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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.

RE: Stone Outlet Design

I would just do a stage-storage that takes into account the 100% void ratio of the forebay and basin, then add the volume (40% voids) of the stone wick. Then the hydrographs will show the time to dewater.

RE: Stone Outlet Design

(OP)
I was thinking of modeling the forebay separately. The basin is so big, the forebay sits up high enough that the peak of the storm will occur before the tail water within the outlet becomes a factor. The invert of the forebay is 5 ft above the invert of the basin. Just looking at the stone spillway, I want to make sure it can handle the flow. However, there will be point when the basin fills up and the tailwater should be taken into account.

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

Can you through a small pipe at the outlet to meter the flow such that you achieve your detention?

RE: Stone Outlet Design

you say the forebay handles 1 inch of rainfall. then you say you are routing the 100-year through it. I assume that is much more than 1 inch. so the water rises to a level higher than the stone and then flows over the top. yes you could include some allowance in your basin volume to account for the void ratio of the stone, although that should be minimal. you could route flow through it but should assume the flow velocity through the rock is very low. not one orifice, many small orifices. More like Darcy's flow equation.

https://en.wikipedia.org/wiki/Darcy%27s_law

RE: Stone Outlet Design

Your most important design issue is designing the forebay water quality structure to meet its required detention time (12hr, 40hr) and drain in no more than 72 hours for the frequent water quality events. You can't do this with rocks.. you need an engineered outlet device placed within the rocks.

RE: Stone Outlet Design

(OP)
Does anyone have a reliable source on what the permeability of AASHTO #57 stone would be. I have some sources that have it 0.013 ft/s to 0.03 ft/s.

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

Flow through that stone is going to be negligible, and obviously is going to be dependent on length and width. You'll have infiltration working at a faster rate.

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

(OP)
When I use the maximum porosity of the the stone and multiply it by the length and width if a spillway designed according to the manual you find that, your right, the flow rate is negligible compared to the inflow rate. So I'll design the stone outlet for a two year event and design a spillway for the lower frequency storms.

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

The "porosity" of the stone will quickly clog shortly after construction. Need to design an outlet that is maintainable.

RE: Stone Outlet Design

(OP)
The outlet will be designed in accordance with the States sediment trap standard. Agreed that the stone will need to be removed and replaced but the maintenance will be no more than a standard sediment trap. Since this is an item that the regulator specifically requested, I am reluctant to submit anything different.

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