Blockage of cross-drainage hydraulic structures (e.g., bridges, culverts) is reported as an exacerbating factor during flash flooding in Wollongong and Newcastle. Lack of data from flooding events and the fragmentary nature of post-flood data are the factors hindering research in studying the impact of blockage on the performance of hydraulic structures. This paper proposes lab-scale simulations using scaled physical models of culverts to study the behaviour and effects of urban and vegetative debris. The first investigation studies the interaction between specific debris types with culvert inlet geometries and their impact on the hydraulic blockage. In the second investigation, a flood hydrograph is simulated in the laboratory to study complex relationships between blockage-related influential factors and to relate the observed visual blockage and hydraulic blockage. From the results of first investigation, urban debris was reported the main contributor in increasing the hydraulic blockage at structures. Furthermore, the degree of hydraulic blockage was found sensitive to the orientation of the debris. Results from the second investigation reported several insights regarding the complex relationships between blockage-related influential factors. The temporally variable nature of blockage was observed from the experiments that suggested revising the existing constant blockage based Australian Rainfall and Runoff (ARR) guidelines.