This tool is designed to explore the potential implications of tidal inundation (otherwise known as nuisance inundation or sunny day flooding) along the NSW coast. This type of inundation typically occurs during higher tides in some low-lying sites adjacent to estuaries (see Figure 1 for an example)

These inundation events involve minor, short lived (1-3hrs), flooding of roads, paths and driveways usually over successive days of the spring tide, although they suggest more widespread and significant impacts ahead as sea levels rise.

In many instances tidal waters are able to enter the stormwater system and emerge in street gutters flooding streets and paths. Some Councils in NSW already take action to warn of potential flooding associated with predicted king tides. Some have installed tidal gates or flaps to limit ingress of tidal waters into the stormwater system to limit the associated flooding.

The tool leverages the extensive network of tide gauges in NSW and enables users to investigate the potential implications of sea level rise using water level exceedance data from their local tide gauge.

The tool enables users to explore projected future changes to the frequency of inundation above a user defined threshold. By changing the threshold level users are able to explore the potential benefits of various adaptation options (eg raising/filling or building small levee walls).

The tool requires users to set a threshold level at which inundation is expected to occur. There are several options available to do this, including:

• First-hand observation of inundation and cross matching against measured inundation levels on the nearest gauge
• Cross match the date/time of photographs of tidal inundation (for example sourced by undertaking web searches or accessing sites like Witnesskingtides.org) against the measured inundation on the nearest gauge (see Figure 1)
• Local surveying to determine the levels of low points in streets and gutters and overland inundation thresholds
• High resolution mapping using LiDAR point could data which is available for NSW via ELVIS (elevation.fsdf.org.au)

Doing this enables users to understand how frequently inundation above the threshold level is occurring now, including the extent of interannual variability based on observed variability in annual water level exceedance distributions.

It also provides users with projections, at decadal intervals, of future increases to inundation frequency associated with each of the sea level rise scenario’s undertaken for the UN Intergovernmental Climate Change Sixth Assessment Report (AR6). These sea level rise scenarios have been extracted from the nearest grid cell using the NASA AR6 sea level rise projection tool.

Setting higher thresholds (e.g. of low-lying assets or overland inundation thresholds) enables users to explore when assets or overland inundation will begin occurring if initial action is taken to limit high tidal inundation through the stormwater system (eg by installing non return flaps and floodgates).

It also enables users to explore the potential benefits and longevity of other interventions like building small levees to limit overland inundation or raising and filling.

This tool is designed for nuisance inundation planning in the medium-to-long-term (10-50+ years). It uses the latest information from NSW Government's coastal water level gauge network as well as five potential sea-level rise (SLR) scenarios from the latest IPCC report in order to estimate the likelihood of future inundation due to normal tidal processes (i.e. not flood events)

The below image shows a numbered diagram of the tool's interface, with a description of each component following:

1. Station selector: Choose from the dropdown which water level station to use for the forecast. A full list of all stations can be found on the MHL homepage. You can filter sites by typing either the station's number or name in the input field, then select the station by clicking on it.
2. Threshold selector: This field displays the currently selected flood threshold level. The plotted data relates the number of projected days where the tidal level will exceed this level. You can type the desired level into this field or use the slider in the following component.
3. Threshold slider: The slider provides an alternative way to select the desired flood threshold level, displayed in the threshold selector above. You can drag the slider bar up and down (or left and right on mobile) to select the desired flood threshold level. The box next to the slider will update with the currently selected level as you move it around, and the plot will update on release of the slider.
4. Uncertainty selector: With these radio buttons you can elect to only display the projected increase in inundation frequency associated with the median (50%) sea level rise projections, or alternatively, include interannual variability (associated with observed year to year variability in the water level distribution), or the 17-83% confidence interval of the projected sea level rise.
5. Download button: You can click this button to download the currently plotted data in CSV format for your own purposes. You will need to accept the disclaimer and user agreement before downloading will commence.
6. Scenario selector: You can toggle the visibility of each of the various sea level rise scenarios (called SSP’s – Shares Socioeconomic Pathways) on the plot. This does not impact downloading data.