SSA CB Inlet Analysis: Depth / Spread Inconsistencies

[flea21212]

I’ve set up a model to analyze gutter spread for a proposed pipe network.  I’ve compared gutter spread from the Storm and Sanitary Analysis “Output Report”, “Curb & Gutter Inlet Calculator”, and Flowmaster.  I found that the gutter spread calculated by the program appears to be inconsistent with Flowmaster and the Curb & Gutter Calculator.

Using SSA, for a 10-year storm, the peak flow to a particular catch basin is 0.40 cfs (CB-4.D)

Manual Input Values:

Inlet Type: Median & Ditch Inlet

Discharge:                      0.40 cfs

Longitudinal Slope:         1.0%

Bottom Width:                 2.00 ft

Left Side Slope                 1.5% (1:66.67)

Right Side Slope:             1.5% (1:66.67)

Clogging Factor:                    15% (77.90% Flowmaster*)

Grate Width:                     18”

Grate Length:                   18”

Grate Type:                       Reticuline

*Note: Clogging factor was adjusted in Flowmaster in order to match the grate efficiency output from Storm and Sanitary Analysis

Output Values:

Storm and Sanitary Analysis (Output Report)

Gutter Spread During Peak Flow:       5.31 ft

Gutter flow depth during Peak Flow:     0.03 ft

Peak Flow                                                      0.40 cfs

Peak Flow Intercepted                                 0.10 cfs

Peak Flow Bypassing:                                  0.30 cfs

Efficiency:                                                      25.42

Storm and Sanitary Analysis (Curb & Gutter Inlet Calculator)

Top Width:                                                 9.60 ft

Gutter flow depth during Peak Flow:     0.057 ft

Peak Flow                                                      0.40 cfs

Peak Flow Intercepted                                 0.135 cfs

Peak Flow Bypassing:                                    0.265 cfs

Efficiency:                                                      34.00

Storm and Sanitary Analysis (Flowmaster)

Top Width:                                                   9.62 ft

Gutter flow depth during Peak Flow:     0.057 ft

Peak Flow                                                       0.40 cfs

Peak Flow Intercepted                                 0.102 cfs

Peak Flow Bypassing:                                    0.298 cfs

Efficiency:                                                     34.00

As you can see above the gutter depth and spread results from the Curb & Gutter Inlet Calculator and Flowmaster are very similar.  However, there is a drastic difference in the spread and depth from the Sanitary and Storm Analysis Output Report that is generated.

Can anyone explain these discrepancies? 

[flea21212]

I've continued to investigate this matter futher by setting up a real generic project. With two catch basins and one outlet.

I've found that the lower the peak flow, the more inconsistant SSA is in calculating gutter spread and flow depth.  I've attached a PDF showing the comparison between increasing peak flows and gutter spread accuracy.

Can anyone explain how SSA is calculating the low flow spreads and why they differ so much from flowmaster?

[CPeng78]

Has anyone come across a solution for this? I'm running into similar scenrios using SSA 2013. Please advice.

[Matt.Anderson]

SSA 2014 and 2015 have a service pack which, I think, addresses this issue.  

[CPeng78]

Thanks Matt. Does the service pack have to be downlaoded separately? 

[CPeng78]

Sorry hit sent to quickly. 

Does it need to be downloaded separately? Would the software automatically update itself?  My work computer has SSA 2015 installed so I can definetely look into it. If does need to be downlaoded separately, what's the name of the Service Pack so I can search for it? Thanks again

[Matt.Anderson]

Check:  https://knowledge.autodesk.com/support/autocad-civil-3d/downloads

[aszymaszek]

Seems like these issues are still present in SSA 2016. Hopefully the Autodesk team can fix them!