Sump Pump High In-flow

[ChaseH]

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

I'm 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.

 

[psmart]

If you use the shorter duration you are basically selecting a pump that would be able to handle the peak runoff without any water accumulating in the sump. But if you are able to allow storage in the sump, then you would be able to use a smaller pump, since the sump will basically act as a detention pond and attenuate the peak flow. Using the 24-hour duration would be advisable only if you are able to store a considerable fraction of the 24-hour volume. A more detailed hydrograph routing analysis would be required to get a precise value.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[bimr]

Ed noticed that you made a mistake in your calculation, it isn't 7"/hr, it is 0.6" in 5 min.

 

[AJB1262]

I recently had some experience preparing a report which addressed basements flooding out. We used Hydrocad to model the basement sump pumps. We determined that a basic sump alone would not address the basement flooding issue.You will find that during a rainfall event even in a small drainage area a lot of water accumulates quickly. A sump pump may not be the best solution. You may want to look at ways to caputure or divert the water around the tunnel or provide enough storage capacity in a vault which the pump will eventually draw down. Using a 24 hr design storm may more closely simulate actual rainfall conditions. In a modeling package such as Hydrocad you can provide your own storm distribution or custom hydrogaphs which allows you to use historical precipitation data as a check to your design storm. For our project area we found that the rainfall distribution from tropical storm Lee closely approached the 10 year SCS design storm peak flows. Volume as well as flow rates must be considered.

 

[chicopee]

Bimr .6"/5min= 7.2"/hr. being close to 7"/hr.