STORM SEWER SIZING

[rcspiel]

I have been asked to show the 12" gravity storm sewer on site can handle the 4" gravity drain tile discharge around a building. Does anyone know if this is possible? Storm sewer is sized per rational method for the 10yr storm event. Drain tile around building is used for subsurface waters(specifically 2.5' below finished floor for waterproofing drain board against foundation) which in my opinion is very variable and does not discharge at the same time as the storm event. Any thoughts?

 

[cvg]

it is unlikely that it will handle the foundation drain flow during a storm but would the rest of the time. it may back up into the drain tile and saturate your foundation. probably not a good idea.

 

[Pinwards]

How can you say it's unlikely without knowing pipe slopes, storm intensity, depth to groundwater, soil type and transmissivity, etc? There are a lot of variables, your Geotech may be able to help come up with a guesstimate on flow rate to expect from the foundation drains. If it's just seasonal high water, then the rising groundwater should drain out gradually via the foundation drain as surrounding groundwater levels rise. If you have proper grading around the structure, and with gutters on the roof properly piped to downspouts/leaders that drain to a storm system or other disposal (so that roof runoff isn't contributing to your groundwater issue) then the foundation drains might function very well draining to a 12" storm drain with adequate slope. I had nearly that exact situation on a project and the foundation drain is functioning quite well draining to a 12" pipe, although we also had the help of a cut-off drain upgradient that intercepts some of the groundwater flow before it ever reaches the foundation drains.

#

 

[bimr]

The backfilled soils around the building will be more porous than the native soils so your comment "does not discharge at the same time as the storm event" is suspect. Of course, you can add clay to the backfilled soils to prevent this.

I agree with CVG that it is good practice to minimize the possibility of a storm sewer backing up into the foundation drain.

It is a simple exercise to show that there is additional capacity in the 12-Inch storm sewer for full flow from a 4-Inch pipe.

 

[beej67]

"How?"

Add the ten year rational flow to the predicted flow from the drain tile, and check that Q in your 10 inch pipe. If you don't know the flow from the drain tile, calculate the flow in a 4" pipe at capacity and use that.

Or am I not understanding your question?

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