100 year Detention Pond Design

[dabearsfan78]

All,
If I have a site designed with a detention pond sized for a 100 year storm event and the pipes discharging to the pond sized for the 10 year storm event can I account for additional storage in the storm sewer pipes up the the High Water Level? The site is short storage volume after the pond was built, can this extra storage in the pipe be accounted for or is there an issue due to the pipes being used for the 10 year storm? I was thinking they are different events and in the 100 year situation the flow is expected to get to the pond by overland flow and I can count the extra storage. Any thoughts on this would be appreciated. Also any back up text, manuals, websites, etc. to proove to a review engineer would be appreciated as well. Thank you.

 

[SMIAH]

2 things :

Peak Flow and Runoff volume.

Even if you're pipes are designed for a 10-year peak flow, the runoff volume of the 100-year storm will get to your basin.
There will be an attenuation of peak flow. You should consider that in your analysis.

If you want to take in account the storage in pipes, you have to use a rainfall-runoff model with an unsteady flow analysis. This might be an overkill.

 

[gbam]

dabearsfan78 - you should check with your local reviewer/agency as they will have final say on whether they will allow your storm sewer system to be included in the total available storage. I have ran into some that do and some that don't. There is typically no problem with the sizing of the pipes for the 10-year vs the 100 as long as you show the R/O reaching your basin.

 

[beej67]

I don't necessarily agree with SMIAH's first statement.

If your pipes are designed for the 10 year storm, and the 100 year storm hits your site, then it's quite possible that some of the flow is going to happen over land, instead of through the pipes, on its way to the pond. And depending on how your site is graded, it's quite possible that some of that flow will miss the pond entirely, and bypass stormwater management, by jumping a curb or heading out into the street / etc. Most municipalities in the south east require you to show how the 100 year storm gets to your pond. This can simply be by showing the 100 year HGL in the pipe profile to prove it doesn't daylight and makes it to the pond via pressure flow, and by showing the 100 year ponding limits at inlets. In some areas (Durham NC for instance) I've been required to produce a plan showing how the 100 year storm would flow to the pond over land if the entire pipe system was clogged. I consider that to be a little extreme, but they wanted it so I gave it to them.

The easy way to take credit for the storage volume available in your pipe system is simply to add the pipe volume to your pond volume. For most open pond applications, the difference is going to be negligible, and not really worth including. If you're using a complicated unsteady model such as XP-SWMM then that volume may already be included in the analysis whether you choose to include it or not, so don't double count it in the nodes if you're already showing the pipes submerged in the model. This may not be true in simpler models.

In Florida, engineers quite often take credit for the storage volume not only in the pipes, but in the parking lot and in landscaped areas of the site. As you can see on TV, the whole south half of the state is a bit like one big detention pond.


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

http://www.campbellcivil.com

 

[SMIAH]

Even if water bypass stormwater management, the 100-year runoff will get, one day or the other, to the pond (?)
Of course neglecting infiltration, total bypass (in another watershed!?) and permanent detention somewhere else.

Considering that the pipes might "choke" the 100-year peak flow, I would check the effect of an attenuated flow (e.g. 10-year peak flow but volume of the 100-year runoff) in the pond.
No?

 

[SMIAH]

"showing how the 100 year storm would flow to the pond over land if the entire pipe system was clogged"

Why not showing the effects of the storm hitting meanwhile there's an earthquake. Little extreme you say!

 

[beej67]

Well, the point is if your stormwater jumps the curb of your parking lot and gets out into the street, then it never goes through your detention pond at all, which means your hydrologic routing is based on a false assumption.

I'm working on a project right now, in fact, where the reviewer is requiring me to do a model of the roof drains on a flat topped roof, to prove that the 100 year storm is in fact caught by the roof drains and doesn't partially bypass the underground detention vault by flowing over a scupper. Can't say I've ever encountered that one before, but it's what they want, so we give it to them.



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

http://www.campbellcivil.com