Detention pond - No Depth available 1

[SMIAH]

I have to design a sediment pond that will be also used as a retention pond (i.e. after construction 25-year peak flow < actual 25-year Qp).

The concept is similar to dry extended detention pond as the outlet is designed to detain the stormwater runoff from a 2-year storm for some minimum duration (riser, barrel and emergency spillway included to control major event).

Thing is that the available depth from the inlet to the outlet of the pond has now changed and is... of only 1 foot.

Is there a way to get through this? Like using a skimmer (similar to the one on the picture attached).

We are in a cold climate and the pond has to be a permanent measure.

Ideas?

 

[SMIAH]

Well, don't think that the skimmer will help for the 1 foot of depth.

So, any ideas?

 

[gbam]

Do you meet your required volume? What is your specific question regarding, the outlet works or the basin sizing?

 

[SMIAH]

How to design a sediment pond with 1 foot available from the inlet to the outlet... Not a sediment trap.

 

[msquared48]

Raise the site or lower the outlet. I see no other options.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

That is unless you can get around the .1 cfs difference limit in the 100 year storms, not have to detain, but only treat the water.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

OR... You could allow the parking lot to pond to a maximum depth of 6", maybe gaining a foot or two of head, depending on your site layout.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SMIAH]

So raise the site or lower the outlet. Or use pumps...

 

[msquared48]

Pumps could be an option but many municipalities frown on them in situations where the power goes out. They may be allowed though with automatic backup generators.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[beej67]

1) Where's the project located?

2) Sounds like you meant "detention pond" not "retention pond" ..? Presuming that's correct, your only recourse for lack of volume is find more volume. Exercise:

Print the post-development inflow hydrograph. Draw a line across it at your allowable discharge rate. Now draw another line from where it intersects the tail end of the inflow hydrograph back down to the origin of the hydrograph. Shade everything above the slanted line and below the hydrograph with a blue marker. If you do the calculus, the area under a hydrograph is a volume. The blue markered volume is approximately how much detention volume you need, to detain that hydrograph to that allowable discharge rate. If that volume isn't available in your pond, then you need a bigger pond. Raw science says that, there's no tricks you can use to bypass it.

Well, almost none. You could add a retention element to the detention pond to provide more peak discharge capture. Examples:

Your photo looks awfully flat, like Florida flat. I heard some rumblings that NFWMD (once it got going) was planning to allow folks to irrigate out of wet retention ponds, and if you can do a water balance calculation showing your irrigation draws the pond down to a certain level, then you might be able to justify some extra capture volume for your event storm.

Another idea, presuming your pond invert is above the seasonal high water table, is dig the pond out deeper and rig it to an infiltration system. Do the Darcy math to show that the retention volume is recaptured in a minimum time frame (usually 24 to 72 hours in the regs I've seen) and you can credit it as capture volume for your event storm.

Third idea is go underground with it. Stormtech chambers or an HDPE manifold or something, to catch your volume. You can make them cisterns too, again depending on a water balance calculation.

Someone mentioned parking lot storage - I've done that quite a bit in central and southern Florida. It helps that their maximum design storm is the 25 year, and they've all evacuated when it hits anyway.


As far as sediment ponds go, in any sediment pond sizing methodology that's based on Stokes Law (falling velocity of spheres) the volume turns out not to be the controlling factor for sediment trapping efficiency - the surface area is. That's unintuitive, but it has to do with length canceling out when you're calculating flow velocity across the pond. (if I recall correctly anyway, saw it in school)



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[SMIAH]

Thanks for the tips.

Actually. I did a couple of basin similar to this one here. Every of them had a drop from the inlet to the outlet.

I use the commonly-used formula of As = 1.2Q/Vs for the sediment pond design. And I try to be "somewhere near" the estimated surface.
As it's not possible to build a football field here (not using a 0.02 mm particle), I choose to completly detain the runoff volume associated to a 2-year event (calculated with HEC HMS and routing). The release is done via a perforated riser (and I have an overflow for a 10-year and an emergency for the 100-year).

Ok, everything was going pretty well before they dropped the inlet elevation. Now we have to dig the ditch at the outlet and this is not very Environmentally Friendly.

Infiltration... Hum. I'm not a fan but I guess I have to check this out.

 

[SMIAH]

1) Canada. Cold climate (to add a little challenge).

 

[beej67]

Yeah, Canada's definitely outside my comfort zone, both professionally and personally.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com