AutoCAD Civil 3D - Stormwater

AutoCAD Civil 3D - Stormwater

Posts: 1,162
Registered: ‎01-07-2011
Message 1 of 2 (642 Views)

SSA - Surcharged Inlet System Losses

642 Views, 1 Replies
12-11-2012 12:35 PM

I am investigating SSA's capability for my company's use in modeling dual street drainage systems utlizing SSA's Inlet objects.


I created a surcharged flooding condition at an Inlet node location with no surface flows coming into the inlet for the testing. I tested Inlets in both the Sag and OnGrade conditions. The model was ran for 24 hours to allow the system to drain and pass all upstream surcharge events.


I found that SSA lost the surcharged flooding water from the system, generating hydraulic routing continuity errors of greater than 96%.


I have included the two SSA models for your review. There are also 5 screen captures below with numbered descriptions.  I would appreciate any comments.


Sag Inlet Condition:


I provided a large area for the Ponded Area property of the Sag Inlet. The Ponded Area is a constant-area volume mechanism used to store water that has surcharged up through the inlet, to be conveyed back into the inlet node when the system is no longer surcharging at that location. SSA calls the surcharge flooding "upwelling" flow in the Help.


I also conneted a trapezoidal channel link to the Sag Inlet at the Rim elevation to see if SSA would convey the surcharged flows  through this overflow channel. This is a typical model setup in dual street drainage system analysis.


Result: SSA failed to convey any Ponded surcharged or "upwelling" flow back into the system after all surcharging events had passed, and  SSA failed to convey any surcharged flooding flow to the Overflow link, and the flooding flow volume was lost from the system.


OnGrade Inlet Condition:


In SSA there is no Ponded Area property available for an OnGrade inlet. There is a Roadway/Gutter bypass link option available to convey surface flows leaving the OnGrade inlet location.


I chose the same trapezoidal Overflow channel used in the Sag Inlet test that is connected with its invert at the Rim elevation of the inlet, as the Bypass link.


Result: SSA was not able to convey any surcharged or "upwelling" flow into the Bypass link, and it was lost from the system.



To model the Major Storm in dual street drainage analysis, the modeler is required to convey and track the surcharged flow from the storm sewer system on the ground surface. The storm sewer becomes surcharged during the Major Event analysis due to the fact the storm sewer pipe system is typically sized only to accomodate the Minor Storm sewer system without surcharged nodal flooding. 


To model an Existing storm sewer system, the modeler is also required to be able to model surcharged nodal flooding to determine where the system is undersized and problematic.


  • SSA utilizing the Inlet object in its present form, can not be used to analyze either the Major Storm event for a dual drainage storm sewer system or used for an Exisitng sewer system study.


  • SSA utilizing the Inlet object in its present form would be only appropriate to analyze the Minor Storm sewer system events, where there is no nodal flooding.



1. From the SSA Help showing "upwelling" flow or nodal flooding.



2. Plan view of test model.Capture5.JPG


3. Profile of surcharged model with Inlet location circled.



4.  Time series of the links going into "Inlet In" and out: "Inlet Out" and "Overflow". This shows the flow being lost at the Inlet.Capture.JPG



5. Hydraulic routing continuity error.











Fred Ernst, PE
C3D 2015 SP2
Ernst Engineering
Posts: 1,162
Registered: ‎01-07-2011
Message 2 of 2 (603 Views)

Re: SSA - Surcharged Inlet System Losses

12-18-2012 02:55 PM in reply to: fcernst

I received confirmation from Autodesk support that the Inlet objects in SSA can not support surcharged nodal flooding ("upwelling") as they show it can in the Help.


Therefore I can not use the Inlet objects to model dual drainage storm sewer systems for the Major Storm event, or use them to analyze an Existing storm sewer system.



Fred Ernst, PE
C3D 2015 SP2
Ernst Engineering
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