The latest eruptive episode of Merapi volcano, Java, Indonesia, produced block-and-ash flows (BAFs) that affected the densely populated areas on the volcano's southern flank. The flows were not confined to the existing valleys but spilled over the valley sides to create overbank flows that resulted in fatalities and strong damages in the village of Kaliadem, -5 km away from the summit. This testifies the unpredictable behaviour of BAFs, not only at this volcano but also at similar locations around the world, and the need for an improved understanding of the flow transport and depositional mechanisms. In this respect, the new approaches, descriptive schemes and numerical simulations presented in this thesis address some of the key concepts of the dynamics and hazards of BAFs. The 2006 deposits were examined both immediately after flow emplacement and after the first rainy season following the eruption, allowing detailed correlations between their surface characteristics and internal architecture. Two main types of BAFs (short- to mediumrunout and long-runout BAFs) are recognized based on parameters including generation mechanism, flow volume, travel distance, deposit morphology, distribution, lithology and grain size distribution. The effects of topography on flow dynamics have been examined through the development of conceptual models for the two types of BAFs. Integration of high-resolution field-based data into numerical simulations using the Titan2D and VolcFlow models allows the validity of these models to be tested and rapid quantification of best-fit input parameters. Sensitivity analyses and inundation maps based on the probability of impact were used to produce a suite of potentially inundated areas from future collapse events affecting the Gendol valley and adjacent areas on the southern flank. Results provide the basis for defining hazard zonations of key areas at risk from BAFs at Merapi