Wastewater (sewage and stormwater) collection systems are vulnerable to damage and disruption from volcanic ash. Ash can enter sewer lines where there is inflow or infiltration through illegal connections, cross connections, gully traps, manhole covers and cracks in sewer pipework. Cities that have combined sewer and stormwater collection systems are particularly vulnerable as ash will readily enter stormwater drains and can then cross over into sewer lines.
In general, few effects can be expected for very thin ashfalls (<2 mm). At ash depths of >20 mm, effects may include severe intractable blockages and surface flooding which may require extensive repairs and/or maintenance. Factors affecting the severity of impacts include: