Modern sewers carrying storm water are designed to carry the 2-year storm event without surcharging, and not to cause flooding in a less than 30-year event, i.e. in any year, there is a 3% chance of the sewers flooding. Older sewers are generally constructed to a lower standard, and are often combined sewers, carrying both foul and storm water.
Older sewers were generally constructed with Combined Sewer Overflow structures (CSO). The Urban Wastewater Treatment Directive issued by the EU has directed that these overflows should be eliminated where possible, and the spill frequence of those that remain should be reduced. This means that either the capacity of the sewers must be increased, or additional storage must be provided within the sewerage network. In either event, more water is conveyed to treatment.
The analysis of these networks is undertaken using hydraulic modelling as part of a Drainage Area Study. A great deal of work has been carried out, and is currently underway in assessing these overflows throughout the country.
The problem remains at present that we have a legacy of sewers, particularly in the densely urbanised regions, designed many years ago to lower standards, which have been progressively overloaded by continuing development in the upstream catchments as towns have expanded. Tens of thousands of properties remain at risk of flooding from overflowing sewers.