Storm and Sanitary Analysis - HGL & Flowrate issues

[Anonymous]

There are a few things I do not understand with my model that I am hoping someone can help with. 

I am trying to model this as a proposed system that ties into an existing system at MH-3. I have set the outfall fixed elevation to 1' below the Rim of MH-3 to simulate a surcharged condition in the existing pipe. I am using hydrodynamic analysis and rational method. Is there a better way to "fix" the starting HGL?

1.) The HGL seems to peak in MH-3 after 20 seconds but then decreases after. Does that mean its over-capacity? Does it seem like there is an instability in the model with this type of display? 

2.) Why does the flowrate seem to decrease from MH-1 to MH-3? 

Any information would be helpful! Model attached for reference. 

[wpagl452 · Accepted solution]

Hi @Anonymous, thanks for your post!

A quick caveat: We cannot help with engineering design and I do not want you to make design decisions based on conversations with our support team. That being said, I will attempt to help you understand how the model is working as you have currently configured it.

I took a look at your model; thanks for providing it! To answer your questions:

1. MH-3 is flooding and Link-04 is surcharging. This makes sense as you have set the outfall fixed elevation so high. You've modeled a backwater situation where water is flowing back up the system to elevation 69' and mixing with the water coming from the basin which peaks around 20 min as you said. You can see this by looking at the time series plot of Link-04; the negative flow shows the water flowing upstream. If this submerged tailwater condition reflects reality, then so be it. If not, then change the outfall boundary condition.

2. The flowrate is decreasing because the flow rushing downstream from the basin encounters the backwater from the outfall tailwater. 

As a general modeling note, I would change the outfall boundary condition unless that reflects existing conditions. To model existing flow coming into MH-3, the ideal way would be to create an existing model with basins, structures, pipes, etc and connect those to MH-3. If this is not feasible, you could also add a user-defined external inflow to MH-3. You could add a constant flow or add a time-series to reflect the existing time series flowing through MH-3 if you have data for that. 

[cillian]

Hello,

I am having an issue with the outfall boundary condition. Please see attached model. I have set a fixed boundary condition of 5m to simulate flood level in the drain where my outfall is located. However when I run the model and check the profile plot it does not show the desired water level at the outfall. Any ideas would be much appreciated?

[aponchaiTZ3NJ]

Hello 

      I am modeling a system that discharge to existing creek. I always have a problem with maximum HGL at the junction structures. The Maximum HGL line ( Red line ) are not line up with HGL line ( a blue line ) in junctions. It always flood. When I try to raise the rim elevations 10 ft., the maximum HGL always go up to the rim elevation but the HGL line ( the blue line ) stay the same.

      I try to ignore dampen and change time step up to 300 second but nothing change. If you have see the problem, I am more than happy to share my model.

Best regards,

Arnon Ponchai

[namugangajoyd]

hello ,

this is Joydash i am very new to SSA and i have encountered some challenges on how to use and understand the software.

i have attached a stm file that am working on could anyone help me tell which method/way is best for inputing the necessary parameters(as in should i use sub basins or use external inflows for every junction.)

when i used sub basins and assigned areas to them the analysis was sucessful but didnt calculate any of the inlet parameters.

and why would a junction change its color from red to blue after running the analysis

your help will be of great help

[matt_anderson_pe]

@namugangajoyd 
1.  Red is the regular node color.  Blue indicates the analysis floods.

2. STM import should work out of the box.  The question of subbasins vs. external inflows is primarily about how you transform rain - does SSA perform that transform method. 

Matt Anderson

[namugangajoyd]

thanks dear Anderson for clearing the air for me.

currently i am working on about 10 roads that have about 1000 inlets each .Is there an easier way i can input the required parameters like the roadway slope, cross slope, gutter width etc without doing it manually for every inlet.

anyone who can help on this, i will be very grateful