be selected from the library. This library itself is defined under the Network pulldown menu, option Drainage Runoff Library within the Sewer Network Libraries "flyout". Each layer also has hatch settings for drawing the runoff areas. The hatch settings include the layer, color, pattern and scale. The Auto Hatch Scale option will size the hatch scale to fit the runoff area. The Hatch All button will hatch all the runoff areas in the drawing as closed polylines and defined in the list. The Hatch Selected will hatch the area of the currently selected layer from the list. The purpose of the hatch functions are for visual checks that the layers and closed polylines are set right.

With the SCS Method in use, the primary dialog would look something like this:


Adding or editing a layer brings up the next dialog:


The Library button brings up the next dialog, where new Ground Covers can be added from the SCS Method library.  Select a Cover Description line, and then pick a Soil Group button, A, B, C or D.


Using the HydroCAD method, the initial dialog might look something like this:


Note there are no Curve Numbers listed, and the Default Runoff Curve Number field is grayed out.

Editing a layer brings a dialog like this:


Note that you can change the Layer name, but not the description of the Ground Cover.  This is because the specific descriptions provided for HydroCAD Ground Covers must exactly match the description specified within HydroCAD itself.

Picking the Library button accesses the HydroCAD Ground Cover library.


Back in the primary Define Watershed Layers dialog box, the list of layers and other settings can be saved as .RCL files.  It is useful to save and recall the configurations in .RCL files using the SaveAs and Load options.  The currently loaded assignment is applied within the command Watershed Analysis.

The Create Layers button creates the layers from the list in the current drawing.  All layers can be created, or certain layers can be selected before picking the Create Layers button and you can specify to create only the selected layers.

The Color TIN button applies the colors defined for the ground cover areas to colorize a triangulation file which is helpful for visualizing the surface in 3D viewers.

There are settings for the default area name and default coefficient that are used for any part of the drainage area that is not covered by one of the runoff layer polylines.

The Soil layers are optional for finding the soil sub-areas within each runoff sub-area. These Soil layers are used in commands that calculate the runoff sub-areas within a watershed boundary such as the Select Watershed function within the Curve Numbers & Runoff command. When the Soil layers are assigned, the program will get all the linework on the Soil Linework layer to build a topology of the soil areas. The linework does not need to be made of closed polylines but the linework collected together should enclose the soil areas. Then the program takes the text entities in the drawing that are on the Soil Label layer. The text is used to identify the soil group. The first character in the text should be A, B, C or D for the four soil groups. The program looks for the text to be within the soil area to assign the soil group to that area.

The Watershed Linework layer is used in commands for selecting the watershed area by picking a point within the area. For instance, the Select Watershed routine in Curve Numbers & Runoff will prompt whether to select by perimeter (closed polyline) or interior point (watershed layer method), when the Watershed layer is defined. Similar to the Soil Linework layer, the program will get all the linework in the drawing that is on the Watershed layer and build a topology of the watershed areas. The linework does not need to be made of closed polylines but should make closed watershed areas when taken together. When you pick the interior point, the program finds the watershed linework that encloses the point to get the watershed perimeter.

The Watershed Label layer is used by the Hydro Network commands to match the drawing watershed area with the subcatchment node in the network. The match is between the value of a text entity on the watershed layer with the name of the subcatchment node.

The runoff polyline areas use region logic where a polyline inside another on the same layer is used as an exclusion. A limitation is that polylines on the same layer must not intersection each other. For polylines on different layers, there can be polylines within other polylines and for any given point, the smallest enclosing polyline is used to determine the runoff coefficient.

Example 1: In the example below, the site perimeter polyline is on the Regions layer, the building pads are on the Pads layer and the edge of pavement polylines are on the Roads layer. All these polylines are closed polylines. The areas within the buildings are inside both the Region and Pads polylines and the Pads govern because they are the smaller area. Likewise the road areas are governed by the Roads layer and road interior islands are not counted for Roads because the interior Roads polyline acts as an exclusion perimeter. The rest of the area is set to the Regions layer.

Example 2: Consider the subdivision shown below.  

Buildings, roads, driveways, lot lines and wooded areas are in distinct layers.  As soon as the command is selected the dialog below appears.  The applicable layers can then be organized as follows within the command.  Note that the lot lines do not have any hydrology impact and are not included in the layer-runoff coefficient assignment.

Example 1 used the built-in logic to remove closed polylines from outer enclosing closed polylines.  So in the example 2 case, the overall property boundary had a runoff coefficient of 0.2 that was assigned its runoff coefficient by layer, and all other assigned closed polylines found within it (roads, buildings, driveways) will be calculated distinctly.  For example 2, the entire "remainder" area that is not assigned and is given a default runoff coefficient, such as 0.5 shown above. Therefore, within any site perimeter, both the "unassigned" method for remainder areas or the assigned, outer boundary layer method for the remainder areas can by used. When the "Hatch All" button is clicked, the drawing will hatch in the defined colors and layers, as shown below:

Pulldown Menu Location: Watershed
Keyboard Command: define_runoff_layers
Prerequisite: Closed polylines on different layers for the different areas