Rainfall Duration for Design Purposes 1

[ONENGINEER]

I am designing a dry well. The local authorities just recommend to use a 10 year return period for rainfall intensity. As the intensity depends on the rainfall duration i.e. 5, 10, 15, 30, 60 minutes, 1, 2, 6, 12, 24, 36, 72 hours. Which of these duration should be used for a small residential project. Thanks.

 

[jgailla]

I'm with cvg, unless the regulatory agency specifically says to consider infiltration in the well. Even in this case, I would see where the water level backs up to if infiltration does not occur.
Also, I would check a 100 year storm without infiltration to see if any vital site elements are compromised.
In Florida, infiltration would not be considered during the storm, only for storage volume recovery afterwards.

 

[cvg]

a lot more retention storage can be achieved by grading a low area and then placing your rock pit below that. underground chambers or pipes can also be used for retention.

 

[ONENGINEER]

I have read in some city specs that the bottom of drywell should be 4 ft to 10 ft above the groundwater table. Would it not be that if the drywell were connected to the groundwater, it would have been connected to a large more secure aquifer with a high potential of water absorption. Or is the limitation because of the risk of rising groundwater/seasonal variations? Thanks

 

[cvg]

extending the extra 10 feet will not seriously increase percolation rate. however, contamination of groundwater with unfiltered stormwater would be more likely