Coastal erosion and flooding remain unknown hazards and elected officials and coastal managers may have difficulty discussing these phenomena with the general public. However, a better knowledge of the evolution of these hazards, through citizen science, can help raise awareness of coastal dynamics among the general public and thus meet the current and future challenges of coastal management. During the OSIRISC project (2016-2020), funded by the Fondation de France, coastal managers wanted to set up a smartphone app to save time during their field surveys, while allowing citizens to perform some measurements. How to create a citizen science app on coastal hazards that is reliable and intelligible and that guarantees the sustainable support of participants (professionals and non professionals)? Within the framework of the observatory of coastal risks in Brittany (OSIRISC), protocols for measuring hazards 1) with common tools (benchmarks such as the size of one's finger, qualitative data with photographs) or cheap tools (tape measure, laser distance meter...), 2) applicable to all types of coasts (beach, cliff…) have been co-designed with the managers and integrated into the android app called CoastAppli. Co-funded by EUR ISblue (the interdisciplinary graduate school for the blue planet), Sea-Eu and Interreg AGEO projects, CoastAppli is currently being tested for 6 months (until April 2022) in the municipality of Guissény (Brittany, France) by grade school students, coastal managers and citizens. The feedback from this test will allow us to know: i) the reliability of the measurements; ii) whether the app meets the expectations of the coastal managers by saving time for data collection in the field; iii) the app ability to be understood and educative; iv) the sustainability of the commitment of the citizens. These results are essential to validate the application before a future deployment that is intended in Brittany, then in France and beyond.