Applies To
	Product(s):	StormCAD, CivilStorm, SewerGEMS, FlowMaster
	Version(s):	All
	Area:	Calculations
	Original Author:	Jesse Dringoli, Bentley Technical Support Group

 

Problem

How does the GVF-Rational (StormCAD) solver and FlowMaster compute the depth and spread for a grate inlet in sag? How does the clogging factor come into play?
 

Solution

A sag inlet always captures 100% of the approach flow. The numerical solver first determines the head (depth) necessary to push all of this flow through the inlet, using standard HEC-22 methodology. Then, based on this depth and the road cross slope, it finds the spread.  The equation used to figure out the depth is a rearranged form of HEC-22's equation 4-26.   This equation is found in the StormCAD Help under Contents > Theory > Surface Gutter System > Inlet Openings > Inlets in Sag > "Grate inlet in sag". Note that in some conditions (higher depths), orifice flow can occur, in which case a different equation is used (see the help.) These equations from the help are shown below:

The important note when using this equation is that the value for depth, d, is at the middle of the grate. StormCAD and Flowmaster report the depth at the curb. (where the maximum is.) So basically StormCAD uses this equation to find the depth at the middle of the grate, based on the flow, then uses this along with the geometry based on the road cross slope to figure out the depth at the curb, and reports that. Note also that the average depth across the grate is used when computing weir flow over the weir ends (perpendicular to curb).

Meaning, when calculating weir flow, the depth at the front of the inlet is used when considering contribution of weir length L, since this is the actual depth along that side (parallel to curb). The average depth across the grate is used when computing weir flow over the weir ends (perpendicular to curb). If it were to just consider the average depth across the grate for all weir components, as presented in the HEC-22 example, it would overestimate the capacity of the grate. This effect would be more pronounced the longer the grate, or the steeper the cross-slope. This can potentially lead to differences in results when compared to other software.

When using a clogging factor, the Width (W) part of the equation is multiplied. 

It then uses Spread = Depth/Slope to find the spread. (the slope value being in units of ft/ft or m/m such as 0.05 ft/ft)

Detailed Explanation

For grate inlets in sag operating as weir flow, please see more details in the following article: Grate Inlets in sag operating as weir

See Also

How do catalog inlets handle clogging at low depths?

Where is the Gutter Depth measured from?

How does the clogging factor work with catalog inlets?