AutoCAD Civil 3D - Stormwater

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New Member
Posts: 2
Registered: ‎07-19-2012
Message 1 of 2 (694 Views)

SSA CB Inlet Analysis: Depth / Spread Inconsistencies

694 Views, 1 Replies
07-19-2012 02:46 PM

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? 

New Member
Posts: 2
Registered: ‎07-19-2012
Message 2 of 2 (681 Views)

Re: SSA CB Inlet Analysis: Depth / Spread Inconsistencies

07-20-2012 08:16 AM in reply to: 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?

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