Coogee Bay Floodplain Risk Management Study & Plan Presentation to the Committee 4 April 2016
Outline • Project Status • Background • Flood Management Options • Flood Mitigation Options • Property Modification Options • Emergency Response Modification Options • Options Assessment • Modelling Outcomes • Multi-criteria assessment • Recommendations of the Study • Document Review and Public Exhibition
Floodplain Management Process
Project Status • Flood Study review. • Floor level survey & economic damages. • Planning & emergency response review. • Identification of flood management options. • Hydraulic analysis and economic assessment of 4 detailed structural options. • Multi-criteria assessment of all options, including non-structural. • Draft Floodplain Risk Management Study and Plan. • Committee Review. • Public Exhibition.
Study Area Covers parts of the suburbs of Randwick, Coogee, and South Coogee. The largest demographic is aged 20-29 years, comprising 24% of people living in Coogee and South Coogee. In total 79% of the population are aged below 55 years
Data and Consultation Registered surveyors undertook a survey of floor levels of buildings located within the 1% AEP flood event + 0.5 m freeboard in April 2014. A total of 913 properties were surveyed The wider community was advised of the FRMS&P via the distribution of a newsletter on 5 August 2014 Consultation with relevant agencies over the phone and via email including Sydney Water Corporation, NSW Office of Environment and Heritage (OEH), NSW State Transit Authority (STA), NSW State Emergency Service (SES), Council’s strategic planning, and engineering services teams.
Flood Study The Coogee Bay Flood Study was prepared for Randwick Council in 2013. The Flood Study: • Conducted hydrology and hydraulic modelling of the catchment using Tuflow • Calibrated the model to three historical events • Underwent a community consultation process Review of the Flood Study model was conducted in Stage 1 of this project. The use of the Flood Study model as the basis for the FRMS was seen as appropriate based on the review.
Flood Extents
Existing Flooding An existing trunk drainage network lies under the main flowpaths which drains to Coogee Oval from Dolphin Street. Flowpaths often do not have consistently falling grade due to the development of residential properties, and the construction of road embankments across local low points. This leads to excessive ponding at some locations.
Flood Affected Properties
Tangible Flood Damages Existing Flood Damages have been calculated as follows. The biggest contributor to Annual Average Damages was found to be the more frequent flood events in particular the 5% AEP, with the 20% AEP and 1% AEP also contributing significantly. Flood events greater than the 1% AEP contribute only 12% to AAD calculations.
Impact of Climate Change Climate change sensitivity runs were conducted for 10%, 20% and 30% increases in rainfall intensity. The modelling results showed an average increase in flood depths of 0.1, 0.2 and 0.3 metres across the catchment respectively. This resulted in a 37%, 77%, and 111% increase in flood damages respectively.
Flood Management Options • Property Modification Options • Reducing consequences of flood risks. • Modify properties (both existing and future) so that there is a reduction in flood risk. Including planning and development considerations. • Emergency Response Modification Options • Reduce the consequences of flood risks. • Modify the behaviour of people during a flood event. • Floodplain Modification Options • Reducing the likelihood of flood risks. • Modification of the flood behaviour in the catchment. • Data Collection Strategies • Better informs the Floodplain Risk Management Process
Floodplain Modification Options • 16 Options considered. • 4 Options further detailed assessment.
Property Modification Options The following six property modification measures have been assessed for Coogee Bay catchment: • PM1 - House raising; • PM2 - Voluntary purchase; • PM3 - Land swap; • PM4 - Council Redevelopment; and, • PM5 - Review of building and development controls; • PM6 – Public access to flood data PM5 and PM6 were recommendations following review of current policies and procedures. Review of PM1, PM2, PM3, and PM4 found they were not feasible for any locations in the Coogee Bay floodplain.
Flood Planning Level Flood Planning Level = flood level + freeboard. • Majority of development = 1% AEP + freeboard (0.5m for mainstream 0.3m for overland flow). • PMF applied to sensitive land uses. Flood Planning Area defined in LEP as area below the FPL. • 1% AEP (536 properties) • 1% AEP + 0.3m (711 properties) • 1% AEP + 0.5m (724 properties)
Flood Planning Area Trimming of the area below the FPL was conducted using the following rules to remove insignificant flooding: • Flood depths less than 0.15 metres; • Flood extents less than 100 m 2 in area; • Flood affectation deemed unrealistic or insignificant based on site visits by hydraulic engineers In addition property tagging was reviewed to consider excluding marginally affected properties from S149 notification tagging. However this was not applied as the relevant development controls would have negligible impact on marginally affected properties in any case.
PM5 - Planning Review Updating of the LEP to allow for development controls to the PMF level, not just the 1% AEP plus 0.5 metres as is currently the case Minor alterations to the relevant section of the DCP: • Remove requirement for balancing of net floodplain storage if detailed hydraulic modelling shows negligible flood impacts • Include requirements for electrical components to be raised above the Flood Planning Level • Provide greater flexibility for proponents in determining the appropriate flood emergency response for the subject site, including the potential addition of requirements for shelter-in-place • Council should consider the adoption of a PMF design event for safety and evacuation requirements.
PM6 – Public Access to Flood Data • To assist property owners in understanding their flood risk and complying with any flood related development controls, Council is recommended to update the S149 certificates with the relevant flood information. • In addition, Council can make available the flood data from this study for each flood affected property, upon request by the property owner. • Council should also investigate other methods to make flood information for their property more accessible such as on Councils website
Emergency Response Modification Options The following emergency response options were assessed as part of the Study: • EM1 – Public Awareness and Education; • EM2 – School Education Program; • EM3 - Flood markers and signage; • EM4 - Local flood warning systems; • EM5 - Localised evacuation procedures; and, • EM6 - Local flood response plan for the Rainbow Street depression. These options look to either improve flood awareness in the community or develop improved response procedures for specific localities
Option EM1 and EM2 EM1 – Public Awareness and Education, and EM2 – School Education Program both focus on improving flood awareness within the community. There are a broad range of approaches that can be adopted, which should all be done in close consultation with NSW SES: • Develop FloodSafe Brochure and FloodSafe Toolkit; • Hold a FloodSafe launch event; • Develop a flood information package for new residents • The SES has developed a tailored education program for school children in primary schools
Option EM3 Flood Signage proposed for the pink locations to ensure residents do not attempt to drive through floodwaters
Options EM4 and EM5 Options EM4 – Local Flood Warning System, and EM5 – Localised Evacuation Procedures both attempt to improve flood warning time allowing for evacuation. While the flash flooding environment of the catchment means evacuation is difficult, these measures may assist. The relative width of the floodplain is conducive to evacuation (less than 50 metres in most locations)
Rainbow Street Depression • Flows from surrounding catchment converge to one depression • Area is drained by 1.05m diameter pipe • Capacity of pipe is exceeded for large infrequent flooding events • 7 metre depth in the Probable Maximum Flood (PMF) event • Severe emergency response issues Options investigated: • FM15: upgrade trunk drainage line – preliminary assessment found that pipe upgrades to the scale required are unlikely to be feasible given the depth of the pipe, likely through bedrock. • FM16: further investigations into flood risk management works • EM6: further investigation into developing a detailed flood evacuation procedure for residents of Rainbow Street depression.
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