Portland Riverine & Stormwater Flood Risk Investigation

Overview Glenelg Hopkins Catchment Management Authority is seeking proposals from suitably qualified and experienced tenderers to complete new climate ready flood risk mapping for Portland, accounting for riverine, storm tide and stormwater flood risk. About Portland is in Gunditjmara Country on the Henty Highway (A200) in southwest Victoria. With a population of around 10,000 people, it is Glenelg Shire's main population and commercial hub. Parts of the town are prone to flooding driven by riverine, ocean storm tide and stormwater events. The 2011 Portland Flood Study currently provides the best available flood risk mapping and accounts for up to 0.8 metres of sea level rise by the year 2100. This mapping is now out of date given the latest sea level rise information from the Intergovernmental Panel on Climate Change and the latest climate change guidance from Australian Rainfall and Runoff (ARR V4.2). ARR V4.2 makes it clear that increasing rainfall intensity must now also be factored into Portland's flood risk mapping. Additionally, the risks posed by stormwater driven flooding and the interactions of stormwater with riverine and ocean storm tide floods are yet to be considered. Based on the 2011 flood investigation, it is currently thought that 1%AEP riverine floods will produce higher flood levels than 1%AEP ocean storm tides. The 2011 Flood Study Report (available from the CMA's website) provides some background on the study area hydrology, which should be read in conjunction with this brief. Attention should be paid to the challenges associated with the lack of gauging of the riverine inflows and the application of appropriate regional estimation methods to determine inflows to the hydraulic model. Purpose The successful tenderer will apply the latest modelling techniques and climate science to ensure the new mapping provides a sound basis for climate change ready land use and development planning and integrated management of all drivers of flood risk over the coming decade(s). Objectives In broad terms, the objective is to update the area's flood risk mapping and intelligence information to: Comply with current industry best practice, including the latest climate change guidance from Australian Rainfall and Runoff (ARR) Account for the risks posed by riverine, storm tide and stormwater driven flooding, and coincident riverine/storm tide/stormwater events, under current (2030) and future (2100) climate scenarios Provide a sound foundation for adoption, revision, improvement, development or more detailed consideration of strategic plans (including Integrated Water Management) for residential and other forms of growth in and around Portland Provide a sound foundation for statutory land use and development controls in the Glenelg Shire planning scheme Provide a sound foundation for flood event alerting and response arrangements Provide a sound foundation for feasible (in terms of cost versus benefit) options for investment in physical works to mitigate flood risks driven by riverine, storm tide or stormwater events Scope The project must deliver: Comprehensive documentation and analysis of Portland's flood history A comprehensive suite of climate change ready stormwater (50% to 0.2% AEP and PMF), riverine (20% to 0.2% AEP and PMF) and storm tide (20% to 0.2% AEP and PMF) flood risk mapping outputs, accounting for both the present day (2030) level of risk and future (year 2100 and 2120) level of risk for the full suite of design floods A comprehensive suite of flood intelligence outputs to support community education and flood emergency response, including assessment of flood warning/alerting, for the present day (2030) climate 20% to 0.2% AEP design events Draft planning scheme amendment maps delineating where land should be included in a Flood Management Zone and/or a Flood Management Overlay with four levels of hazard Estimation of the potential economic impact of present-day (2030) design floods ranging from the 50% to 0.2% AEP for stormwater and 20% AEP to the 0.2% AEP for riverine and storm tide flooding (or a combination of both if more appropriate) Estimation of the future potential economic impact of 1% AEP floods, accounting for the effect of increased rainfall intensity that could arise if global warming levels of 3.6°C and 4.5°C are reached by the years 2100 and 2120 respectively Pre-feasibility assessment, including cost versus benefit analysis, of structural options for mitigating riverine, storm tide or stormwater flood risks Assessment of the potential impact of full build out of the potential growth area of Wattle Hill Creek and the potential effects of this on flood risk Contractor Requirements Tender submissions must demonstrate a high level understanding of best practice methodologies and approaches to flood modelling that account for the effects of climate change on riverine, storm tide and stormwater flood risk into the future The applied methodology(s) must be consistent with the latest Climate Change Considerations guidance provided by Australian Rainfall and Runoff Enquiries Graeme Jeffery (Enquiries): Office: +61 (03) 55712526, g.jeffery@ghcma.vic.gov.au Jane Blackburn (Contractual): Phone: +61 (417) 264166, jane.blackers@gmail.com