Floodplain Management Plan Report and figures. This Plan provides the basis for the future management of flood liable lands adjacent to the lake in the Lake Macquarie City Council area. The Plan was developed from the Lake Macquarie Flood Study (Part 1 - January 1998 and Part 2 - April 1998) and the Lake Macquarie Floodplain Management Study (August 2000). The Lake Macquarie Flood Study determined design flood levels for the 1%, 2% and 5% AEP floods and an Extreme flood event. It examined the still water level and also the effect of wave runup along the foreshore. The design events represent floods which have a certain probability of occurrence. The “1% AEP” or “1 in 100" flood has a 1 in 100 chance of being equalled or exceeded in any year. On a LONG TERM average it will happen once in every 100 years, but it is wrong to think it can only happen once in a century. Because floods are random events there is still a 1 in 100 chance of such a flood occurring next year no matter what happens this year.

This Floodplain Risk Management Study investigates what can be done to reduce or manage the effects of flooding in the catchment. The Floodplain Risk Management Plan recommends a mix of strategies to manage the risks of flooding. Using the merits-based approach advocated in the NSW State Government’s Floodplain Development Manual (2005) and in consultation with the community, Council and state agency stakeholders, a number of potential options for the management of flooding were identified. These options included: • flood modification measures; • property modification measures; and • emergency response measures. An extensive list of options was assessed against a range of criteria (technical, economic, environmental and social). Hydraulic modelling of some of the flood modification options was undertaken to provide a comprehensive analysis of those options that would involve significant capital expenditure.

This document constitutes the first and second stages of the management process for the North Creek catchment. It presents a compilation of the data collected and has defined flood behaviour and flood risk for the catchment area. This study was commissioned under the 2005 NSW Floodplain Development Manual (Reference 1), however, it is recognised that the 2023 Flood Risk Management Manual (Reference 2) was gazetted shortly after the project commenced. While the study was undertaken in accordance with the 2005 manual, there are elements that are consistent to both the 2005 and 2023 manuals. Where appropriate, the 2023 manual is referenced where project methodology or outputs are consistent with the new 2023 manual

Introduction The Dora Creek Flood Risk Management Plan has been prepared for Lake Macquarie City Council (Council) in accordance with the NSW Floodplain Development Manual (April 2005 - Reference 1) and the August 2010 Flood Risk Management Guide – Incorporating sea level rise benchmarks in flood risk assessment (Reference 4) and: Is based on a comprehensive and detailed evaluation of factors that affect and are affected by the use of flood prone land; Represents the considered opinion of the local community on how to best manage its flood risk and its flood prone land; and Provides a long-term path for the future development of the community. The Dora Creek catchment is located on the western side of Lake Macquarie waterway, 30 km south-west of Newcastle and 120 km north of Sydney. Dora Creek has a catchment area of approximately 238 km2 and is the largest catchment flowing into the Lake Macquarie waterway which has a total catchment of 648 km2 (Figure 1). The study area focuses on the existing township of Dora Creek and includes the townships of Cooranbong and Avondale, where new and planned rural and urban developments are proposed within the floodplain. The upper limit of the study area extends above Freemans Drive to Cooranbong and downstream to the confluence of Dora Creek with the Lake Macquarie waterway. There are three main tributaries within the catchment with Dora Creek being joined by Jigadee Creek at Cooranbong and then Stockton Creek before draining into Lake Macquarie waterway at Bonnells Bay and Lake Eraring. Watercourses within the catchment area and above the tidal limit are narrow; Dora Creek is typically less than 15 m wide upstream of Freemans Drive. Below the tidal limit, the creeks become broader with Dora Creek varying from a width of 50m at the Sydney to Newcastle Freeway to 100m where it forms a delta into Lake Macquarie waterway. Upstream of the delta the Eraring Power station draws cooling water for steam condensation from Bonnells Bay, which passes under Dora Creek through a concrete tunnel and open canal.

Main report

Summary of Management Plan (1) Most of the flood damage reduction measures considered as part of the 1993 Floodplain Management Study cannot be justified on benefit-cost considerations. They could be reconsidered if new information becomes available that leads to substantially increased benefit-cost ratios. Notwithstanding, Option 1E from the 1993 Floodplain Management Study was shown to have a Benefit-Cost ratio of about 0.4 and therefore should be retained within the Plan. This floodplain management strategy involved the voluntary raising of a 39 houses located in areas of Edgeworth, Barnsley and Boolaroo that are potentially flood affected. It is recommended that these houses be re-assessed and prioritised in terms of their suitability for house raising. Once a prioritised list has been determined, steps should be undertaken to implement the strategy. (2) The Flood and Floodplain Management Studies on which this Plan is based should be revisited. The reason for this and the process that should be followed is outlined in Appendix C. The majority of the floodplain management options considered in the 1993 study are either not viable from a cost/benefit perspective or may not be considered appropriate now. Hence, contemporary floodplain management issues need to be determined through consultation with the Committee and key stakeholders within the community. It is not clear whether any consultation took place as part of earlier studies. However, this is considered to be essential to managing future flooding issues that may arise along creek system and its floodplain. This could be achieved by distribution of a brochure similar to that included in Appendix A. (3) Development within areas defined in Figure 4 as 2A, 2B and 2C, should be able to proceed without extensive independent flood investigations, provided all appropriate requirements specified by the current Local Environment Plan are addressed (eg., those related to filling). (4) Development Restrictions Flooding/Tidal Inundation Certificates should be revised to incorporate the predicted peak level of the Probable Maximum Flood or an ‘extreme’ flood. The format of Development Restrictions Flooding/Tidal Inundation Certificates could be modified to provide more information. (5) Further investigations need to be undertaken to determine whether flood damage reduction measures can be implemented for those problem areas listed in Section 4.3.1 (6) Further hydraulic investigations should be undertaken the determine the amount of time before inundation of key areas along Cockle Creek in a typical major flood. (7) A flood liable lands DCP is to be developed specifically for the Cockle Creek catchment. The DCP would effectively serve as a revised Flood Policy and relate to the specifics of flood behaviour in the catchment (as distinct from the lake inundation flood behaviour that applies in other areas of Lake Macquarie City).

The key objective of this Flood Study is to develop a suitable hydrologic/hydraulic model that can project flood and permanent inundation water levels in Lake Macquarie from rainfall, sea level rise and storm surge. These results will be used by Lake Macquarie City Council, in consultation with the community of Lake Macquarie City, to manage flood and permanent inundation risks to low lying land around the Lake Macquarie waterway. The key stages in the process are: Undertaken a comprehensive review of the 1998 Lake Macquarie Flood Study (Part 1 – Reference 1) and develop suitable hydrologic/hydraulic models to define flood behaviour over the full range of design events for existing catchment conditions, Use the hydrologic/hydraulic models to assess various climate change scenarios, including application of the NSW Government’s sea level rise benchmarks, Assess the potential increase in storm surge as a result of climate change and its impact on elevated ocean levels, Review the potential impact of climate change on the local wind/wave climate as this affects the extent of wave runup on the foreshore, Assess the hydraulic and hazard categories for existing and climate change conditions. This report details the results and findings of the above investigations. The key elements include: a summary of available historical flood related data, establishment of the hydrologic and hydraulic models, calibration of the hydrologic and hydraulic models, definition of the design flood behaviour for existing catchment conditions, sensitivity analysis of the design flood behaviour, assessment of the impacts of climate change on the still water and wave runup water levels re-definition of the flood extent and hydraulic and hazard categories mapping for existing and climate change conditions. A Flood Study is a technical document and not easily understood by the general public. A glossary of flood related terms is provided in Appendix A to assist. If more explanation of terms or a better understanding of the approach is required, type “NSW Government Floodplain Development Manual” into an internet search engine and you will be directed to the NSW Government web site which provides a copy of this manual and further explanation. Flood levels given in this report relate only to the water level with the lake itself. Design water levels in the creek systems entering the lake (Cockle Creek, Dora Creek etc.) will be higher than those shown for the lake.

Copy of full Floodplain Risk Management Study and Plan Report and appendices

This report documents the Lake Wyangan Floodplain Risk Management Study and Plan which investigates and presents a flood risk management strategy for the catchment. The study identifies the existing flooding characteristics and canvasses various measures to mitigate the effects of flooding. The end product is the Floodplain Management Plan, which describes how flood liable lands within Lake Wyangan are to be managed in the future.

This study updates the previous Wrights Creek Flood Study (Patterson Britton & Partners, 2007) and provides Council with a suitable platform for undertaking the next steps in the Floodplain Risk Management Process. The XP-RAFTS hydrologic model developed in the 2007 Flood Study has been adopted for this study. A TUFLOW hydraulic model of the catchment has been established using up to date survey of the ground levels and culvert and bridge structures. The TUFLOW hydraulic model has been used to reproduce the historical flood behaviour from events in 1995 and 2002 and to define flood behaviour for a range of design events. Actions undertaken as part of this flood study include: • collection of additional survey, • development of a TUFLOW hydraulic model, • definition of the design flood behaviour, • assessment of provisional hydraulic hazard, • assessment of hydraulic categorization, • assessment of the model sensitivity to parameter changes, • assessment of climate change impacts on model results, • assessment of sensitivity of model results to the new ARR 2016 design inputs, • community consultation.