Modeling catchbasins and inlets in SWMM

Early hydrologic/hydraulic numerical modelling efforts of urban drainage system performance focused on the minor system (i.e. storm sewer only). Over the past two decades (+\-) there has been an increased focus on concurrent evaluation of the conveyance capacity of the major system (i.e. overland flow – roadways). Increasingly, these two systems are being modelled dynamically, using a linked approach known as dual drainage modelling. Dual drainage modelling allows for an improved assessment of deficiencies in both systems, through integrated assessments which consider the interaction between the two systems.

Of key interest in dual drainage models is the representation of inlet capture functions (catchbasins and ditch inlets), since these intrinsically link the two parallel flow conveyance systems.As part of this presentation, various methods of modelling these functions are assessed under differing conditions, using ‘real world’ models developed for several urban communities. The relative benefits and drawbacks of each method are presented and discussed.The simulated results are also compared to available literature values on inlet capacity, including guidelines prepared by regulatory agencies.

The various approaches to inlet capacity modelling are compared to assess current issues in urban drainage system design. Differing regulatory guidelines for roadway catchbasin spacing are assessed and compared against simulated results, as well as standard assumptions with respect to flow capture. Further, the simulated performance of rear-yard catchbasins are also assessed based on actual project examples, and compared against original design assumptions and expected performance.