Lahars are fast moving mixtures of debris and water. They have been responsible for 40% of all volcanic fatalities over the past century. Development of numerical models capable of accurately simulating these complex debris flows requires a good understanding of sediment transport processes and flow mechanical properties. To provide constraints on these characteristics, we use hydraulic data measured in a river channel at Semeru volcano during real lahars. In addition, rheological tests have been carried out in laboratory to infer the rheological behavior of lahar constitutive material, while the hydraulic relationship between flow height and discharge in control sections is investigated. These elements are then implemented in a code relying on shallow-water equations to simulate the propagation of gravity-driven surges on real topographies and with real hydrograph inputs.