With the capability to penetrate cloud cover in extreme weather conditions, Synthetic Aperture Radar (SAR) is more suitable for flood mapping in comparison to the optical sensor. In this paper, we develop a series of processing operators covering the steps of flood mapping using Sentinel-1 Ground Range Detected (GRD) images. Operators are published as OGC WPS services to facilitate interoperability and are further chained as a geo-processing workflow for automatic flood mapping. The workflow includes five steps: data access, preprocessing, flood detection, post-processing, and map production. The experiment in the response of the flood event of Poyang Lake demonstrates the availability. With the automatic execution by the scientific workflow engine, the manual labor is greatly reduced and the Sentinel-1-based flood mapping is effectively improved.