External drain overflow

An external drain overflow occurs when wastewater rises out of an outside gully, inspection chamber or manhole cover, usually because the drain downstream is blocked or surcharged. The visible overflow indicates the highest point at which the water can escape and provides a useful clue to engineers about where the obstruction lies. In Greater London, combined sewers serving Victorian terraces are particularly vulnerable during heavy rainfall, when stormwater overwhelms capacity and pushes foul flows back to the surface. Building Regulations Part H1 and BS EN 752 set out design standards intended to prevent surcharge in normal conditions. Engineers usually address common causes by rodding or jetting from the nearest accessible chamber, then carrying out a CCTV inspection. Reporting external overflow to Thames Water is advisable where a public sewer may be involved.