The Scoping Document (Appendix A) detailed the intention of this study to consider trigger points in relation to hazards, rather than tying scenarios to a specific year. This includes producing maps that show potential inundation extents based on various sea-level rise scenarios, regardless of a year. This is in contrast to producing hazard maps that appear to ‘predict’ the position of the shoreline in a particular year, or the frequency and extent of inundation in a particular year. Generally, the better the background data, the better and more certain the assessment can be. For the Bellarine area, there are few thorough and consistent background datasets available, making a move to trigger points more reasonable and flexible. Although this is not common for hazard assessments, uncertainty related to the lack of background data and future rates of sea-level rise means that the prediction of a shoreline position or frequency of inundation in future years is too uncertain. This use of trigger points also allows for new data, particularly new rates of sea-level rise, to be incorporated as they become available. This will increase the certainty of assessments and provide additional confidence to managers that the planning decisions they are making are appropriate.
The triggers presented are tailored to each location based on the nature of the site, and knowledge of the site. The triggers are likely to be superseded when risk, options and mitigation studies are undertaken, or will remain valid for locations that are of lower priority, where subsequent studies won’t be undertaken for a number of years. It is recommended that risk assessment and planning be undertaken before the triggers are met.
The triggers are categorised into planning/investigation triggers and physical triggers. Planning triggers relate to an inundation extent under a particular sea-level rise scenario that alerts to a need for additional investigation and a potential management response. This is to ensure that land managers have a reasonable indication of when work needs to be started to address a forthcoming hazard. The physical triggers are to initiate a response to an imminent hazard, this may override a management trigger, e.g. if a storm of a magnitude greater than the modelled scenario was to occur, this may initiate an immediate emergency management response. In this context, actions are ways to mitigate against the effects of storms and increases in sea-level, either works or planning and management actions.
The aim is to allow managers to better prioritise responses, to make best use of coastal management funds and resources.
There are large areas of low-lying land throughout the study area. At present, these areas are only inundated under extreme events. With increases in sea-level, the frequency of inundation is likely to increase, with some areas likely to become tidal. Land managers require information as to when they should start taking action to mitigate the effects of this potential inundation. Note, that this is based on the occurrence of a 1% AEP event, the triggers provided do not account for events in excess of a 1% AEP. The assessment presents two inundation triggers:
Inundation trigger A - this is to trigger a management response when the measured sea-level rise reaches a certain threshold. Investigation or action should be taken to mitigate against the potential effects of the increase in sea-level. In this context, actions are ways to mitigate against the effects of storms and increases in sea-level, either works (e.g. protection) or planning and management actions (e.g. retreat).
When assessing inundation risk to assets (subsequent to this study), frequency of inundation (i.e. the likelihood) and the consequences will be considered. For example, a property that is inundated, on average, once every 100 years (i.e. in a 1% AEP event) may be considered to be a tolerable risk, however, a property that was inundated yearly, would not. Therefore it is appropriate to determine under which sea-level rise scenario the hazard becomes too great and needs to be dealt with. This will be different for different types of assets. The trigger tables give an indication of when action is required by land managers related to increases in local measured increases in sea-level. It is also noted, that the difference in saline and freshwater inundation should be considered when addressing inundation of certain assets, particularly natural assets, i.e. habitats, as resilience will be dependent on physiological tolerances. This finer detail will be part of the scope of subsequent studies that arise; however, this study initiates the process by identifying which events and sea-level rise scenarios trigger a response. Following from this, risk assessments and determination of adaptation responses will combine all relevant hazard and asset information to address the issues and environment as a whole.
Interpreting the Hazard Results
Prediction of Inundation Events
