Storm Sizing Criteria

[tbonebanjo]

The National Standard Plumbing Code requires main roof drains to be sized for 1 hour 100 year storm. The secondary drainage must be sized per the 15 minute 100 year storm, which is about three times as much flow.

If there is an areawell with drains discharging to a pump in the basement, should the pump be sized based on the 1 hour storm or the 15 minute storm? In my case, the 1 hour storm is 130 gpm and the 15 minute storm is 310 gpm.

 

[Artisi]

For such a wide flow rate I would suggest 2 pumps minimum and level control in the collection pit to start the pumps at different levels of inflow - you could have a common low level for shut down or phased level if you want. Also the pumps could be set for alternating starts.
You also neeed to address pump maintenance - don't expect the pumps to sit round for months on end doing nothing and then come online during a major down-pour - the recipe for a flooded basement.

 

[TrippL]

Not sure about the Standard Plumbing Code, but the International Code requires Secondary Roof/Storm drain systems to discharge to a visible location in order to alert occupants to a problem with the storm drain system. Also, it is required to be piped completely separately of the primary system. Piping the secondary system to the same pumps as the primary does not seem to be a completely separate system. The Secondary System is an emergency drainage system that keeps the roof from filling with water and possibly collapsing the structure (not good). MAKE SURE! you design the Secondary System with this in mind. It is potentially a life safety system!

Read the code!!!, National Plumbing Code does require the same things as International Code in most places.

 

[lma1]

The BCPC in our Province (British Columbia, Canada) only requires us to use the 15-min duration 10 –year event data for roof and on-site drainage and in Metro Vancouver, the RI (rainfall intensity) is about 10 mm (0.394”). Comparing with the 1-hr figure it is about 1.5 to 2.5 times more.

If any storm water needs to be pumped around the building, we calculate the load based also on the same RI.

 

[TrippL]

I want to make clear that the Secondary roof drainage system is an emergency system and is required to be routed to a noticeable location, separate from the primary system, that will call immediate attention to a problem. The secondary system should not depend on a pumping system!

 

[tbonebanjo]

I think my question is generally being misunderstood.
The worst case for designing an areawell drainage system which is being pumped (nothing to do with primary/secondary issues) is the 15 minute storm. In this event, for the area I have involved, there would be 310 gpm peak flow using a 15 minute storm. The code does not specifically address design for an areawell. It will never collapse because it is basically a hole in the ground lined with concrete, and therefore does not require a secondary drainage system. Technically, by code, it could be designed for the one hour storm which is only 130 gpm for the area I have involved. It seems to me that sizing for this storm data would create a potential for flooding since the 15 minute storm data is greater flow and there is not enough storage capacity in a basin for the volume over 15 minutes (310 gpm x 15 min = 4650 gal). If I size per the code for roof drainage, the pump would be 130 gpm so it could pump out 1950 gpm in 15 minutes, leaving 2700 gallons of excess water to flood the basement. I don't want to oversize the system if not necessary, but I also do not want flooded basements on my projects. It seems necessary to size for the 15 minute data. Has anyone given thought to this?

 

[TrippL]

tbonebanjo,

Ok, I think you are referring to an "Areaway" not "areawell", corrrect? I'm picturing all of your roof drain water discharging into an open well in the basement. Obviously a bad idea.

Is there any possible way to drain this outside of the building either to a storm water sewer system or some type of cistern pumping system? I think it is a bad idea to pipe storm water into a building and relying on pumps to get it out. That is asking for the basement to become an indoor swimming pool. If you do have to pipe into the building you do not want the system to be open. I would think you would want a sealed sump with pumps designed to expel the maximum 310 GPM. I recommend having more than one pump as Artisi suggested.

Sounds like the invert of the storm sewer system in the area, if there is one, is way too shallow. Have you discussed addressing this from the civil infrastructure side? Such as providing a deeper storm sewer with a pump station. Seems this would be the ideal solution, and if there are going to be multiple buildings with this problem I would push it even harder. Get the water out of and away from the building as quickly as possible. Asking for major problems otherwise.