ChaseH
Civil/Environmental
- May 2, 2017
- 4
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.
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.