Augmenting Operational Flood Forecasting by Assimilating Satellite Earth Observation-derived Flood Probability Maps into a Large-Scale Hydrodynamic Model

The event of Hurricane Harvey generated a high-frequency and high-resolution stream of satellite EO data over several weeks. Rapidly interpreting the obtained satellite imagery provided critical information on the large-scale flooding that was unfolding. This study presents an application of a recently introduced near real-time flood forecasting system that makes use of regularly acquired satellite EO data to keep model predictions on track. We take advantage of satellite EO data processing algorithms enabling an efficient and fully automated generation of flood probability maps using sequences of Sentinel-1 Synthetic Aperture Radar (SAR) images acquired during the flooding event. Given a SAR image of backscatter values, it is possible to decompose the total histogram of backscatter values into probability distribution functions of backscatter values associated with flooded (open water) and non-flooded pixels, respectively. These distributions are used to estimate for each pixel, its probability of being flooded.

We further demonstrate that the sequential assimilation of these data sets into the 2D LISFLOOD FP hydraulic model of the Colorado River (Texas) can substantially reduce the uncertainty of the flood extent and water elevation predictions over subsequent time steps. The flood probability maps are assimilated into the model using a variant of the Particle Filter. The particle ensemble of flood extent maps coinciding with the satellite overpasses was generated by propagating rainfall uncertainty through the flood forecasting system. The results of the assimilation experiment are evaluated using independent optical satellite imagery as well as water level recordings from gauge stations distributed along the stream.

We show that the proposed approach holds promise for improving flood forecasts at locations where observed data availability is limited, but satellite coverage exists.