Detention Pond Inside Floodplain Zone AE

[edilpena]

I'm designing a detention pond for a 8 acre project that part of the site lies on the limits of the AE Zone, but outside the delineated floodway of a river. Of course the lowest point on my site and the only one suitable to establish the pond is right inside the very limits of the floodplain. The problem is that the top of my pond will be 1.6 meters below the BFE. Rising the top of the pond is not an option, because that will cause to rise all grading the same amount, too expensive plus the hydrologic implications to the floodplain.

The question is, it is correct the assumption that it doesn't matter that the pond is on the floodplain? Because anyway the pond will be controlling all storm events from 1 to 100 years, because the time of concentration of my basin is very small in comparison with the river basin, so during the event the pond will peak and will be at full capacity (100yr volume) before the flood reaches, the pond area.

It is correct to assume that? If yes, how do I justify it without a full floodplain analysis witch is completely out of my scope of work.

 

[cht13er]

Let's say the river flows from west to east.

What will happen when a slow-moving storm event travels towards your site from the west? I'm proposing that even if the time of concentration of your basin is very small in comparison with the river basin, you may not be correct in assuming that during the event the pond will peak and will be at full capacity before the flood reaches the pond area.

 

[edilpena]

Ok! thanks, I appreciate your opinion, I understand what you are saying, let's say that the Storm approaches from either way, and that affects the river so the flood is traveling in front of the storm. Then the flood reaches the site before the storm. At the time the storm hits, my site is underwater, so, I don't need a pond at that moment in time. What you think?
The purpose of the pond is to prevent the excess runoff generated at the site to contribute to the flooding. When the flood reaches the site, then I don't need to control the runoff anymore. That is what I believe, but maybe I'm wrong. I appreciate your opinion.

 

[francesca]

Calculate the Tc on the river. Compare it to the Tc on your site. If the Tc on the river is >24 hours you're probably safe as detention calculations are based (usually) on a 24-hr storm.

Remember that there will be small, regional storms, close to your site that may have a small Tc to your site and generate substantial runoff. Find the size of the drainage basin that has a 24 hr Tc to your site and calculate the runoff peak flow added to the base flow in the river. Then calculate the water surface elevation and compare to your detention pond OUTLET elevation. If your detention pond outlet is underwater, then you have to size your detention pond using that tail water elevation. If it all works under those conditions then you're safe saying "it doesn't matter that I'm in the flood plain."

 

[JALeng]

The goal of detention pond is attenuate the pick discharge by means of detention volume caused by development. If we suppose that your detention pond will not within floodplain, but your outlet discharge to AE zone, once the WSE reach the BFE your pond will not mitigate the discharge.

Due to detention pond is within the floodplain you might verify that the proposed structure doesn't make an encroachment in the floodway.

See the following forum:

http://www.eng-tips.com/viewthread.cfm?qid=51034&page=56

 

[beej67]

It is not correct to simply assume that you'll still get detention in your pond without doing a hydrograph timing analysis of the flow in the river vs the stage in the detention pond. Francesca gives some good rules of thumb, but your reviewer is probably going to want to see an analysis showing it.

The first thing I would do in a situation like you describe, if this truly is a "river" as you state, is ask if they'll waive detention entirely. If the river is that big in comparison to your site, you're not really effecting the flood elevations in the river, so detention doesn't really do you any good. In fact, it might aggravate the issue by delaying your peak to coincide closer with the peak of the river.

I had one project, where we did two analyses, one with detention and one without, and showed a higher peak flow in the river with detention. Reviewing agency waived the detention requirement, and we got another two lots in our subdivision out of the deal.

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

http://www.campbellcivil.com

 

[amb2002]

I have had similar results regarding the two analysis approach that beej67 describes.

A good reference for this (IMO) is the Georgia Stormwater Manual, particularily this section:

http://www.georgiastormwater.com/vol2/2-1.pdf

Jump to section 2.1.9 on page 54.

 

[CivilEngMatt]

I have had several sites with this same scenerio here recently. My approach is as follows:

Step 1: Determine the time to peak for your site at the confluence with the stream. For small basins using the SCS method this is typically around 12 hours.

Step 2: Determine the time to peak of the stream.

Depending on where your site is within the stream drainage basin, you may be able to waive detention. I have found that 3 square miles is enough drainage area to allow detention to be waived. If the stream driange basin is large enough there will be enough lag time that will allow for your site to peak prior to the flood wave associated with the stream reaching your site. By utilizing a detention pond you may attenuating the flows so that your adding to the flood peak flow. Which is making the flooding situation worse.