This document provides a technical foundation for the Australian Rainfall and Runoff (ARR) guidelines, specifically focusing on flood hydraulics and numerical modelling. It outlines the administrative structure and funding of the revision projects while offering detailed engineering principles for open channel hydraulics and hydraulic structures like bridges and culverts. A significant portion of the text evaluates various unsteady flow models, comparing the accuracy and limitations of 1D, 2D, and coupled simulations. Furthermore, the sources establish critical safety design criteria by classifying flood hazards based on their impact on people, vehicles, and buildings. Practical examples and  ...