Basements in residential buildings are often subject to flooding. It is common practice to connect basement drainage by gravity to the stormwater system which means that if there is any surcharge caused by capacity constraint, constriction, blockage or partial blockage in the system downstream of where the basement drain connects, then the basement is at risk of flooding. This surcharge can lead to water backing up in the drainage pipe and entering the basement.
The flood risk associated with basement drainage is well recognized anecdotally; an internet search will return thousands of sites. Basement drainage problems are associated with property damage, loss of basement contents and damp houses which have associated health risks.
Basement flooding can occur in any house if the basement drainage system is not designed or constructed appropriately. There also seems to be a lack of regulation and guidance for basement drainage.
Based on Australian standards and design guidelines, complying solutions to providing safe basement drainage include:
- Untrapped floor drains
- Surcharging outlets
- Reflux valves
- Installing a basement sump and pump
Basement drainage is best addressed at the time of preparing a stormwater plan for a development but retrofit solutions may also be required. Where a basement drain is connected to the stormwater system, redevelopment of a site or modification of the stormwater pipes may increase flood risk. If the catchment area is enlarged because of increased roof or paved areas, the risk of surcharging will increase. A basement drain that previously had acceptable performance may fail unexpectedly.
Retrofit solutions are likely to be complex and costly. Sealing the existing floor drain and installing a sump and pump may be feasible but is likely to require indoor excavation works. Similarly installing a reflux value may require exposure of the existing under-floor plumbing. Enlarging drainage capacity downstream of the basement drainage connection to the stormwater system may also be an option.
The most cost effective way of reducing this risk is to provide an appropriate solution at the time of development that prevents surcharged stormwater entering the basement.
For more information see our paper: Ladson, A. R. and Tilleard, J. W. (2013) Reducing flood risk associated with basement drainage. Australian Journal of Water Resources 17(1): 101-104 (abstract) (accepted paper)