Drain and Sewer Flooding

Primary reference(s)

Priestly, S., 2016. Sewer flooding. House of Commons Library. London. Accessed 30 October 2020.

Additional scientific description

Drain and/or sewer flooding is a condition where wastewater and/or surface water escapes from or cannot enter a drain or sewer system and either remains on the surface or enters buildings. Drains and/or sewers can also undergo surcharge which is a condition in which wastewater and/or surface water is held under pressure within a gravity drain or sewer system but does not escape to the surface to cause flooding (Document Center, 1995).

The United Nations Educational, Scientific and Cultural Organization (UNESCO) Intergovernmental Hydrological Programme (IHP) Urban Water Series, comprising a set of books on urban water management, addresses fundamental issues related to the role of water in cities and the effects of urbanisation on the hydrological cycle and water resources. Focusing on integrated approaches to sustainable urban water management, the Series provides valuable scientific and practical information for urban water practitioners, policymakers and educators throughout the world (UNESCO, 2011).

Water governance is usually highly decentralised, with overall responsibility held at the level of local government. However, there are still countries where the federal level or the state plays a key role in day-to-day operations (UNESCO / ARCEAI IdF, 2016).

Metrics and numeric limits

Not identified.

Examples of drivers, outcomes and risk management

Urban flooding caused by severe pluvial rainstorms has been reported for an increasing number of locations throughout the world (Grüning and Grimm, 2015; Schmitt and Scheid, 2020). For many years the design of urban drainage systems had been based upon concepts of design storm frequencies. Surface runoff and sewer flow values have been computed for site‐specific rainfall intensities provided by national weather services or databases (IDF curves) and required sewer pipe slope and diameters have been selected. Little consideration had been given to the possible occurrence of sewer flooding and resulting damage, in general and in terms of site‐specific factors of flood hazards (Schmitt and Scheid, 2020).

In 1970, the United Nations identified three megacities. This number rose to 10 in 1990 and 28 in 2014. According to projections, there will be 41 megacities by 2030, many located in the world’s least developed countries. Throughout history, these cities have often lacked both the time and the means to develop their urban services, including those relating to access to water, sanitation and rainwater drainage. This situation creates profound vulnerabilities and complex challenges. It is crucial that megacities share their experiences, to develop services capable of meeting the expectations of their inhabitants (UNESCO / ARCEAI IdF, 2016).

The study of hydrological extremes should consider that humans are part of the hydrologic system, both as agents of change and as beneficiaries of ecosystem services. While there have been significant advances in coupling hydrological and biogeophysical models over the past decade, these advances remain inaccessible to resource management decision-makers and other water sector professionals. With most river basins being no longer ‘natural’ as humans live and interact with the continuously changing hydrological system, there remains a need for better understanding of the coupled human-ecosystems (UNESCO IHP, 2019).

Sewage contains harmful microorganisms such as bacteria, viruses and protozoa. This contributes significantly to the spread of diseases such as typhoid and cholera and may increase the likelihood of contracting worm infections from soil contaminated by faeces. Flooding itself may displace populations and lead to further health problems (Howard et al., 2002). The greatest danger is not the risk of disease, but the risk of electrocution or explosion. People need to avoid entering a flooded basement or lighting matches until the utility companies have shut off the gas and electric service.

Flooding in urban drainage systems as may occur at different stages of hydraulic surcharge depending on the drainage system (separate or combined sewers), general drainage design characteristics, as well as specific local constraints. When private sewage drains are directly connected to the public sewer system without backwater valves, the possible effects of hydraulic surcharge depend on the levels of the lowest sewage inlet inside the house (basement), the sewer line, and the water level during surcharge, respectively. Whenever the water level in the public sewer exceeds the level of gravity inlets in the house below street level, flooding inside the house will occur due to backwater effects. In such a case flooding is possible, as well as surface flooding. In the same way, hydraulic surcharge in the sewer system might produce flooding on private properties via storm drains, when their inlet level is below the water level of the surcharged storm or combined sewer. In both cases, the occurrence of flooding, being linked directly to the level of inlets versus water level (pressure height) in the sewer can be easily predicted by hydrodynamic sewer flow simulations, assuming the availability of physical data for the private drains and public sewer system (Schmitt et al., 2002).