This command uses layers to assign Rational Method runoff coefficients to closed polylines or to polylines that end on their original starting point. 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 coefficent (C factor) such as 0.85 and wooded areas defined by closed polylines can be assigned a low runoff coefficient such as 0.20. These runoff coefficient area polylines are used to determine the weighted runoff coefficients for drainage areas in 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.
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.
Layers and their runoff coefficient assignments can be edited and deleted. The assignment files can vary from project to project, so it is useful to save and recall the assignments into ".rcl" files using the SaveAs and Load options. The currently loaded assignment is applied within the command Watershed Analysis.
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 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
diffferent areas
File Name: \lsp\cntr_grd.arx