Flood Mapping: What a Flood Study Needs From Site Data

If you came here to find out whether a particular address floods, this is not the page, and we would rather say so in the first line than waste your time. Official flood mapping in Australia is published by councils and state floodplain authorities, not by aerial operators. The next section points you at the right door.

If you are on the other side of it — an engineer, developer, council officer or consultant needing accurate terrain data to feed a flood study or overland flow assessment — that is what this page is about. We capture and process the elevation data underneath the modelling. We do not do the modelling.

Where to find official flood mapping for an address

Published flood information comes from the authority responsible for the floodplain, and it is usually free or close to it.

Start with the planning scheme. In Victoria, flood controls appear as overlays — Land Subject to Inundation, Special Building and Floodway — visible on VicPlan or in the free planning property report for the address. Other states publish equivalent controls through their own portals.

Then the floodplain authority. Across greater Melbourne that is generally Melbourne Water; in regional Victoria the relevant catchment management authority; elsewhere the equivalent state agency. They hold the flood studies the overlays are drawn from, and most will issue a property-specific flood level on request. If you need a number for a permit, this is where it comes from.

Your council. Planning and drainage teams hold local drainage and overland flow information that never makes it onto a state map, and they assess your application. VICSES local flood guides add context on how a catchment behaves in a real event, though they are emergency documents rather than design inputs.

None of those sources need us. Everything below is for when the published mapping is not enough.

Published flood overlay vs site-specific survey data

These two get confused constantly, and the confusion costs people time.

A published flood overlay is a regulatory, catchment-scale product, derived from a model covering a whole catchment and built on the best terrain available when the study was done — often state-wide LiDAR captured years ago at a coarse grid. It answers a planning question across thousands of properties: is this land subject to inundation, and what controls apply. It does that job well.

What it is not designed to do is describe one site to the tolerance a drainage design needs. At catchment scale, a 300 mm kerb upstand, a raised crossover, a new retaining wall or a levelled building pad is not represented. At site scale, those features frequently decide where water goes.

Site-specific survey data is the opposite: fine-grained, current, captured for one project. It does not replace the overlay — that remains the regulatory position. It gives your engineer a surface accurate enough to model what happens on the ground, and shows how the site has changed since.

What a flood study needs from terrain data

Whatever software your engineer uses, a flood or overland flow model is only as good as the ground surface underneath it. Four things matter.

Bare earth, not the top of everything

A digital surface model (DSM) is the top of the visible world — canopy, roofs, vehicles, stockpiles. A digital terrain model (DTM), or bare earth, has those stripped away so only ground remains. Both are types of digital elevation model, which is why “send us the DEM” is an ambiguous request and the right follow-up is always: surface, or bare earth?

Flood modelling needs bare earth. Run a 2D overland flow model on a DSM and water routes over hedges, sheds and parked cars, ponds behind vegetation with no hydraulic effect, and finds paths that do not exist. The result looks plausible and is wrong.

Grid resolution and contour interval

A terrain model’s cell size sets the finest feature it can represent. A coarse grid averages a kerb-and-channel into a gentle slope. On a flat site — where 100 mm decides which way a flow path turns — that averaging is the difference between water running down the driveway and water entering the building.

The same applies to a contour survey: a wide interval suits a steep rural site and is useless on a flat suburban block. Interval and grid resolution should be set by what the study needs to resolve — a conversation to have before the flight, not after delivery.

Breaklines and the features that redirect overland flow

This is the part most often left out of a brief. A surface built from points alone quietly rounds off every sharp edge on the site. Breaklines are hard lines forced into the surface where the ground changes slope abruptly — the face of a kerb, the invert of a table drain, the lip of a driveway crossover, the top and toe of a batter, the base of a retaining wall.

For overland flow work these edges are the controls, not decoration. What steers water across a suburban site in a real storm is kerbs, crossovers, thresholds, fence plinths, garden edging, raised slabs and the fall of a paved area. A model built on a smoothed surface loses all of them and produces sheet flow where reality produces a channel. Tell us what matters hydraulically and we will pick it up as breaklines and 3D strings.

Currency

Regional flood mapping is updated periodically, not continuously, while the site keeps changing — earthworks, a new pad, a retaining wall, a filled hollow, a new estate upstream. Terrain that predates those changes describes a site that no longer exists. If the site or its surrounds have been worked since the regional data was flown, that alone is usually reason enough to capture current terrain.

What we supply, and what we don’t

We supply the terrain layer:

• DEM / DTM raster (GeoTIFF) — a gridded elevation surface at an agreed cell size, bare earth or surface, in your project datum. This is what loads into hydraulic modelling software.
• Contours and 3D strings (DXF/DWG) — contours at the interval you specify, plus spot levels, breaklines and feature strings for Civil 3D, 12d or AutoCAD.
• Point cloud (LAS/LAZ) — the full measured dataset, so your team can reclassify or re-derive surfaces.
• Orthomosaic backdrop — a geometrically corrected, scaled aerial image. Under a flow-path plan it makes the result legible to a client or council officer, because everyone can see the fence, the driveway and the pit the flow path runs past.

We do not do the rest. We are not hydrologists and we do not perform flood modelling, produce flood studies, issue flood certificates or advise on flood risk. A civil or hydraulic engineer does the modelling; a licensed land surveyor certifies levels where certification is required. Our job is to make sure the surface they start from is accurate, current and in the format they asked for.

When aerial capture suits this work — and when it doesn’t

It suits open and lightly vegetated sites: subdivisions, development sites, industrial land, road corridors, drainage reserves, car parks, sporting fields, construction sites and rural blocks with scattered trees. It suits large or awkward-access areas, and repeat capture before and after works.

It does not suit four cases worth knowing before you engage anyone:

• Dense continuous canopy. Photogrammetry measures what the camera can see. Under thick cover the bare-earth surface is interpolated from ground visible nearby rather than measured — and that matters here, because a flow path through vegetated riparian land may be the most important part of the model. Laser scanning handles it better. On a partly treed site we say which areas are measured and which are inferred.
• Water surfaces. Open water does not photogrammetrically reconstruct. It moves and reflects, offering nothing consistent to match between images, so dams, creeks and retarding basins come out as noise. Anything needing bathymetry — the ground under water — needs a different method.
• Certified levels for a statutory submission. If your submission needs levels certified by a licensed surveyor, engage one. Aerial data sits alongside that work; it does not substitute for it.
• Pits and underground drainage. Floor levels, pit inverts and pipe details are measured on the ground, not from the air.

The arrangement we see most often: a licensed surveyor picks up certified levels and critical hydraulic structures, we fly the dense surface in between, and the engineer models on the combined dataset.

Briefing a capture

Four things told to us up front save rework: the coordinate system and datum, the grid resolution and contour interval, the hydraulic features to capture as breaklines, and the software the deliverables are loading into. If your engineer is engaged, put us in touch and let the two of us settle the specification. We are a CASA-licensed operator based in Hampton Park, south-east Melbourne; raise airspace early, since sites near an aerodrome or in controlled airspace need approval under CASA rules before anything flies.

General information only — not engineering, hydrological or planning advice. Flood risk decisions should be made with a qualified civil or hydraulic engineer and the relevant council or floodplain authority.

Frequently asked questions

Where do I find official flood mapping for my property in Australia?

Start with your state planning portal and council planning property report — in Victoria, VicPlan shows the Land Subject to Inundation, Special Building and Floodway overlays. For a property-specific flood level, contact the floodplain authority: Melbourne Water across greater Melbourne, the relevant catchment management authority in regional Victoria, or the equivalent state agency elsewhere.

Can a drone survey replace a flood study?

No. A flood study is hydraulic modelling performed by a qualified engineer or hydrologist. Aerial capture supplies one input — a current, accurate bare-earth terrain model. The engineer combines it with rainfall data, catchment parameters, drainage information and floor levels. Terrain data does not produce a flood study on its own.

What is the difference between a DSM and a DTM for flood modelling?

A digital surface model (DSM) includes everything visible from above — canopy, roofs, vehicles, stockpiles. A digital terrain model (DTM), or bare earth, has those removed so only ground remains. Flood and overland flow modelling needs the DTM: run a model on a DSM and water routes over hedges and sheds and follows paths that do not exist.

Why isn’t the council flood overlay enough for a site design?

The overlay is a regulatory, catchment-scale product, often built on terrain captured years earlier at a coarse grid. It answers a planning question well. It does not resolve the 100–300 mm features — kerbs, crossovers, thresholds, retaining walls, recent earthworks — that frequently decide where water goes across a single site.

Do you provide flood certificates or flood risk advice?

No. We are a CASA-licensed aerial data capture operator, not hydrologists, engineers or licensed surveyors. We do not perform flood modelling, issue flood certificates or advise on flood risk. Flood levels and certificates come from your council or floodplain authority; modelling comes from a civil or hydraulic engineer. We supply the terrain data underneath.