What is the "Depth (Surcharged)" field in the properties of a manhole or catchbasin, and how is it different from using a Surface Storage Method? (Ponded Area)
The "Depth (Surcharged)" field is only used with the Explicit (SWMM) solver in SewerGEMS and CivilStorm, and represents the depth (above the rim elevation) above which overflow occurs (and it currently only applies when NOT using the Ponded Area surface storage method). So, for example if water can pond up above the rim elevation to a maximum depth of 1 m, above which overflow occurs, then you would set this field to 1 m. The extra storage is simulated by simply extending the manhole size (diameter) vertically.
The other option for modeling surface overflow is to set the "Surface Storage Type" to "Ponded Area"(this is the only surface storage method currently available with the SWMM solver.) With the Ponded Area method, the "Depth (surcharged)" does not apply and is not used. Meaning, there is no "top" to the ponded area surface storage. Any overflow reported when using this method represents flow out of the node and into the surface storage. Note that the "Allow ponding" calculation option also needs to be enabled, or the Ponded Area option for nodes will be ignored.
If you need to model overflow above the top of surface storage, you may want to set the "surface storage type" to "no storage" and set the "Depth (surcharged)" to the surface depth above which overflow occurs. The other option would be to use the Implicit solver, in which case a different surface storage method can be used, such as Surface Depth-Area Curve (where overflow occurs above the top of the maximum depth on the curve).
Note: a future release may enhance the Explicit (SWMM) solver to allow overflow (per the surcharge depth field) when using the ponded area surface storage - reference #570416
The below example model demonstrates the use of both the Ponded Area and the "Depth (surcharged)" field for modeling surface overflow:
Example model - SWMM ponded area vs surcharge depth.zip
Note: this model was saved in version 10.00.00.4X and can only be opened in a version equal to or greater.
Interpreting results when using manhole or catchbasin Surface Storage
Modeling surface ponding for overflow above multiple inlets