A field study of manhole cover leakage

Leaking manhole covers add inflow to sanitary sewer systems. These are the most accessible components of the sewer system, and so potentially the cheapest to repair or modify, to reduce inflow. There is, however, very little data regarding manhole cover leakage available to evaluate the cost-benefit of such an approach, and there is no field data. So, a multi-year field study of inflow was conducted. Two-hundred and fifty manholes in New York City and surrounding towns were tested. A key field observation in this study was that a dust and grit seal typically built up between the cover and the rim. This seal had a big impact on leakage rates. This made testing in-situ a necessity, and the test method had to preserve this seal. Due to the presence of the dust and grit seal, approximately 80% of the manholes tested showed no leakage. A further 15% had slow leaks. Only 5% had significant leaks.