Flooding from Public Sewers
Introduction
Flooding can also be caused by the sewer system. In Fingal, the majority of the sewer network is a separate system. This means that there are two separate networks, one for surface water and and the other for foul effluent. Surface water would consist mainly of runoff from roofs, hard standing areas, driveways and gardens. These sewers normally outfall to the sea, nearest river or ditch. Foul water typically consists of flows from baths, toilets, dishwashers and trade effluent. The would drain to the nearest treatment plant and then be discharged to a watercourse.
The sewer network can be further divided into two types, public and private. Fingal County Council provides and maintains the public sewer system. Private sewers do connect into the public sewer, however the responsibilty of maintenance remains with the owner. The sewer pipes connecting individual houses to the public sewer are classed as private.
Surface Water Sewer Flooding
The development of catchments results in a increase in surface water runoff. In the past the surface water sewer network was designed to direct the runoff to the nearest watercourse with little consideration of the downstream impact . More recently, in order to alleviate the effects of the accelarated runoff associated with hard standing areas and roofs, large developments are installing attenuation features. These facilities are designed to slow down the runoff and help prevent downstream flooding.
Localised flooding from the surface water network normally occurs as a result of defects in the network, associated with blockages or collapsed pipes. Streams that have been culverted will have trash screens at the inlet in order to prevent debris becoming lodged in the culvert. The trash screens are important as most culverts are very difficult to access and the removal of internal blockages is problematic. These are maintained along with the rest of the public network by the operational staff of the Council. If defects are noted they should be referred to the relevant Area Engineer (See Section on Contact Names and Numbers).
Installation of an underground attenuation tank, Swords
Foul Sewer Flooding
Flooding from the foul sewer can occur as a result of a blockage in the pipes or insufficient capacity due to high inflow and infiltration of surface water. The foul network is designed with a margin of safety to cater for surface water ingress. During extremely heavy rainfall however, the sewer can still be overwhelmed, consequently overflows are incorporated which divert the excess flow to rivers or other watercourses. Typically, severe foul flooding is normally restricted to the trunk sewers and individual houses are infrequently affected.
When flooding does occur in houses, it is normally associated with basements (with a toilet or bath at a low level) connected directly to the foul sewer. During periods of extreme rainfall, the foul network can surcharge and as a result it is possible that flows can back up through toilets and plugholes.
When the local network connecting houses to the public sewer floods, it would more than likely be as a result of blockages caused by fat/grease, paper/cloth, siltation or tree root ingress. The onus rests with the house owner to have these cleared as it is in a private sewer.
When defects are noted in the public sewer, they should be referred to the local Area Engineer for action. See the Contacts Section for Names and Numbers
After a foul flooding incident it is important that the area is properly cleaned and sterilised as it can present considerable risk to public health.
Foul Sewer Flooding
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