Monitoring sewer sediments is necessary to better understand sedimentation and erosion processes. Sonar is one of the available techniques to proceed to sewer sediment measurements. Extraction of numerical data, implementation of a new algorithm to identify the water-sediment interface, laboratory and field experiments have been done to evaluate the device, to quantify uncertainties and to test the sonar under various conditions. Results demonstrate that: 1) uncertainties in repeatability are less than 4%, 2) the sonar delivers accurate results under various conditions (small to large sewers and grit chambers), and 3) in situ measurements are affected by uncertainties, mainly due to the fact that the sensor is not in a fixed position but is floating on the free surface in the sewer. This device is useful and accurate for semi-automatic measurement but further research and improvements should be done to better know the position of the device in the section.