Australian flood data coverage is the hard part

Bushfire mapping in Australia is national and reasonably consistent. Coastal change is measured from satellite imagery with uniform coverage. Heat projections come from the same CMIP6 ensemble everywhere.

Flood is different, and it is the hazard finance teams ask about first.

Why it is uneven

Flood studies are commissioned by local councils, usually catchment by catchment, on their own schedules and budgets. A well-resourced metropolitan council may have probabilistic modelling covering every watercourse, updated within the last five years. A neighbouring shire may have a single 1990s study covering one creek, never digitised.

The studies that do exist are not directly comparable. Some publish flood planning area boundaries, some publish design flood extents at specific annual exceedance probabilities, some publish depth grids. A flood planning area is a planning instrument with freeboard built in; a 1% AEP extent is a modelling output. Treating them as the same layer produces a confidently wrong answer.

What we do about it

Three rules, applied consistently:

Name the study. Every flood result records which study produced it, its adoption year, and what kind of layer it is. If two sites in the same portfolio were assessed against different study types, the appendix says so.

Do not interpolate across gaps. Where a site sits in an LGA with no usable study, it is flagged unassessed. It does not get the state average, and it does not get "low" by default. In a recent 240-site run, 51 sites came back unassessed for flood — a fifth of the portfolio, and a fact the reporter needed to know.

Distinguish the layer types in the output. Planning-instrument boundaries and modelled extents are reported as separate columns rather than merged into one rating.

Why this is the right trade

It is tempting to fill the gaps. A complete-looking table is easier to put in front of a board than one with a column of blanks. But an unassessed site is a real finding — it tells you where to spend money on a study, and it is honest about the limits of what public data can tell you.

An assessment that hides its gaps is not a better assessment. It is the same assessment with the uncertainty moved somewhere you cannot see it.