Minimum Scouring Velocity & Pipe Slope 1

[CivilG]

I have a question about designing a storm drain pipe using my local state standards. I am designing an storm drain system using RCP. My current state standards say to design using Manning's equation and assume a gravity flow condition that is just full (NO Pressure flow). The standards say the slope must be enough to provide 3 fps for proper scour at just full pipe using the design storm. Using manning's equation provides me with a minimum slope; however, I am concerned that the fact we rarely see a design storm intensity and that the pipe will rarely see a full pipe flow or half full pipe flow to provide proper scour. Most storms are smaller and will provide small flows that will rarely cause the flow to rise much higher than the bottom of the pipe. As you know when the flow is low in the pipe the hydraulic radius is low and causes a lot of major loss which slows velocity. I have been around a lot of existing storm drain pipes and have seen them half full or more of sand.

How do you as other engineers handle this?

I have one more question. Is my understanding of the hydraulic elements chart correct when I believe it says that the velocity of gravity flow water in a pipe under just full conditions is the same as when the pipe is half full.

thanks,
CivilG, P.E.

 

[TerryScan]

I work in a coastal area. We need to provide full flow velocity slope even if our design is never full. It is often not feasible to go steeper as one has limited fall from site elevations to an outfall elevation.

While these pipes will not be naturally self cleaning, the consolation is that one can hopefully flush sediment manually if sedimentation becomes a problem as long as there is sufficient flow in the pipes with induced flow.

 

[cvg]

a lot of things can affect the operation of the storm drain, the flow / velocity standard is just a minimum requirement. but you cant possibly consider them all.

few things to consider

design velocity is based on design n-value. What is the actual n-value?

the velocity is based on clean water, presumably you will have a mixture of water and sand. which will almost certainly have different flow characteristics.

it is based on the geometry of the pipe, not assuming that the bottom is full of sediment which may not be moving.

most sediment is carried into the pipe under the peak flow conditions, it drops out when the flow begins to drop off

3 fps is just barely able to convey sediment, is that enough?

what is the velocity when the drain is surcharged? does it clear out the sediment? so if the capacity is reduced due to sediment and it then becomes surcharged, does the velocity increase enough to unplug it?

 

[beej67]

There's an interesting little feedback effect at work here.

The more a pipe silts in, the lower its cross sectional area of conveyance. The lower its area of conveyance, the higher its velocity. The higher its velocity, the more silt is washed out. So in a way, all pipes are self cleansing at a certain depth of sedimentation. That's not to say that pipes can't get clogged, but it's still an interesting little byproduct of the math. Nature works towards equilibria in mysterious ways.

I wouldn't worry about it too much if I were you. It's not uncommon where I do most of my work to have a 3 ft/sec minimum design velocity, but also have a pipe size minimum that makes that 3 ft/sec unachievable.

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

http://www.campbellcivil.com

 

[Spartan5]

Your understanding of the hydraulic elements chart is correct.

What's your 10 year storm intensity vs. your 1 year storm intensity? In my area (mid-atlantic) the 10 year storm is less than twice the 1 year storm. So if you're getting 3 fps at full during the 10 year storm, you'll exceed that with a pipe slightly more than half full with the 1 year storm. What's your design storm intensity?

Also take a look at the NOAA Point Precipitation Frequency Estimates to get a better handle on the design intensity vs. intensities of more frequent, but shorter duration storms. Select your area and select Precipitation Intensity under Data Type at the top of the page to see what I am talking about:

http://hdsc.nws.noaa.gov/hdsc/pfds/

Here's a random example: