The manhole cover seal process from Inflow Seal is dependent upon the weight of the cover and a clean seat on the frame. If debris enters this seat, the cover is lifted, creating a gap and allowing surface water to flow at often more than 30 gpm into the manhole. Manhole cover gaskets, bolt down covers, or dishes can help, but they are still dependent upon a clean seat. Manhole covers are heavy, and every time you remove and install the cover, you bring debris into the frame seat. A solution is to install a vertical seal on the frame that seals out the inflow and up to 3/8 inch of debris in the frame seat. 800-490-5325; www.inflowseal.com.