Floatation of a Subsurface Detention\Retention Bed

[ropotto]

Hi All,

I am currently working on Subsurface Detention\Retention Bed that is located on property in the 100-yr. floodplain.

The reviewing Engineer has requested calculations for assurances the facility is designed to resist floatation during a 100 yr. storm event.

Please can anyone advise me or point in right direction.

Thanks

 

[WL123]

Tell us a little more about the Detention/Retention bed. What is it made of? Cross-section?

If the weight of the material(empty) and soil above it are greater than the water that displaces it, it will not float. Also, if it is perforated all around and vented, it should not float, depends on how fast the flood plain rises, plus it will probably fill with water prior to the flood plain rising and won't float anyway.

But it would be helpful to know more about it construction.

 

[beej67]

Start with Archimedes principle and determine the upward force from flotation by calculating the volume of water displaced and multiplying that by the weight of water. Then calculate an equivalent weight of concrete and put that in the bottom of your design as an anti-flotation block, if necessary. Add in a factor of safety.

My first question, though, is whether you're supposed to be detaining for the 100 year storm. Obviously you can't detain for the 100 year storm in the 100 year flood plain, because of backwater. If your design storm is a lesser storm, then are you sure you're out of *that* floodplain with your system?

 

[ropotto]

I found the attached document in the Technical Notes on the ADS Pipe Website.

http://www.ads-pipe.com

It covers the underground pipe systems that are subject to flotation and how to proper install and/or anchor pipe in these situations. This document provides an analysis on minimum cover heights required to prevent pipe flotation for HDPE pipe.