SubCatchment Generator

  • Details

  • Contact

  • Documents

IMA are focused on delivering tangible benefits for clients and there is nothing we like more than a being set a challenge.  Subcatchment generator is exactly that, a challenge laid down when someone said “this is an art rather than science and cannot be automated”.

The result is an automated tool which set out to automate a process 80%. Results have shown the tool has achieved 95% success and has significantly reduced manual involvement in the creation of subatchment polygons.


Need: When building a sewer system network model (e.g. For Drainage Area Plans) one of the most time consuming and difficult task is defining the catchment areas otherwise known as subcatchments and the percentage of impermeable surfaces  within them.

Challenges: The task of creating subcatchments involves the division of complex network catchment into smaller manhole based subcatchments. A complex set of rules is used to define divisions and determine impermeable areas. This can be an extremely time consuming task when carried out using traditional manual methods.

Solution: A toolbox was created which uses set inputs and applies hierarchical set of business rules to generate model ready subcatchments. Analysis is undertaken to build a subcatchment based upon all roofs, roads, paths, property boundaries and any other semi/ impermeable areas within a manhole drainage area. Subcatchments are automatically measured and codes assigned creating a ‘network model ready’ file


  • Time savings of >80% – the model rather than the modeller generates subcatchments
  • Improved modelling –more detailed catchments providing a better representation of run-off
  • Consistency – Judgement is removed so all catchments based on the same rules
Key benefits

The traditional approach is to hand digitise each subcatchment, picking out boundaries and making decisions as you go. A single subcatchment may take a minute or two to create. That might not sound like much but some catchments may contain 10,000 subcatchments. The 1 to 2 minutes soon add up to weeks, months or even years. This level of expenditure means models go unbuilt or once built are not kept up to date. This impacts the models effectiveness

  • Time consuming to the extent it prevents the task being done
  • Boundaries based on judgement so not always correct / the same / understandable

Subcatchment generator takes moments to set up and is then left to run without user intervention. Boundaries are formed based on a complex rule base relying on OS MasterMap to make decisions.

  • Requires virtually no manual input significantly reducing costs
  • Boundaries based on rules ensuring the same decision is made each and every time



Location Analytics Capability Sheet
Key features

Subcatchment generator is a tool build to answer a seemingly impossible question. The tool was built and has been used to generate over 1 million subcatchments for clients in the UK at a fraction of the cost to do this manually


Network modelling, hydraulic modelling, problem solving, geospatial tool building