This was the first presentation I gave about my PhD work as part of the 2019 Coasts & Ports conference in Hobart, Tasmania. The slides I used are below as well as an outline of my talk. If you found this interesting or have any questions or ideas, please reach out!
- This drone footage shows damage caused at Sydney’s Collaroy beach in 2016 due to a large East Coast Low.
- Houses were put at risk due to ongoing erosion from a combination of large waves and high water levels.
- If we can predict where damage is going to occur, we can give Early Warning and start taking actions to minimize damage.
- We can use morphodynamic models (e.g. XBeach) to predict how severe erosion and flooding hazards may be.
- However, XBeach needs a lot of computational power to run and a lot of data to properly calibrate.
- Storms can affect 100-1000 km’s worth of coastline (regional-scale), so we need another approach to estimate where damage might occur.
- We’re developing a two tier trial storm impact Early Warning System to see if we can forecast damage at the regional scale.
- The first tier is a fast, simple, preliminary assessment of which areas may be affected by large hazards.
- The second tier uses more sophesiticated morphodynamic models at chronic hotspots to get a more detailed understanding of hazards.
- There are many ways of doing a fast, preliminary assessment of storm impacts.
- One option is the Storm Impact Scale which compares the Total Water Level to the dune toe and crest elevation.
- This approach is used by the USGS Coastal Change Hazard Portal for the U.S. East coast.
- The USGS also run a second product in this area, called the Operational Total Water Level and Coastal Change Forecasts which shows how the water level varies over time.
- The slide shows the animated forecast for Hurricane Dorian on the Florida coastline.
- The USGS approach is a great place to start if we want similar capabilities in Australia.
- However, the U.S. East coast and the S.E. Australian coastlines are quite different.
- The U.S. East coast can experience large storm surges, but wave energy is usually dissipated across the continental shelf causing flooding.
- On the S.E. Australian coast however, the storm surges are relatively small, but more wave energy is allowed to reach the coastline causing erosion.
- We need to see if the Storm Impact Scale works well on the SE Australian coast where hazards are more to do with erosion than flooding.
- Let’s use the Storm Impact Scale and hindcast severity of erosion and flooding hazards from the 2016 storm and see if it produces a useful forecast.
- The 2016 East Coast Low caused significant erosion on the South-East Australian coastline.
- The severe erosion was mostly caused by the anomalously easterly wave direction combined with the fact that the peak of the storm coincided with Highest Astronomical Tides.
- Immediately prior and after the event, UNSW flew airborne LIDAR over 177 km of shorelines between Sydney and Nambucca Heads.
- Cross shore beach profiles were extracted at 100 m alongshore spacings for this analysis.
- Several wave buoys and tide gauges were also operational during the storm within the surveyed region.
- To establish the observed Storm Impact Regime (either swash, collision, overwash or inundation), the pre and post storm surveys were compared.
- It was assumed the point where the pre and post-storm surveys started to differ was the highest point reached by the Total Water Level.
- We then hindcast the Storm Impact at each beach transect.
- Wave runup was estimated using the equation provided by Stockdon et al. (2006) with modelled waves at the 10m depth contour.
- Dune toe and crest elevations were manually extracted from the survey data.
- Comparing the hindcast regime to the observed regime, we see that the correct regime was selected around 65% of the time.
- But, 97% of the beach profiles either fell into the swash or collision regime.
- If most of the profiles fall into only one of two categories, it is difficult to find which areas are most at risk.
- The figure on the left shows Collaroy beach and the observed storm impact at each transect.
- The yellow dots indicate dune collision occurs at the majority of the beach.
- At the northern end, dune collision is relatively minor, but at the southern end, houses are at risk of damage.
- We need a better way to describe the severity of erosion at each location.
- For surge-dominated coastlines where there can be a large range of water levels, such as on the U.S. East coast, all four Storm Impact Regimes can be commonly observed.
- However, for wave-dominated coasts, such as those in SE Australia, water levels don’t vary as much.
- This means, most of the time we are either in the swash or collision regime.
- There can be two different ways of thinking about storm impacts.
- Flooding hazards apply to the vertical axis, i.e. the height of the water level is very important.
- Erosion hazards apply to the horizontal axis, i.e. the width of the beach which is lost due to erosion is of more concern.
- Therefore, a new scale to better describe erosion hazards is being developed.
- In the figure, each coloured line represents a post-storm profile.
- We can classify erosion hazards into four levels based on how eroded our beach and dune are.
- Applying the new scale to Collaroy becah, the southern end of the beach experiences a more severe hazard.
- This is an improvement compared to the Storm Impact Scale which just assigned collision at both locations.
- Work is still ongoing to see how the Erosion Hazard Scale would perform over the entire NSW coastline.
- In summary, we can do a reasonable job of predicting storm impacts at a regional-scale.
- If our impacts are primarily erosion, it might be better to use a different framework to describe those impacts.
- This framework could be used by an EWS to alert the community and authorities before a storm so they can take actions to minimize potential damage.
