Large Flow into Sump

[ChaseH]

First, thank you in advance for all those who take the time to answer these questions. Y'all really help out.

I'm a civil engineer who is designing a pedestrian tunnel under a roadway and I have to incorporate a sump pump to remove the water that will collect in the low point of the tunnel. The issue I am having is my flow (Q) is becoming too large when I use the intensity from my time of concentration calculation. Givens are as follows:

C = 0.98 (concrete sidewalk)
TC = 5 minutes
I for 10 yr event, 5 minute storm = 7.34 in/hr
A = 0.27 acres
Q = CIA = 0.98 * 7.34 in/hr * 0.27 acres = 1.942 cfs = 871.6 gpm

This was much too large (according to my local pump guy, I honestly had no idea as this is my first time handling pumps). When we change the intensity to the 10 year, 24 hr storm (I = 0.25 in/hr), Q = 29.7 gpm, which he states is much more reasonable.

My question is this: Is the second application, using the 24 hour storm intensity the more correct method for design? If so, why?
We believe it to be for the simple reason that this flow is much more in line with what he has experienced in the past and we really only will need a small pump to drain the tunnel after storm events. The rest of the design we have under control, just looking for someone to help justify our reasoning.

 

[LittleInch]

This isn't strictly a pump problem and you may get better responses in the civil or water treatment forums, but I think you're taking a very simplistic view of the issue and ignoring the time taken for the rain to actually arrive at the sump, the hold up amounts on the sidewalk, in the pump sump etc, then you're looking at a much smaller flowrate. Volume might end up being the same, but you don't need to look at it as instantly hovering up the entire rainfall.

I think your local pump guy is more realistic - the issue for me would be long periods of heavy rain where sure, over time, the flowrate of the water you need to get rid of will equal what is falling out of the sky.

however this is really an input question and I'm no expert on storm drainage so can't really say which is best.

For pump out conditions for such a location you might be better off with two smaller pumps, say 20gpm and if level rises above a certain amount, kick both off.

For such a variable flow you'll never get the right pump, but if it's too big it will cycle on and off for a less intense storm, but too small and the tunnel will flood. Hence some sort of variability of flow would be good.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ChaseH]

Thanks for the comment. I'll post over in the civil forum as well.

 

[EdStainless]

You made a mistake in your calculation, it isn't 7"/hr, it is 0.6" in 5 min.
This is a balance between sump size and pump capacity.
The idea of 2 x 20-25gpm with staged trips is very good, it also gives you a backup.
You use sump size to buffer for the very intense short term events.

0.98 x 11800 sq ft x 0.05 ft of rain = 578 cu ft = 4325 gal in 5 min
If you can store this water while pumping out at a reasonable rate then you should have a plan.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[LittleInch]

The calculation, despite the mix of units, looks correct as an instantaneous flow rate, however that is just never going to translate to an actual required flow rate given the limited time span and exceptional cloudburst nature of 7.5 inches per hour.

I don't think anyone expects any system like an underpass tunnel to cope with that and at worst, what is the size and depth of the puddle?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Dustin Maki]

I was agreeing with LittleInch, then his question "what is the size and depth of the puddle?" made me think. You will not have a static puddle. You will have a high volume and rate of flow over a pedestrian surface from both ends toward the center, and the larger your sump pump, the worse the man trap flow rate becomes. We are answering the wrong question, or at least we need to ask a second question and choose the smaller pump between the 2 answers.

Assuming infinite sump capacity and no pump, given tunnel dimensions, what flow rate into the sump will knock pedestrians off their feet? It might be significantly less than your calculations, and there is no point going larger than that, because now the tunnel is no longer a safe pedestrian space and the entire tunnel becomes the sump.

Consider the floor design and purpose of this tunnel. Is it critical infrastructure that must remain open and useable during a 10 year storm, or is it possible to restrict access when conditions exceed safe limits?

If it is a critical tunnel, consider the entire concrete floor the sump area and specify a raised grate walkway. Some other constraint will determine the maximum height of that walkway. You choose your pump size to keep the top of it dry in your worst case instantaneous flow.

I have walked through concrete floor pedestrian tunnels exposed to runoff and after a few years it is difficult to walk on because the surface gets coated in perpetual slime puddles full of oil, insect larvae, and dirt.

Dustin Maki