There is considerable debate at present regarding appropriate advice on design blockages that should be assumed for various hydraulic structures in urban drainage systems. While a number of studies commissioned by Wollongong City Council in response to the widespread blockage of hydraulic structures during the 1998 major flood in Wollongong have developed criteria for the assessment of blockage for new hydraulic structures, these studies have not been universally accepted. Hence, there is a need to extend these previous studies and to validate their suitability so that appropriate guidance on design blockage for hydraulic structures can be developed. For the purposes of this project, hydraulic structures refer to culverts and small bridges over drainage channels (rather than major bridge structures) and to inlet structures (i.e. pits) to urban drainage systems.