Gutter Spread Confusion

[Anonymous]

I am modeling a highway drainage scenario in SSA, and I am having some trouble with the gutter spread that is in the results of my model.  I am comparing the data from the same project done in a different program and I noticed some difference in the results, specifically at one inlet. 

Here are the results that I receive from SSA:

Peak flow:  .21 cfs

Bypass:      .17 cfs

Efficiency:  83.66 %

Gutter Spread:  1.18 ft

Flow Depth:  .06 ft

Since I was comparing to already known results, the other program showed a gutter spread of 3.04 ft.  Now I know there might be some small differences but this is an extreme difference.

So I did some more looking and I used the Curb & Gutter Inlet Calculator in SSA to see what I would get.  When I enter all of the same information in as far as slopes and etc. as the inlet in the model, I get these results:

Gutter Spread:  3.05 ft

Flow Depth:  0.173 ft

And these are the numbers that I also get by hand using the HEC-22 eqn's.  So my question is what might be going wrong with the inlet in my model?  I am using a HEC-22 generic grate inlet and put that same exact information into the Curb & Gutter Inlet Calculator.  Why are there different results from my model in SSA and the Curb & Gutter Inlet Calculator in SSA???  Please help I am really confused by this.

Thanks!

[Hidden_Brain]

my first guess, try a "ponded area" of 100 sq.ft for this inlet and see if makes any difference, if you have not done that already.

[Anonymous]

The inlet that I was having issues with is an inlet on grade and thus does not have ponded area as an option.  I did adjust the sag inlet on the system to have a ponded area of 100 sf; however it did not affect the inlets upstream.

[Charlie.Ogden]

Could you post your file to the forum (or at least the inlet properties/screenshot in order to recreate)... and/or open a support request?

Thanks!

[Anonymous]

I have attached screenshots of my inlet properties and the results from the curb and gutter calculator as well.  I am not sure how to attach my SSA file as it will not allow me to attach that file type but I can email it or if there is a work around that I can attach it just let my know.  Any help with this would be very much appreciated!  Thanks!

[Anonymous]

Was this issue ever resolved for the original person that posted the question?  I had similar questions. 

Can you provide the equation that SSA is utilizing for spread?  In addition, can you verify which reported flow value is being utilized for the calculation (before or after the interception has been considered - peak flow or peak flow bypassing).  I have already tried to verify results utilizing HEC-22 equations.  I am assuming that I am just missing some piece of information, but I was unable to find any support information in the User's Guide.  

In the model I am looking at, the inlets were intentionally setup so that the gutter and road cross slopes are the same, so that the equation is a little faster to replicate across multiple inlets (to avoid having to utilize the method for composite gutter sections in HEC-22). 

Thanks

[Hidden_Brain]

I am sure I am missing something, I just set up a very simple model in SSA 2013 with uniform gutter section, and the gutter spread (and spread depth) calculations from SSA match my calculations in Excel using HEC-22 gutter spread equation:

http://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/page04.cfm

I also get comparable results using the "Curb and Gutter Inlet Calculator" tool in SSA. please see images attached.

[Anonymous]

I.  Is the inlet you compare with in the another software (not SSA) is similar to FHWA HEC-22 Generic?

2. You use sort of "extreme " conditions - 0.001 longitudinal slope, it is too flat, I do not think any formula should be trusted, even for comparison only

[Hidden_Brain]

pardon my ignorance, but i do not think the gutter spread calculations will be affected by the inlet type, unless there is bypass flow involved. inlet type will affect Q-capture, which will affect Q-bypass and gutter spread.  even when I involve inlet bypass in the mix for IN-2 (next downstream), the results between SSA and hand calculations are comparable (see GUTTER4.png).

as for the extreme flat area, calculations for IN-1 are revised attached (see GUTTER3.png), results are similar between SSA and hand calculations.

good luck!

[Anonymous]

Is there a solution to this discrepancy of spread?  I also noticed the same issue.  The spread calculated in the SSA model is less than the spread calculated using the Curb & Gutter Inlet Calculator in SSA.  The Curb & Gutter Inlet Calculator results also match FlowMaster results and HEC-22 manual check, but not the SSA spread results in storm drain model.