The Dual Drainage Storm Water Management Model (DDSWMM) is a comprehensive model which can be used either for the design of new urban drainage systems or the evaluation of existing ones. It represents a new generation of its predecessor, OTTSWMM which has been widely used in Canada since the mid-1980's. It has also been in use in the U.S., Europe and South America. DDSWMM represents a complete overhaul of OTTSWMM. It was first released in March 1996, and has undergone many improvements in the current release 2.1.

DDSWMM is a fairly flexible and user-friendly model which can be applied for separate or combined sewage systems under many largely varying flow conditions, from the extreme 1:100 year storms and larger, to the common design storms involved with the common design of urban drainage systems. It is fully compatible with the various versions of the EXTRAN model. DDSWMM is accompanied with a detailed reference manual which explains both the theoretical formulation and the practical application of the model, with detailed input data description and examples.