What is a "Catchment" or "Sub-catchment"?

Catchments are defined in CD 535 as “A group of all the drainage systems and the adjacent land they drain, between two geographical high points of the highway network.”.

Sub-catchments are defined in CD 535 as “The separate carriageways of a catchment, used primarily for flood risk assessment.”

Catchments and sub-catchments are a re-segmented version of the road network model, with the catchment boundaries being based on high points on the road network. Each catchment includes one or more sub-catchment, usually so each carriageway is a separate sub-catchment. Each catchment is associated with one Area:

Catchments and their sub-catchments are defined within GDMS and are read only.