Developing a guideline for modelling the interaction of catchment and coastal flooding for Victoria - why do we need one and what does it cover?

Much is at stake in balancing land use and development decisions with flood risks. This is especially so in coastal areas with high property values. Flood risk mapping underpins floodplain development controls applied in Local Government planning schemes and is the foundation for balanced decisions. It must therefore be robust enough to justify planning scheme controls on coastal floodplain development. It must also be robust enough to support confident decisions on flood mitigation and adaptation investment. Flooding in coastally connected waterways (estuaries and coastal lagoons) can be driven by inland catchment rainfall events or ocean storm tides and can happen due to both mechanisms occurring at the same time, caused by the same meteorological event. Understanding the interaction between catchment and ocean (coastal) flood mechanisms is therefore very important in understanding overall coastal waterway flood risk. The influence of the two mechanisms on risk is complex, so the determination of flood risk with reasonable confidence requires consideration of multiple factors. These include ocean levels (due to both tidal fluctuation and storm tide), estuary, river or creek mouth morphology, waterway/ocean interface dynamics and the nature of the catchment and waterway draining to the coast. Most of Victoria's coastal waterways are wave-dominated estuaries. 90% of these are intermittently closed to the ocean due to a sandbar at the river/creek mouth. The NSW Office of Environment and Heritage's (OEH) 2015 coastal modelling guideline has led the way in providing guidance on how coastal flood risk modelling exercises should account for coastal flood risk complexities. Victoria's soon to be released version accounts for the range of coastal floodplain characteristics likely to be encountered by Victorian coastal flood risk modelling projects. It incorporates technical information from Victoria's Resilient Coast - Adapting for 2100+ program. It starts with the identification of the type of coastally connected waterway under consideration. This then informs the selection of modelling approaches aligned with the project objectives, together with guidance on how different factors such as wave setup, sea level rise, joint probability, and event selection can be appropriately accounted for. Then with increasing sea level rise, there is a clear need to not only assess the risks posed by extreme flooding events, but also to understand the risks that a gradual increase in tide height poses to the liveability of coastal areas. This issue alone (regardless of extreme events) will become increasingly challenging for many communities. The Guide has been endorsed for adoption by Victoria's coastal CMAs (excluding Melbourne Water), and when released will ensure fit-for-purpose coastal flood assessments are available for planning, emergency management, and assessing and managing development impacts. [ABSTRACT FROM AUTHOR]