Hydrology & flood engineering
Hydrologist — Great Southern, Western Australia
Overview
Fiord Group is a hydrology and flood engineering practice based in Denmark, Western Australia, working across the Great Southern — the eleven local government areas running from Denmark and Albany through Plantagenet, Cranbrook, Kojonup and Katanning out to Gnowangerup, Jerramungup and the Kent. Principal Blake Marxsen — MIEAust, BEng (Environmental) with Honours — is a hydrologist and environmental engineer working in flood studies, catchment and floodplain modelling, drainage and surface water design, and the hydrological work that sits underneath DWER, EPA and local government approvals.
Services in Great Southern
- Flood studies and flood impact assessment for development applications — Flood Impact Assessment
- Catchment and floodplain modelling to Australian Rainfall & Runoff (ARR) 2019 — Flood Hydrology & Modelling
- Surface water management plans, drainage design and erosion control — Surface Water Management
- Culvert, levee and basin design through to construction drawings — Hydraulic Structure Design
- Watercourse realignment and creek diversion design — River & Creek Diversions
- Flood immunity assessment for roads, rail and critical infrastructure — Flood Resilience of Infrastructure
- Hydrological input to DWER and EPA Western Australia approvals — Hydrology for Approvals
Most of the Great Southern has no published flood level
The Great Southern is eleven local government areas across about 39,000 square kilometres, with roughly 250 kilometres of Southern Ocean coastline. Hydrologically it is two DWER river basins: the Denmark Coast basin, 2,651 square kilometres draining to Wilson Inlet, and the Albany Coast basin, 22,989 square kilometres running east from Albany to Bremer Bay and containing about fifteen river systems, the largest of them the Pallinup, the Kalgan and the Fitzgerald.
The single fact that matters most to anyone developing here is a gap in the public record. DWER floodplain mapping covers twenty-six study areas across the whole of Western Australia. Four of them are in the Great Southern: the Denmark River at Denmark, Willyung Creek and Yakamia Creek at Albany, and the Gordon River at Tambellup. Mount Barker, Katanning, Kojonup, Cranbrook, Gnowangerup and Jerramungup have none. DWER states the position plainly in the dataset itself — information on flooding or flood levels outside those areas is not currently available — and adds that the mapping which does exist addresses major river flooding only, with stormwater drainage expressly outside its scope.
That gap has become a planning obligation. State Planning Policy 2.9 Water, operational from 18 December 2025, requires a proposal to identify flood-prone land, to keep development out of a defined floodway, to maintain the free passage and temporary storage of floodwaters, and to set habitable floor levels at least 0.5 metres above the 1% AEP flood. Where flooding is likely and no mapping exists, the policy requires DWER advice. On most sites in this region the 1% AEP flood level does not exist as a published number — it has to be derived, site by site, before any of those tests can be applied.
The physical setting rewards local knowledge in two ways. Rainfall falls away sharply from the coast — Albany averages 921 millimetres over a record running from 1877, Mount Barker 721 millimetres, Katanning 478 millimetres, and Jacup in the Shire of Jerramungup 435 millimetres — so a design rainfall carried inland from a coastal gauge is wrong by a factor approaching two. And the landscape is not static: clearing has driven stream salinity to 16,800 mg/L on the Pallinup and 4,700 mg/L on the Kalgan against 750 mg/L on the far less cleared Denmark, on catchments 81, 62 and 18 per cent cleared respectively. Rainfall itself has stepped down — the Bureau records a 10 to 20 per cent fall in winter rainfall across the south-west since around 1970, and describes it as a step change rather than a gradual trend.
There is also very little warning infrastructure to lean on. The Bureau operates information locations on the Denmark River but no forecast service; the Kent, the Frankland and the wider Albany Coast are data collection only. The riverine warning service is scoped to catchments where rain-to-flood times run six hours or more. 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 30 millimetres in half an hour to an hour at many sites. That is a stormwater and flash-flood problem, not a riverine one, and it sits outside both the warning service and the mapping.
The region has almost no published flood information and almost no resident specialist capacity to produce it, which is why most Great Southern flood work is currently commissioned from Perth. Being based in the catchment is the difference between knowing that a rainfall gauge exists and knowing whether the number it produced is the one your site actually sees.
Common questions
- Is there a hydrologist in the Great Southern?
- Yes. Fiord Group is a hydrology and flood engineering practice based in Denmark, Western Australia, working across the Great Southern. Principal Blake Marxsen (MIEAust, BEng Environmental Honours, Murdoch University) is a hydrologist and environmental engineer specialising in surface water — flood hydrology, catchment and floodplain modelling, surface water management and hydraulic design.
- Does my Great Southern property have a mapped flood level?
- Probably not. DWER floodplain mapping in the Great Southern covers only the Denmark River at Denmark, Willyung Creek and Yakamia Creek at Albany, and the Gordon River at Tambellup. DWER states that flood level information outside its mapped areas is not currently available, and that the mapping addresses major river flooding only — not stormwater drainage. Everywhere else, the 1% AEP flood level has to be derived for the site.
- What does State Planning Policy 2.9 require for flooding?
- SPP 2.9 Water, operational from 18 December 2025, requires proposals to identify flood-prone areas, avoid rezoning or intensifying land use within a defined floodway, maintain the free passage and temporary storage of floodwaters, set habitable finished floor levels a minimum of 0.5 metres above the 1% AEP flood event, and obtain DWER advice where flooding is likely and no flood mapping exists.
- Which shires does Fiord Group cover in the Great Southern?
- All of them — Albany, Denmark, Plantagenet, Cranbrook, Katanning, Kojonup, Broomehill-Tambellup, Gnowangerup, Jerramungup, Woodanilling and Kent — from the practice base in Denmark. Work also extends west to Walpole, east to Esperance, and across Western Australia and the Northern Territory on larger flood and water projects.
- What is the difference between a hydrologist and a hydrogeologist?
- A hydrologist works with surface water — rainfall, runoff, catchments, rivers, flooding and drainage. A hydrogeologist works with groundwater — aquifers, bores, water table behaviour and groundwater licensing. Fiord Group is a surface water practice. If your question is about a bore, a groundwater licence or soil testing for effluent disposal, you want a hydrogeologist.
Sources
Every catchment figure on this page comes from a published agency record. Where two agencies publish different numbers for the same catchment, the more recent is used and the earlier is stated alongside it rather than quietly dropped.
- DWER — FPM Map Index (floodplain mapping coverage)
- WAPC — State Planning Policy 2.9 Water (2025)
- DWER Healthy Rivers — Albany Coast and Denmark Coast basins
- Stream salinity status and trends in south-west WA (SLUI 38)
- Bureau of Meteorology — Western Australia in 2025
- Great Southern Development Commission — our region