Restoration of the South Florida ecosystem is a major undertaking for the U.S. Army Corps of Engineers and the South Florida Water management District. The Biscayne Bay Coastal Wetlands (BBCW) Project is one component of more than 60 restoration plans and has a goal to restore or enhance freshwater wetlands, tidal wetlands, and near shore bay habitat. The primary purpose of the BBCW project is to redistribute runoff from the watershed into the Biscayne Bay, away from the canal discharges that exist today and provide a more natural and historical overland flow through the existing and/or improved coastal wetlands. In an effort to restore wetlands, several structures, and management plans and scenarios are considered. One of the plans is to deliver fresh water from the existing canals through a shallow spreader swale system that is to distribute fresh water through wetlands into the Biscayne Bay. To achieve this, a tool is needed to design this complicated shallow spreader swale system. This paper presents how a spreader swale system, which includes 1D canal network routing, 2D overland flow, 3D subsurface flow, and flow through the interface of any two sub-domains of the spreader system, is simulated with the WASH123D computer code. Brief physics-based mathematical statements and numerical strategies of the model will be given. A hypothetical example that uses topographic data for the project area will be provided to demonstrate how WASH123D can help the design of a spreader swale system. Some issues that concern the numerical convergence of the coupled flow model will also be discussed in this paper.