Sediment pond - Am i using doing it the good way?

[SMIAH]

I recently had to design a couple of sediment pond for the construction of an highway (major project). The basin will be let in place after the construction phasis and supposedly maintenance will keep up after that (I have some doubts).

Used the commonly-used formula for sizing basin area of As = 1.2Q/Vs. Assuming Q as the average storm water flow (actually used some criteria written in Erosion and sediment control handbook, Goldman, 1986). Vs for a 0.02 mm particle and some other criteria for the shape of the pond.

I decided to go for a complete retention of a 2-year runoff volume flood (routing a 3-hour duration rainfall through the pond). This to maximize the sedimentation process of most flood. Then used a perforated riser for the evacuation of this volume and a weir for a 25-year flood aswell as providing an additional emergency weir for 100-year flood.

Voila.

Obviously, some sed pond are quite large (and I didn't try yet with Vs associated to clay).

Now, last week I had a meeting with another Engineer working for another firm (They are building ponds on this project too but not using this method). Instead, they are using a pre-shaped pond which is way smaller than what I've recommended so far (it looks more like a sediment trap to me as the outlet is a granular berm which is already clogged from what I saw).

Thing is, I just hope that I didn't "overkill" it and now I have to back up the method choosen so I don't look like a fool in front of a client who sees so much difference between two consultants on the same project.

Am I using a good method? Am I missing something?

 

[francesca]

What you're using is what's required in Tennessee for drainage areas >5 acres. What they're using is acceptable for <5 acres... BUT, as you said, theirs is already clogged, which will be a maintenance nightmare during construction to remain in compliance with their SWPPP. (The SWPPP should require clearing it out when it's at 50% capacity, with inspections weekly/after every rain event leading to it being cleaned pronto.) Yours will function well and probably won't need much maintenance during construction.

 

[SMIAH]

Thanks, will also take a look at Tennessee Erosion & Sediment Control Handbook.

 

[beej67]

What you're using sounds like what's required in Georgia for 1 acre and above.

The whole thing boils down to Stokes Law, which is a highly questionable design assumption in clays, but that's a rant for another day.

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