Above-ground manholes still need to be vented to lessen corrosion due to the presence of hydrogen sulfide gas and to lengthen the life of the sewer system
by letting oxygen into the pipes.
Additionally, the inflow report* by the Neenah Foundry Company shows that even a splashing rain on a 24” manhole lid with one just 1” hole lets in 4.34
This means that a town that has 400 manholes that are raised above ground can still expect an inflow of 416,640 more gallons of water during a four hour
rainstorm. A city with 2000 manholes can expect 2,082,000 more gallons.
The more storm water you stop, the less you will pay for pumping and treatment as well as repairing flooded homes and cleaning up SSOs.
* Neenah Foundry Company, “A Report on Inflow of Surface Water through Manhole Covers,” date unknown. http://www.nfco.com/municipal/engineering-literature-videos/pdf/report-inflow-brochure.pdf. Accessed April 10, 2014.