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When using the Implicit or Explicit solver, when the HGL in a catchbasin reaches the rim elevation, how is the overflow rate calculated?
When the inlet is surcharged (i.e., the inlet node hydraulic grade line is above the ground rim elevation), the Saint Venant equations based dynamic calculation is applied instead of the HEC-22 based calculation. The physical size of the inlet opening is not used in this calculation; the overflow rate is primarily driven by the HGL and the physical properties of the adjacent link elements (no orifice/weir equation used). Imagine it as the water in the catchbasin vault spilling over into the cross sectional geometry of the downstream gutter or channel.
Note that catchbasin overflow will pass down a connected downstream gutter, or a channel link element if the invert is set to the rim elevation.
With the Explicit (SWMM) solver, a calculation option called "Inlet Transition Depth" is available, to help achieve a smooth transition in switching between the HEC-22 and dynamic equations when a surcharge condition is reached. The model needs a small transition depth within which the equations are gradually switched. The smaller the transition depth is, the more accurate results are but the model will be less stable. This calculation option gives user a control over the transition depth. Typical values are on the order of 0.1-0.5 feet and a default value of 0.5 has been proven to be satisfactory for most conditions.
Interpreting results when using manhole or catchbasin Surface Storage