Flood map (Wales) indicating flood risks from rivers, sea, reservoirs and surface water.
There is no risk in Blaenavon from reservoirs.  There is flood risk to areas from the Afon Llwyd, and some risks from surface water.

Sustainable Drainage Systems (SuDS) are designed to reduce the potential impact of new and existing developments with respect to surface water drainage discharges.
The idea behind SuDS is to try to replicate natural systems that use cost effective solutions with low environmental impact to drain away surface water run-off through collection, storage, and cleaning before allowing it to be released slowly back into the environment, such as into water courses. This is to counter the effects of conventional drainage systems that  can result in flooding, pollution of the environment with the resultant harm to wildlife and contamination of groundwater sources used to provide drinking water.