10-year storm capacity 1

[David Krugler PE]

So a discussion came up in our office that I am requesting what others are doing in the Municipal world.

We normally design our storm pipes (Size and slope) for the 10-year event. However, we had not taken into account that the outfall had any water standing. For instance, we have an outlet of our storm pipes into a detention pond. We normally had the pipe invert placed at the "Normal Water Level". This assumption does not apply as our pond will have water elevation higher than the NWL in a 10-year event (say NWL+0.8') thereby decreasing the capacity of the storm system.

Am I getting too far into the weeds for concern, or is the normal operation to place the pipe invert at the pond's 10-year event so that the pipe flows at the capacity for the 10-year event?

Thanks in advance.

 

[gbam]

It is normal practice to use the appropriate tailwater for your system design regardless of system type (culvert or storm drain). One should assess the probability of coincidental occurrences also, FHWA HEC 22, see link below, provide general guidance on that. There are time you will need to use the 100-Year tailwater on a 10-year SD, for instance. To ignore or improperly estimate the tail water can result in undersized systems.

FHWA HEC22 -

https://www.fhwa.dot.gov/engineering/hydraulics/pubs/10009/10009.pdf

Hope this helps.

 

[GeoEnvGuy]

The inlet should be above the spillway elevation.

Also a detention pond for a 10 year storm? Is this for a commercial property only. Most municipalities I am familiar with size storm pipes to 25 or 50 year for primary roads which are the only things that would get detention ponds.

 

[David Krugler PE]

Thanks for the input so far. Let me elaborate a bit more

If you look at the diagram, I have an outfall to a nurp pond that is designed for a 100 year event, the storm pipe is designed for the 10 year. Normally we were placing the invert of the storm pipe at the NWL of the pond. A recent issue with tolerance has raised a question, how much capacity is lost in the pipe when a 10 year event occurs since water is not free flowing out of the pipe at the 10 year event. HydroCAD is reporting to us what the level of the water is during a 10 year event, which brought up the question 'Is the capacity of the 15" pipe adequate for the 10 year event when 1/2 is submerged' We went through a calculation to show that the pipe was at 85% capacity with our example by just using the 'area reduction', but it was more 'Should we be putting the storm pipe invert at the 10 year event, not the NWL so that the pipe maintains its capacity during a 10 year event. I know there is an energy gradient that is pushing the water into the pond, but that can "surcharge" the pipes upstream.

Just wondering what others are doing, if I'm too deep in the weeds, should we be putting the invert at the 10 year event and rip rap down to the nwl, or some other BMP?

 

[gbam]

Not at all too far in the weeds, we all follow some sort of logic to design these systems.

So if this is a wet pond without outlet for the NWL then one needs to assess the condition above the NWL. If not, set your TW to the half pipe and run the SD hydraulics. Review the HGL to see if the system fails. If it does, upsize or raise outlet invert. I know that some of the storm drain design software can "design" based on constraints one sets in the model. I personally have spreadsheets that I use to preliminarily size the pipes based on flow rates I provide and then use that to set the system. Knowledge of hydraulics is paramount to good design that way one can eliminate design iteration.