This is a backup post to go with the EPA hydrology manual (Manual I) which can download in PDF format here
There are three runoff surfaces on a SWMM5 Subcatchment: Impervious with depression storage, Impervious without depression storage and pervious area with depression storage (Figure 1). The depression storage can be zero in all cases.
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| Figure 1. Three Runoff Areas for Subcatchments in SWMM5 |
You can see the flow from all three areas (Figure 2). If you look at the following graph the flow in the impervious and pervious area with depression storage is delayed as the depth in the area has to increase enough to reach the depression storage as you can see in Figure 3-2 from the EPA Manual.
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| Figure 2. Graphs of Three Runoff Surfaces or Areas + Total Runoff |
You can see the depth from all three areas (Figure 3). If you look at the following graph the flow in the impervious and pervious area with depression storage is greater than the depth in the impervious area with depression storage as the runoff commences right away in the area without depression storage.
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| Figure 3. Graphs of Three Runoff Surfaces or Areas
In Figure 4 you can see the Runoff from the Pervious area does not occur until the depth in the pervious area is greater than the depression storage of 1 inches.
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