SSA - HGL at Inlets

[Anonymous]

I have a question regarding how SSA models inlets and calculates the subsequent HGL:

We are modeling an existing system which is significantly undersized.  When we set up the model using actual inlets in sag conditions at the nodes, the HGL tops out at the the grate elevation and the information given regarding any ponding is limited to a gutter flow depth of 0.15' and a spread of 0.0'.  There is no indication of what depth the water is rising to due to the surcharge in the pipeline.  If there is a depth of ponding for inlets not related simply to a gutter flow calc, I can't find it presented in output results.

A more accurate portrayal of the depth of ponding is to change these same inlets to junctions as the limiting factor in the system is the pipe size, not the inlet size.  Doing this we get what I would consider to be closer to the true condition (that the water ponds up to a 1' deep above the inlet.)

Could someone please comment on if SSA accounts for water ponding at inlets due to inadequate pipe size or if the water is 'lost'?  And if it does, why is this not shown in the HGL profile?

Thanks!

[Matt.Anderson]

Did you apply any ponded area to the Inlet?

[Anonymous]

Yes, we provided between 100 and 1000 sq.ft. of ponding area at the inlets.  We then applied these same ponding areas to the second model where we changed the inlets to junctions.

[trippiv]

I'm curious if you ever found an acceptable solution for this. I'm dealing with the same problem, only I've got a mix of sag and inline inlets. I've set them up as junctions for now to avoid the issues with bypass.

If I turn off the "Enable overflow ponding" and then set a ridiculously high Surcharge elevation, then my HGL lines came out like I somewhat expected, around 30' to 35' above the rim elevation due to the capacity issues.

As soon as I turn on the "Enable overflow ponding" the HGL's shoot up to +2000' over the rims, using a small ponded area, 5 sqft. If I increase the ponded area to around 60 sqft (5' wide X 12' lane width) then the HGL's move down to around +11 or 12' above the rims.

I have not gone thru the process of hand calculating the HGL's, but I'm going to do them to see what matches up.

Thanks,

Robert T

[Matt.Anderson]

I assume you are running hydrodynamic analysis, so, try to take a look at the interial variables.

[trippiv]

Yes I am. Let me adjust the variables and see what happens and compare.

I've been running with Dampen, Water surface slope & Froude number and no other adjustment to time steps.

Robert T

[jalneal]

I used to use inlets, but I have a few issues with them:

1) The inlet calculator in SSA matches up pretty good with hand calcs, but when using actual inlets, I sometimes get different results from the SSA inlet calculator and hand calcs;

2) I haven't found a good way to get the peak overflow rate and volume when they surcharge; and

3) The ponding depths as you have mentioned don't usually seem terribly accurate - storage options are limited as you can't use a stage/storage data pair at a sag inlet.

In most cases, I'd recommend modeling inlets on grade as inlets if they are not surcharging, and model inlets in sag as a storage node for the surface, and an orifice, weir, or other conveyance links into a junction to model the inlet capacity. This provides much better reporting information on the water elevation at the inlet.  Weirs or conveyance links (depending on model stability this can sometimes take some work)  between ponding areas can be used for overflows to normalize a flow depth when two inlets pond so high they become one as well, something that I haven't been able to do with a sag inlet.

I would love to see stage/storage pairs added for sag inlets, as well as better reporting on HGL and overflows with inlet structures. 

[fcernst]

No it does not model "upwelling flow" up through the Inlet Objects.

Frederick C. Ernst, P.E. | iPhone Transmittal |