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.
Satellite gaging reaches are polygonal land areas encompassing river and floodplain reaches where total surface water area expands and contracts as river discharge varies. Traditional gaging stations measure water level, or stage, as a surrogate for discharge. Such stations are commonly located where discharge changes are primarily accommodated by stage and not width changes. In contrast, we identify for testing purposes 191 gaging reaches distributed worldwide where multi-temporal remote sensing demonstrates significant water surface area variability. Typical reach lengths are 30 km and reach widths average 10-30 km. The gaging reaches are sufficiently wide to accommodate the largest floods. Measured water surface areas using the MODIS sensor 250 m resolution bands are converted to river characteristic widths (water area/river length) and, in the U.S., are compared to adjacent gaging station data. Preliminary results indicate that, along many U.S. gaging reaches, MODIS-derived widths are robust predictors of discharge. As is the case for in situ gaging stations, local width/discharge relations vary, and some reaches record discharge changes with greater precision than others. Time series of MODIS river characteristic widths can provide hydrologists and water resource managers with a geographically extensive and economical river monitoring capability
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