The study was commissioned by Bayside Council, with the assistance of the NSW Government Office of Environment and Heritage (OEH). Additional information has been provided by Sydney Water Corporation (SWC). The Flood Study comprises the development of computational hydrologic and hydraulic models that define design flood behaviour for the 20% AEP (0.2 EY), 10% AEP, 5% AEP, 1% AEP and 0.5% Annual Exceedance Probability (AEP) design storms and the Probable Maximum Flood (PMF) in the Bardwell Creek and Wolli Creek catchments.

The flood study carried out for Medowie identifies flood liable land for a range of floods and finds that many properties and approximately 20 residences, are likely to be flood liable during the 1% AEP event. Recommendations going forward are as follows: • In some locations existing drains are incapable of carrying 20 – 10% AEP (and in some cases even lower) flows. An example of this is the open drain which passes through Ballet Close from north to south. Enhancing the capacity of these trunk drainage systems should be examined as this may achieve a degree of flood relief for those properties which are exposed to flooding on a regular basis. Such an examination should however ensure that flooding issues are not simply transferred downstream; • Houses in County Close which remain subject to inundation should be closely monitored. It is likely that local flows cause inundation of these low lying properties and minor works by Council may alleviate this circumstance for impacted residents; • More generally there a number of locations where, due to the limited range of design flood behaviour (flood depths do not change markedly for variable AEP), minor works such as bunding could provide good levels of flood protection for individual properties. A good example of this is on Kirrang Drive (western side) where houses are currently impacted by flows from the swale at the front of their properties; • Generally Council needs to recognise the main overland flow paths identified in the study and move to restrict development in these “corridors” in the future; • In order to assess the issue in the CDIA with regard to upstream development potentially increasing flooding downstream of Ferodale Road, satisfactorily answer questions in the CDIA in regard to the impact of proposed development on the wet sequences which often deny property owners of access/enjoyment of their land, Council need to have a water balance type of exercise carried out. This will address the kinds of events which impact on inundation within the CDIA but which do not have any relevance to the definition of peak flood levels. It is noteworthy that during the Management Study process it will also be necessary to ensure mitigation works designed to alleviate flooding liability do not negatively impact on long term inundation for water sequences in the CDIA. The Management Study should consider a range of solutions but ensure that water quality is either maintained or improved; • Council should take care not to raise those sections of roads currently inundated as doing so will directly impact on peak flood levels. This is due to the fact that much of the 1% AEP flow moves over roads rather than through corresponding culverts; and • During the Management Study it is recommended that runs are carried out which examine the impact of removing the pinch which currently exacerbates the duration, extent and magnitude of inundation further upstream in the CDIA; and • Pinch plus pump capacity enhancement runs be carried out. Note unless pinch works are modelled, nothing will be achieved by looking at pumping rate enhancement alone.

Flood modelling of local catchment runoff was completed the Empire Bay catchment for a range of annual exceedance probabilities of storms from 100% AEP to 0.5% AEP and up to a PMF event. In a 1% AEP event, the modelling shows that some properties and roads may be inundated up to 0.5m. This occurs notably around Gordon Road, Boongala Avenue, Rickard Road, and Greenfield Road in the main residential area of Empire Bay and around the main drainage channels at Pomona Road and Empire Bay Drive to Palmers Lane in the rural residential area. In the 1% AEP event, the modelling showed that 22 houses are flooded above the floor level when the storm runoff is combined with a 1% probability of exceedance level in the estuary. Mapping of high provisional hazard in the catchment for the 1% AEP event shows that it is limited to the channel behind Myler Avenue and the channel north of Pomona Road which conveys runoff to Allawa Close. Scattered occurrences of high hazard also feature adjacent to parts of Empire Bay Drive. Hydraulic categorisation mapping for the 1% AEP shows floodway areas along the main watercourses from Pomona Road across Empire Bay Drive, and also some road near the main residential areas of Empire Bay and Bensville. Flood storage areas are identified in some properties, roads, and the open space / vegetated areas within the catchment. Increases to sea levels due to climate change have the potential to significantly affect flood impacts, particularly in the low elevation areas of the Empire Bay catchment. The Floodplain Management Authority’s Prioritisation Ranking table for the Empire Bay catchment is included as Appendix D.

The objective of the study was to assess the impacts of climate change on the baseline 1% AEP flood condition within the Williamtown / Salt Ash Flood Study area. Central to this was the development of a new two-dimensional hydraulic model of the study area, in order that the impacts could be properly assessed. In completing the flood study review, the following activities were undertaken: • Review of relevant studies regarding flood conditions and climate change impacts within Port Stephens; • Site inspection to confirm the presence and configuration of key hydraulic structures; • Merging of the existing Williamtown / Salt Ash Flood Study and Williams River Flood Study modelled to produce a composite model capable of properly assessing the impact of climate change in the study area; • Updating of model topography with available LiDAR survey data; • Calibration of 1% AEP design event flood levels with the 4.84m AHD level from the Flood Frequency Analysis at Raymond Terrace; • Prediction of design flood conditions in the catchment using the developed model; and • Production of design flood mapping series. The climate change scenarios that were considered were combinations of 2050 and 2100 sea level rise conditions with baseline, +10% and +30% flood flows. A sea level rise of 0.4m by 2050, results in around a 0.2m increase to the 1% AEP flood level in Fullerton Cove. A sea level rise of 0.9m by 2100, results in around a 0.6m increase to the 1% AEP flood level in Fullerton Cove. For the 1% AEP event peak flood levels in Fullerton Cove increase by around 0.1m and 0.3m for the 10% and 30% flow increases respectively. The dominant flooding mechanism (in terms of peak design water levels) for the Williamtown / Salt Ash locality is mainstream Hunter River flooding. Under these conditions, Hunter River flooding results in Fullerton Cove filling and discharging into the Tilligerry Creek floodplain, under crossdrainage structures and through overtopping of Nelson Bay Road. The baseline flood level within the Tilligerry Creek floodplain is increased from 1.2m AHD to 2.6m AHD, under the worst case climate change scenario. The flood levels along Windeyers Creek are driven by flow conditions in the Hunter River. Hunter River flood water provides a backwater influence in Windeyers Creek, which fills the storage area to the east of the Pacific Highway. The total volume of water flowing from the Hunter River along Windeyers Creek determines the flood level reached in the storage area. A higher flood level in the Hunter River will result in a higher flood level in the storage area. At this location, the sea level rise scenarios have little impact on peak flood levels. There is only a small difference between flood levels for the baseline condition and the 2100 scenario. However, the increased flood flow scenarios do have a significant impact, with peak flood levels increasing by around 0.2m and 0.6m for the 10% and 30% flow increases respectively. The baseline flood level at this location is increased from 4.4m AHD to 5.2m AHD, under the worst case climate change scenario. The flood study review will form the basis for the subsequent floodplain risk management activities, being the next stage of the floodplain management process.

The Flood Study had three main components:,

The flood study carried out for the Griffith CBD catchment identifies flood liable properties for a range of design floods and finds approximately 168 properties that are flood liable during the 1% AEP event. The Griffith CBD and upstream areas have experienced flooding in the past. This has been attributed to the insufficient capacity of the existing pipe drainage system, the flat topography across much of the area and the impediment to overland flow paths posed by the Main Branch Canal and the Temora-Roto Railway Line that passes to the immediate north of the CBD. In recognition of these flood issues, Griffith City Council (Council) have appointed WMAwater to carry out an Overland Flow Flood Study for the Griffith CBD catchment under the guidance of Council’s Floodplain Risk Management Committee. The main objective of this study is to: Define the overland flow flood behaviour within the Griffith CBD catchment over the full range of design flood events; Detail the number of properties at flood risk, extent of flood damage, and provide Council with the technical basis to prepare a Floodplain Risk Management Study & Plan; Prepare a modelling system suitable for use in the Floodplain Risk Management Study and Plan, for assessing flood mitigation works; and Provide Council with a modelling tool for the pit and pipe drainage system. The study has established suitable hydrologic and hydraulic modelling tools, demonstrated their capacity to emulate local flood behaviour via calibration/validation (as data allows) and then applied these tools to establish the existing flood risk for a range of design flood event probabilities in conjunction with a range of event durations. This study has assessed major overland flow flooding throughout the Griffith CBD catchment in general and in the commercial areas in particular, providing Council with a sound understanding of the full range of potential flood risks and enabling Council to determine an optimum mix of works and measures to manage these risks.

An update of the modelling outputs for the Williams River Flood Study tying the model into the Williamtown Flood Study model.

Potential Flood areas for all Hobart catchments. METHODOLOGY: Modelled Flood Areas - 2100 1% Annual Exceedance Probability (Modelled areas of all Hobart waterways including Hobart, New Town and Sandy Bay rivulets and minor drainage lines). Additional areas may be subject to flooding. Site specific investigation may be required for each property. DISCLAIMER: This map should be read in conjunction with the Understanding Flood Risk page on the Hobart City Council web site. While all reasonable care has been taken to ensure the accuracy of the information portrayed in this data, its purpose is to provide a general indication of the location of Council services. The information provided may contain errors or omissions and the accuracy may not suit all users. A site inspection and investigation is recommended before commencement of any project based on this data. The Hobart City Council does not warrant that the information contained on this plan is correct and a field survey is to be conducted before the information contained in this plan is relied upon.