AutoCAD Civil 3D - Stormwater

AutoCAD Civil 3D - Stormwater

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Registered: ‎05-08-2012
Post 1 of 2

Simple Backwater Analysis with SSA

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07-26-2012 09:29 AM

I am attempting to run some basic backwater calculations using SSA and would appreciate some insight.  My hope is to be able to replace the current program I am using with SSA but I first need to have confidence in the results from SSA to do so.


I'm working on a project in King County, Washington.  They have their own hydrologic model that is required and that same model is used to calculate conveyance flows.  Due to this fact, I do not need or want to use any of the hydrologic functions in SSA.


In the past I have used StormCAD for this exact purpose.  I build my pipe network, set the outlet tailwater elevation to the 100-year water surface produced by the King County model and introduce 100-year peak flows to each catchbasin.  The model runs and provides me an HGL at each catchbasin for that 100 year event.  The required criteria is typically that the 100-year peak flows not overtop any catchbasin.


This is a simple model and that seems like SSA should be able to replicate.  However I am having very little success.  I built in SSA a piping network that was modeled in StormCAD so that I could compare the results.  The results seems to be similar but I have no confidence in what is coming out of SSA because it seems to change in ways that do not make logical sense to me.


For instance, I built a model with 2 junctions and one outfall.  I set the outfall tailwater to an elevation that would surcharge all pipes, say arbitrarily elevation 100.  I didn't add any other flows to the model.  So logically, the Max HGL for all catchbasins should be the 100 elevation, as the model is static.  The average HGL at all catchbasins was indeed 100, very promising.  However, the max HGL at the catchbasins were higher than 100.  With no additional flow, this seems illogical.


Back to the larger model I created (which I have attached changing the extension to a .dwg from .spf).  I do not want to run a series of flows through the model, I want to run specifically the 100 year flow only.  I basically want to only use the hydraulic portion of SSA and not the hydrologic portion.  However, SSA makes me pick a length of time to run the model for and when I change the length of time from say 5 minutes to 1 day, the maximum and average HGLs change.  This doesn't make sense to me as I am not attempting to introduce different flows based on time but one instance where the flow is very specific.


So my general question is:  Can I route a single flow event through a pipe network and get the HGL for that one set of flows consistently.


The only way I will have confidence in using SSA is if I can compare it to programs I have been using for years (StormCAD and King County's Backwater Analysis model) and get similar numbers.  I don't expect to get the exact same numbers due to calculation methods and headloss analysis differences, but nearly the same is important.


I've read nearly 100 posts on these boards and haven't come across anyone doing exactly the same thing I am attempting, yet it seems like it should be something fairly common, at least in the Seattle area.


If anyone has done something like this I'd appreciate some help.

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Post 2 of 2

Re: Simple Backwater Analysis with SSA

08-11-2012 07:13 AM in reply to: hansenbk

You are trying to force a Dynamic model to behave like a Static model, then compare results. Totally different methodoligies : St Venant equations (Dynamic) vs. HEC 22 standard step algorithim (Static) .


Hydraflow Storm Sewers is the comparable model to StormCAD, both claiming to use the static HEC 22 methodology. I would suggest your time would be better spent trying to calibrate Storm Sewers results to your old StormCAD runs.


Hope this helps.





Fred Ernst, PE
C3D 2015 SP2
Ernst Engineering
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