What is the background to this project?
The State government’s Flood Prone Land Policy 2005 requires that the management of flood liable land remains the responsibility of local government. The state government subsidises flood mitigation works to alleviate existing problems and to provide specialist technical advice to assist Councils in the discharge of their floodplain management responsibilities. Under this process, Councils must prepare a Flood Study followed by a Floodplain Risk Management Study (FRMS) and Floodplain Risk Management Plan (FRMP). Once a flood study and plan is completed and adopted, Councils can then seek funding from State and Federal governments to implement the Plan.
Coffs Harbour has a long history of flooding. The floods of March 2009 and November 1996 are among the largest on record. The Coffs Creek catchment is prone to severe flash flooding as it is a relatively small catchment with steep upper slopes, a high level of urban development on the floodplain and the tendency for high rainfall.
Previous flood studies have been undertaken for different sections of the Coffs Creek catchment. These studies are 10 to 20 years old. Data collection in the intervening period provides an improved understanding of extreme rainfall conditions in the catchment. More accurate terrain data has also been obtained and the sophistication of computer flood modelling has significantly improved. Extensive flood mitigation works have also been completed in recent years.
Tell me about the Flood Study
The draft Coffs Creek and Park Beach Flood Study – February 2018 uses improved rainfall data, digital terrain data, and 2D computer modelling to assess the impact of the detention basins and other recent works and refine our understanding of flood behaviour in the Coffs Creek catchment as a whole. The study establishes the basis for improved assessment of future floodplain management options. The study also provides more accurate GIS-based flood mapping which will be used to update Council’s online mapping resource.
The new flood model provides a good match between estimated levels and observed flood levels from 1996 and 2009 with consideration of the impacts of the basins. The model has found the detention basins are typically providing flood level reductions in the order of 200mm – 600mm in sections of the mid-upper reaches of Coffs Creek in the vicinity of the basins; and reductions in the order of 200-400mm around the Coffs CBD.
Tell me about the Flood Maps.
Draft Coffs Creek and Park Beach Flood Study – Mapping Compendium – February 2018 contains detailed mapping of flood extents, levels, depths, and velocities for the 1 in 20, 50, 100, 200 and 500 year flood events, as well as the PMF (Probable Maximum Flood) event. The compendium also includes flood hazard mapping for the 1 in 100 year event.
Tell me about the Park Beach Management Options Assessment.
Draft Park Beach Management Options Assessment – February 2018 utilises the modelling prepared in the overall draft flood study to assess drainage modifications for reducing flood risk within Park Beach. The area acts like a basin which fills and drains slowly under limited outlets. The modifications address these limitations and include: upgrading the pipe network leading to the outlet at Macauleys Headland; upgrading culverts beneath the railway line; and upgrading other key drainage lines leading to the various outlets. The study assesses the reduction in flood damage costs provided by each option and calculates the subsequent benefit cost ratio.
The outcomes of the study will be used to prioritise drainage improvement works in this area.
What are the key issues with this project?
Coffs Creek Catchment
The new draft Coffs Creek flood model provides a good match between observed flood levels and estimated levels, meaning that it is a robust model. The study has identified that some levels in the catchment were previously quite conservative, and provides a best practice model. The study also identifies some areas where flood level estimates have increased compared to previous study estimates which underestimated levels. This needs to be communicated as part of the public exhibition process to make the community aware that, in limited cases, the estimated flood level has increased on some properties.
It should be noted this is a flood study and not a floodplain management plan. Modelling has identified that the recently constructed detention basins won’t make much difference in flood levels for some inner city areas, such as Gundagai Place. It will therefore be very important to revisit the remaining flood mitigation options in the existing Floodplain Risk Management Plan (FRMP) for Coffs Creek. The Flood Study recommends that Council proceed to a review of options for the whole of the Coffs Creek catchment – resulting in the preparation/review of the FRMP.
Park Beach Catchment
Several management options have been provided in the study. Economic performance of management options are not that favourable if assessed in the traditional way. The Benefit Cost Ratio is realistically probably higher than that identified as it is likely that the standard approach to estimating flood damages has under-estimated the average annual cost of flood damages in the Park Beach area.