New Findings in Estimating Flow Rates Through Overflowing Manhole Covers

When sewers overflow through manhole covers, the flow rate of the overflow is expected to depend, to some extent, on the head loss through the partly open manhole cover. In this study, the flow rate and head loss was measured through several manhole covers with different weights and number of holes to determine that relationship between head loss and flow. While it is common to model an overflowing manhole as a weir or orifice, this experimental study found that this approximation may not be very accurate. Instead, it was shown that once sufficient head is provided to marginally lift the cover from the casting rim, flows can vary dramatically with only a minor change in head. Experimental results are provided in this paper along with some discussion on the implications for hydraulic models.