This paper describes a sensor-based system for monitoring of sewer flooding. A global trend in Smart Cities is the installation of sensors for surveillance of the city infrastructure. The presented system is mounted in a number of strategic sewers for initial evaluation. In order to monitor the natural variations of the sewer (including daily variations) the system should be able to integrate into normal monitoring/alarm systems. The communication channel and mounting position (sewer well in the ground) put limitations on the system.