Release 2.0

The Dual Drainage Storm Water Management Model (DDSWMM) Manual is a comprehensive reference document on the model. It is comprised of three chapters and three appendices.

Chapter 1 introduces and explains the dual drainage principle, i.e., the two separate and distinct networks a) a minor or convenience system, formed by the storm sewers, and b) a major system, which is formed by the streets. Storm inlets, which intercept surface runoff, represent the main link between the major and the minor systems. Features of the dual drainage system, which are considered in DDSWMM, include inlet control, that is,the limitation of flow access into the sewers for the purpose of limiting sewer surcharge, and separate storage facilities for the minor and major drainage systems (dual storage).

DDSWMM is explained in Chapter 2. The model has four main sub-components a) surface runoff sub-model, b) major system sub-model, c) inlet sub-model and d) minor system sub-model. In addition, DDSWMM has sub-models for the design/analysis of storage facilities for the purpose of runoff control. DDSWMM can be used for either the design of new urban drainage systems or the evaluation of existing ones. As a design tool, it will determine pipe sizes, storage requirements for runoff control, flow depths on the streets, inlet control requirements, etc. Although the model was initially designed for urban drainage systems which follow the dual drainage principal, it has the facility to simulate existing systems, including the simulation/analysis of depression storage at street sags.

DDSWMM is also designed such that it allows an interface with the EXTRAN Model, either as a replacement of, or in addition to, DDSWMM minor system simulation. The DDSWMM-EXTRAN interface offers the user the facility to analyse the minor system using EXTRAN, with inlet flows calculated by DDSWMM. This allows the user to take advantage of the many useful features of EXTRAN such as the simulation of looped sewer systems and sewer surcharges.

DDSWMM is fairly flexible which allows application to a variety of practical design/analysis situations. Many features which are not explicitly included in the model such as roof storage, roofs directly connected to storm sewers, etc. can be easily handled by DDSWMM.

A number of examples to illustrate the application of the model are given in Chapter 3.

A detailed description of the input data is given in Appendix A. The flow routing technique employed in DDSWMM for the minor and the major system networks is presented in Appendix B. It uses a numerical solution scheme of a specially-weighted finite difference formulation of the kinematic wave model with discharge-dependant wave celerity. The solution is shown to give reasonably accurate results while maintaining the efficiency of the calculation.
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