4/06/2014. Need for Guidance. East Coast Lows. Catchment and coastal flood events can combine! Project 18 ARR Review

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1 Need for Guidance Development of Practical Guidance for Coincidence of Catchment Flooding and Oceanic Inundation Angela Toniato, NSW OEH Duncan McLuckie, NSW OEH Grantley Smith, Water Research Laboratory Catchment and coastal flood events can combine! Project 18 ARR Review East Coast Lows East Coast Lows What if catchment flows and storm surge combined? What if catchment flows and storm surge combined? 1

2 Need for Updated Guidance Large Bays Deep Entrances Recognise that flood behaviour varies from estuary to estuary Update of current guidance: Appendix A of Flood Risk Management Guide: Incorporating Sea Level Rise Benchmarks in Flood Risk Assessments by the NSW Government (DECCW, 2010) Shallow Entrances Shallow Entrances ICOLLs Design Flood Issues Size of the ocean event (AEP) Size of the catchment event (AEP) Location in the estuary Timing of the ocean event compared to the catchment event 2

3 Guidance Needs to be: Flexible Easy to use and understand Proposed Steps 1. Gather and review available information 2. Determine waterway type 3. Select a modelling approach 4. Consider waterway entrance conditions 5. Determine the ocean boundary condition 6. Translate to the study boundary or site location 7. Consider relative timing of catchment and ocean drivers 8. Sensitivity testing Waterway type (Roy et al) Group 1 Oceanic Embayments -e.g. Botany Bay, Jervis Bay); Group 2 Tide Dominated Estuaries - (large, deep entrances with tidal ranges similar to the open ocean, e.g. Port Stephens, the Hawkesbury River); Group 3 Wave Dominated Estuaries - (entrances that are constricted by wave-deposited beach sand and flood-tidal deltas, but are permanently open, e.g. Tweed River, Lake Illawarra). Waterway Type A - Ocean Dominated little ocean tide attenuation and negligible wave set-up Trained, maintained entrances Drain into bays Group 4 Intermittently Closed Estuaries (ICOLLs). e.g. Dee Why Lagoon, Lake Conjola); and Group 5 Freshwater Bodies (coastal water bodies that have rare and occasional connection to the ocean, e.g. Cudgen Lake, Myall Lakes). Waterway Type B Trained but not maintained Ocean tide attenuation Potential for wave set-up Waterway Type C ICOLLs Very shallow entrances that may close from time to time Flood levels controlled by berm Wave set-up likely 3

4 Selecting an ocean level Selecting an ocean level Sydney analysis Fort Denison AEP % Design Still Water Level (m AHD) exceedance per year Selecting an ocean level Applying boundary condition (A) Applying boundary condition (B) Applying boundary condition (C) 4

5 Modelling approach A. Simplistic approach B. General approach C. Detailed approach Simplistic Approach Small scale Site specific development (single property) Cost effective Conservative Steady state ocean boundary 1% AEP Extrapolate existing flood gradients General Approach FRMP or Strategic Planning Unsteady numerical modelling Variable ocean time series Design levels and velocities for a range of AEPs Conservative timing Conservative ICOLL entrance approach General Approach Design Combinations Design 1% AEP oceanic inundation with 5% AEP catchment flooding with coincident peaks, o to test peak levels; Design 5% AEP oceanic inundation with 1% AEP catchment flooding with coincident peaks; o to test peak levels Coincidence of low tide in neap tide cycle with 1% AEP catchment flooding o to test peak velocities. Detailed Approach Site specific boundary analysis Especially for wave set-up Dynamic entrance morphology Updating Guidance Flexible Easy to follow Conclusions Detailed joint probability analysis 5

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