What is overland flow, and is it a flood overlay?

Overland flow is rainwater that runs across the surface of the ground when a heavy downpour is more than the pipes, gutters and drains can carry. It follows the low line of the land through streets and backyards, and it can reach blocks nowhere near a creek. Whether it counts as a flood overlay depends on the council: some map it inside their flood overlay, some as a layer of its own, and some have yet to map it. A mapped path is a warning that water crosses the lot, and it is not a ban on building.

How is overland flow different from a river flood?

A river or creek flood comes from a waterway rising and spreading over the land beside it. Overland flow comes from the rain falling on the suburb itself, so it tends to arrive quickly, run along a path and drain away sooner. Councils can set different rules for each. The post on flood overlays covers the wider picture.

How do I know if my lot has an overland flow path?

Start with the council’s flood mapping or a property report from the council. The flood overlay check reads the mapping from an address. Then look at the ground: a dip running between houses, a stormwater easement, a fence line where water stands after rain. Not every council has studied every catchment, so a lot without mapping can still carry water.

What does an overland flow overlay ask for?

a floor level above the water level the council gives
the flow path kept clear of solid walls, sheds and stored goods
fences that let water through
fill and raised slabs that do not push water onto a neighbour
materials below the water level that can take a soaking

An overlay can also change how the work is assessed, so a job that was simple on another lot may need a council decision here.

Can I build across an overland flow path?

Sometimes. The council weighs what the structure does to the water. An open structure on posts lets water pass where a solid walled shed or a raised slab blocks it, so the two can get different answers. Moving the work out of the path is often the simplest fix. Paving and new hard surfaces matter too, because they send more water off the lot and faster.

What about the neighbours?

Changing how water leaves your lot, so that it now harms a neighbour, can lead to a dispute or a council order.

Reading it for your own lot

RapidSiteplan reads the council and building rules for your lot from its address, checks the design against them as you place it, lets you edit it as often as you like, and prints the site plan. Site plans and assessments in minutes. Where the council’s flood mapping covers overland flow, it is shaded over your lot, so you can move the structure out of the path before the design settles.

Search your address, or see what the plan shows for a shed, carport or other outbuilding. The Brisbane page lists what we hold for that council, and site plans by council covers the rest of Australia.

Common questions

What is overland flow, and is it a flood overlay? Overland flow is rainwater running across the ground when a heavy downpour is more than the drains can carry. Some councils map it inside their flood overlay, others as its own layer. Either way it means water can cross the lot, and it changes what the council asks to see.

What does an overland flow overlay ask for? Usually a floor level set above the water level the council gives, a flow path kept clear, and fences and fill that do not hold water back or push it onto a neighbour. On bigger work the council can ask for a report from a civil engineer.

Can I build across an overland flow path? Sometimes, and the council decides. A structure that lets water pass is easier to support than a solid one that blocks it, and moving the work out of the path is often the simplest answer.