Abstract
Abstract. Remote sensing analysis is routinely used to map flooding extent either retrospectively or in near-real time. For flood emergency response, remote-sensing-based flood mapping is highly valuable as it can offer continued observational information about the flood extent over large geographical domains. Information about the floodwater depth across the inundated domain is important for damage assessment, rescue, and prioritizing of relief resource allocation, but cannot be readily estimated from remote sensing analysis. The Floodwater Depth Estimation Tool (FwDET) was developed to augment remote sensing analysis by calculating water depth based solely on an inundation map with an associated digital elevation model (DEM). The tool was shown to be accurate and was used in flood response activations by the Global Flood Partnership. Here we present a new version of the tool, FwDET v2.0, which enables water depth estimation for coastal flooding. FwDET v2.0 features a new flood boundary identification scheme which accounts for the lack of confinement of coastal flood domains at the shoreline. A new algorithm is used to calculate the local floodwater elevation for each cell, which improves the tool's runtime by a factor of 15 and alleviates inaccurate local boundary assignment across permanent water bodies. FwDET v2.0 is evaluated against physically based hydrodynamic simulations in both riverine and coastal case studies. The results show good correspondence, with an average difference of 0.18 and 0.31 m for the coastal (using a 1 m DEM) and riverine (using a 10 m DEM) case studies, respectively. A FwDET v2.0 application of using remote-sensing-derived flood maps is presented for three case studies. These case studies showcase FwDET v2.0 ability to efficiently provide a synoptic assessment of floodwater. Limitations include challenges in obtaining high-resolution DEMs and increases in uncertainty when applied for highly fragmented flood inundation domains.
Highlights
Flooding is the most destructive natural disaster on Earth
The mean difference in floodwater depths, calculated by averaging the raster values of the [Floodwater Depth Estimation Tool (FwDET) v2.0 – model] map algebra expression, is −0.16 m with a standard deviation of 0.29 m, meaning that FwDET v2.0 slightly underestimates floodwater depth
The mean difference in water depth estimations by FwDET v2.0 and the hydrodynamics model are small, the difference map (Fig. 2d) reveals a heterogenous tapestry of values, some of which are quite considerable and many with sharp transitions, not readily apparent in the water depth maps (Fig. 2a and b). They are attributed to FwDET v2.0 reliance on nearest boundary cell elevation to calculate water depth
Summary
Flooding is the most destructive natural disaster on Earth. About 100 000 people lost their lives due to floods in the last decade of the 20th century (Higgins et al, 2014). The highest loss proportion of the global insured catastrophes in 2017 (USD 144 billion) came from Hurricanes Harvey, Irma, and Maria, resulting in combined insured losses of USD 92 billion (Swiss Re, 2018). Of the global disasters between 1994 and 2013, 43 % were floods, affecting approximately 2.5 billion people (CRED, 2015). S. Cohen et al.: Floodwater Depth Estimation Tool (FwDET v2.0)
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