Depth x velocity flood hazard in Tasmania: reading the model

A published flood model carries more than a single water level. It reports modelled depth, modelled velocity and, in most Tasmanian studies, a combined depth x velocity hazard class drawn from the Australian Rainfall and Runoff (ARR) hazard curves. Depth tells you how high the water reaches. Velocity tells you how fast it moves. The product of the two, depth multiplied by velocity (often written D x V, in square metres per second), is what sorts a site into a hazard category from H1 to H6, because a shallow, fast flow can be as dangerous to people and buildings as deep, still water. For a site in the North Esk floodplain at Launceston or along the Hobart Rivulet, the hazard class often matters more to a design than the flood level alone.

This walkthrough is for people who already read flood studies: surveyors setting a floor level, builders pricing a slab, and planners checking a site against the Tasmanian Planning Scheme. It assumes you know what an AEP is and goes to the detail and the edge cases.

What the three numbers are, and how they relate

A two-dimensional hydraulic model (TUFLOW is the common engine in Tasmanian studies) computes a water surface at every cell in the model grid for a given design event. From that it derives:

• Flood level, the water surface elevation in metres to Australian Height Datum (AHD).
• Depth, the flood level minus the ground elevation at that cell, in metres.
• Velocity, the depth-averaged flow speed, in metres per second.
• Depth x velocity (D x V), the hazard product, in m²/s, which the model maps to a hazard class.

Depth and flood level move together only if the ground is flat. On a sloping Launceston riverbank, two cells a few metres apart can share a flood level but have very different depths because the ground drops away. That is why a floor level set from a single reported level, without checking the depth at the actual building footprint, can be wrong by a significant margin.

The depth x velocity hazard classes

Most recent Tasmanian studies use the ARR 2019 hazard classification. The classes describe the vulnerability of people, vehicles and buildings as the D x V product and depth increase.

Class	Limit (approx.)	What it means on the ground
H1	D x V up to 0.3, depth up to 0.3 m	Generally safe for people, vehicles and buildings
H2	D x V up to 0.6, depth up to 0.5 m	Unsafe for small vehicles
H3	D x V up to 0.6, depth up to 1.2 m	Unsafe for vehicles, children and the elderly
H4	D x V up to 1.0, depth up to 2.0 m	Unsafe for people and all vehicles
H5	D x V up to 4.0, depth up to 4.0 m	Unsafe for people and vehicles; buildings vulnerable to structural damage
H6	D x V above 4.0	Unsafe for all; all building types vulnerable to failure

The limits are not simple thresholds on a single number. The classification takes whichever class is reached first as depth or D x V rises, so a deep, slow backwater and a shallow, fast overland flow can both land in H3 for different reasons. When a study reports a hazard class for a site, read the depth and velocity behind it, not the label on its own.

Why velocity is often the missing number

A council flood certificate or a planning overlay typically gives a flood level and sometimes a depth. Velocity is rarely on either. For most riverine sites on a wide, flat floodplain the velocity is low and the hazard is depth-driven. The exceptions are the ones that catch people out:

• Sites near a channel, a culvert or a constriction, where flow concentrates and speeds up.
• Overland flow paths between buildings, where a Hobart stormwater surcharge can run fast and shallow.
• Breakout flows where a levee or embankment is overtopped.

In these cases the D x V hazard can be a class or two higher than depth alone would suggest, which changes whether the structure needs to be designed for flow forces, not just inundation. The Launceston worked example steps through a site where the velocity, not the depth, set the design constraint.

Present-day versus future-climate runs

Many Tasmanian studies now report two sets of results for the same AEP: a present-day run and a future-climate run that applies a rainfall intensity increase and, for coastal and estuarine sites, a sea level rise allowance. Launceston's situation is particular, because the lower reaches are tidal and a storm surge on the Tamar can coincide with a flood coming down the North and South Esk. A model that couples the two will report higher levels for the future run than a rainfall-only uplift would.

When you pull a floor level or a hazard class, record which run it came from. The Tasmanian Planning Scheme's flood-prone areas provisions and a council's own freeboard policy may key off one or the other. Setting a floor to the present-day 1% AEP level when the controlling document references the future-climate level will leave the building short.

AEP and ARI: read the label

The 1% Annual Exceedance Probability event (a one percent chance of being equalled or exceeded in any year) is the common design event. It is the same event once called the 1 in 100 Average Recurrence Interval (ARI). Older Tasmanian studies and some council documents still use ARI. The two are close but not identical at frequent events, so confirm which convention a study uses before comparing it against another document. A study may also report the 0.5% AEP (1 in 200) and the Probable Maximum Flood (PMF), and the hazard class can jump sharply between them where a floodway is involved.

From the model to a floor level

A habitable floor level is normally the modelled flood level for the design event plus a freeboard allowance, checked at the building footprint rather than at the lot centroid. Three checks separate a defensible level from a guess:

1. Confirm the AEP and whether the level is present-day or future-climate.
2. Pull the depth at the footprint, not just the reported level, so the ground slope is accounted for.
3. Check the hazard class. An H4 or worse footprint may need the slab and structure designed for flow forces and debris, which a floor level alone does not cover.

None of this replaces a site-specific flood report from a hydraulic engineer or a survey of the actual ground level. A published model is a desktop starting point that tells you which event, which run and which hazard class to design against, and where the field work needs to focus.

Common questions

What is depth x velocity flood hazard? Depth x velocity flood hazard is the modelled flood depth multiplied by the flow velocity, in square metres per second, used to classify a site from H1 (safe) to H6 (all building types vulnerable to failure). It captures the danger of fast shallow flow as well as deep still water, which a flood level on its own does not.

What is the difference between flood level and flood depth? Flood level is the water surface elevation in metres to Australian Height Datum, and flood depth is that level minus the ground elevation at a point. On sloping ground two nearby points can share a flood level but have very different depths, so a floor level should be checked using the depth at the building footprint.

Does a flood model cover every Tasmanian property? No. A flood model only covers a property where a council or state study has been published for that catchment. Parts of Hobart, Launceston and Devonport are well modelled, but many rural and smaller-town catchments have no published model, in which case a flood level, velocity and hazard class cannot be reported for the site.

To see whether a published model with depth, velocity and hazard reaches a particular site, you can check whether a flood model covers the address, or if you need the zoning and overlays as well, 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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