I NEED TO DETAIN ABOUT 12 CFS WITHIN A SUBSURFACE PIPE SYSTEM.

[cmm1080]

How do I calculate the storage volume required if I have:

a. my impact, in cubic feet per second, to the site due to development
b. my proposed hydrograph

I NEED TO DETAIN ABOUT 12 CFS WITHIN A SUBSURFACE PIPE SYSTEM. I'm just not sure which time I need to compare that rate to in order to calculate the storage volume necessary.

Thx in advance.

 

[pinkpig]

assume a volume or (area-depth relationship), route the flow thru this storage, to see if the outflow meets your requirements;if not, get another number;any hydrology and hydraulic software should be able to do this for you quickly.

 

[TerryScan]

The time would be your entire length of storm event (you proposed hydrograph). You need to perform a routing calculation of your hydrograph through your storage to an outlet (whether that is infiltration or a control structure). Whenever inflow is greater than outlfow in your routing...storage is needed. Your maximium storage does not necessarily occur at the same time as your peak inflow rate(I assume this is the 12 cfs you speak of). So, the most significant item you have not provided in your problem is the outlet.

 

[cmm1080]

I have calculations for the design of detention/retention basins, but not sure how (or if) I can apply the outflow parameters for a subsurface system.

For a particular portion of the development, I have 13.596 cfs within a pipe. I'd like to decrease this flowrate to about 1.50 cfs before outfall. I have about 10,000 sf to work with under the parking lot, but also know that area will be less due to pipe spacing for adequate compaction.

My peak flow time is at 12.403 min (corresponding to the 13.596 cfs) with a flow end time of 61.449 mins.

 

[gbam]

cmm if you have out flow hydrograph(s) for your system, plot it and draw a straight line from the start to 1.5 cfs on the recession limb. the area between your line and the top of the hydrograph is the minimum storage volume you need. Then you set your underground system with a weir at the outlet. Same procedure as in an open basin. Depending on your governing agency you may or may not be able to use infiltration as a portion of detention. Have fun with it.

 

[beej67]

If you're doing this by hand, here's the process:

Pick an outlet control that gives you your target discharge at your target head. I suggest an orifice.
Generate a stage-discharge relationship from your chosen outlet control.
Lay out a sample pipe system.
Generate a stage-storage relationship for your pipe system.
Derive a Storage-Indication Curve, like they taught you in hydrology class. (you have had that, right? if not go back to school for a semester, or buy a textbook and teach yourself)
Use the SIC to route the inflow hydrograph through the facility.
See if it pegs out the facility. If so, add more pipes and recalc your stage-storage relationship.



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

http://www.campbellcivil.com