Introduction to Rainfall-Runoff Modelling with SWMM

MIKE+ provides a versatile set of tools and computational models for modelling surface storm runoff and infiltration on urban and semi-rural catchments. The User can quickly prepare a precipitation-runoff model setup of desired level of detail (in terms of spatial discretization and input data) and use the computed runoff as a load to the collection network.

Various hydrological processes that produce runoff from urban areas can be modelled by SWMM. These include:

• Time-varying rainfall
• Evaporation of standing surface water
• Snow accumulation and melting
• Rainfall interception in depression storage
• Infiltration of rainfall into upper soil layers
• Percolation of infiltrated water into groundwater layers
• Interflow between groundwater and the conveyance system
• Nonlinear reservoir routing of overland flow

Spatial variability in these processes is achieved by dividing a study area into a smaller collection of homogeneous areas. In literature, different terms are used to describe the delineation of large study areas into smaller sub-areas each containing its own fraction of pervious and impervious areas.

The following terms for the delineation of areas are widely used in North America:

• Basin
• Sub-basin
• Watershed/Sewershed
• Catchment

Throughout this documentation the term catchment will be used as the term for the smallest hydrological unit of land.

Overland flow can be routed between catchments or to load points of a drainage conveyance system.