As the discussion indicates, both SWMM and MOUSE have dynamic hydraulic engines (in that they solve the full Saint Venant equations of gradually-varied, unsteady flow). For SWMM the engine is EXTRAN and for MOUSE the engine is MOUSE HD (hydrodynamic module). They differ in the numerical techniques used in the hydraulic engines: EXTRAN uses an explicit technique while MOUSE HD uses an implicit technique. Speed is the basic issue here with implicit being faster, generally. One reason MOUSE has a different solution technique (and other time- saving techniques) is to better accommodate continuous simulations which seem to be the norm in many countries.

Regarding the spiking discussion: MOUSE does not use any forced or artificial smoothing techniques. The bulk of the reason that EXTRAN and MOUSE HD seem to perform differently in the same situation is the difference in numerical solution techniques (explicit vs. implicit).

Both have the ability to model hydrology. SWMM does this with the RUNOFF block and MOUSE has an add-on module (NAM). RUNOFF and NAM share some approaches and differ in others.

In general terms (not knowing the specifics of your application), both will model combined sewers with storm overflow chambers.

MOUSE, developed by the Danish Hydraulic Institute in 1985 and subsequently improved, has several additional capabilities that have been developed largely in response to European efforts. These include the ability to model sediment transport and deposition in collection systems, ability to model water quality components in-system, ability to model Real Time Control techniques, and integration with receiving water quality and wastewater treatment works models. MOUSE also includes an add-on module to integrate with Geographic Information Systems (MOUSE GIS). MOUSE has a performance track record on par with SWMM having been applied on thousands of projects over the past 14 years (DHI has over 1000 licensed users of MOUSE worldwide).

DHI's user interface for MOUSE is called MIKE and includes extensive graphical pre- and post-processing capabilities to build model files and interpret model results. With the database and presentation capabilities available on the desktop today, the user interface for hydrologic/hydraulic models is increasingly important to the process of modeling. This is where proprietary models such as MOUSE have excelled over the last few years.

As Chris Ward alluded to, Camp Dresser & McKee is collaborating with DHI to develop a full version of their MIKE interface for SWMM which will include the GIS interface capabilities of MOUSE GIS. This product, MIKE SWMM, will be formerly introduced later this month (at WEFTEC) and will be available in December. MIKE SWMM will use the public domain version of SWMM and will be a full 32-bit interface (Windows 95&NT). We think this provides the best of both worlds for users who would like a state-of-the-art interface for the public domain version of SWMM as well as seamless linkage to MOUSE. CDM will continue to support and enhance the public domain version of SWMM as we have in the past.

Regarding the debate of proprietary vs. public domain engines: I propose that people on both sides of this issue are going to have to agree to disagree. European interests decided a long time ago that engines and user interfaces could be proprietary and that users would decide promote the best models with their purchase (survival of the fittest). North America has always viewed the engines as something to be advanced in the public domain, although user interfaces and model variants (XP) are often proprietary.

The applicability of a particular model depends on many things specific to the user and proposed application. A decision regarding public domain vs. proprietary must be made while weighing other factors and needs. The only universal truths are that the model must be proven (both MOUSE and SWMM are) and the model developed must be calibrated and verified.

Robert W. Brashear
brashearrw@cdm.com