StormCAD - Modeling Gutter and Conduit Flow

[sabz04]

Hi all,

I am trying to understand why StormCAD is calculating flows in the manner it is. The situation I have modeled is in a particular area, there are two Catch Basins connected to each other through gutter flow and conduit flow. The conduit was added as a "dummy conduit" since StormCAD won't allow only gutter flow leaving a catch basin. Ordinarily, I understand that an inlet would flow into a storm drain system and when it holds it's maximum capacity, gutter flow would then come into play, but this is a special case since I am not able to model a split gutter flow in StormCAD. Since I wanted the program to use the majority of the flow (55.87 cfs) I've manually entered as gutter flow, I modeled the conduit as a 0.1" pipe, but when I ran the model, the gutter flow between these 2 catch basins is shown as zero, and the conduit is shown to have an excess capacity of -56.87 cfs, which makes sense since the negative number indicates that all that flow isn't getting into the pipe, but how come the gutter flow is indicated as zero cfs?

Thanks!

 

[zabrab]

I'm not sure I understand the actual situation you are trying to model, but 56 cfs would be an excessive flow to capture with an inlet or two and may not be able to be safely conveyed within your roadway section to actually reach the inlets.

That being said, how do you have the catch basin defined in StormCAD? If you have the catch basin’s inlet type as “Percent Capture” with capture efficiency of 100%, all flow that reaches the inlet will be captured and added to the total flow for the catch basin, which could explain the results.

 

[beej67]

I don't know what question you're trying to evaluate, but I highly suspect the answer to that mystery question is either "add more inlets" or "catch some of this water before it gets into the road."

I don't think StormCAD does what you're thinking it does. I believe StormCAD determines how much flow stays in the gutter and how much flow goes in the inlet by the efficiency of the inlet itself, as determined with HEC-22 calculations involving gutter parameters such as longitudinal slope, cross slope, inlet opening, grate type, etc. Then if the flow captured by the inlet doesn't fit through the pipe it just tells you the pipe is too small, not how much then is rediverted back to the gutter by the flow obstruction of the smaller pipe. That's from memory, I don't have the software in front of me to check.

If all you want to do is a quick gutter spread calculation, without any culverts in the first place, then don't use StormCAD at all. Use this thing instead: (free)

http://www.fhwa.dot.gov/engineering/hydraulics/software/toolbox.cfm

But what that thing is going to tell you, is that 56 cfs is way too much flow to carry safely in a gutter near vehicle traffic. I could maybe see some instances where it'd be okay at the back of an industrial truck court with an elevated curb or something, but even then only if the client is trying to be a real cheapskate. For example, a 2% longitudinal sloped gutter with 2% road cross slope and 56 cfs gives you almost 7 inches of depth at the curb, and almost 30 feet of spread, with 80% of the flow being carried out in the road itself instead of the gutter. That's typically not desirable.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

a bit off topic, but for 100-year flow, the criteria is usually to convey the flow within the R/W. In that case, 7 inches deep and 30 feet spread is perfectly acceptable.

calculating gutter flow and spread is just as easy in Excel as anything else. I have never used Stormcad to do it

 

[beej67]

Around here you're almost always held to one half of the travel lane for your spread limits, for a five or ten year storm, depending on the local regs. There's often no criteria for spread in parking lots, other than to make sure the water goes where you want it to go. If you've got 7 inches of depth at the curb, and the curb is 6 inches, it's not going where you want it to go.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[TerryScan]

sabz04-

I believe you are attempting to misuse the software. Your understanding of how it should work is flawed. I believe StormCad calculates like this:

The gutter flow approaching an inlet is computed or input and the program calculates the amount captured (dependent on inlet configuration) the captured amount is added to the conduit flow and the remainder is bypassed to the next inlet. (this is pretty standard in calculating roadway hydraulics re: FHWA HEC-12, HDS-22)

It sounds like your configuration has 100% inlet capture. The software does not account for surcharged flows from the conduit to re-enter gutter flow.

What are you really trying to accomplish? Calculating gutter flow between two points can be accomplished simply with Manning's equation and the channel geometry.