DATA

Topographic LiDAR

The most recent LiDAR available was flown in 2007. The data are available at 1 m horizontal resolution, and were re-processed from the original level 2 classification into level 3 for input to a higher accuracy Intergovernmental Committee on Surveying & Mapping (ICSM) Level 3 classification.

The horizontal accuracy of the LiDAR data is +/- 35 cm, the vertical accuracy +/- 10 cm (RMSE 68% conf.). The implications of this are that the height of any given point could on average be approximately 10 cm above or below the height stated in the dataset. The implication is that, potentially, cross-shores profiles extracted based on the LiDAR could be incorrect, and flood and erosion extents could be over or underestimated.

- Locations that have been significantly modified through works (e.g. Queenscliff marina) will not show within the LiDAR data. 

Elevations were checked against survey data or construction drawings (where available) for vertical accuracy and manually checked and modified where possible for horizontal alignment. 

Bathymetric data

The most recent bathymetric (LADS) data was collected in 2009 /  2010. The data set has some gaps within the study area, these are largely due to turbidity of the water during data capture (the LADS laser could not penetrate the water to the seabed).

The horizontal resolution of the data is 2.5 m, the horizontal accuracy is +/- 1.62 m, the vertical accuracy is +/- 0.26 m at 68% confidence, or better. The implications of the data are similar to those documented for the Topographic LIDAR data.

The depth and slope for the nearshore zone has an effect on the wave modelling outputs, and therefore has an effect on the erosion modelling. A deeper or steeper bathymetry would mean wave conditions would be underestimated, and modelled erosion extents could possibly be further inland than presented. 

The data gaps are quite vast in the nearshore zone.

Some hydrographic data exists for the offshore areas, however this is rather old and relatively uncertain in itself. The bathymetric data was given consistency checks, and where data was missing, profiles were checked against hydrographic charts (where possible) or profiles linearly interpolated to fill data gaps.   

Waves

Measured wave data is available for two locations:

- Point Nepean has 10 years of relatively continuous Wave buoy data (PoMC)

- 1 year of measured wave height information was gathered using a pressure gauge near Portarlington (Water Technology, 2008).

There is very little data to validate wave models and the time span is too short to obtain more certain extreme events.

Pressure gauge wave data is relatively uncertain, and also has no associated  directions. Data set not likely to be appropriate for validation of PPB model.

No additional data at nearby locations is available to validate / compare the local measured data.

Tidal

20+ years of tidal data exists for the main tidal stations. 

The data record is insufficient to provide fully certain values for extreme storm-tides.

The locations of some tide gauges can introduce some uncertainty, e.g. Point Lonsdale is behind a rock shelf within the PPB entrance therefore is not representative of the open coast conditions. 

Extremes determined using the tidal data were compared to known storm-tide events for consistency.

Point Lonsdale open coast STLs are based on tides at Rip Bank (i.e. outside the entrance) rather than the Point Lonsdale tide gauge. 

Coastal protection structures

Coastal protection structures were captured by the DSE Future Coasts Program, using DSE aerial imagery. It was identified that some protection structures were not captured initially. This was most likely because they could not be identified on the available aerial photography.

Structure crest elevation, residual life and general condition is also uncertain / unknown in some locations.

Missing structures or structures that are present but in poor condition (that is unknown) will affect the inundation modelling, possibly flooding areas unnecessarily, or not allowing areas to flood that in reality might.

Uncertainty about elevation will have similar implications to the above.

Members of the project reference group assisted with adding additional information.

Structures with no crest elevation information were check with LiDAR and local stakeholders.

Aerial imagery

The most recent aerial imagery was flown in 2012. Historical imagery also exists for some areas from as early as the 1930s.

The ortho-rectification process and digitising of shorelines can introduce error. The level of error depends on the accuracy of ortho-rectification and scale at which shorelines are digitised. Ortho-rectification quality depends to some extent on the number and quality of reference points

Depending on the degree of horizontal error, this has an effect on the position of the shoreline at the time of capture and therefore values used for calculating coastal change.

It must also be noted that historical aerial images used in this study provide only a snapshot in time. Typically, it is not known if the image was taken during a calm or stormy period or before or after a coastal storm. This could lead to a false impression of a ‘stable’ shoreline.

Images were provided by the client (DELWP) and checked against existing information, and between images of different years.

No significant issues could be identified and images were utilised as supplied.

Geology

Surface Geological Mapping is available at 1:250,000 scale for the entire Bellarine Peninsula.

The accuracy of the dataset and level of information available for sub-surface geology does not allow for a fully accurate assessment of the possible available erodible (sand) volume of the open coast dunes/cliffs.

All available information, such as reports containing more localised information (e.g., Rosengren, 2010) were utilised in the assessment. This included information about calcarenite along the open coast between Barwon Heads and Point Lonsdale.

Geotechnical information

A number of geotechnical assessments have been undertaken for the study area previously.  The scope of these studies differs from the current study, therefore the information within has limited use as part of this assessment. 

The studies have been carried out for the ‘present day’, no sea-level rise scenarios have been considered. It may be more appropriate to link future geotechnical hazards to inspection by professional engineers, once significant change is determined through regular impaction by land managers. 

Geotechnical assessment is beyond the scope of this project, however where necessary it will be noted that future hazard and risk will be linked directly to engineer inspections. 

MODELLING

SWAN Modelling

Bass Strait

Modelling of waves in Bass Strait for relationship with Point Nepean measurements depended on bathymetry beyond the LADS coverage area and may not be accurate. Hindcast for extreme events has limited validation and only spans 33 years. There is 10 years measured wave data from Point Nepean.

There is moderate to high uncertainty in the extreme wave conditions, the limited data used could over or underestimate the extreme values calculated, which would have an effect on the erosion hazards determined. More data would increase confidence levels.

The values used were checked using the limited measured data, but this has too short a span to provide high certainty.

There are no data to validate the modelled relationships with the Point Nepean measurements and the study area. Both remain sources of uncertainty.

SWAN Modelling

Port Phillip Bay

No detailed data for directional waves available to validate modelling.

Hindcasting has been used to obtain extreme events, but that is limited since there are only 22 years of wind fields available at Point Wilson.

Waves are fetch limited, but effects of bottom friction and depth-limited wave breaking provide significant uncertainty. Therefore, the extreme conditions could be over or underestimated in individual locations, which would have an effect on the erosion hazards determined.  

Consistency checked (not validated) against Water Technology (2008) measured waves at Portarlington.

Inundation Modelling

(Static)

Depends on the accuracy of the LiDAR DEM and potential flow paths.

Depends on accuracy of design water levels.

Results likely to be conservative (overestimate inundation) for given water levels, as overland flow etc. are not considered.

Manual checking where possible of elevations and extents, cross-referenced against anecdotal information.

Inundation Modelling

(Dynamic)

Depends on the accuracy of the LiDAR DEM and structure information. Requires friction factors for overland flow.

Depends on accuracy of design water levels.

Results likely to be less conservative than the static model method.  

Manual checking where possible, cross-referencing against anecdotal information. Timing of inundation is a good addition of information to aid in subsequent risk assessments.

Storm events

Only one AEP event has been considered as the design scenario (a 1% AEP). This is a rare and significant event. More frequent events with a lesser impact are likely to occur, however, the implications of these, although possibly also significant, are not presented.

More frequent events, although smaller in magnitude have the potential to have a significant effect, especially in very low-lying or exposed areas. The concentration on a large infrequent event limits the amount of information available for short term management planning.

Where there is significant impact under a 1% AEP event with 0.0 m SLR, it may be necessary to assess more frequent return periods in subsequent assessments, especially where consequential risk may be significant. 

Mapping

Inundation hazard maps (static)

The inundation layers present the maximum extent based on the LiDAR topography, the detail of this is limited by the background data as well as the processing of data.

Where areas of inundated land at relevant elevations are located but not connected (flood path), these are edited out, however not at a resolution to remove all. Only large significant areas were removed. 

Some low-lying areas not connected could appear to be flooded with no connection to the ocean. In reality, these may be filled with runoff, so the significance is potentially low, but an uncertainty nonetheless. 

Maps are checked for connectivity, large obvious unconnected inundated areas are removed.

Inundation hazard maps (hydro-dynamic)

The areas are gridded to cells; therefore the extent is based on a series of grid-sized square cells. 

May be some slight elevation differences within the gridded cells, however only significant for the lower resolution models with larger grid cell sizes.

A prioritising exercise was undertaken early in the project, where the more detailed areas were targeted for a higher resolution model assessment (smaller grid cells).