The Lower Clarence Flood Model extends from Mountain View, upstream of Grafton, to the Ocean at Yamba and is used to simulate the flood behaviour of the lower Clarence River and to determine design flood levels which are used for planning purposes. Clarence Valley Council’s previous adopted model was last updated for Council in 2013. Since publication of that study, more recent versions of the model were developed for specific infrastructure projects, most notably the second crossing of the Clarence River at Grafton and the Pacific Highway upgrade.

Following the significant flood event of February/March 2022, Council engaged BMT to update and recalibrate the Lower Clarence Flood Model. The model update accounts for notable changes to the floodplain such as the Pacific Highway upgrade and incorporates the latest available datasets. The update also takes advantage of significant advancements in modelling software and computing power since the time of the 2013 study, allowing the floodplain to be modelled and mapped to a higher resolution than was previously feasible. The update also presented an opportunity to revisit some of the modelling assumptions thereby ensuring that the model is compatible with current guidelines and accepted best practice. The modelling documented in this report covers Clarence River flood events. Tributaries of the lower Clarence River are only represented in the model in so far as allowing backwater from the Clarence River to extend into the tributary catchments.

The key updates with regards to the design flood events are as follows: • Revisions to the design storm tide boundary so that it is consistent with boundaries derived for coastal-specific assessments undertaken for Council by others and compatible with current guidelines. • The flood frequency assessment at Grafton has been updated to account for the full period of record to the present day, including the events of 2009, 2011, 2013 and 2022. • Additional design floods have been modelled including the 0.5% and 0.2% AEP events. • The assumptions regarding the extreme flood event have been updated to accord with current best practice. • Two climate change scenarios have been modelled representing an intermediate (CC1) and a worst-case (CC2) scenario. The scenarios include 12% and 21.5% increases in rainfall for CC1 and CC2 respectively and allowances for sea level rise.