Problem

How do FlowMaster and StormCAD handle clogging at low depths, for an equal-length (curb opening length equals grate length) combination inlet in sag?

Why does depth decrease as clogging increases, for a combination inlet in sag?

Solution

FlowMaster and StormCAD use the HEC-22 standard for inlet calculations. In HEC-22 for combination inlets with equal length openings, when the depth is such that the orifice equation is controlling, the inlet capacity is the grate capacity plus the curb opening capacity.

HEC-22 also states that for weir flow depths, capacity is essentially the same as the grate alone.

However, if the software followed HEC-22 and did not consider the curb opening at all for weir depths, there would be an abrupt transition when orifice flow begins to control. So, FlowMaster employs a "weir/orifice factor" in the calculations. This is the ratio of the weir flow solution to the orifice flow solution for the grate. This factor, when less than 1 (corresponding to weir control for the grate) is multiplied by the weir equation for the curb opening. Thus, as the flow transitions toward orifice control depth, more of the curb inlet is considered, until you actually reach orifice control (weir/orifice factor greater than 1), when 100% of the curb is considered.

If there is clogging, FlowMaster compares the weir/orifice factor to the clogging factor (as a decimal), and uses the larger of these to multiply by the weir equation for the curb opening. Because of this logic, you can sometimes run into these non-intuitive relationships between clogging and depth/length, depending on the geometry of the inlet. 

Note: In some cases you might see higher efficiency being calculated for higher clogging factors (like 99% clogging) which doesn't make sense. This is because, the weir / orifice factor gets compared with the clogging factor (as a decimal) and the higher of the two is used to multiply by the weir equation solution which results in skewed results. With that being said, FlowMaster and StormCAD gives you the option to use only the grate inlet in calculations. So, if you would like to strictly follow the HEC-22 method whereby the curb is not considered at low, weir-flow depths:

FlowMaster: click the "Options" button and choose to consider the grate only.
StormCAD: use "Grate only" for the "Active components..." options in the Inlets section of the Calculation Options

You should then see the expected relationship between clogging and required length.