This is not correct in the SSA help. The basin Width drains to the outlet channel.
The Width should be parallel to the outlet channel. The Length should be the distance down to the outlet channel.
This image is updated vestiage of the old SWMM 4 runoff block "idealized subcatchmetn-gutter arrangement illustrating subcatchment width" to which the 2 * subbasin lenght = width.
The goal of the image is to make clear that Area divided by Width is subbasin lenght for Tc.
The only thing I might change would be twist flow arrows to flow less perpendicular to channel.
No, you're making the mistake in that diagram of including the gutter as a component of the idealized subcatchment runoff calculation parameters. Below is what you should be using.
An initial estimate of the characteristic width is given by the subcatchment area divided by the average maximum overland flow length. The maximum overland flow length is the length of the flow path from the the furthest drainage point of the subcatchment before the flow becomes channelized.
The idealized subcatchment in SWMM is simply a non-linear reservoir model.
The goal was not to make it "idealized" but rather to provide how the software "views" the subbasin in Figure 9.4.
The source of the Figure 9.3 in the SSA and original StormNET manuals comes from this image which is the SWMM 4 runoff block section.
Reading further In the SSA help, following Figure 9.3, the help file shows and discusses Figure 9.4 using an irregular basin. It then explains:
In order to develop this equivalent rectangular subbasin, these methods use an
equivalent (or sometimes called characteristic) width of the assumed
rectangular subbasin in order to internally compute the overland flow path
and the corresponding time of concentration. An initial estimate of the
characteristic width is given by the subbasin area divided by the average
maximum overland flow length, as shown in the following figure.
Using the method as described in Figure 9.4 to compute an equivalent width,
the software then develops a rectangular subbasin in which overland flow
contributions from both pervious and impervious areas are idealized as
running down-slope off the subbasin, as shown in Figure 9.3, to the subbasin
The maximum overland flow length is the length of the flow path from the
inlet to the furthest drainage point (sometimes called a concentration point or
spill point ) of the subbasin. Maximum lengths from several different possible
flow paths should be averaged. These paths should reflect slow flow, such as
over pervious surfaces, more than rapid flow over pavement, for example.
WIth both images, together with the narrative following, they properly describe Width and Lenght and the role of Area plays in establishing Lenght for an overall area. In the figure you provided, you have two subbasins.
Should the order of the image be swapped for clarity? Maybe... Should the "gutter" portion of the ground be removed for clarify making the two sides a simple fold to avoid the appearance of "channelized" flow rather than just make it appear as the cocentration point ...? Probably in light of Figure 9.4...
I think what you are expecting is something that represents the single flow plane like the following which is in the original BOSS StormNET Techincal Reference with the original source being the EPA SWMM manual image.
I have no problem having the Equivalent Width descriptions re-arranged for clarity.
Old Dominion has the older version of the SWMM 4 manual:
Here are the figures sources:
This is the source of the SSA Figure 9.4:
I like this one from your reference for demonstrating the SWMM flow width parameter.... A, B and C are good... and D&E are unacceptable (channelization would occur prior to these lengths).
I think the second image, of section view, I show from SWMM 5 is very important to grasp for the subcatchment computation conceptualization...