Draining over 725 square kilometers, the Rio Grande de Loiza’s watershed is the largest in Puerto Rico. During flood conditions, the lower 14 kilometers of the river spills over its banks and spreads out in multiple directions, flooding an area of over 100 square kilometers. It is a challenge to use traditional mapping and modeling techniques to accurately depict this flooding process. As part of the Risk Assessment, Mapping, and Planning Partners (RAMPP) joint venture project with URS and ESP Associates, we met these specialized needs by applying detailed two-dimensional unsteady flow study methods to compute new flood elevations, floodplain boundaries, and floodway, as well as completed new rainfall-runoff analyses to provide the necessary inflow hydrographs.