Spatial Morphology Dynamics Analysis of 3D Physical Hydraulics Model: Case study of Leprak River, Rejali Watershed, Mt. Semeru, Indonesia

This paper aims to analyse the performance of the 3D physical hydraulic modelled sabo-structures in controlling river bed deformations and how the model results may vary across repetitions, with the case study of sabo-structures in Leprak River, Rejali Watershed, Mt. Semeru, Indonesia in 1:100 scale, which includes 2 (two) diversion dams and a consolidation dam. The model was tested with 3 (three) constant discharges (5 l/s, 7 l/s, and 9 l/s), each for 600 seconds. The scenario of 9 l/s was repeated for 7 (seven) times, specified that the difference of river gully boundaries could vary between 1 cm and 10 cm (means 1 m to 10 m in the field scale). The modelled flood scenarios (considering the sabo-structures) had altered the river bed morphology, with the depth averages of gully-erosion from about 0.8 m (case 9 l/s) to 1.6 m (case 5 l/s) in the field scale. The larger modelled discharge (9 l/s) tends to bring more depositions than the lower ones, with the differences in the net deposition had reached from about 2×10 5 m 3 to 5×10 5 m 3 in the field scale. The hydraulic physical modelling of sabo designs is important for technical verifications, providing conceivable result variations for the decision makers. • Little was known about the effect of diversion dams and consolidation dams to the morpho-dynamics of lahar rivers. • The 3D hydrodynamics model was elaborated to mimic the characteristics of lahar flows in the study river located in Mt. Semeru, Indonesia. • The model provided technical perspectives on how the predicted lahar flow affects the sediment balance, accompanied by the possible uncertainties.