A HYDRAULIC MULTIMODEL ENSEMBLE FRAMEWORK FOR VISUALIZING FLOOD INUNDATION UNCERTAINTY 1 Christopher M. Zarzar , Hossein Hosseiny, Ridwan Siddique, Michael Gomez, Virginia Smith, Alfonso Mejia, and Jamie Dyer 2 ABSTRACT: While deterministic forecasts provide a single realization of potential inundation, the inherent uncertainty associated with forecasts also needs to be conveyed for improved decision support. The objective of this study was to develop an ensemble framework for the quantification and visualization of uncertainty associ- ated with flood inundation forecast maps. An 11-member ensemble streamflow forecast at lead times from 0 to 48 hr was used to force two hydraulic models to produce a multimodel ensemble. The hydraulic models used are (1) the International River Interface Cooperative along with Flow and Sediment Transport with Morphological Evolution of Channels solver and (2) the two-dimensional Hydrologic Engineering Center-River Analysis Sys- tem. Uncertainty was quantified and augmented onto flood inundation maps by calculating statistical spread among the ensemble members. For visualization, a series of probability flood maps conveying the uncertainty in forecasted water extent, water depth, and flow velocity was disseminated through a web-based decision support tool. The results from this study offer a framework for quantifying and visualizing model uncertainty in fore- casted flood inundation maps. (KEY TERMS: flooding; uncertainty mapping; risk assessment; decision support system; ensemble modeling; uncertainty analysis.) Zarzar, Christopher M., Hossein Hosseiny, Ridwan Siddique, Michael Gomez, Virginia Smith, Alfonso Mejia, and Jamie Dyer, 2018. A Hydraulic Multimodel Ensemble Framework for Visualizing Flood Inundation Uncer- tainty. Journal of the American Water Resources Association (JAWRA) 1–13. https://doi.org/10.1111/1752-1688. 12656 INTRODUCTION Floods are one of the costliest natural disasters in the United States (U.S.) (Pielke and Downton 2000). Each year floods are responsible for numerous weather-related fatalities alongside billions of dollars in damages to properties (NWS 2015). Effective and efficient flood preparation efforts rely heavily on the accuracy and communication of forecasted flood risks. Forecasted flood inundation maps are typically based on deterministic model output generated using a hydraulic model forced with a single discharge output from a hydrologic model (e.g., Merwade et al. 2008; Grimaldi et al. 2013). These deterministic flood inundation maps are subject to uncertainty due to imperfect forcing, initial conditions, model parameteri- zations, and numerical limitations (Merwade et al. 1 Paper No. JAWRA-17-0040-P of the Journal of the American Water Resources Association (JAWRA). Received April 13, 2017; accepted April 24, 2018. © 2018 American Water Resources Association. Discussions are open until six months from issue publication. 2 Department of Geosciences (Zarzar, Dyer), Mississippi State University, Mississippi State, Mississippi, USA; Department of Civil and Environmental Engineering (Hosseiny, Smith), Villanova University, Villanova, Pennsylvania, USA; and Department of Civil and Environ- mental Engineering (Siddique, Gomez, Mejia), Pennsylvania State University, University Park, Pennsylvania, USA (Correspondence to Zarzar: chris.zarzar@gmail.com). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION JAWRA 1 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION AMERICAN WATER RESOURCES ASSOCIATION