Calculation of Side Flow factor with grate width less than gutter width


Product(s):FlowMaster, StormCAD, SewerGEMS, CivilStorm
Version(s):08.11.01.03 (FlowMaster) and higher, 10.00.00.40 (others)
Area: Modeling

Problem

When a grate inlet's grate width is less than the depress gutter width, the Side Flow factor (Rs) and other results do not match hand calculations.

How does FlowMaster and the GVF-Rational solver in StormCAD, SewerGEMS or CivilStorm calculate the side flow factor when the grate width is less than the gutter width?

Solution

In general, the Discrepancy is likely due to the fact that the program uses the gutter slope to calculate the side flow factor when the grate width is less than the gutter width, for grate inlets. Traditional calculations often use the road cross slope in all cases, which may not be appropriate if the gutter slope is significantly higher than the road slope and the gutter flow is within the depression.

Equation for reference:

Rs = (1/ (1+ (Kcs * V^1.8)/(Sx * L^2.3))

Where :
Rs = Side Flow Factor
Kcs = 0.0828 SI and 0.15 U.S. Customary
L = Grate Length
V = Velocity
Sx = road cross slope or gutter cross slope  - see below.

When the grate width is less than the depressed gutter width...

In FlowMaster, a grate inlet on grade will use the gutter cross slope for the calculation of the side flow factor (and ultimately capture efficiency). For a combination inlet on grade, the road cross slope will be used. There is currently no option available to change this behavior.

In StormCAD (and with the GVF-Rational solver in SewerGEMS and CivilStorm), for a grate inlet on grade, the side flow factor calculation uses the road slope or gutter slope depending on how the Neglect Gutter Cross Slope for Side Flow switch in Calculation Options is set. If the setting is False, gutter slope is used. If the setting is True, road slope is used. For a combination inlet on grade, the inlet side flow factor (and ultimately capture efficiency) calculation uses the road slope. The Neglect Gutter Cross Slope for Side Flow switch in Calculation Options does not have an impact in this case.

When the grate width is greater than or equal to the depressed gutter width...

In both FlowMaster and in StormCAD (and with the GVF-Rational solver in SewerGEMS and CivilStorm), both grate inlets and combinations inlets on grade use the road cross slope to calculate the side flow factor.


See Also

Purpose of "Neglect side flow?" and "Neglect Gutter Cross Slope for Side Flow?" options