MOUNTS BAY, MARENGO IS BACKSHORE SAND RENOURISHMENT A VIABLE OPTION FOR MANAGING COASTAL HAZARD RISK? Report Prepared For: Department of Environment, Land, Water and Planning, Victoria Report Prepared By: ENVIRONMENTAL GEOSURVEYS P/L (Neville Rosengren) ASMINER GEOTECHNICAL (Tony Miner) V5: October 2018 1 Contents 1 INTRODUCTION AND BACKGROUND ................................................................................. 1 2 SCOPE OF WORKS .............................................................................................................. 3 3 CONTEXT: GEOLOGY AND GEOMORPHOLOGY .................................................................. 4 3.1 Otway Ranges .............................................................................................................. 4 3.1.1 Terrain .................................................................................................................. 4 3.1.2 Geology ................................................................................................................ 5 3.1.3 Structural Geology ............................................................................................... 6 3.2 Bass Strait: Submarine Geomorphology ..................................................................... 7 3.2.1 Otway Escarpments ............................................................................................. 8 3.2.2 Bass Strait Seabed ................................................................................................ 9 3.2.3 Sediment Dynamics ........................................................................................... 10 3.3 Coastal Geomorphology ............................................................................................ 11 3.3.1 Coastal Geomorphic Compartments ................................................................. 11 3.4 Coastal Processes ...................................................................................................... 14 3.4.1 Swell and Waves ................................................................................................ 14 3.4.2 Wind ................................................................................................................... 15 3.4.3 Tides ................................................................................................................... 16 3.5 Apollo Bay- Marengo Precinct (Storm Point to Wild Dog Creek North) ................... 16 4 MOUNTS BAY AND LOWER BARHAM RIVER .................................................................... 18 4.1 Geology ..................................................................................................................... 18 4.2 Geomorphology ........................................................................................................ 18 4.3 Bathymetry ................................................................................................................ 19 4.4 Subtidal Surface ......................................................................................................... 21 4.5 Beach and Intertidal Surface ..................................................................................... 22 4.6 Backshore: Mounts Bay Barrier ................................................................................. 24 4.6.1 Main Barrier ....................................................................................................... 24 4.6.2 Barrier Spit ......................................................................................................... 27 4.7 Sediment and Landform Dynamics of Mounts Bay and Barriers .............................. 28 4.7.1 Rates of sediment transport .............................................................................. 29 4.7.2 Dredging Records ............................................................................................... 29 4.7.3 Sediment Sources and Pathways: Mounts Bay and Apollo Bay ........................ 29 4.8 Backshore: Barham River Valley ............................................................................... 30 i 4.8.1 Implications for Age and Origin of Mounts Bay Barrier ..................................... 31 5 PREVIOUS STUDIES: AIMS & CONCLUSIONS .................................................................... 33 5.1 Purpose...................................................................................................................... 33 5.2 Vantree Pty Ltd, May 1997: Mounts Bay Beach. Report on Coastal Erosion. .......... 33 5.2.1 Aims.................................................................................................................... 33 5.2.2 Findings .............................................................................................................. 33 5.2.3 Recommendations ............................................................................................. 33 5.2.4 Comments .......................................................................................................... 34 5.3 Coastal Engineering Solutions, August 2005: Apollo Bay Sand Study Final Report. 34 5.3.1 Aims.................................................................................................................... 34 5.3.2 Findings .............................................................................................................. 34 5.3.3 Recommendations ............................................................................................. 34 5.3.4 Comments .......................................................................................................... 35 5.4 Water Technology October 2012: Coastal Hazards Management Plan Marengo to Skenes Creek ................................................................................................................ 35 5.4.1 Aims.................................................................................................................... 35 5.4.2 Findings .............................................................................................................. 35 5.4.3 Recommendations ............................................................................................. 36 5.5 Water Technology October 2016: Marengo to Wild Dog Creek Sand management program........................................................................................................................ 36 5.5.1 Aims.................................................................................................................... 36 5.5.2 Findings .............................................................................................................. 36 5.5.3 Recommendations ............................................................................................. 37 5.6 ASMiner Geotechnical: Evaluation of Vegetation Line on Aerial Photographs ........ 37 5.6.1 Indicators of Shoreline Position ......................................................................... 37 5.6.2 The Vegetation Line ........................................................................................... 38 5.7 ASMiner Geotechnical: Ground & Aerial Monitoring 2017, 2018 ............................ 40 5.7.1 Aims.................................................................................................................... 40 5.7.2 Programme ........................................................................................................ 40 6 PROGRAMME OF WORKS 2017 ....................................................................................... 41 6.1 Design profiles adopted ............................................................................................ 41 6.1.1 Works Completed .............................................................................................. 41 7 POST-WORKS CHANGES ................................................................................................... 44 7.1 Summary of Changes ................................................................................................. 44 ii 7.2 Causes of Changes ..................................................................................................... 47 7.2.1 Metocean Conditions ......................................................................................... 47 7.3 Was Loss of the Renourished Backshore a Result of Unusual Events in 2017? ........ 50 8 IS BACKSHORE SAND RENOURISHMENT A VIABLE OPTION FOR MANAGING COASTAL HAZARD RISK AT MOUNTS BAY, MARENGO ........................................................................ 51 8.1 Background ................................................................................................................ 51 8.2 Key Factors ................................................................................................................ 51 8.3 Major knowledge gaps .............................................................................................. 53 8.4 Potential Management Responses & Implications for Future Conditions of Mounts Bay ................................................................................................................................ 53 8.4.1 Do Nothing ......................................................................................................... 53 8.4.2 Repeat the Nourishment ................................................................................... 54 8.4.3 Other nourishment Options ..............................................................................