When using the Implicit or Explicit solver, when the HGL in a catchbasin or unbolted manhole reaches the rim elevation, how is the overflow rate calculated?
The answer depends on whether there is an attached gutter.
Catchbasin with attached surface gutter link
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.
Catchbasin with no surface gutter, or unbolted manhole
Depending on the dynamic solver being used, the overflow from an unbolted manhole, or catchbasin with no downstream gutter link, is treated as follows:
(a) Implicit dynamic solver (DW)
For catchbasins with no downstream gutter link element, or for manholes with the "is bolted?" set to "false", overflow is calculated when the hydraulic grade exceeds the rim elevation. The overflow from the node ground is similar to flow over a weir, so a weir equation is used as follows (as also mentioned in the help topic "Pressure (Surcharged) Flow and Overflow (Street Flooding)"):
Where Qover is the overflow discharge, Lw is the overflow length, C is the discharge coefficient and C=3.0 is used, H is the head over the overflow elevation.
Note: The volume of water that overflows out of the manhole (or catchbasin with no attached downstream gutter) is lost from the system and is accounted for in the "total overflow volume" portion of the mass balance as seen in the Calculation Summary. If you need to store the overflow in the above-surface terrain, you would need to use one of the surface storage options, as explained in the article in the "See also" section below.
(b) Explicit dynamic solver (SWMM)
When using the Explicit (SWMM) solver, it truncates (limits) the node water elevation at the ground (rim) level and overflow is determined by the total inflows minus the total outflows which is based on the enforced node elevation.
Therefore, the SWMM solver will not allow the node water elevation to rise above the rim/ground and the Implicit solver has the node water elevation normally slightly above the ground. As a result, the SWMM solver normally calculates a little more overflow than the Implicit solver.
Both dynamic solvers will handle concurrence of overflow and storage above the ground.
Note: The volume of water that overflows out of the manhole (or catchbasin with no attached downstream gutter) is lost from the system and is accounted for in the "total overflow volume" portion of the mass balance as seen in the Calculation Summary or the Hydraulic Reviewer found under the Analysis Tab. Just select the catch-basin tab and run to see the overflow volume. If you need to store the overflow in the above-surface terrain, you would need to use one of the surface storage options, as explained in the article in the "See also" section below.
Interpreting results when using manhole or catchbasin Surface Storage
Zero overflow at manhole despite HGL above rim.