The ACT Government is currently undertaking detailed design of a major upgrade to the spillway at Isabella Weir. The weir is located on Tuggeranong Creek in the southern suburbs of Canberra, and is being upgraded to ensure that it can safely pass extreme flood events. HARC was engaged by the ACT Government to quantify the risks of flooding of the work site during construction. As part of this project, we analysed the seasonal flood risk and demonstrated that the site is heavily influenced by summer thunderstorms. We also produced concept designs for temporary works to mitigate the impacts of flooding during construction.