Reading a Launceston flood model for one property: a worked example

To read a published flood model for a single Launceston property, you work through it in a fixed order: confirm which model reaches the lot, then read the modelled flood level, modelled depth, velocity and the depth x velocity hazard for each event the model reports, and finally compare the present-day run against any future-climate run. The model is not the planning overlay and it is not a council flood certificate. It is the engineering output the overlay was drawn from, and it carries the numbers a builder needs for floor levels and an owner needs for an insurance conversation.

What follows is an illustration. The property, the figures and the floor level are invented to show the method. No Launceston address, flood level or price in this article should be taken as real, and nothing here is engineering advice for a specific lot. Launceston is the right setting because much of the city sits on the floodplain where the North Esk and South Esk rivers meet the Tamar, behind a levee system, and because the published flood modelling there is detailed enough to show exactly how to read a model.

The site in this example

Imagine a single-storey weatherboard house on a flat block in one of the lower-lying Launceston suburbs near the river flats, inside the levee-protected area. The buyer wants to know two things: how exposed is this lot to river flooding, and what would that mean if they later lifted the floor or extended. We are not relying on the planning overlay alone. We want the model behind it.

Step 1: confirm a model actually reaches the lot

The first question is whether a published flood model covers this address at all. Coverage follows where a model has been built and released, not council or municipal boundaries, so a neighbouring street can be mapped while a pocket a few hundred metres away is not. For our example lot, assume a riverine flood model for the Tamar and Esk system covers it. If no model reached the lot, there would be no modelled depth or level to read, and the honest answer would be that the published modelling does not extend here.

This is also the point to separate the model from the Tasmanian Planning Scheme flood overlay. The difference between a flood certificate and a flood model report matters here: the overlay is a planning trigger drawn at one threshold, usually the 1% AEP extent, while the model holds results for several events and several outputs.

Step 2: read the flood level for each event

The model reports results by event, described two ways that mean the same thing. AEP is the annual exceedance probability, the chance the flood is equalled or exceeded in any one year. ARI is the average recurrence interval, the long-run average spacing between such floods. A 1% AEP flood is the 1 in 100 ARI flood.

The flood level is the modelled water surface elevation in metres to the Australian Height Datum (AHD), the national vertical reference. For our example lot, assume the model reports these levels at the property.

Event	AEP	ARI	Modelled flood level (m AHD)
Frequent	20%	1 in 5	below ground level
Moderate	5%	1 in 20	at or near ground level
Major	1%	1 in 100	above ground level
Rare	0.5%	1 in 200	further above ground level

The flood level is the number that drives floor levels. A builder compares the 1% AEP flood level in m AHD against the floor level in m AHD, adds the freeboard the planning scheme or council requires (freeboard is a safety margin above the modelled level), and reads off how high a new floor must sit. Depth near the house is a consequence of that comparison, not a substitute for it.

Step 3: read the modelled depth

Depth is the flood level minus the ground surface at a point, so it changes across the block. On our flat example lot the depth is fairly even; on a sloping block it would be deeper at the low corner. For the 1% AEP event, assume the model reports water over the yard and lapping the existing floor. That single figure reframes the buyer's question from "is it in the flood overlay" to "how far below the flood does the current floor sit, and by how much would it need to lift."

Step 4: read velocity and the depth x velocity hazard

Velocity is how fast the modelled water moves, in metres per second. Depth alone can be survivable; depth moving quickly is not. The model multiplies the two into a depth x velocity product, in square metres per second, and most Australian flood hazard frameworks sort that product into hazard categories from low through to unsafe for vehicles, then people, then buildings.

Behind the Launceston levee the picture has two faces. While the levee holds, velocities near this lot are low, because the floodplain fills slowly rather than flowing through. The hazard in that state may read as low to moderate even where depth is significant. The model should also be checked for how it treats a levee overtopping or failure scenario, because velocity and hazard change sharply once water enters the protected area. For our example, assume the standard run shows low velocity and moderate hazard at the 1% AEP event, with a separate note that the levee provides that protection up to a design standard.

Step 5: compare present-day and future-climate runs

Many published models now hold a present-day run and one or more future-climate runs that add rainfall intensity, sea level rise, or both. For a lot near the Tamar estuary, sea level rise matters because higher tailwater holds flood levels up for longer. Assume the future-climate run in our example lifts the 1% AEP flood level by a meaningful margin and increases the depth at the house.

That comparison changes the floor-level decision. Designing a new floor to the present-day 1% AEP level plus freeboard is one answer; designing to the future-climate level is a more durable one, and it is the number worth raising with a builder and with an insurer. If estuarine influence is the driver, the tide gauge sea level rise projections for the address give the context the flood model assumes.

What the model changed about the decision

Starting from "is this lot in the flood overlay," the model moved the buyer to concrete figures: a 1% AEP flood level in m AHD above the current floor, a modelled depth at the house, a hazard category that is moderate only because the levee holds, and a future-climate level higher again. Those are the numbers that let a builder price a floor lift and let an owner have a specific conversation with an insurer, rather than a vague one about being "in a flood zone."

Two limits stay in view. The model is regional-scale and does not survey the floor level of this house, so a floor-to-flood comparison needs the actual floor level in m AHD measured on site. And landslip, not flood, governs parts of Launceston's slopes; a lot on the valley sides would need the Tasmanian landslip hazard check instead of, or alongside, the flood model.

If you want to run this check on a real address, you can check whether a flood model covers the address, or if you need zoning, overlays and site context as well as flood, get a Property Report instead.

LayeredGeo's Flood Model Report collects every published flood model that reaches a property in the areas we hold across Queensland, New South Wales, Victoria, the ACT and Tasmania, with modelled depth, velocity and flood level for each event.

About this article. Published by LayeredGeo. It is general information about how property and ground conditions are assessed in Australia, not engineering, planning, legal or financial advice, and it is not specific to any property. Check anything that matters against the source dataset or a suitably qualified professional before you rely on it. If you spot something wrong, tell us at hello@layeredgeo.com.au and we will fix it.

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