Storm Water Subsurface Drain Issues

[nstessen]

I am currently working on a project of a newly constructed building with a ground-level deck flooding during heavy rain events. This area has some walkway surface-level drains that drain into stormwater plumbing. Then along the walkway edges, there is also a 10" wide strip of gravel along the concrete curb with a 10" subsurface drain in the concrete slab below the gravel. There are high points on either side of the subsurface drain acting like a trough. I am concerned this gravel above the drain in the trough is impeding the flow into the drain and could be causing the flooding of this area. Could this be the case and what would the stormwater infiltration rate be through the gravel? Stormwater and plumbing is not my specialty so any help is appreciated!

 

[cvg]

gravel strips / french drains are not generally suitable for handling the high peak runoff from rain and roof runoff. infiltration rate through the gravel is not the problem. the problem is volume of water which quickly fills the trough and cant flow away fast enough

 

[oldestguy]

The gravel idea is fine when it works. However in time there can be leaves, dirt, etc clogging the voids. I'd look at a trench drain with a grate on top. Take a look at the drain in the center of a garage maintenance area where you take your car for servicing for what I am talking about. Use a big trough.

 

[nstessen]

That is very helpful thank you. Is there any guidance or specification for the type/size of stone that should be used in this situation since it technically falls under the building code and was done by the landscape architect? Would this be considered a type of french drain even through it is simply a sloped trough with a drain in the middle covered by gravel?

 

[oldestguy]

As to spec for gravel, I'd check with your local ready-mix guys and they may have stocked some, such as uniform between 3/4" and 1", or maybe 1" - 1-1/2". Sieving out a closer size probably can be done by changing their screens and that will cost..

 

[cvg]

open graded / uniform graded crushed rock will have larger void space and will flow better. it will however also plug with sand, silt and debris eventually. long term drainage performance of the gravel strip will not be good. a properly designed pipe or trench drain will have a lot more capacity and less likely to plug up