Flood mapping
DWER’s floodplain mapping tool shows the defined flood event for land use planning — in most cases the 1% AEP flood — for twenty-six mapped study areas across Western Australia. It shows nothing anywhere else, and DWER says so explicitly: information on flooding or flood levels outside those areas is not currently available. Within the mapped areas, the advice relates to major river flooding only; stormwater drainage, environmental and ecological considerations are not addressed. Both limits matter more than the maps do.
What the tool covers
The mapping is published as a set of interlocking datasets rather than a single layer, and a full view of a mapped area requires twelve of them loaded together. The main ones are the floodplain area, the extent of flooding, the 1% AEP floodway and flood fringe areas, and flood level contours and points in metres AHD. The scenarios run from the 10% AEP event through to the probable maximum flood.
The two designations that carry planning weight are floodway and flood fringe. A floodway is the river channel and the portion of floodplain where significant flow or storage occurs during floods — obstructive development there raises upstream flood levels, so new buildings are not acceptable. A flood fringe is the rest of the 1% AEP floodplain, typically still or slowly moving water, where development can be permitted if it is compatible with the flood hazard. Some mapped reaches have a floodplain but no floodway and flood fringe split at all.
The twenty-six mapped areas
They are, in DWER’s own index: the Avon River from Toodyay to Beverley; the Blackwood River at Augusta, Bridgetown, Nannup and Boyup Brook; the Bow River at Bow Bridge; the Capel River at Capel; the Chapman River at Geraldton; Cohn Creek at Merredin; the Collie River at Collie; Corrigin townsite; the Denmark River at Denmark; the Fitzroy River at Fitzroy Crossing; the Gascoyne River at Carnarvon and the lower Gascoyne; the Gordon River at Tambellup; the Greenough River from the Indian Ocean to Walkaway; Gribble Creek at Kalgoorlie; the Harding River at Roebourne; the Irwin River at Dongara; the Margaret River at Margaret River; the Murray River from Ravenswood to Blythewood; the Nullagine River at Nullagine; the Preston River at Boyanup and Donnybrook; the Serpentine River and the Peel Inlet and Harvey Estuary; the Swan and Canning rivers and tributaries from Fremantle to Walyunga; Toby Inlet at Quindalup; the Vasse-Wonnerup estuaries, Broadwater and New River at Busselton; and Willyung Creek and Yakamia Creek at Albany.
Read that list against a map of Western Australia and the shape of the problem is obvious. The mapping is townsite-scale and major-river-focused. It was never designed to be landscape coverage, and it is not.
What that means in the Great Southern
Four of the twenty-six sit in the Great Southern: the Denmark River at Denmark, Willyung Creek and Yakamia Creek at Albany, and the Gordon River at Tambellup. So in the City of Albany, mapped floodplain covers two creeks and nothing else — the Kalgan, the King, the Hay and the Oyster Harbour catchments are unmapped. In the Shire of Denmark, only the Denmark River is mapped. In the Shire of Plantagenet, nothing at all: Mount Barker, Kendenup, Narrikup, Porongurup and Rocky Gully have no published flood level.
Walpole has none. Esperance has none. Katanning, Kojonup, Cranbrook, Gnowangerup and Jerramungup have none. For most sites in this region, the honest answer to "what is the 1% AEP flood level here" is that nobody has calculated it.
The scope note that catches people inside the mapped areas
Every DWER floodplain dataset carries the same qualification: the advice is related to major river flooding only, and other issues such as stormwater drainage and environmental and ecological considerations are not addressed. That is not boilerplate. It means the mapping answers one mechanism and is silent on the others.
On the south coast the others are not marginal. Estuarine flooding from a closed sandbar, tidal and storm-surge inundation from the seaward side, and overland stormwater flow from an intense short-duration storm all put water on the ground independently of riverine flood levels. On 14 March 2025 Albany recorded 126.4 millimetres in a day — its highest daily fall in a record beginning in 1877 — with the Bureau reporting many sites taking 30 millimetres in half an hour to an hour. That is a stormwater event. No riverine flood map would have described it, and the Bureau’s riverine flood warning service, scoped to catchments where rain-to-flood times run six hours or more, would not have warned of it either.
DWER also states that the maps should not be used by the public to make decisions during a flood emergency, and that they are intended for general interest and as a land use planning guide. Site-specific flood advice comes from DWER at flood@dwer.wa.gov.au — within the areas where DWER holds information.