Introduction to Water Quality Modelling with SWMM

SWMM models in MIKE+ can model both the quantity and quality of runoff generated within catchments, as well as the flow rates, depths, and water quality in pipes and channels over a simulation period.

Water quality routing within conduits assumes that the conduit behaves as a continuously stirred tank reactor (CSTR). Although a plug flow reactor assumption might be more realistic, the differences will be small if the travel time through the conduit is on the same order as the routing time step. The concentration of a constituent exiting the conduit at the end of a time step is found by integrating the conservation of mass equation, using average values for quantities that might change over the time step such as flow rate and conduit volume.

Water quality modelling within storage unit nodes follows the same approach used for conduits. For other types of nodes that have no volume, the quality of water exiting the node is simply the mixture concentration of all water entering the node. The production and fate of pollutant loads associated with runoff is also modelled.

The following processes can be modelled for any number of user-defined water quality constituents:

• Dry-weather pollutant buildup over different land uses
• Pollutant washoff from each land use during storm events
• Direct contribution of rainfall deposition
• Reduction in dry-weather buildup due to street cleaning
• Reduction in washoff load due to BMPs
• Entry of dry weather sanitary flows and user-specified external inflows at any point in the conveyance system
• Routing of water quality constituents through the pipe/channel network
• Reduction in constituent concentration through treatment at junction nodes or in storage units.