Stone Outlet Design
Stone Outlet Design
(OP)
Hey All,
I have a project where the State Regulator is requiring a a Forebay for a large basin. The Forebay has to completely dewater in 72 hours (Township Code) and there is Zero Percolation on the site. The Forebay has been designed to accommodate 1" of runoff from the contributing area. In order to completely dewater the facility, I have the floor of the Forebay sloped at to a stone outlet as per guidance from the State regulator.
Now the Question is... How to I design the stone outlet? I'd like to route the 100 year storm through the stone outlet assuming a void ratio of 40%. My first thought is to model it like a rectangular orifice that goes from the invert to the top of the stone with the dimensions of the orifice equal to 40% of the surface area of the entire stone spillway. Does this approach seem to be acceptable?
Another option is to install and underdrain system with some amended soils on top and assume a percolation rate. However, when the client heard underdrains, they say too many dollars and prefers the stone outlet.
Thanks in advance.
I have a project where the State Regulator is requiring a a Forebay for a large basin. The Forebay has to completely dewater in 72 hours (Township Code) and there is Zero Percolation on the site. The Forebay has been designed to accommodate 1" of runoff from the contributing area. In order to completely dewater the facility, I have the floor of the Forebay sloped at to a stone outlet as per guidance from the State regulator.
Now the Question is... How to I design the stone outlet? I'd like to route the 100 year storm through the stone outlet assuming a void ratio of 40%. My first thought is to model it like a rectangular orifice that goes from the invert to the top of the stone with the dimensions of the orifice equal to 40% of the surface area of the entire stone spillway. Does this approach seem to be acceptable?
Another option is to install and underdrain system with some amended soils on top and assume a percolation rate. However, when the client heard underdrains, they say too many dollars and prefers the stone outlet.
Thanks in advance.





RE: Stone Outlet Design
RE: Stone Outlet Design
BTW....I directed this same question to the regulators and they can't seem to answer the question. They responded without really answering.
RE: Stone Outlet Design
RE: Stone Outlet Design
https://en.wikipedia.org/wiki/Darcy%27s_law
RE: Stone Outlet Design
RE: Stone Outlet Design
I just talked to the regulator on the phone and I have to provide a discharge analysis for the site while under construction. Somebody must have upset the regulator. Anyway, I have a embankment sediment trap with a stone outlet that I also need to route. I suppose I could go back and re-design the trap with a riser outlet but I guess this goes back to my original problem. I asked the regulator, a licensed P.E., how she would route a storm through a stone outlet and she told me I'm the engineer not her. Gee Thanks.
I'm already providing 1.5 times the water quality needed for the State requirement as a result of the Township. the township doesn't care about the forebay but the State regulator wanted it for some reason. I'm getting a ton of pressure from the client about costs. I really don't want to show the client a bigger forebay than what was discussed at the meeting with the regulator.
RE: Stone Outlet Design
Isn't there a state E&S Manual that tells you what they are looking for in the sed trap design?
If you're crunched for time, maybe just throw a skimmer in there during the E&S phase and call it a day.
RE: Stone Outlet Design
Yea there's an manual that provides the design parameters but it only tells you how determine the dimensions of the spillway. They don't give any information beyond that.
Thanks all. Great answers all around.
RE: Stone Outlet Design
RE: Stone Outlet Design