Bendalong Coastal Hazard Mapping
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Bendalong Coastal Hazard Mapping Preliminary Draft Report September 2017 Level 17, 141 Walker St North Sydney NSW 2060 Australia 301311-13501-002 www.advisian.com Shoalhaven City Council Bendalong Coastal Hazard Mapping Preliminary Draft Report Synopsis This report describes the derivation of the coastal hazard for Bendalong Boat Harbour beach, based on survey information and Council’s Sea Level Rise projections. Disclaimer This report has been prepared on behalf of and for the exclusive use of Shoalhaven City Council, and is subject to and issued in accordance with the agreement between Shoalhaven City Council and Advisian. Advisian accepts no liability or responsibility whatsoever for it in respect of any use of or reliance upon this report by any third party. Copying this report without the permission of Shoalhaven City Council and Advisian is not permitted. Project No: 301311-13501-002 – Bendalong Coastal Hazard Mapping: Preliminary Draft Report Advisian Rev Description Author Review Approval Date A Draft for Internal 7/9/17 Review C Adamantidis A Nielsen B Draft for Customer 11/9/17 Review C Adamantidis A Nielsen C Adamantidis Advisian ii Shoalhaven City Council Bendalong Coastal Hazard Mapping Preliminary Draft Report Table of Contents 1 Introduction ........................................................................................................................... 5 2 Study Area .............................................................................................................................. 6 2.1 Shoalhaven Coastal Sediment Compartments ............................................ 6 2.2 Bendalong Boat Harbour Beach ....................................................................... 8 2.3 Available data ........................................................................................................ 14 3 Coastal Hazard Assessment Methodology ............................................................... 16 3.1 Short Term Coastal Erosion .............................................................................. 16 3.1.1 Creek entrance instability .................................................................................20 3.2 Long Term Coastline Recession ...................................................................... 20 3.3 Future Beach Recession - Sea Level Rise ..................................................... 23 3.3.1 Estimation of Closure Depth ...........................................................................24 3.3.2 Sea Level Rise Projections ................................................................................26 3.3.3 Future Recession due to Sea Level Rise......................................................27 3.3.4 Discussion on application of the Bruun Rule ............................................29 3.4 Inundation .............................................................................................................. 30 3.4.1 Coastal Inundation..............................................................................................30 3.4.2 Wave Transformation Modelling...................................................................32 3.4.3 Boundary Conditions .........................................................................................36 3.4.4 Model Validation .................................................................................................37 3.4.5 Model Results .......................................................................................................37 4 Coastal Hazard Assessment Results ............................................................................ 40 4.1 June 2016 storm event ....................................................................................... 40 4.2 Short Term Erosion .............................................................................................. 41 4.3 Long Term Recession .......................................................................................... 41 Advisian iii Shoalhaven City Council Bendalong Coastal Hazard Mapping Preliminary Draft Report 4.4 Inundation - Wave Runup Calculation ......................................................... 42 4.5 Summary ................................................................................................................. 42 5 Discussion on Management Measures ...................................................................... 46 5.1 Stormwater Management ................................................................................. 46 5.2 Coastal Erosion Management.......................................................................... 47 5.2.1 Beach Scraping .....................................................................................................47 5.2.2 Specific Recommendations for Bendalong Boat Harbour ...................50 6 Conclusion ............................................................................................................................ 55 7 References ............................................................................................................................ 56 Appendix List Appendix A Coastal Hazard Maps Advisian iv Shoalhaven City Council Bendalong Coastal Hazard Mapping Preliminary Draft Report 1 Introduction Coastal hazards of the Shoalhaven coastline were mapped and identified based on coastal engineering studies undertaken as part of the risk assessment for the current Shoalhaven Coastal Zone Management Plan (Umwelt Australia, 2012). That mapping took into account sea level rise benchmarks under the previous NSW Government Sea Level Rise Policy Statement (Department of Environment Climate Change and Water, DECCW 2009). However, it did not include the beach at Bendalong Boat Harbour. Following an East Coast Low that occurred in June 2016, Bendalong Beach (together with other north-facing embayments along the Shoalhaven coastline) experienced severe erosion. In light of the severe erosion experienced at Bendalong, the revised sea level rise projections adopted by Council, and a new Landscape Master Plan for the area, Council has undertaken bathymetric and beach surveys of Bendalong in December 2016, to enable a coastal hazard assessment for Bendalong to be carried out and understand the level of risk to public infrastructure along the Bendalong beachfront. This report presents the derivation of the refined coastal hazards at Bendalong Boat Harbour based on the latest available data and Council’s adopted sea level rise projections. It describes the coastal processes and the impact of these processes on the areas where infrastructure is at risk. The assessment quantifies the observed long-term beach changes with reference to updated beach survey transects, LiDAR data, as well as estimating the beach recession that may be caused by sea- level rise as a result of climate change. The risk to infrastructure is defined in terms of the present day risk, the risk by 2030, the risk by 2050 and the risk by 2100. Simple, practical measures for dealing with the beach erosion risk are provided also. Advisian 5 Shoalhaven City Council Bendalong Coastal Hazard Mapping Preliminary Draft Report 2 Study Area The study area for this investigation covers Bendalong Boat Harbour Beach, the location of which is shown in Figure 2-1. An aerial photograph of the site is shown in Figure 2-2. Site visits were undertaken on 20 March 2017 and 3 August 2017. A major east coast low occurred on June 5 – 6, 2016, the effects of which could still be seen in the erosion escarpment along the back of the beach area in March 2017, despite beach scraping works which were undertaken in November 2016. Immediately prior to the site visit in March 2017, 117 mm of rainfall was recorded at the nearest Bureau of Meteorology weather station at Bendalong STP within a single day on 17 March (which was the highest daily rainfall total recorded since the June 2016 East Coast Low). This rainfall event resulted in severe localised erosion on the beach caused by runoff from overland flow cascading down the road and the embankment behind the road, as well as severe scour in the vicinity of the beach stormwater outlets. A brief description of the coastal compartments of the Shoalhaven coast, a description of the beach, as well as a summary of available data for hazard analysis is provided below. 2.1 Shoalhaven Coastal Sediment Compartments Carvallo and Woodroffe (2015) have undertaken a study of the coastal compartments of the eastern coast of NSW. They considered sediment compartments as subdivisions of the coast separated by major obstacles such as headlands, which interrupt the transport of littoral drift alongshore. Compartments were delineated based on physical characteristics as well as a review and interpretation of hydrologic, geomorphic and sedimentological data. The following primary sediment compartments were identified for the Shoalhaven coast: A compartment centred on the Shoalhaven River estuary A compartment centred on Jervis Bay A compartment between Bannisters Point and Jervis Bay, encompassing Bendalong Boat Harbour beach A compartment between Warden Head and Bannisters Point, encompassing Ulladulla Harbour, Collers and Mollymook beaches. These compartments have been subdivided further into secondary and tertiary sediment compartments by the presence of smaller headlands, as based on sediment characteristics and transport. McPherson et al. (2015) have delineated the coastline into similar sediment compartments when compared with