Puerto Rico’s Rio de la Plata River Basin is the island’s largest watershed. The river flows from the central mountain ridge northward and discharges into the Atlantic Ocean west of San Juan. Along its path lie a number of towns and other population centers. Rainy season stormwater flows cause the river to overflow and flood the coastal floodplain project area.In 2007, the Commonwealth of Puerto Rico (Commonwealth) contracted Taylor Engineering to redesign the project based on current natural conditions and land uses and to obtain a U.S. Army Corps of Engineers (USACE) permit for the project. Since the original design, long-term average rainfall had increased and agricultural drainage works keeping the floodplain dry had been largely abandoned. Taylor Engineering developed the engineering design and the successful USACE permit application, which included a supplemental environmental assessment (SEA) to assess potential impacts of channelizing the river. These included direct impacts of construction, potential impacts from additional freshwater moving downstream to nearshore coral reefs and potential movement of salinity upstream into freshwater wetlands.Preliminary design alternatives existed for this project. Taylor Engineering’s tasks included an engineering evaluation of several of these alternatives, and recommendations for improvements, if possible. Taylor Engineering conducted all the hydrologic, hydraulic modeling, flow-way and levee design analysis, stormwater analysis and design, construction design documents, and all USACE environmental analysis and permitting.Modeling efforts included an analysis of new rainfall data in the basin, provided by updated NOAA rainfall atlas, updating topographic data with LiDAR and field surveys, and historic data analysis. Modeling for the main flow-way, conducted in two phases, included 1- and 2-D modeling of the flow-way and 1-D ICPR modeling of inflow storwmater basins. Taylor Engineering modeled the stormwater drainage and designed flood control alternatives to safely pass the flows into the main flow-way. Taylor Engineering considered an array of hydraulic design alternatives in an attempt to maximize flood damage reduction benefits while remaining economically and environmentally sound.