Hydrology & flood engineering
Insights
Overview
Answers to the questions that come before a commission. Every piece here is written against a checkable regulatory or hydrological fact, with the primary sources listed at the foot of each page, and every question is answered on the page rather than gated behind an enquiry form.
Why this section exists
The reason this section exists is narrow. In Western Australia the honest answer to most flood and drainage questions is that it depends — and almost nobody publishes what it depends on. State planning policy changed at the end of 2025 and renamed the entire water management report family. DWER floodplain mapping covers twenty-six areas in a state of two and a half million square kilometres, and says so only inside a dataset description. Whether a farm dam needs a licence turns on a phrase — no sensible reduction — that is a hydrological calculation wearing legal clothing.
The same is true one level up, in environmental impact assessment. The EPA assesses water under a factor called Inland Waters, whose objective is a single sentence and whose issues section names the specific things it keeps encountering — buffers to wetlands and waterways in the South West, surplus water discharged to creeks, potash proposals on salt lakes, pit lakes, and a forty-year decline in South West runoff and recharge that every baseline now has to be stated against. That guidance is public, current, and almost never quoted back in plain language.
None of that is secret. It is just scattered across agency PDFs, superseded guidance and scheme text, in a form that assumes you already know which document to open.
Articles
All insights
- Do I need a flood study for a development application in Western Australia?ApprovalsIn Western Australia there is no single lot size or setback distance that triggers a flood study. A flood study is required, in practice, when one of four things is true: the site sits in or near a mapped floodway or flood fringe; the local planning scheme prohibits building on land the local government defines as liable to flooding or inundation and the site may be such land; the proposal changes drainage, fill levels or overland flow paths; or State Planning Policy 2.9 Water applies and flooding is likely at a site with no published flood mapping, in which case DWER advice must be obtained.
- What the WA floodplain mapping tool shows — and where it shows nothingFlood mappingDWER’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.
- SPP 2.9 Water: what changed, and what an LWMS is called nowPolicyState Planning Policy 2.9 Water was published on 16 December 2025 and became operational on 18 December 2025. It supersedes nine instruments, including Better Urban Water Management (2008), SPP 2.9 Water Resources, the Government Sewerage Policy and the flooding provisions of SPP 3.4.
- Albany’s record rainfall, 14 March 2025 — and what it means for designGreat SouthernOn 14 March 2025 the Bureau of Meteorology’s Albany station recorded 126.4 millimetres in twenty-four hours. That is the highest daily rainfall in a record that began in 1877, beating the previous all-time figure of 113.6 millimetres set on 21 November 2008 and dwarfing the previous March record of 89.7 millimetres from 1917.
- Do you need a licence for a farm dam in Western Australia?Rural waterIn Western Australia a landholder may generally build a dam that captures runoff without a licence, provided four conditions hold: the water is not stored on a watercourse or wetland; there is no sensible reduction in the volume of any watercourse or wetland; there are no significant environmental effects; and local by-laws are met. Inside a proclaimed surface water area, taking water requires a licence.
- Hydrologist or hydrogeologist? Which one you actually needGetting the right helpA hydrologist works with surface water: rainfall, runoff, catchments, rivers, flooding and drainage — everything that happens on top of the ground. A hydrogeologist works with groundwater: aquifers, bores, water table behaviour, groundwater licensing and contamination transport — everything below it.
- The tailwater problem: why south coast flood models fail reviewMethodOn the Western Australian south coast, most estuaries are bar-built and seasonally closed: a sandbar shuts the mouth for much of the year and is breached, often artificially, when levels rise. That makes the downstream boundary condition of any hydraulic model a variable rather than a constant.
- What is the EPA’s Inland Waters factor, and when does it apply to a proposal?EPA approvalsInland Waters is one of the environmental factors the Environmental Protection Authority uses to assess proposals in Western Australia. The EPA defines it as the occurrence, distribution, connectivity, movement and quantity of inland water — the hydrological regimes — together with its chemical, physical, biological and aesthetic characteristics, which is the quality.
- Can you discharge surplus mine water to a creek in Western Australia?Mining & inland watersNot as a first option, and the EPA says so directly. Its stated position is that disposal of excess water to waterways should only be undertaken after other potential uses have been maximised — including mitigating environmental impacts, fit for purpose on-site activities, meeting the demands of other water users, and aquifer reinjection.
- How big does a buffer to a wetland or waterway have to be in Western Australia?WetlandsThere is no default distance. The EPA publishes no buffer width in its Inland Waters guideline — it asks proponents to apply the mitigation hierarchy to any proposal near a wetland or waterway, and to conduct appropriate buffer studies to justify whatever buffer they propose.
- Potash on a salt lake: the inland waters questions that decide the approvalSalt lakesThe EPA has assessed a run of referrals from proponents seeking to abstract potash-rich brines from the surface water and groundwater associated with salt lakes in the more arid parts of Western Australia, and it names three issues of particular interest. Disturbance of the lake surface that may change flooding regimes, leading to inundation of areas outside the lake surface, with saline water, that are not normally inundated.
- What the Coastal Great Southern drought plan says about farm waterDrought resilienceThe Coastal Great Southern Regional Drought Resilience Plan covers the City of Albany, the Shires of Denmark and Plantagenet, and the Frankland River sub-region. It is jointly funded by the Australian Government’s Future Drought Fund and the Department of Primary Industries and Regional Development, and delivered by the Great Southern Development Commission with DPIRD.
- Does a mine need a works approval in Western Australia?Works approvalsIt depends on whether any part of the operation is a prescribed premises, and that is a question about categories and thresholds rather than about scale or intent. Schedule 1 of the Environmental Protection Regulations 1987 lists the categories, each with a production or design capacity at which the premises becomes prescribed.
- Why does an EPA approval not let you start building?Approvals sequenceBecause the two approvals answer different questions, and only one of them is about the thing being built. An assessment under Part IV of the Environmental Protection Act 1986 decides whether a proposal may be implemented and subject to what conditions.
- Why does a water storage sized at referral stage not fit the approved footprint?Mine water designBecause a volume is not a footprint. A storage sized at referral stage is usually sized volumetrically — operating volume plus an allowance for the design storm — and that number is what the clearing figure gets drawn around.
- Which flood event should a site be designed for?Design eventsThere is no default flood event. The 1% AEP is the most commonly quoted because planning policy attaches habitable floor levels to it, but it is a planning threshold, not an engineering answer, and it governs only the assets whose consequence of failure matches it.
- How do access tracks and haul roads change flood behaviour?Linear infrastructureOn a site where the built footprint is spread thinly across a large area — a wind farm, a solar farm, a mine haul route, a rural service corridor — the flood risk is rarely to the structures. It is to the linear infrastructure connecting them, and to the flow paths that infrastructure crosses, often hundreds of times.
- When should the water modelling happen on a subdivision?Urban waterBefore the lot layout is drawn, not after. A water management plan prepared once the layout is fixed can only demonstrate compliance with a design that has already been committed to, which means every storage, swale and flow path has to be fitted into land that was allocated to something else.
- What does El Niño mean for hydrology in Western Australia?Climate driversLess than the headlines suggest, and not the thing most people assume. El Niño’s clearest Australian rainfall signal is a drier winter and spring across the eastern and northern parts of the continent, and the Bureau of Meteorology’s own statement of El Niño impacts does not extend that signal to the south-west.