Enhancement of Urban Floodwater Mapping From Aerial Imagery With Dense Shadows via Semisupervised Learning

Timely and accurate mapping of floodwater in urban areas from aerial imagery is critical to support emergency response and rescue work. However, massive shadows cast by buildings and trees over dense built-up urban areas can cause a significant underestimation of flood outcomes, and few studies for flood monitoring explore this in current state-of-the-art approaches. Meanwhile, recent deep learning (DL) algorithms have reported superior performance in flood mapping over conventional machine learning methods. Nevertheless, acquiring a large amount of training data remains challenging in the DL paradigm. In this study, to exploit the potential of the DL algorithm in detecting all visible (including shadowed and nonshadowed) floodwater with limited training samples from aerial imagery, we designed a modified fully convolutional network and combined it with a deep semisupervised learning framework integrating consistency regularization and RandMix strategy into the floodwater detection workflow. Besides, the test-time augmentation technique was leveraged to improve the model performance in the evaluation phase. Extensive experiments on the 2013 Calgary flood demonstrated the effectiveness of our approach on extracting visible floodwater in urban areas with dense shadows. Notably, our method could accurately detect more floodwater with a largely reduced number of labeled training samples, which is a considerable enhancement in the applicability and availability of DL algorithms for flood monitoring in densely shadowed urban areas.