Estimating floodwater depths from flood inundation maps and topography

Remote sensing analysis is routinely used to map flood inundation during flooding events or retrospectively for planning and research activities. Quantification of the depth of floodwater is important for emergency response, relief operations, damage assessment etc. The Floodwater Depth Estimation Tool (FwDET) calculates water depth based on topographic analysis using standard GIS tools within a Python script. FwDET's low input requirements (DEM and inundation polygon) and high computational efficiency lend it as a useful tool for emergency response and large-scale applications. Operational use of FwDET is described herein as part of emergency response activation of the Global Flood Partnership (GFP) during the 2017 USA Hurricane Season and May 2018 flooding in Sri Lanka. Use of FwDET during Hurricanes Harvey (Texas and Louisiana), Irma (Florida) and Maria (Puerto Rico) demonstrated its utility by producing large-scale water depth products at near-real-time at relatively high spatial resolution. Despite FwDET's success, limitations of the tool stemmed from bureaucratic disallowance of non-governmental remote sensing products by U.S. federal emergency response agencies, misclassified remotely sensed floodwaters and challenges obtaining global high resolution DEMs specifically for the aforementioned Sri Lankian flooding. While global-scale DEM products at 30m resolution are freely available, these datasets are of integer precision and thus have limited vertical resolution. This limitation is significant primarily in flat (e.g. coastal) locations and flooded domains comprised of relatively small patches of water.