Update: UNSW s Research Program for Extreme Waves on Fringing Reefs Matt Blacka,Kristen Splinter, Ron Cox
Overview Research Area 1: Extreme wave and water level processes for fringing reef coastlines Research Area 2: Coastal protection design for fringing reef coastlines
Background Cook Islands PASAP Coastal Project 2012/2013 Pacific Adaptation Strategy Assistance Program (PASAP) Project to investigate vulnerability of infrastructure and development to coastal hazards along the north coast of Rarotonga Results presented at 2013 STAR conference in Rarotonga Modelling and Mapping the Impacts of Storm Surge and Waves on the Avarua Coastline Arorangi Lagoon, Rarotonga
Rarotonga, Cook Islands
Reef Survey Data Collection, RTK GPS
Avarua, Rarotonga, Cook Islands TC Meena, 2005 Predicted Storm Surge/Wave Affects for Direct Impact
Avarua, Rarotonga, Cook Islands 100 Year ARI Predicted Storm Surge/Wave Impacted Areas In excess of 700 properties impacted In excess of $100M damage to properties
UNSW Research Program for Reef Mediated Coastlines Research Area 1: Extreme Waves and Water Levels Assess the applicability and capability of existing empirical prediction methods for extreme wave conditions (design) Improve simple empirical methods for predicting wave heights, wave setup, extreme water levels and wave runup levels (preliminary/first pass design estimates) Raise awareness of the complexities of fringing reef processes, and limitations with available prediction/modelling tools
Research Approach Focus has been on using wave tank physical modelling to develop improved understanding and simple empirical prediction methods for: Wave height decay and extreme wave distribution Wave setup Surf beat and extreme (1% exceedance) water levels Physical modelling of selected case study sites to simulate wave processes on site specific bathymetric profiles. Sites selected on the basis of available reef profile data, as well as observations and/or data from extreme cyclonic events
44 m long, 1.2 m wide, 1.6 m deep wave tank Waves up to ~0.5 m high Largest wave flume facility in Australia/Pacific region
Wave Flume Scale Model of Reef/Lagoon/Foreshore Wave Flume Physical Model
Wave Flume Physical Modelling Data collected during the physical model testing included: Wave heights in deep water, across the reef rim and throughout the lagoon; Water surface elevations at several locations throughout the lagoon; Video footage of lagoon wave processes such as surf beat and stacking of bores; Reflection coefficients. Results compared with similar previous investigations such as: Gourlay (1996) - both monorchromatic and irregular non extreme wave conditions Seelig (1983) irregular non extreme wave conditions
Depth (m) 10 0-10 -20-30 -40-50 -60-70 -80 1000 900 800 700 600 500 400 300 Cross Reef Distance (m)
Reeftop Hydrodynamic Processes
Reeftop Hydrodynamic Processes
Flume Modelling Results Reef Top Dynamics Offshore Water Surface Reef Top Water Surface
Example Infragravity/Surf Beat Surge
Flume Modelling Results Reef Top Wave Heights Reef Top Wave Height γ r = 0.35760.439.
Comparison with Previous Investigations Wave Setup Gourlay (1996) Data Good agreement Dimensionless wave setup, N! # $ %& $, For irregular waves: 0.00130.0191 10.
Comparison with Previous Investigations Surf Beat Gourlay (1996) Data Low agreement Surf beat underestimated by previous empirical equation
Comparison with Previous Investigations Extreme WLs Gourlay (1996) Data Low agreement Extreme water levels underestimated by previous empirical equation
Key Points During extreme conditions reef/lagoon systems dominated by: Steep reef face - results in higher than expected waves passing over outer reef Wave/Surf Beat and Wave/Wave interaction results in complex and unpredictable behaviour, altered wave height distributions, higher wave crest levels and higher wave runup levels Sustained (average) wave setup well predicted by existing methods Infragravity, surf beat - results in extreme water levels that are higher than predicted by existing methods, revised methods now published Very limited extreme event data for waves and water levels on fringing reefs. Nikao Lagoon, Rarotonga
Summary and Ongoing Work Detailed results published at: Australasian Coasts and Ports Conference, Blacka et. al (2015) Estimating Extreme Wave Heights and Water Levels on Fringing Reefs ICCE (2016), Splinter et. al (2016) The Influence of Wave Period, Wave Height and Water Levels on Reef-top Hydrodynamics During Extreme Wave Conditions Expanding modelling data set with additional flume testing in July that will include: Increased range of reef profile characteristics Increased range of incident wave conditions Future will include: Modelling of specific events/locations with observed extreme events Improvements of empirical equation parameters from additional data, and understanding of reef top processes such as wave-wave interactions, reef width effect, wave runup
UNSW Research Program for Reef Mediated Coastlines Research Area 2: Coastal Protection Review of affordable coastal protection options for reef mediated coastlines Identify and assess the applicability of existing design methods Establish improved design equations Develop guidance on selection and design of coastal protection
PRIF - Affordable Coastal Protection Study Project undertaken by T&T and UNSW for the Pacific Regional Infrastructure Facility Objective: Build on existing knowledge to develop innovative solutions for coastal protection that maximise the use of local materials and labour, whilst minimising the requirements for imported materials and equipment. Regional scale desktop review and modelling assessment. Vaimaanga Lagoon, Rarotonga
PRIF - Affordable Coastal Protection Study Phase 1: Desktop Review (almost complete) Identification of Pacific Island Countries with limited supply of suitable rock for coastal protection Cataloguing of alternative coastal protection options Critical evaluation of options using a multi-criteria analysis considering technical, financial, environmental and material supply constraints Provides recommendations for preferred coastal protection approaches as a function of location and/or improvement of existing solutions. Phase 2: Development of Guidance Document (yet to start) Development of guidance around the preferred coastal protection design approach including the use of physical hydraulic model testing (where necessary) to confirm design parameters.
Coastal Protection Design for Reef Mediated Sites New research project starting July 2016: Wave flume modelling program to investigate rock revetment/seawall design specifically for reef mediated sites: Rock armour stability and size requirements Wave runup/overtopping characteristics Revetment slope effects Comparison of results from reef mediated site with existing design equations for open coast sites Van der Meer and Hudson Equations for armour size Eurotop for overtopping Avatiu Western Basin Breakwater
Questions?