Simple Backwater Analysis with SSA

[hansenbk]

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.

[fcernst]

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.

[gng]

hansenbk:

I'm in WA and running into the same issue.  If you're running the static method you are choosing a duration.  1 day as a defaut to trick SSA to run a duration when all you want is a moment in time, peak flow rate.  So you should only be concerned about your flow rates in your outputs and not your volumes, for the volumes accounts for the day of peak flow.  This is how I'm using it.  Someone let me know if I'm reading it incorrectly.

I would love to talk you hansenbk to see how you resolved this, for I'm working through this now.

Regards,

Gabe

206-718-5252

gng@amorterra.com

[Mike_G123]

Did you guys ever figure this one out?

I'm trying to do something to the same extent. Except I have specified inflows from pumps at a reservoir I need to add to the storm system basin, these are essentially a static flow 24/7/365. I don't see any way to enter a static flow to the model. Local Jurisdiction wants us to prove that it does not exceed the capacity for a given storm event.

I tried a work around, using a large 'dummy' basin, then splitting the flows to achieve only my desired pumping flow. However, the flow diversion tool can only be used with the Kinematic Routing method, which in turn is not suitable for backwater analysis! I also have negative slopes in the system so the program will not except my model on top of that when using Kinematic routing...

So much for the work around...