Modelling the behaviour of floodways subjected to flood loadings

This research investigates the behaviour of floodways when subjected to extreme flood loadings. Queensland floods of 2010 / 11 and 2013 indicated many floodways failed to meet performance requirements to withstand such events. Current floodway design guidelines primarily focus on hydraulic design aspects for determining a floodways capacity, similar to that of a broad crested weir. This approach fails to consider additional loadings such as drag, debris, impact and lifting forces. Therefore, the loadings utilised in this research are adapted from AS 5100.2-2004: Bridge Design. Strand7 software is used to perform a 2D plane strain finite element analysis to identify the potential failure mechanisms and areas of vulnerability within floodway structures and surrounding soils. This analysis focused on the Left Hand Branch Road (LHBR) floodway located in the Lockyer Valley region, one of the worst-affected areas in Queensland. Due to limited historical flood data available for this region, a parametric study was conducted and identified the worst loading combination with respect to flow velocities and flow depths. Analysis concluded the stress imposed by the worst load combination did not exceed the 32 MPa compressive strength of the concrete used in the LHBR structure even once a damage simulation had been performed. Therefore the floodway is adequate to withstand all stresses resulting from flow velocities less than 10 m/s, however displacement within the structure, surrounding soils and rock protection appeared to be of more concern. Areas of vulnerability and displacement magnitudes have been identified, however, quantifying the significance of this displacement is difficult without an Australian Standard for floodway design to compare to. Based on the structural adequacy of the floodway, the most critical failure mechanisms are most likely attributed to erosion or scour in and around the immediate area of the floodway